CA1280493C - Radio direction-finding using time of arrival measurements - Google Patents
Radio direction-finding using time of arrival measurementsInfo
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- CA1280493C CA1280493C CA000539346A CA539346A CA1280493C CA 1280493 C CA1280493 C CA 1280493C CA 000539346 A CA000539346 A CA 000539346A CA 539346 A CA539346 A CA 539346A CA 1280493 C CA1280493 C CA 1280493C
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Abstract
ABSTRACT:
RADIO DIRECTION-FINDING USING TIME OF ARRIVAL MEASUREMENTS
A method and a system for radio direction-finding by measuring the Time of Arrival (ToA) of the leading edge of signals from a distant source at two relatively closely spaced receiving elements.
In order to give a good degree of immunity to multipath, the times at which the instantaneous detected amplitudes of the received signals first exceed a minimal threshold value such that received signals can be satisfactorily distinguished from noise is measured in such a manner that the measured time is not affected by multipath which involves more than a few metres additional path length for the indirect, delayed signal. A suitable timing circuit is disclosed.
By making ToA measurements on three coplanar, non-collinear receivers, directions of incidence in three dimensions can be determined.
A method and a system using both ToA and phase-difference measurements can provide the accuracy of interferometry but be simpler and cheaper.
RADIO DIRECTION-FINDING USING TIME OF ARRIVAL MEASUREMENTS
A method and a system for radio direction-finding by measuring the Time of Arrival (ToA) of the leading edge of signals from a distant source at two relatively closely spaced receiving elements.
In order to give a good degree of immunity to multipath, the times at which the instantaneous detected amplitudes of the received signals first exceed a minimal threshold value such that received signals can be satisfactorily distinguished from noise is measured in such a manner that the measured time is not affected by multipath which involves more than a few metres additional path length for the indirect, delayed signal. A suitable timing circuit is disclosed.
By making ToA measurements on three coplanar, non-collinear receivers, directions of incidence in three dimensions can be determined.
A method and a system using both ToA and phase-difference measurements can provide the accuracy of interferometry but be simpler and cheaper.
Description
~28~3 RADIO DIR~CTION-FINDING USING TIME OF ARRIVAL MEASUREHENTS
The invention relates to a method and to a system for determining the direction of incidence of electromagnetic ~ave signals from a distant source by determining the difference between the time of arrival (ToA) of the leading edge of wave siynals received respectively from the source at the two elements of at leas~ one pair of mutually-spaced wave-receiving elements.
The invention furthex relates to a method and to a system for determining the direction of incidence of electromagnetic wave signals from a dis~ant source using both ToA
and phase-difference measurements.
ToA (also known as Time Difference of Arrival, TDOA) direction-finding (DF) with a long baseline, i.e. wherein the wave-recelving elements are spaced miles apart, is known from, for example, the article "Passive Direction Finding and Signal Location" by A.R. Baron et al, Microwave Journal, September 1982, pages 59-76~ see particularly page 59 and pages 66-70. A major disadvantage of ToA DF using a long baseline in many practical situations is that if, as is normally the case, the direction of incidence is to be determined over a substantial ran~e of directionæ, there is a substantial interval of time over which signals from the same source may arrive at one wave-receiving element of the pair relative to the other, the difference between the ToAs depending on the position of the source relative to the pair of elements. If there is a plurality of sources from which signals may be received, for example sources emitting pulsed signals with a subs~antial pulse repetition frequency (PRF), then "" ~LZI 3~93 there is a significant probability that the signals whose ToAs at the two elements are compared come from difierent sources rather than the same source; the greater the spacing between the elements, and consequently the longer the above-mentioned period, the greater is the probability. It is then necessary to compare one or more characteristic parameters of the ~``` gL21~14~3 signals received at the two elements, for example frequency, PRF or pulse length, to ascerta;n whether they come from the same or d;fferent sources. Not only does th;s requ;re substant;al further equ;pment, but ;t substant;ally ;ncreases the t;me taken to ascerta;n the d;rect;on of ;nc;dence of the s;gnals.
ToA DF us;ng a short basel;ne, for example 24 feet, ;s known from US patent 3 936 83~. The use of a short basel;ne has the advantage ~although there ;s no ment;on of ;t ;n the US patent) that the above-ment;oned ;nterval with;n wh;ch s;gnals from the same source can arrive at the two elements is so small that there ;s a h;gh probab;lity ;n pract;cal s;tuat;ons that s;gnals from a d;fferent source will not arr;ve in that period~ However, the use of a short basel;ne imposes the d;ff;culty of determ;ning time differences of the order of tens of nanoseconds or less. The above-ment;oned US patent proposes a system wherein a capac;tor is charged at a fast, linear rate from a constant-current source, charg;ng be;ng started by the arr;val of a pulse signal at one element and stopped by the arrival of a pulse signal at the other element; the time d;fference ;s then effectively multipl;ed by transferr;ng the voltage on the capac;tor to a further capac;tor wh;ch is d;scharged at a much slower constant rate. However, the ~ c;rcu;try disclosed for performing these funct;ons would not ;n ; practice be suitable for the very short t;me d;fferences ;nvolved.For example, the current from the constant-current source could not be sw;tched between zero and ;ts full value ;n a t;me wh;ch ;s short compared w;th the t;me d;fference ;nvolved. Moreover, F;gure 5 of the US patent, wh;ch ;s a graph of a count representat;ve of the measured t;me difference against time delay ~actual t;me d;fference), shows a predom;nantly substantially rect;linear variation from about 2000 nanoseconds down to about 150 nanoseconds;
at th;s po;nt, there ;s an abrupt change of slope, with what appears to be a hypothet;cal extrapolat;on to the or;g;n of the graph. Th;s ;nd;cates that the c;rcu;t would not in fact operate as ;ntended for the t;me differences of 0-50 nanoseconds that would actually need to be measured~
-` 12~ 93 It is an object of the invention to provide an improved method and system for short-baseline ToA DF. It is a further object of the invention to provide an improved timing circuit.
According to a first aspect of the invention, a method of de~ermining the direction of incidence of electromagnetic wave signals from a distant source from the time of arrival of the leading edge of the wave signals comprlses:
receiving said signals at a plurality of mutually spaced wave-receiving elements, detecting the respective instantaneous amplitude of the signal received a~ each element, measuring the times at which the detected amplitudes of wave signals received respectively at at least two of said elements first exceed a minimal threshold value such that signals can be satlsfactorily distinguished from noise and which threshold value is substantially less than the minimum peak value of signals whose direction of incidence is to be determined by said method, the time being measured ln suah a manner that the measured time is generally unaffected by multipa~h propagatlon, determining the difference between the measured times in respect of one palr or of a plurali~y of pairs of said elements, wherein the two elemen~s of said one pair or of each of at least two of said plurality of pairs are sufficiently close together tha~ the length of the interval of time within which signals from : the same source must arrive at ~he two elements is so short that there is a high probability in operation that no signals from another source will arrive in that interval, and 30~3 20104~8325 deriving a representation of the direction of incidence from the time difference(s) utilising the relationship cos ~ - c ~t~d where is the angle between the direction of incidence of the signals and the line joining the two elements of a said pair, d i5 ~he distance between those two elements, ~ t is the time difference between the signals at that pair of elements first exceeding the minimum threshold value, and c is the free-space velocity of electromagnetic waves.
According to a second aspect of the invention, a system for determining the direction of incidence of electromagnetic wave signals from a distant source from the time of arrival of the leading edge of the wave signals, comprises:
a plurality of mutually spaced wave-receiving elements, means for detecting the respective instantaneous amplitude of the signal received at each element, means for measuring the times at which the detected amplitudes of wave signals reaeived respectively at at least to of said elements first exceed a minlmal threshold value such that signals can be satisfactorily distingulshed from noise and which threshold value is substantially less than the minimum peak value of signals ~hose direction of incidence is to be de~ermined by said method, the time being measured in such a manner that the measured time is generally unaffected by multipath propagation, means for determining the difference between the measured times in respect of one pair or of a plurality of pairs of said elements, wherein the two elements of said one pair or of each of .
2010~-8325 at least two of said plurality of pairs are suificiently close together that the length of the interval of time within which signals from the same source must arrive at the two elements i5 SO
short that there is a high probability in operation that no signals from another source will arrive in that interval, and means ~or derivin~ a representation of the direction of incidence from the time difference(s) utilising the relationship cosc~ - c S t~d where c~ is ~he angle between the direction of incidence of the I.0 signals and the line joining the two elements of a said pair, d is the distance between those two elements, S t is the time difference between the signals at that pair of elements first exceeding the minimum threshold value, and c is the free-space velocity of electromagnetic waves.
These aspects of the invention involve the recognition that in contrast to other methods and systems ~or direction-finding such as in~erferometry, ToA DF can provide a good degree of immunity to multlpath propagation, provided that the time of arrival is measured 4a ~28[)~93 1 early on the leading edge of the s;gnal and ;n such a manner that the measured t;me ;s not substant;ally affected by mult;path propagation wh;ch involves more than a short add;t;or,al path length for the ;nd;rect, delayed signal. If the ToA ;s not measured early on the lead;ng edge of a s;gnal, multipath can s;gn;f;cantly d;stort the shape of the lead;ng edge as the s;gnal approaches its peak value~ leading to a substantial error ;n a measured t;me difference and hence in the der;ved d;rect;on of ;nc;dence. As w;ll be described below, the ToA may be measured ;n respect of a threshold wh;ch is well below the m;nimum peak ampl;tude and in such a manner that the measured time is unaffected by variations in signal ampl;tude w;thin a period of, for example, not more than about 10 nanoseconds after the threshold is first exceeded, so that the system is immune to mult;path propagation which ;nvolves more than about three metres add;t;onal path length, as w;ll generally be the case for a distant source.
The above-mentioned US patent pays particular attention to attempting to elim;nate the effects of pulse amplitude on ToA
measurement by us;ng a so-called Normalizer, but makes no reference to the poss;ble effects of mult;path. In the f;rst of two Normal;zer processes described with reference to Figure 7 of the patent, the value of the s;gnal ampl;tude at wh;ch the s;gnal ;s t;med ;s dependent on the rate of ;ncrease of the ampl;tude, and since th;s value would necessar;ly have to be at a m;n;mal threshold for sat;sfactory d;st;nct;on of rece;ved s;gnals from no;se when the rate of ;ncrease ;s at ;ts lowest acceptable value, the amplitude value at wh;ch the s;gnal ;s t;med w;ll generally be above the m;n;mal threshold value and hence not as early on the lead;ng edge as ;t m;ght be~ In the second Normalizer process descr;bed w;th reference to Figure 8 of the patent, a second threshold well above a first threshold ;s used. The Normal;zer processes are therefore inherently more suscept;ble to multipath wh;ch affects the shape of the lead;ng edge of the s;gnalO Moreover, ;n v;ew of the relat;vely slow-act;ng circu;try descr;bed ;n the patent, the Normal;zers would necessar;ly requ;re the t;med po;nt on a pulse to be well after the ~ ~28~493 1 start of the pulse. The present ;nvent;on ;nvolves the recogn;t;on that ;n pract;ce, it is more des;rable to obv;ate the effect of most ; mult;path propagation: t;m;ng errors due to multipath w;ll generally be worse than errors due to d;fferent s;gnal ampl;tudes. Moreover, by mak;ng the ToA measurement early on the lead;ng edge, ;naccuracies due to different signal amplitudes may ;n any case be reduced.
Ow;ng to the difficulty of measur;ng very short time differences accurately~ there ;s ;n pract;ce l;kely to be a significant uncertainty ;n the difference between measured ToAs, and therefore ToA DF with a short basel;ne ;s l;kely not to be very accurate, although a s;ngle ToA d;fference measurement can g;ve an unamb;guous ind;cat;on of d;rect;on .
~etter accuracy ;s achievable w;th interferometersO
Direction-finding using interferometers ;s well known. The d;fference ;n phase between s;gnals rece;ved respect;vely from a d;stant source at two mutually spaced wave-rece;ving elements is representat;ve of the angle between the d;rect;on of ;nc;dence of the s;gnals and the l;ne jo;n;ng the two elements. The greater the spac;ng between the elements, the more accurately the angle of ;nc;dence can be determ;ned, but the smaller ;s the unamb;guous range of directions of ;nc;dence. To resolve ambiguity, a rect;linear array of wave-rece;v;ng elements ;s used to prov;de a series of pairs of elements w;th progress;vely smaller spac;ngs; the w;dest-spaced pa;r prov;des an accurate but amb;guous representat;on~ and the closest-spaced pa;r prov;des a coarse but unamb;guous representation. With successive spacings in a suitable rat;o, the ambigu;ty ;n the phase measurement on the w;dest-spaced pa;r can be resolved by reference to the successively more closely-spaced pa;rs of the ser;es. However, to prov;de good accuracy may requ;re a substantial amount of equ;pment s;nce each element ;s associated w;th a respect;ve receiver, and with N
elements, at least (N-1) phase discr;minators are needed. Such a system may consequently be expensive.
It ;s also well known to determine the direction of ;nc;dence 8~493 of rad;o ~aves by compar;ng the ampLitudes of the signals received by t~o adjacent similar antennae whose respect;ve main-beam axes are ;nclined to one another. If the d;rection of inc;dence lies between the axes, the rat;o of the amplitudes is representat;ve of the angle between the direct;on of ;ncidence and either of the axes. This arrangement provides an unambiguous but reLatively ;naccurate measurement of d;rect;on.
Accord;ng to a th;rd aspect of the ;nvent;on, a method of determ;ning the direct;on of ;ncidence of electromagnetic wave s;gnaLs from a distant source compr;ses:-rece;v;ng sa;d signals at each of a plural;ty of mutually spaced wave-receiv;ng elements, measur;ng the phase d;fference between the signals rece;ved respect;vely at the two elements of one pa;r of sa;d elements or the respect;ve phase d;fferences between the signals rece;ved respectively at the two elements of each of a plurality of substant;ally collinear pairs of said elements with different respective spacings, wherein the phase measurement on said one pair or on the closest-spaced of said pairs is amb;guous ;n the operating range of directions of incidence and the operating frequency range, determining by a method embodying the first aspect of the invention the approximate direction of incidence of said signals from the times of arrival of the leading edges of corresponding wave signals received respectively at two of sa;d plurality of elements, the l;ne jo;ning wh;ch two elements is parallel to and substantially coincident ~ith the line joining said one pair or said plurality of substantially collinear pairs, wherein the range of possible values of the actual time difference due to the uncertainty in the d;fference between the measured t;mes corresponds to a range of angles of ;ncidence whose magnitude is not greater than the magn;tude of the range of angles of ;nc;dence corresponding to the unambiguous range of phase difference measurement on said one pair or said closest-spaced pair, and resolving the ambiguity in said ambiguous phase measurement by comparing the possible directions represented thereby with the ~~28~1493 1 approx;mate direct;on determ;ned from the d;fference between the measured t;mes.
