GB2309842A

GB2309842A - Radio direction finder and beacon

Info

Publication number: GB2309842A
Application number: GB9602005A
Authority: GB
Inventors: Stephen Joseph Robinson; Angle Measurement Co Cambridge
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-03-07
Filing date: 1996-02-01
Publication date: 1997-08-06
Anticipated expiration: 2016-02-01
Also published as: GB9602005D0; GB2309842B

Abstract

Ground plane referenced triplet receiving or transmitting antenna arrays (figure 2), are used to measure the direction of propagation of radio signals. The arrays may be optimised for high accuracy and high integrity elevation measurements over selected angular sectors, notably at low angles in the presence of ground reflections. Various receiving arrangements may be embodied in equipment compatible with transmitting, reflecting and transponder based systems; and may be used with identity and range information therefrom, to determine the height of objects. A particular array is described, and included in a system (figure 6), for monitoring the height of aircraft near to airfields. This system may "stand alone" or be fully integrated with standard secondary surveillance radar equipment in the "Airfield Management" or wider "Air Traffic Control" system. A transmitting arrangement is also described which will reduce the cost of standard I.L.S. glidepath installations.

Description

RADIO ANGLE MEASURING INSTRUMENT The invention relates to arrangements of antenna elements to measure the direction of propagation of electromagnetic radiation, and their embodiment in systems to determine the height of objects, such as low flying aircraft.

The directional properties of antennas and the processing of phase and amplitude information from separate antenna elements has long been used for determining the angle of arrival of signals, and thus determining the direction of the source transmitter. However, low elevation angles are difficult to measure because of interference from strong unquantifiable reflections from the ground. Moreover, radio direction finders and navigation systems are constrained by weather conditions, to use relatively low operating frequencies, where ground reflections cannot be excluded by the natural directivity of practical antennas.

A unique equi-spaced triplet arrangement of similar antenna elements was identified in U.K. Patent No.1449196, which when placed orthogonal to a reflecting ground plane, could be used to determine a function of elevation angle which was independent of the unknown ground reflection coefficient. Patent No. 1449196 describes the use of a triplet to define fixed angles in the elevation plane, within which interferometer angle measurements were made. This invention extends these ideas to optimise angle measurements using these triplets, notably over selected sectors at of low elevation angles.

Figure 1 shows a single equi-spaced triplet for which the two R.F. outputs A+C and B are shown in the above patent to be; A+C=2f(a,p,r,h,R,#)cos(2.#.d.sin #/#)..... ... ...........(1) B=f(a,p,r,h,R,#).................................... ........... .(2) a represents the antenna element pattern.

p represents the equivalent transmitted power.

r represents the range to the transmitter.

h represents the mid-height of the triplet.

R represents the ground reflection coefficient.

# represents the elevation angle.

d represents the triplet spacing.

and # represents the wavelength of the transmitted signal.

Whence, the quotient of (1) and (2) above is independent of a,p,r,h and R. The quotient of other functions of (1) and (2), for example the square or the modulus, is also independent of a,p,r,h, and R. The square of the signal amplitude is the natural output of a radio receiver, and takes only positive values, which is advantageous for post detection signal processing.

At low elevation angles, R approximates to -1, and in this case the function (f) simplifies to; f'(a,p,r)sin(2.ll.h.sin/ A )....................................(3) so that h may be chosen to maximise the amplitude of (f).

Measurement of elevation angle can only be made, when amplitude(f), or amplitude(f) squared, is non-zero, and is best made when the aplitude is changing slowly, with angle, near its maximum value.

It will also be noted that when d = A/2, (A+C)/2B takes values from 1 to -1 as the elevation angle changes over 90 degrees, Typically, from the horizontal to the vertical, and that larger values of d and other functions of (A+C)/2B may increase the sensitivity with which the elevation angle can be measured.

However, for a single triplet, the angle measurement may become ambiguous over the angular range of interest.

According to this invention, a general set of these triplets, as shown in figure 2, may be deployed, in which d(a), d(b),..d(q)...

and h(a), h(b),..h(q),... may be chosen independently to suit the angular accuracy, angular resolution, angular coverage and measurement integrity required.

