CA2097976A1 - Two-frequency transmitting apparatus with tone-modulation phasing for an instrument landing system - Google Patents
Two-frequency transmitting apparatus with tone-modulation phasing for an instrument landing systemInfo
- Publication number
- CA2097976A1 CA2097976A1 CA002097976A CA2097976A CA2097976A1 CA 2097976 A1 CA2097976 A1 CA 2097976A1 CA 002097976 A CA002097976 A CA 002097976A CA 2097976 A CA2097976 A CA 2097976A CA 2097976 A1 CA2097976 A1 CA 2097976A1
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- frequency
- audio
- transmitting apparatus
- modulation
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S1/00—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith
- G01S1/02—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith using radio waves
- G01S1/08—Systems for determining direction or position line
- G01S1/14—Systems for determining direction or position line using amplitude comparison of signals transmitted simultaneously from antennas or antenna systems having differently oriented overlapping directivity-characteristics
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- Engineering & Computer Science (AREA)
- Remote Sensing (AREA)
- Aviation & Aerospace Engineering (AREA)
- Computer Networks & Wireless Communication (AREA)
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Radar, Positioning & Navigation (AREA)
- Traffic Control Systems (AREA)
- Radar Systems Or Details Thereof (AREA)
Abstract
Abstract Two-Frequency Transmitting Apparatus with Tone-Modulation Phasing for an Instrument Landing System Two-frequency transmitting apparatus (S1, S2, LA) for instrument-landing systems is disclosed which is in-sensitive to DDM (difference in depth of modulation) distortions caused by reflections of the clearance sig-nal from obstacles (H) located near the runway (RW).
This insensitivity is achieved by providing different phase shifts of the equal modulation frequencies (90 Hz and 150 Hz) used for the course and clearance signals.
The different phase shifts correspond to a phase shift of a common fundamental frequency (30 Hz). The two-frequency transmitting apparatus can be used to specify a localizer course or a glide path.
Fig. 1
This insensitivity is achieved by providing different phase shifts of the equal modulation frequencies (90 Hz and 150 Hz) used for the course and clearance signals.
The different phase shifts correspond to a phase shift of a common fundamental frequency (30 Hz). The two-frequency transmitting apparatus can be used to specify a localizer course or a glide path.
Fig. 1
Description
~9~7~
Two-Frequency Transmitting Apparatus with Tone-Modulation Phasing for an Instrument Landing System The present invention relates to two-frequency transmitting apparatus as set forth in the preamble of claim 1 and as is used in instrument landing systems ~ILS), ma;nly for carrying out so-called Category III landings.
Two-frequency instrument landing systems are described, for example, in a book by E.Kramar, "Funksysteme fur Ortung und Navigat;on", Verlag Berliner Union GmbH, Stuttgart, and Verlag W. Kohlhammer GmbH, Stuttgart, ~erlin, Koln, Mainz, 1973, particularly ;n Sect;on 5.9.2, pp. 196 et seq.
The ground equipment of a two-frequency instrument land-ing system consists of a localizer portion for guiding an aircraft to an airport and for providing azimuth guid-ance during Landing, a glide-slope portion for prov;ding vertical guidance until touchdown on the runway, and two marker beacons for transmitting coarse distance informa-tion. At least the localizer portion consists of two separate transmitters operating ~ith a slight difference ;n their carrier frequencies (two-frequency system).
Frequently, the glide-slope portion is aLso designed as ~797~
such a two-frequency system.
Accord;ng to the regulations of the International Civil Aviation Organization (ICAO), one of the Localizers in the two-frequency localizer equipment rad;ates a so-called clearance signal of a predeterm;ned m;nimum field strength within -35 from the (extended) runway centerline, and the other localizer radiates a sharply defined course signal ;n the direction of the runway centerline. The two signals differ slightly in carrier frequency and are each modu-lated with two audio frequencies t90 Hz, 150 Hz). The audio frequencies used for modulat;on are equal for the clearance and course signals and are generally in phase.