Accord;ng to a fourth aspect of the ;nvent;on, a system for determin;ng the direct;on of ;nc;dence of electromagnetic wave signals from a distant source comprises:-a plural;ty of mutually spaced wave-rece;ving elements, means for measur;ng the phase difference between the s;gnals received respectively at the two elements of one pair of said elements or the respective phase differences between the signals received respectively at the two elements of each of a plurality of substant;ally coll;near pa;rs of sa;d elements ~;th d;fferent respect;ve spacings, where;n the phase measurement on said one pa;r or on the closest-spaced of said pa;rs is amb;guous in the operat;ng range of directions of incidence and the operat;ng frequency range, means, comprising a system embodying the second aspect of the invention, for determin;ng the approx;mate d;rect;on of inc;dence of sa;d s;gnals from the times of arrival of the lead;ng edges of correspond;ng signals rece;ved respect;vely at two of sa;d plural;ty of elements, the l;ne jo;n;ng wh;ch two elements ;s parallel to and substant;ally co;nc;dent ~ith the l;ne jo;ning sa;d one pa;r or sa;d plurality of substant;ally collinear pa;rs, where;n the range of poss;ble values of the actual t;me d;fference due to the uncertainty in the d;fference between the measured times corresponds to a range of angles of incidence whose magn;tude is not greater than the magn;tude of the range of angles of ;ncidence correspond;ng to the unamb;guous range of phase d;fference measurement on sa;d one pa;r or sa;d closest-spaced pair, and means for resolving the amb;gu;ty ;n said amb;guous phase measurement by comparing the possible d;rections represented thereby w;th the approx;mate direct;on determined from the difference between the measured t;mes.
~y using a ToA DF arrangement to resolve ambiguity in the phase measurement on the s;ngle pa;r or the closest-spaced pa;r of elements of an ;nterferometer DF arrangement, the need for one or more ;nterferometer channels wh;ch would merely be needed for -` 12~30493 1 resolving ambiguity but ~hich would not increase the accuracy of the direction-finding is avoided, and the combined arrangements may be cheaper and simpler than a purely interferometric one.
To determine directions of incidence over a ~ide range of directions~ particularly d;rections which are not restricted substantiaLly to a plane including the elements, a system embody;ng the fourth aspect of the invention may comprise phase-d;fference measur;ng means, approximate-direction-determing means and ambiguity-resolving means operable ;n respect of a f;rst pa;r or a f;rst plural;ty of substant;ally coll;near pairs of the elements and of a second pair or a second plurality of substantially collinear pairs of the elements to derive first and second unambiguous phase measurements, wherein the line joining the elements of said first pa;r or said f;rst plurality of pa;rs and the line joining the elements of said second pa;r or said second plurality of pairs are substantially coplanar and ;ncl;ned to one another, sa;d f;rst and second phase measurements being representative of the angle d between the direct;on of ;nc;dence and the l;ne joining the elements of the respective pair(s), and further comprising means for deriving a representation of the angle 0 and/or a representat;on of the angle ~, where 0 ;s the angle between the d;rect;on of ;nc;dence projected ;nto the plane of the l;nes and the normal to a respect;ve one of sa;d l;ne ;n said plane and where ~ ;s the angle between the d;rect;on of ;nc;dence and sa;d plane, from the f;rst and second unamb;guous phase measurements ut;l;s;ng the relat;onsh;p s;n (90 degrees - ~ ) = s;n ~ cos ~ .
For s;mpl;city, said lines may be mutually perpendicular.
In a particularly s;mple system, the approx;mate-direction-determining means may be operable in respect of the times of arrival at a common element and at each of t~o elements respectively on the two l;nes, and furthermore the phase-difference measuring means may be operable to measure the phase differences bet~een said common element and each of two elements respectively on the t~o lines.
As an alternat;ve, a system may comprise three or more mutually 1 ;ncl;ned successively adjacent pairs or plurality of pairs of elements, means for measuring the ampl;tude of wave signals received at one or more elements of each of said three or more pairs or plural;ty of pa;rs, and means for select;ng as sa;d f;rst pair or plural;ty of pa;rs one of sa;d three or more pa;rs or plurality of pa;rs ;n respect of ~hich the ampl;tude ;s at least as great as the ampl;tude in respect of each of the remain;ng pa;rs or plurality of pairs and as sa;d second pa;r or plurality of pa;rs a pair or plural;ty of pa;rs adjacent sa;d first pair or plural;ty of pa;rs ;n respect of wh;ch the ampl;tude ;s at least as great as the ampl;tude in respect of any other adjacent pair or plurality of pairs. Suitably, such a system comprises four mutually orthogonal pa;rs or plural;ty of pa;rs of elements~
A method embody;ng the f;rst aspect of the ;nvent;on may ;nvolve using three substantially coplanar but substantially non-collinear elements to form at least two said pairs, and deriving a representation of the angle 3 and/or a representat;on of the angle utilising the relat;onsh;p s;n (90-c~) = sin 0 cos ~
;n respect of each of sa;d at least two pairs, ~herein 0 is the angle between the d;rect;on of ;nc;dence projected ;nto the plane of the three elements and the normal ;n sa;d plane to the l;ne jo;n;ng the two elements of a sa;d pa;r, and ~ ;s the angle between the d;rect;on of inc;dence and sa;d plane~ D;rect;ons of ;nc;dence ;n three d;mens;ons may thus be determ;ned from ToA measurements at three elements.
The method may further compr;se determ;n;ng a parameter representat;ve of the rate of ;ncrease of the detected amplitude of the respect;ve s;gnal received at at least one of the elements ;n the reg;on of sa;d threshold value, and determining the direction of ;nc;dence of rece;ved s;gnals only if sa;d parameter sat;~f;es a criterion representing a m;nimum rate of ;ncrease ;n sa;d reg;on.
S;gnals whose d;rection of incidence cannot be determined with sat;sfactory accuracy can thereby be d;scarded. Such a method may compr;se measuring the time at wh;ch the detected amplitude first ~2~ 493 exceeds an adjacent further threshold value, wherein said parameter is the difference between the measured times in respect of the two threshold values, and wherein said criterion is that said parameter does not exceed a maximum value. Alternatively, such a method may comprise di~ferentiating the increasing detected amplitude at least in said region, wherein said parameter is the rate of increase in detacted amplitude derived by differentiation, and wherein said criterion is that said parameter exceeds a minimum value.
A system embodying the second aspect of the invention may comprise means for performing optional features of a method embodying the first aspect o~ the invention, as set forth in ~laims 7 to 10.
An embodiment of the means ~or measuring the times may comprise a clock pulse generator, a tapped delay device having a plurality of n mutually spaced taps, a latch coupled to the delay device for latching any signal on each o~ the n taps, and a decoding device coupled to the latch for producing a time representation from the signal(s) latched from the n taps, is characterized in that an input signal to be timed is coupled to the input of the delay device, in that the clock pulse generator is normally operable to clock the latch, in that the circuit comprlses inhibiting means responsive to the presence of a signal on at least one of the n taps when the latch is clocked to inhibit further clocking of the latch, and in that the decoding device is operable to produce a representation of the interval between the time that said input signal reaches the tap nearest the input o~
~IL2~0~93 the delay device and the preceding clock pulse.
Said interval may be represented as zero for the case in which the input signal has reached the tap furthest from the input when the latch is clocked, and other intervals represented accordingly.
A timing circuit embodying the third aspect of the invention may be contrasted with the timing circuit disclosed in GB 2 132 043 A and EP 113 935 A, in which the clock pulse generator is coupled to the input of the delay line, and the input signal to be timed is used ~o latch the latch.
lla ~;; 8~ 3 1 Su;tably, the circuit further comprises a counter for counting the pulses of the clock pulse generator, ~herein the inh;bit;ng means are further operable to inh;bit further count;ng of the clock pulses, the outputs of the decod;ng device and the counter being concatenated.
In order to be able to produce representat;ons of intervals over the majority of the per;od of the clock pulse generator, the period of the clock pulse generator may be not substantially less than the t;me delay bet~een the tap nearest to and the tap furthest from the input of the delay device. ~uitably, sa;d per;od ;s substant;ally equal to sa;d time delay.
To make good use of the delay dev;ce and to prov;de representations of integral multiples of a fraction of the period of the clock pulse generator, the time delay between each adjacent pair of the n taps may be the same, being equal to T, and the period of the clock pulse generator be nT.
Where the circuit ;s to be used to t;me the beg;nning of signals ~hich persist for at least the per;od of the clock pulse generator, the ;nh;bit;ng means may be respons;ve to the presence of a s;gnal on the tap nearest the ;nput of the delay device when the latch ;s clockedR Th;s helps to d;st;ngu;sh true signals to be t;med from no;se ;n the case where a s;gnal ;s present on a tap beyond the tap nearest the ;nput of the delay dev;ce ~hen the latch ;s clocked, since such a s;gnal m;ght be due to noise. As a further safeguard aga;nst false measurements due for example to no;se, the decoding device may be operable not to produce said time representat;on unless when the latch is clocked a s;gnal ;s present on each of the n taps between the ;nput of the delay device and the tap furthest from the ;nput of the delay dev;ce on ~hich a signal is present~
It has been found that attempting to operate a timing circuit of the k;nd d;sclosed ;n the above-ment;oned GB and EP publ;shed appl;cations to measure time to a resolution of about 1 nanosecond produces difficulties in synchron;sing the fine count produced by the decoding device and the coarse count produced by the counter. A
` ~80~93 timing circuit embodying the third aspect of the present invention can be both simpler and more reliable. Furthermore, it has been found advantageous to use the threshold crossing merely to feed an input signal to the delay device, rather than to use it to control gates: the latter is liable in practice to produce undesired di~tortion of the signal.
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Figure 1 is a block diagram of a ToA DF system embodying the inven~ion and comprising one pair of wave-receiving elements;
Flgure 2 is a block diagram of a timing circuit suitable for UBe in a ToA DF system embodying the invention;
Figure 3 is a block diagram of a ToA DF system embodying the invention and comprising three collinear wave-receiving elements;
Figure 4 illustrates the disposition of three non-collinear wave-receiving elements for an omnidirectional ToA DF
system embodying the invention:
Figure S illustrates schematically processing to calculate an angle using the elements of Figure ~;
Figure 6 is a schematic diagram of a simple DF system of a kind using both ToA and phase-difference measurements;
Figure 7 is a schematic diagram of a particularly simple omnidirectional DF system of this kind;
Figure 8 illustrates the disposition of ToA and inter~erometer antenna arrays for a more complex omnidirectional ~28049:~
DF system of this kind, and Figure 9 illustrates processing for a DF system using the arrays of Figure 8.
Figure 1 shows a simple system comprising one pair of wave-receiving elements. The system comprises two similar channels A and B respectively. Each of the channels comprises in succession an antenna AN'r, an RF amplifier RFA, a detector D, a video amplifier VA, and a timing circuit TC. The antennae may be omnidirectional, or may be directional with their axes substantially parallel. The 13a 28~g3 spacing d between the antennae is chosen to be sufficiently small that the length of the interval of t;me with;n wh;ch s;gnals from the same source must arr;ve at the two elements is so short that there ;s a h;gh probab;l;ty in operation that no signals from another source ~;ll arr;ve ;n that ;nterval. If s;gnals may be received from any direct;on, the length of the interval is t~ice the time taken for electromagnetic waves to travel the distance d (;n free space): the limits of this interval are set by the possibil;ty of s;gnals being ;ncident along the line joining the antennae ;n one sense or the other~ ;.e. from left or r;ght ;n Figure 17 The probability of no s;gnals arr;v;ng from another source ;n that ;nterval ~ill depend on the number of sources from which s;gnals can be rece;ved, the frequency with ~hich they em;t fresh s;gnals, and the durat;on of the s;gnals. What probability is sufficiently high w;ll depend on what proport;on of ;ncorrect representat;ons of d;rection, due to measurements hav;ng been made ;n the two channels on signals from d;fferent sources~ ;s cons;dered acceptable. For typ;cal opera~;onal situat;ons of the number of sources em;tt;ng pulsed signals, their PRF and pulse length, a separation d of the order of 50 feet, giv;ng an ;nterval length of about 100 nanoseconds, is considered to give an acceptably high probabil;ty.
When an RF s;gnal ;s ;ncident on the antenna of one of the channels, the RF signal is amplified and de~ected, and the time at which the instantaneous detected amplitude of the signal, after further ampl;ficat;on, f;rst exceeds a threshold value is measured. This threshold value is chosen to be substantially less than the min;mum peak value of signals whose direction of incidence is to be determined, as ~;ll be explained in greater detail belo~; the threshold w;ll usually be much less than the typical peak value.
The timing circu;ts operate w;th a common clock (CLOCK3. A
calculating un;t CALC determines the difference ~ t between the measured t;mes and provides therefrom a representation of the direction of incidence, for example of the angle ~ between the incident signals and the line joining the antennae, utilising the relationship 9~
cos o< = c ~t/d where c is the free-space velocity of electromagnetic waves.
The time at wh;ch the threshold value is first exceeded is measured in each timing circuit TC in such a manner that the measured time ;s unaffected by multipath propagat;on ~here the ; delayed s;gnaL has been reflected ~rom a surface not virtually coincident with the direct path between a distant source and the antenna.
An ECL tem;tter-coupled logic) circuit arrangement, su;table for the tim;ng circuit TC of Figure 1 and capable of measuring to an accuracy of 1 nanosecond, is shown in Figure 2. The detected and amplified video signal from the video amplif;er VA is applied to a very fast comparator COMP ~hose output changes from a log;c "O" to "1" when the input signal exceeds a threshold voltage VT. The t5 comparator output s;gnal ;s fed to the input of a tapped delay line TDL having 8 consecutive taps separated by 1 nanosecond intervals.
The taps are connected to respective ;nputs of an 8-b;t latch LATCH. The latch is clocked at 8 nanosecond ;ntervals by a 125 MHz clock CLOCK v;a a gate G1, the clock signal also being supplied via a further gate G2 to a synchronous counter CNTR ~hich prov;des a coarse measurement of time. The outputs, labelled 0-7, of the latch are fed to a decoding circuit DCDR; the presence o~ a signal on at least one output, in this case the first output, O, ;s also used to control the gates G1 and G2~ the output being connected thereto by a fast ~eedback loop. The outputs of the counter CNTR and the decoder DCDR are concatenated to give a representation of the t;me at wh;ch the output of the comparator COMP changed from O to 1.
In operation, the gates G1 and G2 are normally open. The counter CNTR measures time in 8-nanosecond un;ts, up to a maximum time at least as long as the above-mentioned interval, determ;ned by the antenna spacing d, within which signals from a distant source must be received by both antennae. The latch is s;m;larly clocked at 8-nanosecond intervals, but while the comparator output is 0, there are no signals from the tapped delay line, and the 3~ latch outputs rema;n at zero. ~hen the comparator output changes 80g~93 1 to 1 (which in the case of a true received signal as opposed to noise uill normally pers;st for longer than the period of the clock), the signal travels along the delay line changing successive tap outputs from 0 to 1. When the latch is next clocked, the series of ones and remaining zeroes is held ;n the latch. The presence of a "1" on the first output, 0, of the latch closes the gates G1 and G2, preventing further clocking of the latch and the counter. The output of the latch w;ll be one of the following codes:
1ûOOOOOO
'11 000000 11 100~00 111110nO
1111~111 The first of these codes represents the most recent ToA and the last the earliest ToA since the latch was last clocked; the first ind;cates that 7 nanoseconds should be added to the time represented by the counter CNTR while the last requ;res zero add;t;on. The decoder DCDR transforms that latch output to binary digits which are concatenated with the counter output~
The threshold the cross;ng of wh;ch ;s timed ;s, as prev;ously explained, set at a low value in order largely to avo;d errors due to multipath. Setting the threshold at a level substant;ally below the m;n;mum peak level of signals whose direction of incidence is to be determined, for example 10 dB below the minimum peak level, also provides the advantage of tending to alleviate timing errors ~hich would occur if the signal amplitude crossed the threshold at a slo~
r~ rate because the amplitude ~ reaching its peak level. The lowest level at which the threshold can be set will depend on the noise level in the system: ;f the threshold is set too close to the noise level, the accuracy of t;ming will be degraded by the random fluctuation in the amplitude of desired s;gnal plus noise, and a positive-going threshold crossing may even be caused by noise - ~ 213~9C93 17 PHB 33274 r 1 alone.