The triplet arrays may be used in receiving or transmitting mode, and one application of each will be described by way of example.

As the receiving example, figure 3 shows seven antenna elements deployed as three triplets arranged to operate on responses from secondary surveillance radar interrogations, at a wavelength of about 30 centimetres. It is assumed, for this example, that processing the signals to determine the quotient is best carried out on the signals A+C and B after detection by "square law" detectors.

The design aims in this example, are; to provide unambiguous angle measurements up to about 7 degrees.

to provide measurement accuracy and resolution of the order +/-o.5 degrees from 3 degrees, at which aircraft normally approach to land, down to the lowest angle possible (about +0.5 degrees in this case).

. use the smallest number of antenna elements possible, with the lowest element not so close to the ground, that it might be obscured, and the highest antenna element at an acceptable height.

d is chosen for the elevation angle, at which most measurement sensitivity is required, and h is chosen for the elevation angle at which the maximum value of (f) is helpful. For these criteria values for h and d are calculated from the relationships; 2.n.d.sin / A =(2n+1) fl /4 ......... ......... .... (3) 2.u.h.sin q,/ A =(2n+1) II /2 .... ......... (4) where n is the integer 0,1,2,3,...

In this example values of d and h are optimised for approximately the same elevation angles, so that amplitude(f) is at a maximum and altering slowly at angles where sensitive measurements are needed.

Referring to figure 3, the triplet (n) is arranged to give its best measurements at around one degree and three degrees, the triplet (m), to give its best measurements at around two degrees and the triplet (1), its best at around five degrees. All readings are examined at each elevation angle to decide which triplet measurement is to be preferred, and the extent of the agreement between the three reading is a measure of the integrity of the system.

Typical values that might be chosen for d and h, in wavelengths, are then as follows; d(l)=1.5 h(l)=2.5 d(m)=3.75 h(m)=7. 75 d(n)=7.5 h(n)=15.25 The highest element is at 22.75 wavelengths, which is about 7 metres from the ground.

It will be noted that, subject to a small variation to allow antenna elements to be shared between triplets, h=2d, and that common values of sinv satisfies both equations (3) and (4) for all values of n. Thus, a very long array with a suitable ambiguity resolving system, is capable of giving very high angle measurement accuracy over each of its high amplitude regions.

The thick lines on figure 4 show the preferred measurement ranges for each triplet.

This example is appropriate to the important application of monitoring the height of aircraft approaching to land, which, hitherto, has not been possible with the standard secondary surveillance radar system. However, many other arrangements are possible for civil and military applications, where the measurement of the elevation angle of an emitting or reflecting object is required.

Figure 5 show one of many possible arrangements for measuring the quotient ((A+C)/2B) all squared. Superheterodyne receivers will normally be necessary to achieve the sensitivity required and the dynamic range of function (f), for example, for an aircraft flying from 20,000 metres to 200 metres will be large. However, 2B > (A+C), the squared quotient is always positive and, normally, will be in the range 0.33 to 0.67. In figure 5, a divider is used, where (B squared) sets the gain of two balanced amplifiers (Al and A2), and a timer (T) and comparator (C) measure the decay time of a CR circuit from a charged voltage corresponding to 4{B squared), to the voltage corresponding to ((A+C) squared}. It is well known that the exponential nature of the decay, ensures that the time delay measured is a function of quotient required, and is independent of the absolute amplitude of the signals. In a further refinement the received signals are sampled as quickly as possible after their arrival, so that multi-path interference effects from lateral, and therefore delayed, reflections are minimised.

Figure 6 shows a system in which a directional secondary surveillance radar interrogator (I), measures the range of identified aircraft (T), and triggers a measurement, by the elevation measuring system (E) described, on part or all of the reply message from the selected aircraft. Thus angles are firmly associated with particular identified aircraft, at a known range.

In one possible embodiment, the high directivity of the interrogator antenna (A) is also used to set the seven degree upper coverage limit, by blanking the system, when the signals it receives, fall below those received by the elevation array.