Their respective depths of modulation are initially equal. The transmitt;ng antennas are so designed, however, that the radia-tion fields formed on both sides of the centerline contain one or the other modulation frequency in a higher measure, so that along the centerline and its extension, a ver-tical plane is defined along which the modulation compon-ents of the two audio frequencies are equal, so that their difference becomes zero. On both sides of this plane, a receiver, by comparing the modulation components, can derive a criterion ~DDM = difference in depth of modulation) which indicates on which side of the plane the receiver is lo-cated, and which add;t;onally indicates the angular d;stance to this plane within a small angular range near this plane.
In the receiver, the slight difference bet~een the carrier frequencies of the clearance signal and course signal causes the respective stronger incoming signal to dis-proportionately suppress the weaker incoming signal, which is the so-called capture effect. The field-strength ratio ~7~
between clearance signal and course signal is referred to as "capture ratio" and, according to the current ICA0 rules, must not fall below a value of 10 d~ along the runway centerline.
The capture effect allows the radiation of the course s;gnal to be restricted to a narrow, obstacle-free angu-lar range on both sides of the centerline and to increase the radiated field strength to the point that interference signals, which may be caused, for exampLe, by reflections of the clearance signal from obstacles located on either side of the runway, will be suppressed. In practice, however, the increase in the power of the course-signal transmitter is limited by the transmitter technology used and by the requirement that interference with the ILS
installations at other airports due to nonstandard propa-gation should be avoided.
With the use of larger aircraft and the construction of larger hangars for such aircraft, on the one hand, and because of the frequent lack of space, which forces air-port planners to place buildings closer to the runway, on the other hand, it is quite possible that euen with the use of two-frequency ILS installations, the values required bylthe ICA0 for category III cannot be met, so that a possibly important airport cannot be approved for category III landings.
Interference due to reflection may, in principle, aLso occur along the glide path. If two-frequency transmitting apparatus is used to specify a glide path, reflections of the signal radiated into the wîder angular range below ~79~
an elevation plane conta;ning the glide path from large natural or artif;cial obstacles on the ground may re-duce the field-strength ratio required to utilize the capture effect (capture ratio) to the point that a re-liable specif;cation of the glide path is endangered by excessive DDM distortions.
To improve the suppression of reflected signals, ~ritish Patent 1,062,551, page 2, right-hand column, line 91 et seq., for example,proposes that the localizer transmitting apparatus uses equal audio frequencies (90 Hz and 150 Hz) of the clearance s;gnal and course signal, which are employed for modulation, in quadrature, i.e., that their phases are sh;fted by 90 with respect to each other.
Such a phase shift of +90 or -90 contravenes the regulations of the ICA0, wh;ch, to ensure undisturbed operation of arbitrary receiver types, require common passage of both modulation frequencies through zero in the same direct;on.
It is the object of the invention to provide an improved two-frequency transmitting apparatus which is also insen-sitive to strong reflection-induced interference and meets the relevant regulations.
An apparatus which attains the object of the invention is described by the features set forth in claim 1.
By the different phase differences between the modulation fre-quencies, corresponding to the phase shift of a common funda-mental frequency, suppression of reflected clearance signals is achieved in the region of the runway centerline if the phase shift is introduced in localizer transmitting apparatus, and : .
. -. ". . ..
~:
:: :
~7~7~
a correspond;ng suppression of reflect;ons o~ the glide-path signal rad;ated near the ground ;nto the w;der angular range ;s achieved along the glide path if the phase shift ;s introduced in glide-slope transmitting apparatus. The respec-tive suppression acts in add;tion to the capture effect. r Accord;ng to claim 2, the phase sh;ft ;s opt;mizable by measuring the disturbing influence of a transm;tted signal thus modulated with out-of-phase audio-frequency s;gnals wh;le changing the phase shift.
Such measurements yielded the phase shifts given in claim 3 as values of minimum interference.