The decoder ;s ;n th;s embod;ment arranged to accept only the above-mentioned codes. It consequently accepts only s;gnals whose amplitude remains above threshold long enough to produce a continuous succession of ones in the latch, and rejects any other pattern of zeros and ones which might result from trigger;ng of the comparator by noise spikes or from a received signal w;th a slow rate of ;ncrease of amplitude.
It will be seen that provided the amplitude remains above threshold long enough for this state to be latched, the measurement of ToA will be unaffected by subsequent variat;ons in ampl;tude, ;n part;cular var;at;ons due to a delayed mult;path signal wh;ch arr;ves in phase oppos;tion to the or;g;nal d;rect-path signal and causes the amplitude to fall below t5 threshold~ The max;mum period taken to latch the above-threshold state is in this embodiment the length of the delay line, ;.e. 8 nanoseconds. This time may be much shorter than the t;me taken for the amplitude to reach typical peak value.
A delay not less than and substantially equal to the t;me taken for electromagnetic waves to travel the d;stance d may be ;ncluded in one channel before the timing c;rcuit so that time differences are measured with respect to the time of arrival of a signal in the other channel. Suitably, a time "windw" is used to prevent unnecessary computation on time differences which are too large for the signaLs to have come from the same source. Where the above-mentioned delay is included in one channel, this window may be de~ined as beg;nn;ng w;th a time difference of zero and end;ng with a time d;fference not less than and substant;ally equal to tw;ce the t;me taken for electromagnet;c ~aves to travel the distance d. The use of a time w;ndow also prov;des some protection against random noise signals wh;ch cause the detected amplitude to exceed the threshold from causing false measurements.
At least one, and preferably each, of the channels in the system of Figure 1 may comprise a signal validat;ng c;rcuit ~not shown) to ascertain the rate of increase of the amplitude of the 4~3 1 s;gnal in that channel in the region of the threshold vaLue, and to cause the system not t-o determine the direction of inc;dence unless the rate satisfies a criterion of m;nimum slope. For th;s purpose, the output of the video amplif;er VA may be supplied to a further timing circuit (not sho~n) wh;ch measures the t;me at wh;ch the signal amplitude first exceeds an adjacent further threshold value.
The difference between the times measured by the two circuits ;n a ~hannel may be determined, and the direction of ;nc;dence determ;ned only ;f the difference does not exceed a max;mum value~
0 Alternatively, the ampl;tude ;ncrease may be different;ated and the d;rection o~ incidence determined only if the rate of ;ncrease of a~plitude derived by differentiation exceeds a minimum value.
As a further way of distinguishing signals coming from a distant source from no;se, at least one of the channels may comprise means (not shown) for determining the peak amplitude of a signal wh;ch causes the threshold to be exceeded, and for inhibiting the determ;nation of the d;rect;on of incidence unless the peak ampl;tude is substantially greater than the threshold.
It is cons;dered that a su;table criter;on of minimum slope may be that the steepness of the rising edge should be predom;nantly controlled by the v;deo bandw;dth of the system. Th;s ;nter al;a has the effect of reducing the dependence of the measured t;me on the rate of increase of the RF ampl;tude and hence tends to ach;eve the same object as the Normal;zers ;n the above-mentioned US
nfltent. It may be des;rable for the v;deo bandwidth to be switchable between a broadband value and a narrowband value. The broadband value may allow more accurate t;m;ng of steeply-r;sing lead;ng edges, but the narrowband value may enable acceptable results to be obtained with more slowly-r;s;ng edges, since it may reduce the no;se level in the v;deo c;rcuit and allow the threshold to be set to a lower value and hence to a relat;vely steeper part of the lead;ng edge.
F;gure 3 depicts a modification of the system of F;gure 1 comprising three coplanar and collinearly d;sposed antennae L~ M, N
"
20104-g325 reæpectively in respective channels each channel being the same as each of the channels shown in Figure 1. The spacings of each of the two pairs of elements LM and MN are equal (each being d) and each satisfy the above-mentioned criterion that the spacing is sufficiently small that the length of the interval of time within which signals from the same source must arrive at the two elements of a palr is so short that there is a high probability in operation that no signals will arrive from another source in that interval; the spacing 2d between antennae L and N may however be too large to satisfy khis criterion. Nevertheless, the difference between the times of arrival of signals at antennae L and N may be used to provide a more accurate representation of the direction of incidence than could be provided by the system of Figure 1 if one or more steps are taken to reduce the posslbility that the time difference measured on one of the pairs of antennae LM, MN does not relate to the same source as the time difference measured on the other pair. For example, as indicated in Figure 3, the time differences measured in relation to each pair of antennae, tLM and tMN respectively, may be compared, and only if their values are equal to within a small tolerance is the direction of incidence determined from the difference between the times of arrival at antennae L and N; the probability that signals from different sources should result in substantially equal time differences being measured between the elements of the pairs LM and ~N is small, and even if the signals should have come from different sources, the resultant error in the indicated direction of incidence in relation to the source from which signals were first `- ~2~ 93 received at one pair of antennae will be small.
The systems so far described provide only a representation of the direction of incidence that defines an angle to the line joining a pair of wave-receiving elements, and hence the surface of a cone whose axis is said line. Where sources are known to lie substantially in a single plane including said line and where the wave-receiving elements are directional, this may be sufflcient (although it should be borne in mind that the accuracy with which c~ can be determined decreases as ~ decreases from 90 degrees to 0); however, when signals may be received from each side of said line, and particularly when sources are not restricted to a single plane, it is desirable to perform measurements on at least one further wave-receiving element which is not collinear with the one pair of elements. Figure 4 depicts the deposition of three substantially coplanar but not collinear elements A, B, C respectively, forming an arbitrarily-shaped trian~le. The spacings AB, BC, CA each satisfy the above-mentioned criterion of being sufficiently small. By measuring the times of arrival of slgnals at each elemen~, the direction of lncidence may ~e determined for the general case of distant sources in 3-dimensional space as follows.
Let the spacings AB, BC, CA be dl, d2, d3 respectively.
A
Let the angles CAB and ABC be m and n respectively. Let the length of the perpendicular from C onto AB be a, and the distance from A to the intersection of said perpendicular with AB be b, so that a - d3 sin m and h - d3 cos m. (Thus b is negative if m ~ 90 degrees.~ Let the angle between the direction of incidence and 2 !3(~ 3 the normal to AB in the plane of AB be ~ (so that ~ =
~90 deyrees-~) and coscX = sin ~), the angle between the normal to AB in the plane of ABC and the direction of incidence projected into that plane be ~ (typically the azimuth angle), and the angle between the direction of incidence and the plane o~ ABC be ~
~typically the alevation angle). Let the times oi arrival at A, B, C be tA tB~ tc respectively.
Then sin ~ = C(tA-tB)/~
Now sin ~ a sin ~ CQS
Writin~ x = c(t -t )/d y = c(t -t )/d z = c(tB-tc)/d2, we may put x - sin ~ cos and analogously y = sin ~-m) cos z ~ sin [~-(180-n)] cos -sin (~+n) cos ~.
20a -` ~LZ80493 1 El;minat;ng ~ from either of two pairs of these equat;ons, one obta;ns tan ~ = x sin m/~x cos m - y]
and tan 3 = -x sin n/~x cos n + z].
These expressions have two-fold amb;guity. To distinguish between -90 degrees < 0 ~ 90 degrees and 90 degrees ~ 0 < 270 degrees, one may note that the denominator of for example the first expression for tan ~ may be expanded as sin ~ cos ~ cos m - sin(~-m) cos or cos ~ cos ~ sin m.
Thus the denom;nator is pos;t;ve for -90 degrees ~ O ~ 90 degrees and negat;ve for 90 degrees ~ ~ < 270 degrees.
Re-writ;ng the first express;on for tan ~ in terms of the t;mes of arr;val and mult;plying the numerator and denom;nator by ~5 d1d3 tan O ~ (tA-t~)d3 sin m/C(tA-t~)d3 cos m -(tA-tC)d = - a (tA-tg)/~d1(tA-tC) ~ b~tA tB)J
= - a (tA-tB)/~(d1-b)(tA-tc)+b(t~-tc)~-Writ;ng P = a(tA tB) a = (d1-b) ~tA~tc R ~ b (tB-tC), one obta;ns tan O a - P/~Q+R) or ~ = - Arc tan CP/(Q+R)~
Figure 5 illustrates schematically the processing to calculate O
according to this expression. The differences tA-tp, tB-tC, tA-tC are formed from the measured times tA~ t~, tc and then scaled to produce the quantities P, Q, R. The quantities ~ and R are summed and d;vided into P; the angle whose tangent is equal to minus the quotient is then determ;ned, for example from a look-up table in a PROM~ to obtain an ambiguous value aamb f ~. The sign of (Q+R) is also checked; if (Q+R)> O, a quantity 00 = 180 degrees is produced, otherw;se ~0 = - ~amb and ~0 are summed to produce an unambiguous value of 0. Having calculated ~, the angle ~ may be L2~3~9L9~
1 calculated by subst;tut;ng ~ ;n, for example, the expression x = sin ~ cos ~ .
Alternatively, ~ may be calculated without needing to calculate by elim;nating 0 from a pair of the expressions for x, y, ~.
The calculations and processing may be simplified for particular cases. For an equilateral triangle of side d, a = ~3d/2 and b = d~2, so that the time differences need only be scaled by factors which are independent of du ~lternatively, if m = 90 degrees, a = d3 and b = 0~
so that the quantity R is zero. If d1 = d3, the scaling factors are again independent of the actual value of the spac;ng.
An omnidirectional direction-finding system may comprise four receiv;ng eLements d;sposed at the corners of a parallelogram, or more especially a rectangle and more part;cularly still a square.
The d;rection of inc;dence may be calculated from the times of arrival of the earliest-received three s;gnals wh;ch are of acceptable qual;ty. This allows for the possibil;ty that s;gnals rece;ved at one of the four elements may have been degraded by, for example, an obstruction in the region of the elements.
Comparison of the equations which can be derived from the above two expressions for tan ~ in terms of x and y and of x and z respectively to relate the error in 0 to errors in x, y and z show that the error is not dependent on wh;ch expression is used. The choice of which baselines are considered as primary and secondary baselines for determ;ning ~ is therefore not signif;cant.
As an alternative to a clock common to the t;ming c;rcu;ts of all the channels as depicted in Figures 1 and 3, each channel may have a respective accurate clock and the clocks be kept in synchronism via a lo~-bandwidth link. The time measurements and any other other data may be passed to a central processor and control unit via, for example, an optical fibre l;nk~
The principle of using both ToA and phase-difference . . ., ,~
~80~93 23 PH~ 33274 -measurements will be explained with reference to Figure 6. A ToA DF
system comprises two antennae ANT1, ANT2 separated by a distance L1, and a measuring and calculat;ng un;t MCtTOA) wh;ch determines the difference ~t between the respective times of arrival of the lead;ng edge of an RF s;gnal at the two antennae. The an~le c~
between the direction of incidence of the signal and the line joining the two antennae (the baseline of the system) is given by the equation cos o~ = c ~t/L1 ~13 where c is the free-space velocity of electromagnet;c waves. An interferometer DF system comprises a rectiLinear array of antennae, in this case three antennae ANT3, ANT4, ANTs, so disposed that the l;ne joining the antennae (the baseline of the system) is parallel to and substant;ally coincident ~;th the baseline of the ToA DF
system (in order that the two systems should measure the same angle o~in respect of signals from a distant source). The w;dest-spaced pa;r of antennae of the interferometer system are separated by a distance L2, and the closest-spaced pa;r by a distance L3; the accuracy w;th wh;ch the angle o~ can be determined by the interferometer system depends on the value of L2, and the unamb;guous range of coverage depends on the value of L3. The ;nterferometer system comprises a measur;ng and amb;gu;ty resolut;on un;t MAR~IR) wh;ch measures on each of a plural;ty of pairs of the antennae of different respect;ve spac;ngs, from the w;dest-spaced to the closest-spaced, the phase d;fference between RF signals rece;ved respect;vely at the two antennae of the pa;r, ;n this case the phase d;fference 035 and ~45 between ANT3 and ANTs and between ANT4 and ANTs respect;vely; the phase measurements may be performed after convert;ng the RF signals to an intermediate frequency tIF).
Since the measurement of phase is restr;cted to a range of 2 ~, the measured phases 0 are amb;guous. The actual unamb;guous phase d;fferences may be denoted ~ where = ~ + 2k~
where k is an integer. The unit MAR(IR) in known manner resolves the ambigu;ty in ~35 as far as possible by reference to ~45 (see for B~)~93 - 24 PH~ 33274 1 example Ge 1 337 099). Now cos o~ = C~3s/2 ~fL2 ~2) and cos o~ = C~4s~2 ~fL3 (3) where f is the frequency of the signals and ~35, ~45 are the unambiguous phases. The unamb;guous range of coveraQe of the interferometer system may be obta;ned by ;nsert;ng ;n equat;on 3 values of ~45 separated by 2 n.
Comb;n;ng equations 1 and 3, ~45 = 2 ~f/~t L3tL1 (4) Now ;n order sat;sfactorily to be able to resolve the rema;n;ng amb;gu;ty ;n the measured phases using the d;rect;on of ;ncidence determ;ned by the ToA DF system, the range of possible values of the actual difference between the ToAs of s;gnals at ANT1 and ANT2 due to uncerta;nty ;n the measured time difference should correspond to a range of bC whose magn;tude ;s not greater than the magn;tude of the range of oC correspond;ng to the range of ~45 wh;ch can be determ;ned unamb;guously from 045, ;.e. 2 ~. Thus ;f the uncerta;nty ;n ~t is ~ t, so that the range of possible values of the actual t;me difference is ~ t + ~ t, we obta;n from equat;on 4 2 7r ,~ 2 7r f .2 ~) t ~ L3 /L1 or L1/L3 ~ 2~ t f~ ~6) Relat;onsh;p 6 def;nes the m;n;mum value of the rat;o of the ToA
system basel;ne~ L1, to the closest spac;ng of the ;nterferometer system, L3, wh;ch w;ll enable sat;sfactory amb;gu;ty resolut;on w;th a g;ven uncerta;nty ~ t ;n the measured time difference, at the h;ghest frequency of operation. (The unambiguous angular coverage of the interferometer system ;ncreases as the frequency decreases, whereas the uncerta;nty ;n the angle measured by the ToA system ;s ;ndependent of frequency.) The outputs of un;ts MC(TOA) and MAR(IR) are fed to a calculat;ng un;t CALC wh;ch compares the value of 035, in ~h;ch ambigu;ty has been resolved as far as poss;ble by reference to a45, ~80~3 w;th an approximate but unambiguous value of ~35 derived from ~t in accordance w;th the equat;on ~35 = 2 ~f~ t L2/L1 (7) ~hich is obta;ned by combining equations 1 and 2; the ambiguity is resolved in kno~n manner. A representat;on of ~( ;s then calculated from the accurate value of ~ 5 derived from g35. The accuracy of th;s value of ~ ;s g;ven by differentiating equation 2:
~ G~= -c ~035/(2 ~fL2 s;no~ ) (8) where ~ o( ;s the uncertainty in the calculated value of ~ and ~ 035 is the poss;ble error ;n the measured phase d;fference 035.