In the second example, figure 7 shows the use of a triplet array, as an ILS glide-slope transmitter. The signals generated in space are equivalent to those of the standard ILS elevation guidance system, and there is less need for careful preparation of the ground plane in front of the antenna array.

The simplest ILS glide-slope array is very well known and comprises two elements. The lower one is placed at a height above the ground such that when the ground reflection is taken into account, the maximum of its radiation pattern lies on the chosen glidepath (normally 3 degrees). The higher antenna is placed so that the first null in its vertical radiation pattern also lies along the glidepath. The antennas are vertically polarised and, assuming a ground reflection coefficient of -1, are located, approximately, at heights of 5 wavelengths and 10 wavelengths above the ground, respectively.

In the international standard Instrument Landing System, I.L.S., the lower antenna is caused to transmit the carrier signal and equal sidebands at 90 Hz. and 150 Hz. respectively. Thus an aircraft on the glidepath will be able to receive a large carrier and detect two sidebands of equal amplitude. The higher antenna is caused to transmit the same equal sidebands, but without the carrier, and with the phase of one reversed, so that when the aircraft is below the glidepath is receives a "fly-up" signal, derived from the fact that the composite 150 Hz sideband is larger than the composite 90 Hz. sideband . Similarly, an aircraft above the glidepath receives a "fly-down" signal, for then the 90 Hz. sideband is larger than the 150 Hz. signal. On the glidepath the sidebands are derived from the lower antenna only, and are equal. Thus the difference is zero and no flight correction signal is generated.

Variations in the ground profile, the vegetation and the moisture level in the ground, and in tide levels in sea environments, can alter the position of the null in the pattern of the higher antenna, and thus lead to angular measurement errors in the aircraft.

Many antenna array arrangements have been proposed to reduce these errors, and, in particular, to reduce the area of ground in front of the array which has to be controlled so that such errors may be small and stable. This example of the use of a triplet antenna, relates to a simple array in which the position of the null which determines the angle of the glidepath is not dependent upon the height of the antenna or the fact that the reflection coefficient of the ground is -1.

Figure 7 shows a single equi-spaced triplet, arranged so that the centre antenna element corresponds to the lower antenna of the simplest I.L.S. glidepath system. The two outer antennas are set to produce an interference null at the glidepath angle, and correspond to the higher antenna of the usual I.L.S. array.

With unit amplitude modulation at angular frequencies (e) (say, corresponding to 90Hz.) and (g) (say, corresponding to 150 Hz.), and having in mind that the carrier is suppressed for signals A and C, these signals add together in the aircraft receiver to become; (A+C)+B = 2f[sin(et) - sin(gt)]cos(2.n.d.sin / l ) + f[l+sin(et) + sin(gt)] Thus; A+B+C = f[1 + sin(et)(2cos(2.fl.d.sin q, / A )+l)+sin(gt)(l- 2cos(2..d.sin Q J A )}] from which it is clear that after demodulation to derive sideband signals (e) and (g) the difference between the amplitude of these signals is proportional to; cos(2.ll.d.sin / A this is the "fly-up", "fly-down"control signal and takes the value zero when, 2.n.d.sin v / A = E/2 Thus for a glidepath of 3 degrees: d / l is approximately equal to 5, and it is to be noted that this is independent of values of h and R. However h and R must not take values for which the amplitude of (f) is near or equal to zero. Also (f) must be the same for all elements, so a,p,r,h,R,must have common values for each of the elements, A,B and C.

The frequency band for ILS elevation guidance is at 300MHz., and so the spacing between elements A and C is about 10 metres.

(f) determines the signal carrier level received by the aircraft, which is maximum when; 2.h.sin v / A = E/2 and h / A also equals about 5 when the glidepath is 3 degrees.

Thus antenna element C will be at ground level. However, it is clear that h can vary significantly from the maximum signal value, before the signal strength of the carrier at an aircraft on the glidepath is significantly degraded.

Figure 7 indicates an embodiment of the invention, where d = h and monopole radiators are used. Figure 8 shows dipoles clear of the ground and the mid-point height, (h), of the array at, for example, about 3d/2 which corresponds to the carrier transmission pattern, derived from B, being about 3db. down at 3 degrees.