Values g;ven ;n claim 4 have an added advantage over the other values g;ven ;n cla;m 3 ;n that the ;nfluence of deviations from the predetermined optimum phase-sh;ft angle ;s least there.
Clajm 5 relates to a transmitting apparatus suitable for quick adaptation to different interference situations.
The~jnvention will now be described in detail using a localizer transm;tting apparatus as an example.
F;g~ 1 ;s a block diagram of a test setup, and Fig. 2 is a graph represent;ng a typ;cal test result.
F;g. 1 shows scnematically the far-end port;on of a run-way RW with a test rece;v;ng antenna TA located on the runway centerline CL. The test receiving antenna ;s connected to a test rece;ver TE followed by an output device PL. Located beyond the far end of the runway ;s a localizer antenna LA for a two-frequency instrument "' i , ~7~
landing system which - unlike in conuentional feed sys-tems - is fed here by two separate transmitters S1, S2 which provide the course signal K and the clearance signal R, respectively.
The direction of radiation KR of the sharply focused course signal is the direction of the runway centerline.
The clearance signal is radiated in a wider angle (e.g., 35 on both sides of the runway centerline), and part of the energy is reflected from a hangar H,located in the vicinity of the runway,toward the runway centerl;ne, as indicated by an arrow RR. Part of the clearance sig-nal is also radiated directly ;n the direct;on KR.
Since, in two-frequency instrument landing systems, there ;s a slight difference between the carrier frequencies of the course transmitter and the clearance transmitter, and the field strength of the course transmitter along the runway centerline is h;gher than that of the clear-ance transmitter, the so-called capture effect normally becomes effective, wherein the course signal nearly com-pletely suppresses the clearance signal.
It has turned out, however, that in extreme cases - e.g., if the clearance signal is reflected from large metallic buildings or large aircraft parked near the runway -superpositions of directly radiated and reflected com-ponents of the clearance signaL may occur which de-teriorate the capture ratio, i.e., the field-strength ratio of clearance signal to course signal, in these superposition regions to the point that the suppression of the clearance signal by the capture effect is not - . . ' ~:
: ,.~
~7~
sufficient to guarantee that a stable localizer course is specified along the runway centerline. The clear-ance signal will interfere with the course si9nal, which results in one component of the course signal being weakened or strengthened relative to the other, thus causing a change in the depth of modulation of one audio frequency with re-spect to that of the other audio frequency after demodu-lation (DDM distort;on). Instead of a stra;ght, stable local;zer course, a d;stQrted course line will thus be communicated to the aircraft which does not permit a land;ng ;n poor visibility according to ICA0 regulat;ons.
The ;nventors have found out that such d;stort;ons of the course l;ne can be greatly reduced by sh;ft;ng the phase of the audio-frequency s;gnals used to modulate the clearance-signal transm;tter with respect to the respec-t;ve ident;cal audio-frequency s;gnals used to modulate the course-s;gnal transm;tter. The phase shift must be d;f-ferent for each aud;o frequency (90 Hz and 150 Hz) and must correspond to the same phase angle of a common fundamental frequency (30 Hz) of the t~o audio frequenc;es.
For a system with audio frequencies of 90 Hz and 150 Hz, an18 phase shift of the 30-Hz fundamental frequency, for example, corresponds to a 54 phase shift of the 90-Hz audio frequency and to a 90 phase shift of the 150-Hz audio frequency.
In F;g. 2, distortions of the ~ocalizer course (DDM
d;stortions aDDM) in such a two-frequency instrument landing system, which are measurable at a point on the runway centerline, are plotted as a function of the phase shift ~30 of the 30-Hz fundamental frequency for a field-strength rat;o (capture ratio) of 10 d8 and a ~79~
DDM basic value of 200 mA for the clearance signaL. To adjust the phase sh;fts necessary for the 90-Hz and 150-Hz audio frequencies, digital modu~ators in the two transmitters S1, S2 were driven in a convent;onal manner, nameLy so that the phase shifts (3 X ~30 and 5 X ~30) corresponding to the currently desired phase shift of the fundamental frequency was obtained for the two audio frequencies. The phase sh;fts of the two aud;o frequencies can also be produced, of course, if only one transmitter is employed. This only necessitates giving up the rigid coupLing existing in currently available transmitters, which operate w;thout phase sh;ft, between the equal aud;o-frequency s;gna~s used to modu~ate the clearance signal and course signal, and making available the audio-frequency signals separately w;th the des;red phase shift.