If, for example, ~ 035 is 30 degrees and L2 = 0.66 metres, then from equat;on 8, S~ ;s ~25 degrees at 12 GHz and 0.5 degrees at 6 GHz~ I~ the uncertainty S t in the time difference is 2 nanoseconds, and taking L3 = 0.33 metres, then from relationship 6, L1 should be not less than 16 metres for operat;on up to 12 GHz.
If the frequency of the RF s;gnals is not previously known~ the compos;te To~l;nterferometer system should comprise means for measuring the frequency.
Resolut;on of ambi~u;ty ;n the ;nterferometer system by reference to the ToA system ;s particularly s;mple because both systems determine the direction of incidence with reference to the angle oC ~h;ch define a cone the axis of which is the baseline of the respective system, the baselines of the two systems being parallel and substant;ally co;nc;dent. ~y contrast, for example, an amplitude-comparison DF system locates the d;rection of ;nc;dence substant;ally in a plane normal to the plane of the main-beam axes of the antennae, wh;ch means that such a system is not readily compat;ble ~;th an interferometer DF system.
When the composite DF system is required only to determine the direction of inc;dence of signals from sources on one s;de of the common baseline, and part;cularly when sources can be assumed to be substant;ally ;n an s;ngle plane including the common baseline, calculation of the angle OCmay be suffic;ent to locate the direction of incidence~ The system may in that case use directional antennae which are relat;vely 7nsens;t;ve to s;gnals from the other ~X3~93 26 PHB 33274 ' 1 s;de of the baseline. Where sources l;e substantially ;n said plane but may be on either side of the basel;ne, omnid;rect;onal antennae being used, this ambigu;ty may be resolved by comparing the times of arrival of signals at the t~o antennae of the above-described ToA
system and at a third antenna coplanar but not collinear ~ith the first two~
It may be noted from equation 8 above that the accuracy is greatest ~hen o( = 90 degrees and decreases as o~ decreases to~ards zero. It may therefore be desirable to use a second composite ~ system w;th a common baseline coplanar with but inclined to that of the first system, for example at 90 degrees, to achieve improved accuracy for small values of C~. Such an arrangement may also be used when the direct;on of incidence is not restricted to a singLe plane. It may then be des;red to determ;ne the angle 9 and/or the angle ~ ~here s;n ~ = sin ~ cos ~ (9) where ~ is the angle between the direction of incidence and the normal to one of the common basel;nes ;n the plane ;nclud;ng the direct;on of ;nc;dence ~so that ~ = t90 degrees -C~ ) and s;n ~ = cos 0C), ~ ;s the angle between the d;rection of ;nc;dence projected ;nto the plane of the basel;nes and the normal to the relevant basel;ne ;n that plane, and ~ ;s the angle between the direction of inc;dence and the plane of the basel;nes. Typically, the plane of the basel;nes ;s hor;zontal, so that ~ ;s bear;ng and ~ is elevation.
F;gure 7 ;s a schemat;c d;agram of a part;cularly s;mple omnid;rect;onal DF system us;ng two compos;te ToA/interferometer arrangements ~ith coplanar mutually orthogonal common baselines.
The system comprises an array of seven antennae ANT21-ANT27 ~ith omnidirect;onal responses ;n the plane of th~ arrays, as denoted by a c;rcular symbol. Four antennae, ANT21-ANT2~, are located at the corners of a square; a fifth antenna, ANT2s~ ;s located at the centre of the square, and the remain;ng antennae, ANT26 and ANT27, are respectively d;sposed colL;nearly ~;th the d;agonaLs of the squares~ equ;d;stant from the centre of the square. A f;rst ToA
~8~ 3 system comprises antennae ANT26 and ANT2s, and the assoc;ated first interferometer system compr;ses antennae ANT21, ANT2s and ANT23.
The second ToA system comprises antennae ANT27 and ANT2s, and the assoc;ated second ;nterferometer system compr;ses antennae ANT22, ANT2s and ANT24. The ToA systems comprise respective measurement ar7d calculation units MC(TOA)1, MC(TOA)2 (although the units may be integrated to the extent that onLy a single ToA measurement is required ;n respect of antenna ANT2s), and the ;nterferometer systems comprise respective measurement and ambigu;ty resolution units MAR(IR)1~ MAR(IR~2. The outputs of the assoc;ated ToA and ;nterferometer systems are fed to respective calculating units CALC1, C~LC2 which in th;s embod;ment only determine the respèctive unambiguous phase measurements ~ 2~ referred to the widest-spaced pair of antennae of the respective interferometer, that represent the angles ~1~ DC2 between the direction of ;ncidence of RF signals and the respective common baseline. Having regard to equations 2 and 9 above, one may write ~1 = 2 ~fL2 sin ~ cos ~ /c (10) and analoguously ~2 = -2 ~fL2 cos ~ cos ~ /c~ t11) Solving these simultaneous equations~ one obtains tan 0 = -~ 2 t12) cos ~ 12+~22)1/2 (c/2 ~fLz). ~13) The unabiguous phase angles ~ 2 are supplied to a further calculating unit CALC 3 ~hich calculates ~ and/or ~ in accordance with the above equat;ons~
While the system of Figure 7 is particularly simpLe, it does have the disadvantage that some of the antennae are liable to obstruct signals to others of the antennae when the direct;on of incidence is at only a small angle ( ~) to the plane of the antenna array~ This ;s liable to affect the accuracy of the overall system, since the interferometers are more susceptible to errors due, for example, to mult;path. Thus~ when the plane ;s horizontal, the system is best su;ted to measur;ng direct;ons of ;nc;dence at substantial angles of elevat;ons. F;gure 8 ;llustrates an 8[)~9~
1 alternative antenna arrangement wh;ch ;s better su;ted for small values of ~ as well as larger values. Omn;d;rect;onal coverage ;s ;n th;s case prov;ded by four mutually perpend;cular nterferometer systems, IR1-IR4, d;sposed about a common central po;nt and each compr;s;ng three un;formly spaced antennae wh;ch ;n th;s case each have a substantial response over approximately 180 degrees ;n the plane of the antenna array (as denoted in F;gure 8 by a sem;circular symbol). ~ach of two ToA systems aga;n compr;ses a pa;r of omn;direct;onal antennae wh;ch ;n th;s embod;ment are d;sposed on oppos;te s;des of the common central point. The respective measurement and amb;guity resolut;on units of the ;nterferometers, MAR(IR)1-MAR(IR)4~ in this embodiment compr;se means for measuring the amplitude of the s;gnal received at at least one of the antennae of the respect;ve array. The measured amplitudes, A1-A4, and the amb;guous measured phases, 01-04~ (in which ambiguity has been resolved as far as possible by reference to the closest-spaced pa;r of antennae of the ;nterferometer) from the four ;nterferometers, as well as the time d;fferences measured by the two ToA systems, ~t1 and ~tz, are then processed as will now be descr;bed w;th reference to Figure ~
The amplitudes A1-A4 are suppl;ed to an ampl;tude compar;son and select;on control un;t ACS, and the phases 01-04 are fed to a phase selector un;t PS. The un;t ACS compares the ampl;tudes and selects for further processing two adjacent ;nterferometers, denoted A and B~ At one of these, the ampl;tude ;s at least as great as at each of the rema;n;ng ;nterferometers, and at the other, the amplitude is at least as great as the ampl;tude at the other adjacent interferometer; the basel;nes of ;nterferometers A and B
are respectively parallel to those of the f;rst and second ToA
systems. The phase selector unit PS accordingly selects the amb;guous phases from those two ;nterferometers, 0A and ~, and suppl;es them to a phase calculat;ng un;t CALC0 wh;ch also rece;ves the values of ~tl and at2 measured by the two ToA systems. The un;t CALC0 separately resolves the ambiguity ;n 0A and ~ by reference to ~t1 and ~t2 respectively, as explained above w;th ~Z~)493 1 reference to Figure 1, and produces unambiguous phase angles ~A
and ~B~ These are suppl;ed to a d;rectionaL angle calculating unit, CALC~, ~ which also receives an indication of A and B from the unit ACS. The unit CALC0, ~ calculates the value of O and/or ~ as explained above w;th reference to F;gure 2, also tak;ng ;nto account ;n calculat;ng ~ ~hich two ;nterferometers the phase measurements have been derived from, so as to add an appropriate ;ntegral mult;ple of 90 degrees.
The invention relates to a method and to a system for determining the direction of incidence of electromagnetic ~ave signals from a distant source by determining the difference between the time of arrival (ToA) of the leading edge of wave siynals received respectively from the source at the two elements of at leas~ one pair of mutually-spaced wave-receiving elements.
The invention furthex relates to a method and to a system for determining the direction of incidence of electromagnetic wave signals from a dis~ant source using both ToA
and phase-difference measurements.
ToA (also known as Time Difference of Arrival, TDOA) direction-finding (DF) with a long baseline, i.e. wherein the wave-recelving elements are spaced miles apart, is known from, for example, the article "Passive Direction Finding and Signal Location" by A.R. Baron et al, Microwave Journal, September 1982, pages 59-76~ see particularly page 59 and pages 66-70. A major disadvantage of ToA DF using a long baseline in many practical situations is that if, as is normally the case, the direction of incidence is to be determined over a substantial ran~e of directionæ, there is a substantial interval of time over which signals from the same source may arrive at one wave-receiving element of the pair relative to the other, the difference between the ToAs depending on the position of the source relative to the pair of elements. If there is a plurality of sources from which signals may be received, for example sources emitting pulsed signals with a subs~antial pulse repetition frequency (PRF), then "" ~LZI 3~93 there is a significant probability that the signals whose ToAs at the two elements are compared come from difierent sources rather than the same source; the greater the spacing between the elements, and consequently the longer the above-mentioned period, the greater is the probability. It is then necessary to compare one or more characteristic parameters of the ~``` gL21~14~3 signals received at the two elements, for example frequency, PRF or pulse length, to ascerta;n whether they come from the same or d;fferent sources. Not only does th;s requ;re substant;al further equ;pment, but ;t substant;ally ;ncreases the t;me taken to ascerta;n the d;rect;on of ;nc;dence of the s;gnals.
ToA DF us;ng a short basel;ne, for example 24 feet, ;s known from US patent 3 936 83~. The use of a short basel;ne has the advantage ~although there ;s no ment;on of ;t ;n the US patent) that the above-ment;oned ;nterval with;n wh;ch s;gnals from the same source can arrive at the two elements is so small that there ;s a h;gh probab;lity ;n pract;cal s;tuat;ons that s;gnals from a d;fferent source will not arr;ve in that period~ However, the use of a short basel;ne imposes the d;ff;culty of determ;ning time differences of the order of tens of nanoseconds or less. The above-ment;oned US patent proposes a system wherein a capac;tor is charged at a fast, linear rate from a constant-current source, charg;ng be;ng started by the arr;val of a pulse signal at one element and stopped by the arrival of a pulse signal at the other element; the time d;fference ;s then effectively multipl;ed by transferr;ng the voltage on the capac;tor to a further capac;tor wh;ch is d;scharged at a much slower constant rate. However, the ~ c;rcu;try disclosed for performing these funct;ons would not ;n ; practice be suitable for the very short t;me d;fferences ;nvolved.For example, the current from the constant-current source could not be sw;tched between zero and ;ts full value ;n a t;me wh;ch ;s short compared w;th the t;me d;fference ;nvolved. Moreover, F;gure 5 of the US patent, wh;ch ;s a graph of a count representat;ve of the measured t;me difference against time delay ~actual t;me d;fference), shows a predom;nantly substantially rect;linear variation from about 2000 nanoseconds down to about 150 nanoseconds;
at th;s po;nt, there ;s an abrupt change of slope, with what appears to be a hypothet;cal extrapolat;on to the or;g;n of the graph. Th;s ;nd;cates that the c;rcu;t would not in fact operate as ;ntended for the t;me differences of 0-50 nanoseconds that would actually need to be measured~
-` 12~ 93 It is an object of the invention to provide an improved method and system for short-baseline ToA DF. It is a further object of the invention to provide an improved timing circuit.
According to a first aspect of the invention, a method of de~ermining the direction of incidence of electromagnetic wave signals from a distant source from the time of arrival of the leading edge of the wave signals comprlses:
receiving said signals at a plurality of mutually spaced wave-receiving elements, detecting the respective instantaneous amplitude of the signal received a~ each element, measuring the times at which the detected amplitudes of wave signals received respectively at at least two of said elements first exceed a minimal threshold value such that signals can be satlsfactorily distinguished from noise and which threshold value is substantially less than the minimum peak value of signals whose direction of incidence is to be determined by said method, the time being measured ln suah a manner that the measured time is generally unaffected by multipa~h propagatlon, determining the difference between the measured times in respect of one palr or of a plurali~y of pairs of said elements, wherein the two elemen~s of said one pair or of each of at least two of said plurality of pairs are sufficiently close together tha~ the length of the interval of time within which signals from : the same source must arrive at ~he two elements is so short that there is a high probability in operation that no signals from another source will arrive in that interval, and 30~3 20104~8325 deriving a representation of the direction of incidence from the time difference(s) utilising the relationship cos ~ - c ~t~d where is the angle between the direction of incidence of the signals and the line joining the two elements of a said pair, d i5 ~he distance between those two elements, ~ t is the time difference between the signals at that pair of elements first exceeding the minimum threshold value, and c is the free-space velocity of electromagnetic waves.
According to a second aspect of the invention, a system for determining the direction of incidence of electromagnetic wave signals from a distant source from the time of arrival of the leading edge of the wave signals, comprises:
a plurality of mutually spaced wave-receiving elements, means for detecting the respective instantaneous amplitude of the signal received at each element, means for measuring the times at which the detected amplitudes of wave signals reaeived respectively at at least to of said elements first exceed a minlmal threshold value such that signals can be satisfactorily distingulshed from noise and which threshold value is substantially less than the minimum peak value of signals ~hose direction of incidence is to be de~ermined by said method, the time being measured in such a manner that the measured time is generally unaffected by multipath propagation, means for determining the difference between the measured times in respect of one pair or of a plurality of pairs of said elements, wherein the two elements of said one pair or of each of .
2010~-8325 at least two of said plurality of pairs are suificiently close together that the length of the interval of time within which signals from the same source must arrive at the two elements i5 SO
short that there is a high probability in operation that no signals from another source will arrive in that interval, and means ~or derivin~ a representation of the direction of incidence from the time difference(s) utilising the relationship cosc~ - c S t~d where c~ is ~he angle between the direction of incidence of the I.0 signals and the line joining the two elements of a said pair, d is the distance between those two elements, S t is the time difference between the signals at that pair of elements first exceeding the minimum threshold value, and c is the free-space velocity of electromagnetic waves.