Claims

CLAIMS 1. One or more sets of triplet transmitting or receiving antenna arrays, in which the elements of each triplet are identical, equi-spaced, and set in line orthogonal to a reflecting ground plane, and for which the spacing of each triplet, and the height of the centre of each triplet, may be set independently to optimise the measurement of the angle of arrival of electromagnetic radiation, or of the coding by transmissions of angles in space. 2. One or more sets of triplet antennas, as claimed in claim 1, used with suitable signal processing receivers, to measure the elevation angle of arrival of a radio signal, with selected accuracy and integrity within a range, or ranges, of angles from near zero to near ninety degrees, with respect to the ground plane. 3. One or more sets of triplet antenna arrays, as claimed in claim 1,used with suitable transmitters, to code angles in space, with selected accuracy and integrity over a range, or ranges of angles, within the sector from near zero to near ninety degrees, with respect to the ground plane. 4.0ne or more sets of triplet antenna arrays, as claimed in Claim 1, in which the centre height is set approximately equal to twice the spacing, so that the signal amplitude from the central element of a triplet is non-zero, indeed is near its maximum, when, using square law receivers, the angle measurement capability of the triplet is at its most sensitive. 5. A set of three triplet antenna arrays, in accordance with the positions identified in the specification, to measure the elevation angles of emitters at heights above the ground plane, which subtend a small angle at the array. 6. A system, similar to that described herein, in which an array of the set, claimed in Claim 1, is used with suitable receivers and signal processing circuits, and standard secondary surveillance radar equipment, to provide an aircraft height monitor which may stand alone, but which is fully compatible with, and may be integrated with, air traffic control systems. 7. A transmitting array, typically a single triplet of the type claimed in claim 1, which generates coded signals in space, equivalent to those of the standard glidepath sub-system of the standard Instrument Landing System. 8. An elevation measuring receiving instrument, substantially as herein described, with reference to the drawings. 9. An ILS glidepath transmitting system, substantially as herein described, with reference to the drawings. Amendments to the claims have been filed as follows CLAIMS.

1. An antenna array for operation by radio interferometric techniques comprising; at least four antennae spaced so as to provide at least two equi-spaced linear triplets perpendicular to a ground plane, each triplet having different spacing; radio receiver means to obtain from each antenna of each triplet an information signal of which both amplitude and phase relative to the amplitude and phase of information signals obtained from other antennae of each triplet are functions of the elevation angles, & , with respect to the ground plane, of incidence upon the array of a radio wave arriving from a remote source and of the ground reflection coefficient p; first logic means associated with each triplet to combine vectorially the information signals obtained from the outermost antennae of that triplet to provide a first derived signal of which the amplitude represents the modulus of such combination; second logic means associated with each triplet arranged to derive from an information signal obtained from the centre antenna of each triplet a second derived signal of which the amplitude represents the modulus of that information signal from the said centre antenna; dividing means associated with each triplet arranged to divide one derived signal by the other to provide a quotient signal for each triplet which is a function of & but not of p; and selection means to provide at least one quotient signal which provides a measure of the angle of elevation.

2. An antenna array according to claim 1 in which the selection means is arranged to provide an optimum value of measurement of angle of elevation above the ground plane of received radio waves.

3. An antenna array according to claim 2 in which the optimum value is the measurement having the highest sensitivity and said selection means is arranged to favour the triplet having the most appropriate spacing.

4. An antenna array according to claim 2 in which the optimum value is the measurement having the highest value of strength S and the selection means is arranged to favour the triplet with the most appropriate height above the ground plane.

5. An antenna array according to claim 4 further comprising circuit means to determine the rate of variation of each quotient with variation of & , and the selection means is arranged to favour the quotient having the greatest rate of variation.