, Fig. 2 clearly shows that depending on the phase shift of the fundamental frequency, the DDM distortions (curve M) assume different values, and repeatedly the value zero. The zeros of the curve are at about -18, - 50, -90, -130, and -163 degrees.
It has be~en found that the positions of these zeros also depend on the location of the interfering obstacles with respect to the runway centerline, namely whether they are located on the side on which the ~50-Hz modulation predominates, as in the case underlying the curve of Fig. 2, or on the side on which the 90-Hz modulation pre-dominates. For the latter case, zeros are at phase shifts of the fundamental frequency of -32, +90, and -150 ~not shown ;n F;g. 2).
,:: - - ., -~ ~ ~ 7 ~ r~
_ 9 _ At these zeros, DDM distortions caused by reflections of the clearance signal will, even in an extreme case, be reduced to values far below the limit values pre-scribed by the ICA0 for so-called category ~II landings (+5 mA on the runway).
Fig. 2 also shows that w;th phase shifts corresponding to 18 and 163 of the 30-Hz fundamental frequency and obstacles on the 150-Hz s;de of the runway centerline, the curve goes through zero less steeply than at the other zeros. At these points, deviations from the phase-sh;ft setting, as may be caused, for example, by slight out~of-sync condit;ons and variat;ons of the modulator toLerances, resuLt in a smaller ;ncrease of DDM distortions than at points where the curve crosses the zero line very steeply ~e.g., p30 = 90)~ For ob-stacles on the 90-Hz side, minimally steep zero crossings are observed at 32 and 150 (not shown in the figure).
A phase shift corresponding to one of the above angles of the fundamental frequency, which represent distortion zeros, el;minates the need for many of the convent;onal, generally expensive or otherw;se d;sadvantageous measures for distort;on suppression, and offers a number of add;-tionaL advantages:
For exampLe, a reduction in transmitter power to in-crease the capture ratio or a reduction of the dif-ference in depth of modulation (DDM) for the clearance signal can be dispensed with. Even an increase in the DDM m;nimum ~alue for the clearance signal is possible without increasing the risk of intolerable DDM dis-tortions of the course signal along the runway.
.
, ~97~
~o --The design of the transmitting antennas need not be adapted to the terrain. Since a higher transmitting power of the clearance s;gnal is possible, a greater range of the clearance signal at interference min;ma result;ng from the reflections in the far field is achieved. DDM
distortions (DDM dips) are also reduced in the far field of the clearance-s;gnal transmitter through the possible ;ncrease in the DDM of the clearance s;gnal.
S;nce strong interference sources (large obstacles re-flect;ng the clearance s;gnal) are seldom located on both sides of the runway centerline, optimum ;nterference suppression can be achieved by sett;ng a phase difference wh;ch corresponds, for example, to a 18 phase shift of the 30-Hz fundamental frequency in case of ;nterferences ;ncident from the 150-Hz s;de, and to a -150 phase sh;ft of the 30-Hz fundamental frequency in case of interferences ;ncident from the 90-Hz side. -50 and -163 phase sh;fts of the 30-Hz fundamental frequency for inter-ferences from the 150-Hz s;de and -32 phase shifts for interferences from the 90-Hz side are also effect;ve for suppressing interference in the area of the runway, but they present problems in the transition region be-t~een course signal and clearance signal which make them less suitable than the phase shifts specified above.