These aspects of the invention involve the recognition that in contrast to other methods and systems ~or direction-finding such as in~erferometry, ToA DF can provide a good degree of immunity to multlpath propagation, provided that the time of arrival is measured 4a ~28[)~93 1 early on the leading edge of the s;gnal and ;n such a manner that the measured t;me ;s not substant;ally affected by mult;path propagation wh;ch involves more than a short add;t;or,al path length for the ;nd;rect, delayed signal. If the ToA ;s not measured early on the lead;ng edge of a s;gnal, multipath can s;gn;f;cantly d;stort the shape of the lead;ng edge as the s;gnal approaches its peak value~ leading to a substantial error ;n a measured t;me difference and hence in the der;ved d;rect;on of ;nc;dence. As w;ll be described below, the ToA may be measured ;n respect of a threshold wh;ch is well below the m;nimum peak ampl;tude and in such a manner that the measured time is unaffected by variations in signal ampl;tude w;thin a period of, for example, not more than about 10 nanoseconds after the threshold is first exceeded, so that the system is immune to mult;path propagation which ;nvolves more than about three metres add;t;onal path length, as w;ll generally be the case for a distant source.
The above-mentioned US patent pays particular attention to attempting to elim;nate the effects of pulse amplitude on ToA
measurement by us;ng a so-called Normalizer, but makes no reference to the poss;ble effects of mult;path. In the f;rst of two Normal;zer processes described with reference to Figure 7 of the patent, the value of the s;gnal ampl;tude at wh;ch the s;gnal ;s t;med ;s dependent on the rate of ;ncrease of the ampl;tude, and since th;s value would necessar;ly have to be at a m;n;mal threshold for sat;sfactory d;st;nct;on of rece;ved s;gnals from no;se when the rate of ;ncrease ;s at ;ts lowest acceptable value, the amplitude value at wh;ch the s;gnal ;s t;med w;ll generally be above the m;n;mal threshold value and hence not as early on the lead;ng edge as ;t m;ght be~ In the second Normalizer process descr;bed w;th reference to Figure 8 of the patent, a second threshold well above a first threshold ;s used. The Normal;zer processes are therefore inherently more suscept;ble to multipath wh;ch affects the shape of the lead;ng edge of the s;gnalO Moreover, ;n v;ew of the relat;vely slow-act;ng circu;try descr;bed ;n the patent, the Normal;zers would necessar;ly requ;re the t;med po;nt on a pulse to be well after the ~ ~28~493 1 start of the pulse. The present ;nvent;on ;nvolves the recogn;t;on that ;n pract;ce, it is more des;rable to obv;ate the effect of most ; mult;path propagation: t;m;ng errors due to multipath w;ll generally be worse than errors due to d;fferent s;gnal ampl;tudes. Moreover, by mak;ng the ToA measurement early on the lead;ng edge, ;naccuracies due to different signal amplitudes may ;n any case be reduced.
Ow;ng to the difficulty of measur;ng very short time differences accurately~ there ;s ;n pract;ce l;kely to be a significant uncertainty ;n the difference between measured ToAs, and therefore ToA DF with a short basel;ne ;s l;kely not to be very accurate, although a s;ngle ToA d;fference measurement can g;ve an unamb;guous ind;cat;on of d;rect;on .
~etter accuracy ;s achievable w;th interferometersO
Direction-finding using interferometers ;s well known. The d;fference ;n phase between s;gnals rece;ved respect;vely from a d;stant source at two mutually spaced wave-rece;ving elements is representat;ve of the angle between the d;rect;on of ;nc;dence of the s;gnals and the l;ne jo;n;ng the two elements. The greater the spac;ng between the elements, the more accurately the angle of ;nc;dence can be determ;ned, but the smaller ;s the unamb;guous range of directions of ;nc;dence. To resolve ambiguity, a rect;linear array of wave-rece;v;ng elements ;s used to prov;de a series of pairs of elements w;th progress;vely smaller spac;ngs; the w;dest-spaced pa;r prov;des an accurate but amb;guous representat;on~ and the closest-spaced pa;r prov;des a coarse but unamb;guous representation. With successive spacings in a suitable rat;o, the ambigu;ty ;n the phase measurement on the w;dest-spaced pa;r can be resolved by reference to the successively more closely-spaced pa;rs of the ser;es. However, to prov;de good accuracy may requ;re a substantial amount of equ;pment s;nce each element ;s associated w;th a respect;ve receiver, and with N
elements, at least (N-1) phase discr;minators are needed. Such a system may consequently be expensive.
It ;s also well known to determine the direction of ;nc;dence 8~493 of rad;o ~aves by compar;ng the ampLitudes of the signals received by t~o adjacent similar antennae whose respect;ve main-beam axes are ;nclined to one another. If the d;rection of inc;dence lies between the axes, the rat;o of the amplitudes is representat;ve of the angle between the direct;on of ;ncidence and either of the axes. This arrangement provides an unambiguous but reLatively ;naccurate measurement of d;rect;on.
Accord;ng to a th;rd aspect of the ;nvent;on, a method of determ;ning the direct;on of ;ncidence of electromagnetic wave s;gnaLs from a distant source compr;ses:-rece;v;ng sa;d signals at each of a plural;ty of mutually spaced wave-receiv;ng elements, measur;ng the phase d;fference between the signals rece;ved respect;vely at the two elements of one pa;r of sa;d elements or the respect;ve phase d;fferences between the signals rece;ved respectively at the two elements of each of a plurality of substant;ally collinear pairs of said elements with different respective spacings, wherein the phase measurement on said one pair or on the closest-spaced of said pairs is amb;guous ;n the operating range of directions of incidence and the operating frequency range, determining by a method embodying the first aspect of the invention the approximate direction of incidence of said signals from the times of arrival of the leading edges of corresponding wave signals received respectively at two of sa;d plurality of elements, the l;ne jo;ning wh;ch two elements is parallel to and substantially coincident ~ith the line joining said one pair or said plurality of substantially collinear pairs, wherein the range of possible values of the actual time difference due to the uncertainty in the d;fference between the measured t;mes corresponds to a range of angles of ;ncidence whose magnitude is not greater than the magn;tude of the range of angles of ;nc;dence corresponding to the unambiguous range of phase difference measurement on said one pair or said closest-spaced pair, and resolving the ambiguity in said ambiguous phase measurement by comparing the possible directions represented thereby with the ~~28~1493 1 approx;mate direct;on determ;ned from the d;fference between the measured t;mes.
Accord;ng to a fourth aspect of the ;nvent;on, a system for determin;ng the direct;on of ;nc;dence of electromagnetic wave signals from a distant source comprises:-a plural;ty of mutually spaced wave-rece;ving elements, means for measur;ng the phase difference between the s;gnals received respectively at the two elements of one pair of said elements or the respective phase differences between the signals received respectively at the two elements of each of a plurality of substant;ally coll;near pa;rs of sa;d elements ~;th d;fferent respect;ve spacings, where;n the phase measurement on said one pa;r or on the closest-spaced of said pa;rs is amb;guous in the operat;ng range of directions of incidence and the operat;ng frequency range, means, comprising a system embodying the second aspect of the invention, for determin;ng the approx;mate d;rect;on of inc;dence of sa;d s;gnals from the times of arrival of the lead;ng edges of correspond;ng signals rece;ved respect;vely at two of sa;d plural;ty of elements, the l;ne jo;n;ng wh;ch two elements ;s parallel to and substant;ally co;nc;dent ~ith the l;ne jo;ning sa;d one pa;r or sa;d plurality of substant;ally collinear pa;rs, where;n the range of poss;ble values of the actual t;me d;fference due to the uncertainty in the d;fference between the measured times corresponds to a range of angles of incidence whose magn;tude is not greater than the magn;tude of the range of angles of ;ncidence correspond;ng to the unamb;guous range of phase d;fference measurement on sa;d one pa;r or sa;d closest-spaced pair, and means for resolving the amb;gu;ty ;n said amb;guous phase measurement by comparing the possible d;rections represented thereby w;th the approx;mate direct;on determined from the difference between the measured t;mes.
~y using a ToA DF arrangement to resolve ambiguity in the phase measurement on the s;ngle pa;r or the closest-spaced pa;r of elements of an ;nterferometer DF arrangement, the need for one or more ;nterferometer channels wh;ch would merely be needed for -` 12~30493 1 resolving ambiguity but ~hich would not increase the accuracy of the direction-finding is avoided, and the combined arrangements may be cheaper and simpler than a purely interferometric one.
To determine directions of incidence over a ~ide range of directions~ particularly d;rections which are not restricted substantiaLly to a plane including the elements, a system embody;ng the fourth aspect of the invention may comprise phase-d;fference measur;ng means, approximate-direction-determing means and ambiguity-resolving means operable ;n respect of a f;rst pa;r or a f;rst plural;ty of substant;ally coll;near pairs of the elements and of a second pair or a second plurality of substantially collinear pairs of the elements to derive first and second unambiguous phase measurements, wherein the line joining the elements of said first pa;r or said f;rst plurality of pa;rs and the line joining the elements of said second pa;r or said second plurality of pairs are substantially coplanar and ;ncl;ned to one another, sa;d f;rst and second phase measurements being representative of the angle d between the direct;on of ;nc;dence and the l;ne joining the elements of the respective pair(s), and further comprising means for deriving a representation of the angle 0 and/or a representat;on of the angle ~, where 0 ;s the angle between the d;rect;on of ;nc;dence projected ;nto the plane of the l;nes and the normal to a respect;ve one of sa;d l;ne ;n said plane and where ~ ;s the angle between the d;rect;on of ;nc;dence and sa;d plane, from the f;rst and second unamb;guous phase measurements ut;l;s;ng the relat;onsh;p s;n (90 degrees - ~ ) = s;n ~ cos ~ .
For s;mpl;city, said lines may be mutually perpendicular.
In a particularly s;mple system, the approx;mate-direction-determining means may be operable in respect of the times of arrival at a common element and at each of t~o elements respectively on the two l;nes, and furthermore the phase-difference measuring means may be operable to measure the phase differences bet~een said common element and each of two elements respectively on the t~o lines.
As an alternat;ve, a system may comprise three or more mutually 1 ;ncl;ned successively adjacent pairs or plurality of pairs of elements, means for measuring the ampl;tude of wave signals received at one or more elements of each of said three or more pairs or plural;ty of pa;rs, and means for select;ng as sa;d f;rst pair or plural;ty of pa;rs one of sa;d three or more pa;rs or plurality of pa;rs ;n respect of ~hich the ampl;tude ;s at least as great as the ampl;tude in respect of each of the remain;ng pa;rs or plurality of pairs and as sa;d second pa;r or plurality of pa;rs a pair or plural;ty of pa;rs adjacent sa;d first pair or plural;ty of pa;rs ;n respect of wh;ch the ampl;tude ;s at least as great as the ampl;tude in respect of any other adjacent pair or plurality of pairs. Suitably, such a system comprises four mutually orthogonal pa;rs or plural;ty of pa;rs of elements~
A method embody;ng the f;rst aspect of the ;nvent;on may ;nvolve using three substantially coplanar but substantially non-collinear elements to form at least two said pairs, and deriving a representation of the angle 3 and/or a representat;on of the angle utilising the relat;onsh;p s;n (90-c~) = sin 0 cos ~
;n respect of each of sa;d at least two pairs, ~herein 0 is the angle between the d;rect;on of ;nc;dence projected ;nto the plane of the three elements and the normal ;n sa;d plane to the l;ne jo;n;ng the two elements of a sa;d pa;r, and ~ ;s the angle between the d;rect;on of inc;dence and sa;d plane~ D;rect;ons of ;nc;dence ;n three d;mens;ons may thus be determ;ned from ToA measurements at three elements.
The method may further compr;se determ;n;ng a parameter representat;ve of the rate of ;ncrease of the detected amplitude of the respect;ve s;gnal received at at least one of the elements ;n the reg;on of sa;d threshold value, and determining the direction of ;nc;dence of rece;ved s;gnals only if sa;d parameter sat;~f;es a criterion representing a m;nimum rate of ;ncrease ;n sa;d reg;on.
S;gnals whose d;rection of incidence cannot be determined with sat;sfactory accuracy can thereby be d;scarded. Such a method may compr;se measuring the time at wh;ch the detected amplitude first ~2~ 493 exceeds an adjacent further threshold value, wherein said parameter is the difference between the measured times in respect of the two threshold values, and wherein said criterion is that said parameter does not exceed a maximum value. Alternatively, such a method may comprise di~ferentiating the increasing detected amplitude at least in said region, wherein said parameter is the rate of increase in detacted amplitude derived by differentiation, and wherein said criterion is that said parameter exceeds a minimum value.
A system embodying the second aspect of the invention may comprise means for performing optional features of a method embodying the first aspect o~ the invention, as set forth in ~laims 7 to 10.
An embodiment of the means ~or measuring the times may comprise a clock pulse generator, a tapped delay device having a plurality of n mutually spaced taps, a latch coupled to the delay device for latching any signal on each o~ the n taps, and a decoding device coupled to the latch for producing a time representation from the signal(s) latched from the n taps, is characterized in that an input signal to be timed is coupled to the input of the delay device, in that the clock pulse generator is normally operable to clock the latch, in that the circuit comprlses inhibiting means responsive to the presence of a signal on at least one of the n taps when the latch is clocked to inhibit further clocking of the latch, and in that the decoding device is operable to produce a representation of the interval between the time that said input signal reaches the tap nearest the input o~
~IL2~0~93 the delay device and the preceding clock pulse.
Said interval may be represented as zero for the case in which the input signal has reached the tap furthest from the input when the latch is clocked, and other intervals represented accordingly.
A timing circuit embodying the third aspect of the invention may be contrasted with the timing circuit disclosed in GB 2 132 043 A and EP 113 935 A, in which the clock pulse generator is coupled to the input of the delay line, and the input signal to be timed is used ~o latch the latch.
lla ~;; 8~ 3 1 Su;tably, the circuit further comprises a counter for counting the pulses of the clock pulse generator, ~herein the inh;bit;ng means are further operable to inh;bit further count;ng of the clock pulses, the outputs of the decod;ng device and the counter being concatenated.
In order to be able to produce representat;ons of intervals over the majority of the per;od of the clock pulse generator, the period of the clock pulse generator may be not substantially less than the t;me delay bet~een the tap nearest to and the tap furthest from the input of the delay device. ~uitably, sa;d per;od ;s substant;ally equal to sa;d time delay.
To make good use of the delay dev;ce and to prov;de representations of integral multiples of a fraction of the period of the clock pulse generator, the time delay between each adjacent pair of the n taps may be the same, being equal to T, and the period of the clock pulse generator be nT.