6. An antenna according to any preceding claim comprising nine elements arranged as three linear triplets.

7. An antenna array according to any of claims 1 to 5 comprising seven elements arranged as three triplets.

8. An antenna array for operation by radio interferometric techniques comprising; at least four antennae spaced so as to provide at least two equi-spaced linear triplets perpendicular to a ground plane, the centre antenna of each triplet being at a different height above the ground plane; radio receiver means to obtain from each antenna of each triplet an information signal of which both amplitude and phase relative to the amplitude and phase of information signals obtained from other antennae of each triplet are functions of the elevation angles, & , with respect to the ground plane, of incidence upon the array of a radio wave arriving from a remote source and of the ground reflection coefficient p; first logic means associated with each triplet to combine vectorially the information signals obtained from the outermost antennae of that triplet to provide a first derived signal of which the amplitude represents the modulus of such combination; second logic means associated with each triplet arranged to derive from an information signal obtained from the centre antenna of each triplet a second derived signal of which the amplitude represents the modulus of that information signal from the said centre antenna; dividing means associated with each triplet arranged to divide one derived signal by the other to provide a quotient signal for each triplet which is a function oft but not of p; and selection means to provide at least one quotient signal which provides a measure of the angle of elevation.

9. An antenna array according to claim 8 in which the selection means is arranged to provide an optimum value of measurement of angle of elevation above the ground plane of received radio waves.

10. An antenna array according to claim 9 in which the optimum value is the measurement having the highest sensitivity and said selection means is arranged to favour the triplet having the most appropriate spacing.

11. An antenna array according to claim 9 in which the optimum value is the measurement having the highest value of strength S and the selection means is arranged to favour the triplet with the most appropriate height above the ground plane.

12. An antenna array according to claim 11 further comprising circuit means to determine the rate of variation of each quotient with variation of e , and the selection means is arranged to favour the quotient having the greatest rate of variation.

13. An antenna according to any of claims 8 to 12 comprising nine elements arranged as three linear triplets.

14. An antenna array according to any of claims 8 to 12 comprising seven elements arranged as three triplets

15. An antenna array for operation by radio interferometric techniques comprising; at least four antennae spaced so as to provide at least two equi-spaced linear triplets perpendicular to a ground plane, the centre antenna of each triplet having different spacing; radio transmitter means to obtain from each antenna of each triplet a coded transmission of which both amplitude and phase relative to the amplitude and phase of information signals obtained from other antennae of each triplet at a point in space are functions of the elevation angles, , of that point with respect to the ground plane, and of the ground reflection coefficient p; first decoding and logic means associated with each triplet to combine vectorially the information signals received by a receiver at a point in space, from the outermost antennae of that triplet to provide a first derived signal of which the amplitude represents the modulus of such combination; second decoding and logic means in the said receiver associated with each triplet arranged to derive from an information signal obtained from the centre antenna of each triplet a second derived signal of which the amplitude represents the modulus of that information signal from the said centre antenna; dividing means in the said receiver associated with each triplet arranged to divide one derived signal by the other to provide a quotient signal for each triplet which is a function ofbut not of p; and selection means to provide at least one quotient signal which provides a measure of the angle of elevation.

16. An antenna array for operation by radio interferometric techniques comprising; at least four antennae spaced so as to provide at least two equi-spaced linear triplets perpendicular to a ground plane, the centre antenna of each triplet being at a different height above the ground plane; radio transmitter means to obtain from each antenna of each triplet a coded transmission of which both amplitude and phase relative to the amplitude and phase of information signals obtained from other antennae of each triplet at a point in space are functions of the elevation angles,9, of that point with respect to the ground plane, and of the ground reflection coefficient p; first decoding and logic means associated with each triplet to combine vectorially the information signals received by a receiver at a point in space, from the outermost antennae of that triplet to provide a first derived signal of which the amplitude represents the modulus of such combination; second decoding and logic means in the said receiver associated with each triplet arranged to derive from an information signal obtained from the centre antenna of each triplet a second derived signal of which the amplitude represents the modulus of that information signal from the said centre antenna; dividing means in the said receiver associated with each triplet arranged to divide one derived signal by the other to provide a quotient signal for each triplet which is a function of e but not of p; and selection means to provide at least one quotient signal which provides a measure of the angle of elevation.