If, ;n except;onal cases, strong interference sources should be located on both s;des of the runway centerl;ne, a phase difference correspond;ng to a -90 phase sh;ft o~ the 30-Hz fundamental frequency can prov;de non-opt;-mum, but suff;c;ent interference suppression which is equally well effectiue for reflections from both , ~7g7~
interference sources.
The observed phase-shift values of the common fundamental frequency, which provide DDM interference minima (zeros ;n F;g. 2), apply for the case where exact synchronism exists between the two modulation frequencies~ Even slight inaccuracies ;n synchron;zat;on may change the opt;mum phase-shift values by a few degrees upwards or downwards. Another factor which slightly influences the pos;tions of the opt;mum phase-sh;ft values ;s the ;ntens;ty of the ;nc;dent ;nterference s;g-nal.
Preferably, the transmitting apparatus according to the invent;on ;s so des;gned that besides a sett;ng without phase sh;ft, prepared phase sh;fts are selectable which correspond to phase shifts of the common 30-Hz funda-mental frequency of 18, 90, and 150.
Two-Frequency Transmitting Apparatus with Tone-Modulation Phasing for an Instrument Landing System The present invention relates to two-frequency transmitting apparatus as set forth in the preamble of claim 1 and as is used in instrument landing systems ~ILS), ma;nly for carrying out so-called Category III landings.
Two-frequency instrument landing systems are described, for example, in a book by E.Kramar, "Funksysteme fur Ortung und Navigat;on", Verlag Berliner Union GmbH, Stuttgart, and Verlag W. Kohlhammer GmbH, Stuttgart, ~erlin, Koln, Mainz, 1973, particularly ;n Sect;on 5.9.2, pp. 196 et seq.
The ground equipment of a two-frequency instrument land-ing system consists of a localizer portion for guiding an aircraft to an airport and for providing azimuth guid-ance during Landing, a glide-slope portion for prov;ding vertical guidance until touchdown on the runway, and two marker beacons for transmitting coarse distance informa-tion. At least the localizer portion consists of two separate transmitters operating ~ith a slight difference ;n their carrier frequencies (two-frequency system).
Frequently, the glide-slope portion is aLso designed as ~797~
such a two-frequency system.
Accord;ng to the regulations of the International Civil Aviation Organization (ICAO), one of the Localizers in the two-frequency localizer equipment rad;ates a so-called clearance signal of a predeterm;ned m;nimum field strength within -35 from the (extended) runway centerline, and the other localizer radiates a sharply defined course signal ;n the direction of the runway centerline. The two signals differ slightly in carrier frequency and are each modu-lated with two audio frequencies t90 Hz, 150 Hz). The audio frequencies used for modulat;on are equal for the clearance and course signals and are generally in phase.
Their respective depths of modulation are initially equal. The transmitt;ng antennas are so designed, however, that the radia-tion fields formed on both sides of the centerline contain one or the other modulation frequency in a higher measure, so that along the centerline and its extension, a ver-tical plane is defined along which the modulation compon-ents of the two audio frequencies are equal, so that their difference becomes zero. On both sides of this plane, a receiver, by comparing the modulation components, can derive a criterion ~DDM = difference in depth of modulation) which indicates on which side of the plane the receiver is lo-cated, and which add;t;onally indicates the angular d;stance to this plane within a small angular range near this plane.
In the receiver, the slight difference bet~een the carrier frequencies of the clearance signal and course signal causes the respective stronger incoming signal to dis-proportionately suppress the weaker incoming signal, which is the so-called capture effect. The field-strength ratio ~7~
between clearance signal and course signal is referred to as "capture ratio" and, according to the current ICA0 rules, must not fall below a value of 10 d~ along the runway centerline.
The capture effect allows the radiation of the course s;gnal to be restricted to a narrow, obstacle-free angu-lar range on both sides of the centerline and to increase the radiated field strength to the point that interference signals, which may be caused, for exampLe, by reflections of the clearance signal from obstacles located on either side of the runway, will be suppressed. In practice, however, the increase in the power of the course-signal transmitter is limited by the transmitter technology used and by the requirement that interference with the ILS
installations at other airports due to nonstandard propa-gation should be avoided.