Where the circuit ;s to be used to t;me the beg;nning of signals ~hich persist for at least the per;od of the clock pulse generator, the ;nh;bit;ng means may be respons;ve to the presence of a s;gnal on the tap nearest the ;nput of the delay device when the latch ;s clockedR Th;s helps to d;st;ngu;sh true signals to be t;med from no;se ;n the case where a s;gnal ;s present on a tap beyond the tap nearest the ;nput of the delay dev;ce ~hen the latch ;s clocked, since such a s;gnal m;ght be due to noise. As a further safeguard aga;nst false measurements due for example to no;se, the decoding device may be operable not to produce said time representat;on unless when the latch is clocked a s;gnal ;s present on each of the n taps between the ;nput of the delay device and the tap furthest from the ;nput of the delay dev;ce on ~hich a signal is present~
It has been found that attempting to operate a timing circuit of the k;nd d;sclosed ;n the above-ment;oned GB and EP publ;shed appl;cations to measure time to a resolution of about 1 nanosecond produces difficulties in synchron;sing the fine count produced by the decoding device and the coarse count produced by the counter. A
` ~80~93 timing circuit embodying the third aspect of the present invention can be both simpler and more reliable. Furthermore, it has been found advantageous to use the threshold crossing merely to feed an input signal to the delay device, rather than to use it to control gates: the latter is liable in practice to produce undesired di~tortion of the signal.
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Figure 1 is a block diagram of a ToA DF system embodying the inven~ion and comprising one pair of wave-receiving elements;
Flgure 2 is a block diagram of a timing circuit suitable for UBe in a ToA DF system embodying the invention;
Figure 3 is a block diagram of a ToA DF system embodying the invention and comprising three collinear wave-receiving elements;
Figure 4 illustrates the disposition of three non-collinear wave-receiving elements for an omnidirectional ToA DF
system embodying the invention:
Figure S illustrates schematically processing to calculate an angle using the elements of Figure ~;
Figure 6 is a schematic diagram of a simple DF system of a kind using both ToA and phase-difference measurements;
Figure 7 is a schematic diagram of a particularly simple omnidirectional DF system of this kind;
Figure 8 illustrates the disposition of ToA and inter~erometer antenna arrays for a more complex omnidirectional ~28049:~
DF system of this kind, and Figure 9 illustrates processing for a DF system using the arrays of Figure 8.
Figure 1 shows a simple system comprising one pair of wave-receiving elements. The system comprises two similar channels A and B respectively. Each of the channels comprises in succession an antenna AN'r, an RF amplifier RFA, a detector D, a video amplifier VA, and a timing circuit TC. The antennae may be omnidirectional, or may be directional with their axes substantially parallel. The 13a 28~g3 spacing d between the antennae is chosen to be sufficiently small that the length of the interval of t;me with;n wh;ch s;gnals from the same source must arr;ve at the two elements is so short that there ;s a h;gh probab;l;ty in operation that no signals from another source ~;ll arr;ve ;n that ;nterval. If s;gnals may be received from any direct;on, the length of the interval is t~ice the time taken for electromagnetic waves to travel the distance d (;n free space): the limits of this interval are set by the possibil;ty of s;gnals being ;ncident along the line joining the antennae ;n one sense or the other~ ;.e. from left or r;ght ;n Figure 17 The probability of no s;gnals arr;v;ng from another source ;n that ;nterval ~ill depend on the number of sources from which s;gnals can be rece;ved, the frequency with ~hich they em;t fresh s;gnals, and the durat;on of the s;gnals. What probability is sufficiently high w;ll depend on what proport;on of ;ncorrect representat;ons of d;rection, due to measurements hav;ng been made ;n the two channels on signals from d;fferent sources~ ;s cons;dered acceptable. For typ;cal opera~;onal situat;ons of the number of sources em;tt;ng pulsed signals, their PRF and pulse length, a separation d of the order of 50 feet, giv;ng an ;nterval length of about 100 nanoseconds, is considered to give an acceptably high probabil;ty.
When an RF s;gnal ;s ;ncident on the antenna of one of the channels, the RF signal is amplified and de~ected, and the time at which the instantaneous detected amplitude of the signal, after further ampl;ficat;on, f;rst exceeds a threshold value is measured. This threshold value is chosen to be substantially less than the min;mum peak value of signals whose direction of incidence is to be determined, as ~;ll be explained in greater detail belo~; the threshold w;ll usually be much less than the typical peak value.
The timing circu;ts operate w;th a common clock (CLOCK3. A
calculating un;t CALC determines the difference ~ t between the measured t;mes and provides therefrom a representation of the direction of incidence, for example of the angle ~ between the incident signals and the line joining the antennae, utilising the relationship 9~
cos o< = c ~t/d where c is the free-space velocity of electromagnetic waves.
The time at wh;ch the threshold value is first exceeded is measured in each timing circuit TC in such a manner that the measured time ;s unaffected by multipath propagat;on ~here the ; delayed s;gnaL has been reflected ~rom a surface not virtually coincident with the direct path between a distant source and the antenna.
An ECL tem;tter-coupled logic) circuit arrangement, su;table for the tim;ng circuit TC of Figure 1 and capable of measuring to an accuracy of 1 nanosecond, is shown in Figure 2. The detected and amplified video signal from the video amplif;er VA is applied to a very fast comparator COMP ~hose output changes from a log;c "O" to "1" when the input signal exceeds a threshold voltage VT. The t5 comparator output s;gnal ;s fed to the input of a tapped delay line TDL having 8 consecutive taps separated by 1 nanosecond intervals.
The taps are connected to respective ;nputs of an 8-b;t latch LATCH. The latch is clocked at 8 nanosecond ;ntervals by a 125 MHz clock CLOCK v;a a gate G1, the clock signal also being supplied via a further gate G2 to a synchronous counter CNTR ~hich prov;des a coarse measurement of time. The outputs, labelled 0-7, of the latch are fed to a decoding circuit DCDR; the presence o~ a signal on at least one output, in this case the first output, O, ;s also used to control the gates G1 and G2~ the output being connected thereto by a fast ~eedback loop. The outputs of the counter CNTR and the decoder DCDR are concatenated to give a representation of the t;me at wh;ch the output of the comparator COMP changed from O to 1.
In operation, the gates G1 and G2 are normally open. The counter CNTR measures time in 8-nanosecond un;ts, up to a maximum time at least as long as the above-mentioned interval, determ;ned by the antenna spacing d, within which signals from a distant source must be received by both antennae. The latch is s;m;larly clocked at 8-nanosecond intervals, but while the comparator output is 0, there are no signals from the tapped delay line, and the 3~ latch outputs rema;n at zero. ~hen the comparator output changes 80g~93 1 to 1 (which in the case of a true received signal as opposed to noise uill normally pers;st for longer than the period of the clock), the signal travels along the delay line changing successive tap outputs from 0 to 1. When the latch is next clocked, the series of ones and remaining zeroes is held ;n the latch. The presence of a "1" on the first output, 0, of the latch closes the gates G1 and G2, preventing further clocking of the latch and the counter. The output of the latch w;ll be one of the following codes:
1ûOOOOOO
'11 000000 11 100~00 111110nO
1111~111 The first of these codes represents the most recent ToA and the last the earliest ToA since the latch was last clocked; the first ind;cates that 7 nanoseconds should be added to the time represented by the counter CNTR while the last requ;res zero add;t;on. The decoder DCDR transforms that latch output to binary digits which are concatenated with the counter output~
The threshold the cross;ng of wh;ch ;s timed ;s, as prev;ously explained, set at a low value in order largely to avo;d errors due to multipath. Setting the threshold at a level substant;ally below the m;n;mum peak level of signals whose direction of incidence is to be determined, for example 10 dB below the minimum peak level, also provides the advantage of tending to alleviate timing errors ~hich would occur if the signal amplitude crossed the threshold at a slo~
r~ rate because the amplitude ~ reaching its peak level. The lowest level at which the threshold can be set will depend on the noise level in the system: ;f the threshold is set too close to the noise level, the accuracy of t;ming will be degraded by the random fluctuation in the amplitude of desired s;gnal plus noise, and a positive-going threshold crossing may even be caused by noise - ~ 213~9C93 17 PHB 33274 r 1 alone.
The decoder ;s ;n th;s embod;ment arranged to accept only the above-mentioned codes. It consequently accepts only s;gnals whose amplitude remains above threshold long enough to produce a continuous succession of ones in the latch, and rejects any other pattern of zeros and ones which might result from trigger;ng of the comparator by noise spikes or from a received signal w;th a slow rate of ;ncrease of amplitude.
It will be seen that provided the amplitude remains above threshold long enough for this state to be latched, the measurement of ToA will be unaffected by subsequent variat;ons in ampl;tude, ;n part;cular var;at;ons due to a delayed mult;path signal wh;ch arr;ves in phase oppos;tion to the or;g;nal d;rect-path signal and causes the amplitude to fall below t5 threshold~ The max;mum period taken to latch the above-threshold state is in this embodiment the length of the delay line, ;.e. 8 nanoseconds. This time may be much shorter than the t;me taken for the amplitude to reach typical peak value.
A delay not less than and substantially equal to the t;me taken for electromagnetic waves to travel the d;stance d may be ;ncluded in one channel before the timing c;rcuit so that time differences are measured with respect to the time of arrival of a signal in the other channel. Suitably, a time "windw" is used to prevent unnecessary computation on time differences which are too large for the signaLs to have come from the same source. Where the above-mentioned delay is included in one channel, this window may be de~ined as beg;nn;ng w;th a time difference of zero and end;ng with a time d;fference not less than and substant;ally equal to tw;ce the t;me taken for electromagnet;c ~aves to travel the distance d. The use of a time w;ndow also prov;des some protection against random noise signals wh;ch cause the detected amplitude to exceed the threshold from causing false measurements.
At least one, and preferably each, of the channels in the system of Figure 1 may comprise a signal validat;ng c;rcuit ~not shown) to ascertain the rate of increase of the amplitude of the 4~3 1 s;gnal in that channel in the region of the threshold vaLue, and to cause the system not t-o determine the direction of inc;dence unless the rate satisfies a criterion of m;nimum slope. For th;s purpose, the output of the video amplif;er VA may be supplied to a further timing circuit (not sho~n) wh;ch measures the t;me at wh;ch the signal amplitude first exceeds an adjacent further threshold value.
The difference between the times measured by the two circuits ;n a ~hannel may be determined, and the direction of ;nc;dence determ;ned only ;f the difference does not exceed a max;mum value~
0 Alternatively, the ampl;tude ;ncrease may be different;ated and the d;rection o~ incidence determined only if the rate of ;ncrease of a~plitude derived by differentiation exceeds a minimum value.
As a further way of distinguishing signals coming from a distant source from no;se, at least one of the channels may comprise means (not shown) for determining the peak amplitude of a signal wh;ch causes the threshold to be exceeded, and for inhibiting the determ;nation of the d;rect;on of incidence unless the peak ampl;tude is substantially greater than the threshold.
It is cons;dered that a su;table criter;on of minimum slope may be that the steepness of the rising edge should be predom;nantly controlled by the v;deo bandw;dth of the system. Th;s ;nter al;a has the effect of reducing the dependence of the measured t;me on the rate of increase of the RF ampl;tude and hence tends to ach;eve the same object as the Normal;zers ;n the above-mentioned US
nfltent. It may be des;rable for the v;deo bandwidth to be switchable between a broadband value and a narrowband value. The broadband value may allow more accurate t;m;ng of steeply-r;sing lead;ng edges, but the narrowband value may enable acceptable results to be obtained with more slowly-r;s;ng edges, since it may reduce the no;se level in the v;deo c;rcuit and allow the threshold to be set to a lower value and hence to a relat;vely steeper part of the lead;ng edge.
F;gure 3 depicts a modification of the system of F;gure 1 comprising three coplanar and collinearly d;sposed antennae L~ M, N
"
20104-g325 reæpectively in respective channels each channel being the same as each of the channels shown in Figure 1. The spacings of each of the two pairs of elements LM and MN are equal (each being d) and each satisfy the above-mentioned criterion that the spacing is sufficiently small that the length of the interval of time within which signals from the same source must arrive at the two elements of a palr is so short that there is a high probability in operation that no signals will arrive from another source in that interval; the spacing 2d between antennae L and N may however be too large to satisfy khis criterion. Nevertheless, the difference between the times of arrival of signals at antennae L and N may be used to provide a more accurate representation of the direction of incidence than could be provided by the system of Figure 1 if one or more steps are taken to reduce the posslbility that the time difference measured on one of the pairs of antennae LM, MN does not relate to the same source as the time difference measured on the other pair. For example, as indicated in Figure 3, the time differences measured in relation to each pair of antennae, tLM and tMN respectively, may be compared, and only if their values are equal to within a small tolerance is the direction of incidence determined from the difference between the times of arrival at antennae L and N; the probability that signals from different sources should result in substantially equal time differences being measured between the elements of the pairs LM and ~N is small, and even if the signals should have come from different sources, the resultant error in the indicated direction of incidence in relation to the source from which signals were first `- ~2~ 93 received at one pair of antennae will be small.
The systems so far described provide only a representation of the direction of incidence that defines an angle to the line joining a pair of wave-receiving elements, and hence the surface of a cone whose axis is said line. Where sources are known to lie substantially in a single plane including said line and where the wave-receiving elements are directional, this may be sufflcient (although it should be borne in mind that the accuracy with which c~ can be determined decreases as ~ decreases from 90 degrees to 0); however, when signals may be received from each side of said line, and particularly when sources are not restricted to a single plane, it is desirable to perform measurements on at least one further wave-receiving element which is not collinear with the one pair of elements. Figure 4 depicts the deposition of three substantially coplanar but not collinear elements A, B, C respectively, forming an arbitrarily-shaped trian~le. The spacings AB, BC, CA each satisfy the above-mentioned criterion of being sufficiently small. By measuring the times of arrival of slgnals at each elemen~, the direction of lncidence may ~e determined for the general case of distant sources in 3-dimensional space as follows.
Let the spacings AB, BC, CA be dl, d2, d3 respectively.
A
Let the angles CAB and ABC be m and n respectively. Let the length of the perpendicular from C onto AB be a, and the distance from A to the intersection of said perpendicular with AB be b, so that a - d3 sin m and h - d3 cos m. (Thus b is negative if m ~ 90 degrees.~ Let the angle between the direction of incidence and 2 !3(~ 3 the normal to AB in the plane of AB be ~ (so that ~ =
~90 deyrees-~) and coscX = sin ~), the angle between the normal to AB in the plane of ABC and the direction of incidence projected into that plane be ~ (typically the azimuth angle), and the angle between the direction of incidence and the plane o~ ABC be ~
~typically the alevation angle). Let the times oi arrival at A, B, C be tA tB~ tc respectively.
Then sin ~ = C(tA-tB)/~
Now sin ~ a sin ~ CQS
Writin~ x = c(t -t )/d y = c(t -t )/d z = c(tB-tc)/d2, we may put x - sin ~ cos and analogously y = sin ~-m) cos z ~ sin [~-(180-n)] cos -sin (~+n) cos ~.
20a -` ~LZ80493 1 El;minat;ng ~ from either of two pairs of these equat;ons, one obta;ns tan ~ = x sin m/~x cos m - y]
and tan 3 = -x sin n/~x cos n + z].
These expressions have two-fold amb;guity. To distinguish between -90 degrees < 0 ~ 90 degrees and 90 degrees ~ 0 < 270 degrees, one may note that the denominator of for example the first expression for tan ~ may be expanded as sin ~ cos ~ cos m - sin(~-m) cos or cos ~ cos ~ sin m.