With the use of larger aircraft and the construction of larger hangars for such aircraft, on the one hand, and because of the frequent lack of space, which forces air-port planners to place buildings closer to the runway, on the other hand, it is quite possible that euen with the use of two-frequency ILS installations, the values required bylthe ICA0 for category III cannot be met, so that a possibly important airport cannot be approved for category III landings.
Interference due to reflection may, in principle, aLso occur along the glide path. If two-frequency transmitting apparatus is used to specify a glide path, reflections of the signal radiated into the wîder angular range below ~79~
an elevation plane conta;ning the glide path from large natural or artif;cial obstacles on the ground may re-duce the field-strength ratio required to utilize the capture effect (capture ratio) to the point that a re-liable specif;cation of the glide path is endangered by excessive DDM distortions.
To improve the suppression of reflected signals, ~ritish Patent 1,062,551, page 2, right-hand column, line 91 et seq., for example,proposes that the localizer transmitting apparatus uses equal audio frequencies (90 Hz and 150 Hz) of the clearance s;gnal and course signal, which are employed for modulation, in quadrature, i.e., that their phases are sh;fted by 90 with respect to each other.
Such a phase shift of +90 or -90 contravenes the regulations of the ICA0, wh;ch, to ensure undisturbed operation of arbitrary receiver types, require common passage of both modulation frequencies through zero in the same direct;on.
It is the object of the invention to provide an improved two-frequency transmitting apparatus which is also insen-sitive to strong reflection-induced interference and meets the relevant regulations.
An apparatus which attains the object of the invention is described by the features set forth in claim 1.
By the different phase differences between the modulation fre-quencies, corresponding to the phase shift of a common funda-mental frequency, suppression of reflected clearance signals is achieved in the region of the runway centerline if the phase shift is introduced in localizer transmitting apparatus, and : .
. -. ". . ..
~:
:: :
~7~7~
a correspond;ng suppression of reflect;ons o~ the glide-path signal rad;ated near the ground ;nto the w;der angular range ;s achieved along the glide path if the phase shift ;s introduced in glide-slope transmitting apparatus. The respec-tive suppression acts in add;tion to the capture effect. r Accord;ng to claim 2, the phase sh;ft ;s opt;mizable by measuring the disturbing influence of a transm;tted signal thus modulated with out-of-phase audio-frequency s;gnals wh;le changing the phase shift.
Such measurements yielded the phase shifts given in claim 3 as values of minimum interference.
Values g;ven ;n claim 4 have an added advantage over the other values g;ven ;n cla;m 3 ;n that the ;nfluence of deviations from the predetermined optimum phase-sh;ft angle ;s least there.
Clajm 5 relates to a transmitting apparatus suitable for quick adaptation to different interference situations.
The~jnvention will now be described in detail using a localizer transm;tting apparatus as an example.
F;g~ 1 ;s a block diagram of a test setup, and Fig. 2 is a graph represent;ng a typ;cal test result.
F;g. 1 shows scnematically the far-end port;on of a run-way RW with a test rece;v;ng antenna TA located on the runway centerline CL. The test receiving antenna ;s connected to a test rece;ver TE followed by an output device PL. Located beyond the far end of the runway ;s a localizer antenna LA for a two-frequency instrument "' i , ~7~
landing system which - unlike in conuentional feed sys-tems - is fed here by two separate transmitters S1, S2 which provide the course signal K and the clearance signal R, respectively.
The direction of radiation KR of the sharply focused course signal is the direction of the runway centerline.
The clearance signal is radiated in a wider angle (e.g., 35 on both sides of the runway centerline), and part of the energy is reflected from a hangar H,located in the vicinity of the runway,toward the runway centerl;ne, as indicated by an arrow RR. Part of the clearance sig-nal is also radiated directly ;n the direct;on KR.