Thus the denom;nator is pos;t;ve for -90 degrees ~ O ~ 90 degrees and negat;ve for 90 degrees ~ ~ < 270 degrees.
Re-writ;ng the first express;on for tan ~ in terms of the t;mes of arr;val and mult;plying the numerator and denom;nator by ~5 d1d3 tan O ~ (tA-t~)d3 sin m/C(tA-t~)d3 cos m -(tA-tC)d = - a (tA-tg)/~d1(tA-tC) ~ b~tA tB)J
= - a (tA-tB)/~(d1-b)(tA-tc)+b(t~-tc)~-Writ;ng P = a(tA tB) a = (d1-b) ~tA~tc R ~ b (tB-tC), one obta;ns tan O a - P/~Q+R) or ~ = - Arc tan CP/(Q+R)~
Figure 5 illustrates schematically the processing to calculate O
according to this expression. The differences tA-tp, tB-tC, tA-tC are formed from the measured times tA~ t~, tc and then scaled to produce the quantities P, Q, R. The quantities ~ and R are summed and d;vided into P; the angle whose tangent is equal to minus the quotient is then determ;ned, for example from a look-up table in a PROM~ to obtain an ambiguous value aamb f ~. The sign of (Q+R) is also checked; if (Q+R)> O, a quantity 00 = 180 degrees is produced, otherw;se ~0 = - ~amb and ~0 are summed to produce an unambiguous value of 0. Having calculated ~, the angle ~ may be L2~3~9L9~
1 calculated by subst;tut;ng ~ ;n, for example, the expression x = sin ~ cos ~ .
Alternatively, ~ may be calculated without needing to calculate by elim;nating 0 from a pair of the expressions for x, y, ~.
The calculations and processing may be simplified for particular cases. For an equilateral triangle of side d, a = ~3d/2 and b = d~2, so that the time differences need only be scaled by factors which are independent of du ~lternatively, if m = 90 degrees, a = d3 and b = 0~
so that the quantity R is zero. If d1 = d3, the scaling factors are again independent of the actual value of the spac;ng.
An omnidirectional direction-finding system may comprise four receiv;ng eLements d;sposed at the corners of a parallelogram, or more especially a rectangle and more part;cularly still a square.
The d;rection of inc;dence may be calculated from the times of arrival of the earliest-received three s;gnals wh;ch are of acceptable qual;ty. This allows for the possibil;ty that s;gnals rece;ved at one of the four elements may have been degraded by, for example, an obstruction in the region of the elements.
Comparison of the equations which can be derived from the above two expressions for tan ~ in terms of x and y and of x and z respectively to relate the error in 0 to errors in x, y and z show that the error is not dependent on wh;ch expression is used. The choice of which baselines are considered as primary and secondary baselines for determ;ning ~ is therefore not signif;cant.
As an alternative to a clock common to the t;ming c;rcu;ts of all the channels as depicted in Figures 1 and 3, each channel may have a respective accurate clock and the clocks be kept in synchronism via a lo~-bandwidth link. The time measurements and any other other data may be passed to a central processor and control unit via, for example, an optical fibre l;nk~
The principle of using both ToA and phase-difference . . ., ,~
~80~93 23 PH~ 33274 -measurements will be explained with reference to Figure 6. A ToA DF
system comprises two antennae ANT1, ANT2 separated by a distance L1, and a measuring and calculat;ng un;t MCtTOA) wh;ch determines the difference ~t between the respective times of arrival of the lead;ng edge of an RF s;gnal at the two antennae. The an~le c~
between the direction of incidence of the signal and the line joining the two antennae (the baseline of the system) is given by the equation cos o~ = c ~t/L1 ~13 where c is the free-space velocity of electromagnet;c waves. An interferometer DF system comprises a rectiLinear array of antennae, in this case three antennae ANT3, ANT4, ANTs, so disposed that the l;ne joining the antennae (the baseline of the system) is parallel to and substant;ally coincident ~;th the baseline of the ToA DF
system (in order that the two systems should measure the same angle o~in respect of signals from a distant source). The w;dest-spaced pa;r of antennae of the interferometer system are separated by a distance L2, and the closest-spaced pa;r by a distance L3; the accuracy w;th wh;ch the angle o~ can be determined by the interferometer system depends on the value of L2, and the unamb;guous range of coverage depends on the value of L3. The ;nterferometer system comprises a measur;ng and amb;gu;ty resolut;on un;t MAR~IR) wh;ch measures on each of a plural;ty of pairs of the antennae of different respect;ve spac;ngs, from the w;dest-spaced to the closest-spaced, the phase d;fference between RF signals rece;ved respect;vely at the two antennae of the pa;r, ;n this case the phase d;fference 035 and ~45 between ANT3 and ANTs and between ANT4 and ANTs respect;vely; the phase measurements may be performed after convert;ng the RF signals to an intermediate frequency tIF).
Since the measurement of phase is restr;cted to a range of 2 ~, the measured phases 0 are amb;guous. The actual unamb;guous phase d;fferences may be denoted ~ where = ~ + 2k~
where k is an integer. The unit MAR(IR) in known manner resolves the ambigu;ty in ~35 as far as possible by reference to ~45 (see for B~)~93 - 24 PH~ 33274 1 example Ge 1 337 099). Now cos o~ = C~3s/2 ~fL2 ~2) and cos o~ = C~4s~2 ~fL3 (3) where f is the frequency of the signals and ~35, ~45 are the unambiguous phases. The unamb;guous range of coveraQe of the interferometer system may be obta;ned by ;nsert;ng ;n equat;on 3 values of ~45 separated by 2 n.
Comb;n;ng equations 1 and 3, ~45 = 2 ~f/~t L3tL1 (4) Now ;n order sat;sfactorily to be able to resolve the rema;n;ng amb;gu;ty ;n the measured phases using the d;rect;on of ;ncidence determ;ned by the ToA DF system, the range of possible values of the actual difference between the ToAs of s;gnals at ANT1 and ANT2 due to uncerta;nty ;n the measured time difference should correspond to a range of bC whose magn;tude ;s not greater than the magn;tude of the range of oC correspond;ng to the range of ~45 wh;ch can be determ;ned unamb;guously from 045, ;.e. 2 ~. Thus ;f the uncerta;nty ;n ~t is ~ t, so that the range of possible values of the actual t;me difference is ~ t + ~ t, we obta;n from equat;on 4 2 7r ,~ 2 7r f .2 ~) t ~ L3 /L1 or L1/L3 ~ 2~ t f~ ~6) Relat;onsh;p 6 def;nes the m;n;mum value of the rat;o of the ToA
system basel;ne~ L1, to the closest spac;ng of the ;nterferometer system, L3, wh;ch w;ll enable sat;sfactory amb;gu;ty resolut;on w;th a g;ven uncerta;nty ~ t ;n the measured time difference, at the h;ghest frequency of operation. (The unambiguous angular coverage of the interferometer system ;ncreases as the frequency decreases, whereas the uncerta;nty ;n the angle measured by the ToA system ;s ;ndependent of frequency.) The outputs of un;ts MC(TOA) and MAR(IR) are fed to a calculat;ng un;t CALC wh;ch compares the value of 035, in ~h;ch ambigu;ty has been resolved as far as poss;ble by reference to a45, ~80~3 w;th an approximate but unambiguous value of ~35 derived from ~t in accordance w;th the equat;on ~35 = 2 ~f~ t L2/L1 (7) ~hich is obta;ned by combining equations 1 and 2; the ambiguity is resolved in kno~n manner. A representat;on of ~( ;s then calculated from the accurate value of ~ 5 derived from g35. The accuracy of th;s value of ~ ;s g;ven by differentiating equation 2:
~ G~= -c ~035/(2 ~fL2 s;no~ ) (8) where ~ o( ;s the uncertainty in the calculated value of ~ and ~ 035 is the poss;ble error ;n the measured phase d;fference 035.
If, for example, ~ 035 is 30 degrees and L2 = 0.66 metres, then from equat;on 8, S~ ;s ~25 degrees at 12 GHz and 0.5 degrees at 6 GHz~ I~ the uncertainty S t in the time difference is 2 nanoseconds, and taking L3 = 0.33 metres, then from relationship 6, L1 should be not less than 16 metres for operat;on up to 12 GHz.
If the frequency of the RF s;gnals is not previously known~ the compos;te To~l;nterferometer system should comprise means for measuring the frequency.
Resolut;on of ambi~u;ty ;n the ;nterferometer system by reference to the ToA system ;s particularly s;mple because both systems determine the direction of incidence with reference to the angle oC ~h;ch define a cone the axis of which is the baseline of the respective system, the baselines of the two systems being parallel and substant;ally co;nc;dent. ~y contrast, for example, an amplitude-comparison DF system locates the d;rection of ;nc;dence substant;ally in a plane normal to the plane of the main-beam axes of the antennae, wh;ch means that such a system is not readily compat;ble ~;th an interferometer DF system.
When the composite DF system is required only to determine the direction of inc;dence of signals from sources on one s;de of the common baseline, and part;cularly when sources can be assumed to be substant;ally ;n an s;ngle plane including the common baseline, calculation of the angle OCmay be suffic;ent to locate the direction of incidence~ The system may in that case use directional antennae which are relat;vely 7nsens;t;ve to s;gnals from the other ~X3~93 26 PHB 33274 ' 1 s;de of the baseline. Where sources l;e substantially ;n said plane but may be on either side of the basel;ne, omnid;rect;onal antennae being used, this ambigu;ty may be resolved by comparing the times of arrival of signals at the t~o antennae of the above-described ToA
system and at a third antenna coplanar but not collinear ~ith the first two~
It may be noted from equation 8 above that the accuracy is greatest ~hen o( = 90 degrees and decreases as o~ decreases to~ards zero. It may therefore be desirable to use a second composite ~ system w;th a common baseline coplanar with but inclined to that of the first system, for example at 90 degrees, to achieve improved accuracy for small values of C~. Such an arrangement may also be used when the direct;on of incidence is not restricted to a singLe plane. It may then be des;red to determ;ne the angle 9 and/or the angle ~ ~here s;n ~ = sin ~ cos ~ (9) where ~ is the angle between the direction of incidence and the normal to one of the common basel;nes ;n the plane ;nclud;ng the direct;on of ;nc;dence ~so that ~ = t90 degrees -C~ ) and s;n ~ = cos 0C), ~ ;s the angle between the d;rection of ;nc;dence projected ;nto the plane of the basel;nes and the normal to the relevant basel;ne ;n that plane, and ~ ;s the angle between the direction of inc;dence and the plane of the basel;nes. Typically, the plane of the basel;nes ;s hor;zontal, so that ~ ;s bear;ng and ~ is elevation.
F;gure 7 ;s a schemat;c d;agram of a part;cularly s;mple omnid;rect;onal DF system us;ng two compos;te ToA/interferometer arrangements ~ith coplanar mutually orthogonal common baselines.
The system comprises an array of seven antennae ANT21-ANT27 ~ith omnidirect;onal responses ;n the plane of th~ arrays, as denoted by a c;rcular symbol. Four antennae, ANT21-ANT2~, are located at the corners of a square; a fifth antenna, ANT2s~ ;s located at the centre of the square, and the remain;ng antennae, ANT26 and ANT27, are respectively d;sposed colL;nearly ~;th the d;agonaLs of the squares~ equ;d;stant from the centre of the square. A f;rst ToA
~8~ 3 system comprises antennae ANT26 and ANT2s, and the assoc;ated first interferometer system compr;ses antennae ANT21, ANT2s and ANT23.
The second ToA system comprises antennae ANT27 and ANT2s, and the assoc;ated second ;nterferometer system compr;ses antennae ANT22, ANT2s and ANT24. The ToA systems comprise respective measurement ar7d calculation units MC(TOA)1, MC(TOA)2 (although the units may be integrated to the extent that onLy a single ToA measurement is required ;n respect of antenna ANT2s), and the ;nterferometer systems comprise respective measurement and ambigu;ty resolution units MAR(IR)1~ MAR(IR~2. The outputs of the assoc;ated ToA and ;nterferometer systems are fed to respective calculating units CALC1, C~LC2 which in th;s embod;ment only determine the respèctive unambiguous phase measurements ~ 2~ referred to the widest-spaced pair of antennae of the respective interferometer, that represent the angles ~1~ DC2 between the direction of ;ncidence of RF signals and the respective common baseline. Having regard to equations 2 and 9 above, one may write ~1 = 2 ~fL2 sin ~ cos ~ /c (10) and analoguously ~2 = -2 ~fL2 cos ~ cos ~ /c~ t11) Solving these simultaneous equations~ one obtains tan 0 = -~ 2 t12) cos ~ 12+~22)1/2 (c/2 ~fLz). ~13) The unabiguous phase angles ~ 2 are supplied to a further calculating unit CALC 3 ~hich calculates ~ and/or ~ in accordance with the above equat;ons~
While the system of Figure 7 is particularly simpLe, it does have the disadvantage that some of the antennae are liable to obstruct signals to others of the antennae when the direct;on of incidence is at only a small angle ( ~) to the plane of the antenna array~ This ;s liable to affect the accuracy of the overall system, since the interferometers are more susceptible to errors due, for example, to mult;path. Thus~ when the plane ;s horizontal, the system is best su;ted to measur;ng direct;ons of ;nc;dence at substantial angles of elevat;ons. F;gure 8 ;llustrates an 8[)~9~
1 alternative antenna arrangement wh;ch ;s better su;ted for small values of ~ as well as larger values. Omn;d;rect;onal coverage ;s ;n th;s case prov;ded by four mutually perpend;cular nterferometer systems, IR1-IR4, d;sposed about a common central po;nt and each compr;s;ng three un;formly spaced antennae wh;ch ;n th;s case each have a substantial response over approximately 180 degrees ;n the plane of the antenna array (as denoted in F;gure 8 by a sem;circular symbol). ~ach of two ToA systems aga;n compr;ses a pa;r of omn;direct;onal antennae wh;ch ;n th;s embod;ment are d;sposed on oppos;te s;des of the common central point. The respective measurement and amb;guity resolut;on units of the ;nterferometers, MAR(IR)1-MAR(IR)4~ in this embodiment compr;se means for measuring the amplitude of the s;gnal received at at least one of the antennae of the respect;ve array. The measured amplitudes, A1-A4, and the amb;guous measured phases, 01-04~ (in which ambiguity has been resolved as far as possible by reference to the closest-spaced pa;r of antennae of the ;nterferometer) from the four ;nterferometers, as well as the time d;fferences measured by the two ToA systems, ~t1 and ~tz, are then processed as will now be descr;bed w;th reference to Figure ~
The amplitudes A1-A4 are suppl;ed to an ampl;tude compar;son and select;on control un;t ACS, and the phases 01-04 are fed to a phase selector un;t PS. The un;t ACS compares the ampl;tudes and selects for further processing two adjacent ;nterferometers, denoted A and B~ At one of these, the ampl;tude ;s at least as great as at each of the rema;n;ng ;nterferometers, and at the other, the amplitude is at least as great as the ampl;tude at the other adjacent interferometer; the basel;nes of ;nterferometers A and B
are respectively parallel to those of the f;rst and second ToA
systems. The phase selector unit PS accordingly selects the amb;guous phases from those two ;nterferometers, 0A and ~, and suppl;es them to a phase calculat;ng un;t CALC0 wh;ch also rece;ves the values of ~tl and at2 measured by the two ToA systems. The un;t CALC0 separately resolves the ambiguity ;n 0A and ~ by reference to ~t1 and ~t2 respectively, as explained above w;th ~Z~)493 1 reference to Figure 1, and produces unambiguous phase angles ~A
and ~B~ These are suppl;ed to a d;rectionaL angle calculating unit, CALC~, ~ which also receives an indication of A and B from the unit ACS. The unit CALC0, ~ calculates the value of O and/or ~ as explained above w;th reference to F;gure 2, also tak;ng ;nto account ;n calculat;ng ~ ~hich two ;nterferometers the phase measurements have been derived from, so as to add an appropriate ;ntegral mult;ple of 90 degrees.
Claims (26)
1. A method of determining the direction of incidence of electromagnetic wave signals from a distant source from the time of arrival of the leading edge of the wave signals, wherein the method comprises, receiving said signals at a plurality of mutually spaced wave-receiving elements, detecting the respective instantaneous amplitude of the signal received at each element, measuring the times at which the detected amplitudes of wave signals received respectively at at least two of said elements first exceed a minimal threshold value such that signals can be satisfactorily distinguished from noise and which threshold value is substantially less than the minimum peak value of signals whose direction of incidence is to be determined by said method, the time being measured in such a manner that the measured time is generally unaffected by multipath propagation, determining the difference between the measured times in respect of one pair or of a plurality of pairs of said elements, wherein the two elements of said one pair or of each of at least two of said plurality of pairs are sufficiently close together that the length of the interval of time within which signals from the same source must arrive at the two elements is so short that there is a high probability in operation that no signals from another source will arrive in that interval, and deriving a representation of the direction of incidence from the time difference(s) utilising the relationship cos .alpha. = c.delta. t/d where .alpha. is the angle between the direction of incidence of the signals and the line joining the two elements of a said pair, d is the distance between those two elements, .delta. t is the time difference between the signals at that pair of elements first exceeding the minimum threshold value, and c is the free-space velocity of electromagnetic waves.
2. A method as claimed in Claim 1 using three substantially coplanar but substantially non-collinear elements to form at least 30a two said pairs, the method comprising deriving a representation of the angle .theta. and/or a representation of the angle .beta. utilising the relationship sin (90-?) = sin .theta. cos .beta.
in respect of each of said at least two pairs, wherein .theta. is the angle between the direction of incidence projected into the plane of the three elements and the normal in said plane to the line joining the two elements of a said pair, and .beta. is the angle between the direction of incidence and said plane.
in respect of each of said at least two pairs, wherein .theta. is the angle between the direction of incidence projected into the plane of the three elements and the normal in said plane to the line joining the two elements of a said pair, and .beta. is the angle between the direction of incidence and said plane.
3. A method as claimed in Claim 1 which further comprises determining a parameter representative of the rate of increase of the detected amplitude of the respective signal received at at least one of the elements in the region of said threshold value, and determining the direction of incidence of received signals only if said parameter satifies a criterion representing a minimum rate of increase in said region.
4. A method as claimed in Claim 3 which comprises measuring the time at which the detected amplitude first exceeds an adjacent further threshold value, wherein said parameter is the difference between the measured times in respect of the two threshold values, and wherein said criterion is that said parameter does not exceed a maximum value.
5. A method as claimed in Claim 3 which comprises differentiating the increasing detected amplitude at least in said region, wherein said parameter is the rate of increase in detected amplitude derived by differentiation, and wherein said criterion is that said parameter exceeds a minimum value.
6. A system for determining the direction of incidence of electromagnetic wave signals from a distant source from the time of arrival of the leading edge of the wave signals, wherein the system comprises:-a plurality of mutually spaced wave-receiving elements, means for detecting the respective instantaneous amplitude of the signal received at each element, means for measuring the times at which the detected amplitudes of wave signals received respectively at at least two of said elements first exceed a minimal threshold value such that signals can be satisfactorily distinguished from noise and which threshold value is substantially less than the minimum peak value of signals whose direction of incidence is to be determined by said method, the time being measured in such a manner that the measured time is generally unaffected by multipath propagation, means for determining the difference between the measured times in respect of one pair or of a plurality of pairs of said elements, wherein the two elements of said one pair or of each of at least two of said plurality of pairs are sufficiently close together that the length of the interval of time within which signals from the same source must arrive at the two elements is so short that there is a high probability in operation that no signals from another source will arrive in that interval, and means for deriving a representation of the direction of incidence from the time difference utilising the relationship cos .alpha. = c .delta. t/d where .alpha. is the angle between the direction of incidence of the signals and the line joining the two elements of a said pair, d is the distance between those two elements,.delta. t is the time difference between the signals at that pair of elements first exceeding the minimum threshold value, and c is the free-space velocity of electromagnetic waves.
7. A system as claimed in Claim 6 comprising three substantially coplanar but substantially non-collinear elements disposed to form at least two said pairs, wherein said means for deriving a representation of the direction of incidence comprises means for deriving a representation of the angle .theta. and/or a representation of the angle .beta. utilising the relationship sin (90-.alpha.) = sin .theta. cos .beta.
in respect of each of said at least two pairs, wherein is the angle between the direction of incidence projected into the plane of the three elements and the normal in said plane to the line joining the two elements of a said pair, and .beta. is the angle between the direction of incidence and said plane.
32a
in respect of each of said at least two pairs, wherein is the angle between the direction of incidence projected into the plane of the three elements and the normal in said plane to the line joining the two elements of a said pair, and .beta. is the angle between the direction of incidence and said plane.
32a
8. A system as claimed in Claim 6 comprising means for determining a parameter representative of the rate of increase of the detected amplitude of the respective signal received at at least one of the elements in the region of said threshold value, and means for inhibiting the determination of the direction of incidence if said parameter does not satisfy a criterion representing a minimum rate of increase in said region.
9. A system as claimed in Claim 8 wherein the parameter-determining means comprise means for measuring the time at which the detected amplitude first exceeds an adjacent further threshold value, said parameter being the difference between the measured times in respect of the two threshold values, and said criterion being that said parameter does not exceed a maximum value.
10. A system as claimed in Claim 8 wherein the parameter-determining means comprise means for differentiating the increasing detected amplitude at least in said region, said parameter being the rate of increase in detected amplitude derived by differentiation, and said criterion being that said parameter exceeds a minimum value.
11. A method of determining the direction of incidence of electromagnetic wave signals from a distant source, the method comprising:-receiving said signals at each of a plurality of mutually spaced wave-receiving elements, measuring the phase difference between the signals received respectively at the two elements of one pair of said elements or the respective phase differences between the signals received respectively at the two elements of each of a plurality of substantially collinear pairs of said elements with different respective spacings, wherein the phase measurement on said one pair or on the closest-spaced of said pairs is ambiguous in the operating range of directions of incidence and the operating frequency range, determining by a method as claimed in any one of Claims 1 to 3 the appropriate direction of incidence of said signals from the times of arrival of the leading edges of corresponding wave signals received respectively at two of said plurality of elements, the line joining which two elements is parallel to and substantially coincident with the line joining said one pair or said plurality of substantially collinear pairs, wherein the range of possible values of the actual time difference due to the uncertainty in the difference between the measured times corresponds to a range of angles of incidence whose magnitude is not greater than the magnitude of the range of angles of incidence corresponding to the unambiguous range of phase difference measurement on said one pair or said closest-spaced pair, and resolving the ambiguity in said ambiguous phase measurement by comparing the possible directions represented thereby with the approximate direction determined from the difference between the measured times.
12. A method of determining the direction of incidence of electromagnetic wave signals from a distant source, the method comprising performing a method as claimed in Claim 11 in respect of a first pair or a first plurality of substantially collinear pairs of the elements to derive a first unambiguous phase measurement, performing a method as claimed in Claim 11 in respect of a second pair or a second plurality of pairs of substantially collinear pairs of the elements to derive a second unambiguous phase measurement, wherein the line joining the elements of said first pair or said first plurality of pairs and the line joining the elements of said second pair or said second plurality of pairs are substantially coplanar and inclined to one another, said first and second phase measurements being representative of the angle .alpha.
between the direction of incidence and the line joining the elements of the respective pair(s), and deriving a representation of the angle .theta. and/or a representation of the angle .beta. , where .theta. is the angle between the direction of incidence projected into the plane of the lines and the normal to a respective one of said lines in said plane and where .beta. is the angle between the direction of incidence and said plane, from the first and second unambiguous phase measurements utilising the relationship sin (90 degrees - .alpha.) = sin .theta. cos .beta. .
between the direction of incidence and the line joining the elements of the respective pair(s), and deriving a representation of the angle .theta. and/or a representation of the angle .beta. , where .theta. is the angle between the direction of incidence projected into the plane of the lines and the normal to a respective one of said lines in said plane and where .beta. is the angle between the direction of incidence and said plane, from the first and second unambiguous phase measurements utilising the relationship sin (90 degrees - .alpha.) = sin .theta. cos .beta. .
13. A system for determining the direction of incidence of electromagnetic wave signals from a distant source, comprising:-a plurality of mutually spaced wave-receiving elements, means for measuring the phase difference between the signals received respectively at the two elements of one pair of said elements or the respective phase differences between the signals received respectively at the two elements of each of a plurality of substantially collinear pairs of said elements with different respective spacings, wherein the phase measurement on said one pair or on the closest-spaced of said pairs is ambiguous in the operating range of directions of incidence and the operating frequency range, means, comprising a system as claimed in any one of Claims 6 to 10, for determining the approximate direction of incidence of said signals from the times of arrival of the leading edges of corresponding signals received respectively at two of said plurality of elements, the line joining which two elements is parallel to and substantially coincident with the line joining said one pair or said plurality of substantially collinear pairs, wherein the range of possible values of the actual time difference due to the uncertainty in the difference between the measured timescorresponds to a range of angles of incidence whose magnitude is not greater than the magnitude of the range of angles of incidence corresponding to the unambiguous range of phase difference measurement on said one pair or said closest-spaced pair, and means for resolving the ambiguity in said ambiguous phase measurement by comparing the possible directions represented thereby with the approximate direction determined from the difference between the measured times.
14. A system as claimed in Claim 13 comprising phase-difference measuring means, approximate-direction-determining means and ambiguity-resolving means operable in respect of a first pair or a first plurality of substantially collinear pairs of the elements and of a second pair or a second plurality of substantially collinear pairs of the elements to derive first and second unambiguous phase measurements, wherein the line joining the elements of said first pair or said first plurality of pairs and the line joining the elements of said second pair or said second plurality of pairs are substantially coplanar and inclined to one another, said first and second phase measurements being representative of the angle .alpha. between the direction of incidence and the line joining the elements of the respective pair(s), and further comprising means for deriving a representation of the angle .theta. and/or a representation of the angle .beta. , where .theta. is the anglebetween the direction of incidence projected into the plane of the lines and the normal to a respective one of said line in said plane and where .beta. is the angle between the direction of incidence and said plane, from the first and second unambiguous phase measurements utilising the relationship sin (90 degrees - .alpha. ) = sin .theta. cos .beta. .
15. A system as claimed in Claim 14 wherein said lines are mutually perpendicular.
16. A system as claimed in Claim 14 or 15 wherein the approximate-direction-determining means are operable in respect of the times of arrival at a common element and at each of two elements respectively on the two lines.
17. A system as claimed in Claim 16 wherein the phase-difference measuring means are operable to measure the phase differences between said common element and each of two elements respectively on the two lines.
18. A system as claimed in Claim 15 comprising three or more mutually inclined successively adjacent pairs or plurality of pairs of elements, means for measuring the amplitude of wave signals received at one or more elements of each of said three or more pairs or plurality of pairs, and means for selecting as said first pair or plurality of pairs one of said three or more pairs or plurality of pairs in respect of which the amplitude is at least as great as the amplitude in respect of each of the remaining pairs or plurality of pairs and as said second pair or plurality of pairs a pair or plurality of pairs adjacent said first pair or plurality of pairs in respect of which the amplitude is at least as great as the amplitude in respect of any other adjacent pair or plurality of pairs.
19. A system as claimed in Claim 18 comprising four mutually orthogonal pairs or plurality of pairs of elements.
20. A system as claimed in Claim 6 or 13 wherein said means for measuring the times comprises a clock pulse generator, a tapped delay device having a plurality of n mutually spaced taps, a latch coupled to the delay device for latching any signal on each of the n taps, and a decoding device coupled to the latch for producing a time representation from the signals (s) latched from the n taps, wherein an input signal to be timed is coupled to the input of the delay device, the clock pulse generator is normally operable to clock the latch, the circuit comprises inhibiting means responsive to the presence of a signal on at least one of the n taps when the latch is clocked to inhibit further clocking of the latch, and wherein the decoding device is operable to produce a representation of the interval between the time that said input signal reaches the tap nearest the input of the delay device and the preceding clock pulse.
21. A system as claimed in Claim 20 further comprising a counter for counting the pulses of the clock pulse generator, wherein the inhibiting means are further operable to inhibit further counting of the clock pulses, the outputs of the decoding device and the counter being concatenated.
22. A system as claimed in Claim 21 wherein the period of the clock pulse generator is not substantially less than the time delay between the tap nearest to and the tap furthest from the input of the delay device.
23. A system as claimed in Claim 22 wherein said period is substantially equal to said time delay.
24. A system as claimed in Claim 23 wherein the time delay between each adjacent pair of the n taps is the same, being equal to T, and the period of the clock pulse generator is nT.
25. A circuit as claimed in Claim 20 wherein the inhibiting means are responsive to the presence of a signal on the tap nearest the input of the delay device when the latch is clocked.
26. A circuit as claimed in Claim 20 wherein the decoding device is operable not to produce said time representation unless, when the latch is clocked, a signal is present on each tap between the input of the delay device and the tap furthest from the input of the delay device on which a signal is present.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000539346A CA1280493C (en) | 1986-06-10 | 1987-06-10 | Radio direction-finding using time of arrival measurements |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8614107 | 1986-06-10 | ||
| GB8614108 | 1986-06-10 | ||
| CA000539346A CA1280493C (en) | 1986-06-10 | 1987-06-10 | Radio direction-finding using time of arrival measurements |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1280493C true CA1280493C (en) | 1991-02-19 |
Family
ID=4135863
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000539346A Expired - Lifetime CA1280493C (en) | 1986-06-10 | 1987-06-10 | Radio direction-finding using time of arrival measurements |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1280493C (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN108169710A (en) * | 2017-11-16 | 2018-06-15 | 捷开通讯(深圳)有限公司 | Localization method and alignment system based on reconfigurable antenna |
| CN112129833A (en) * | 2016-01-05 | 2020-12-25 | 雅马哈精密科技株式会社 | Ultrasonic inspection method |
-
1987
- 1987-06-10 CA CA000539346A patent/CA1280493C/en not_active Expired - Lifetime
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112129833A (en) * | 2016-01-05 | 2020-12-25 | 雅马哈精密科技株式会社 | Ultrasonic inspection method |
| CN108169710A (en) * | 2017-11-16 | 2018-06-15 | 捷开通讯(深圳)有限公司 | Localization method and alignment system based on reconfigurable antenna |
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