Since, in two-frequency instrument landing systems, there ;s a slight difference between the carrier frequencies of the course transmitter and the clearance transmitter, and the field strength of the course transmitter along the runway centerline is h;gher than that of the clear-ance transmitter, the so-called capture effect normally becomes effective, wherein the course signal nearly com-pletely suppresses the clearance signal.
It has turned out, however, that in extreme cases - e.g., if the clearance signal is reflected from large metallic buildings or large aircraft parked near the runway -superpositions of directly radiated and reflected com-ponents of the clearance signaL may occur which de-teriorate the capture ratio, i.e., the field-strength ratio of clearance signal to course signal, in these superposition regions to the point that the suppression of the clearance signal by the capture effect is not - . . ' ~:
: ,.~
~7~
sufficient to guarantee that a stable localizer course is specified along the runway centerline. The clear-ance signal will interfere with the course si9nal, which results in one component of the course signal being weakened or strengthened relative to the other, thus causing a change in the depth of modulation of one audio frequency with re-spect to that of the other audio frequency after demodu-lation (DDM distort;on). Instead of a stra;ght, stable local;zer course, a d;stQrted course line will thus be communicated to the aircraft which does not permit a land;ng ;n poor visibility according to ICA0 regulat;ons.
The ;nventors have found out that such d;stort;ons of the course l;ne can be greatly reduced by sh;ft;ng the phase of the audio-frequency s;gnals used to modulate the clearance-signal transm;tter with respect to the respec-t;ve ident;cal audio-frequency s;gnals used to modulate the course-s;gnal transm;tter. The phase shift must be d;f-ferent for each aud;o frequency (90 Hz and 150 Hz) and must correspond to the same phase angle of a common fundamental frequency (30 Hz) of the t~o audio frequenc;es.
For a system with audio frequencies of 90 Hz and 150 Hz, an18 phase shift of the 30-Hz fundamental frequency, for example, corresponds to a 54 phase shift of the 90-Hz audio frequency and to a 90 phase shift of the 150-Hz audio frequency.
In F;g. 2, distortions of the ~ocalizer course (DDM
d;stortions aDDM) in such a two-frequency instrument landing system, which are measurable at a point on the runway centerline, are plotted as a function of the phase shift ~30 of the 30-Hz fundamental frequency for a field-strength rat;o (capture ratio) of 10 d8 and a ~79~
DDM basic value of 200 mA for the clearance signaL. To adjust the phase sh;fts necessary for the 90-Hz and 150-Hz audio frequencies, digital modu~ators in the two transmitters S1, S2 were driven in a convent;onal manner, nameLy so that the phase shifts (3 X ~30 and 5 X ~30) corresponding to the currently desired phase shift of the fundamental frequency was obtained for the two audio frequencies. The phase sh;fts of the two aud;o frequencies can also be produced, of course, if only one transmitter is employed. This only necessitates giving up the rigid coupLing existing in currently available transmitters, which operate w;thout phase sh;ft, between the equal aud;o-frequency s;gna~s used to modu~ate the clearance signal and course signal, and making available the audio-frequency signals separately w;th the des;red phase shift.
, Fig. 2 clearly shows that depending on the phase shift of the fundamental frequency, the DDM distortions (curve M) assume different values, and repeatedly the value zero. The zeros of the curve are at about -18, - 50, -90, -130, and -163 degrees.
It has be~en found that the positions of these zeros also depend on the location of the interfering obstacles with respect to the runway centerline, namely whether they are located on the side on which the ~50-Hz modulation predominates, as in the case underlying the curve of Fig. 2, or on the side on which the 90-Hz modulation pre-dominates. For the latter case, zeros are at phase shifts of the fundamental frequency of -32, +90, and -150 ~not shown ;n F;g. 2).
,:: - - ., -~ ~ ~ 7 ~ r~
_ 9 _ At these zeros, DDM distortions caused by reflections of the clearance signal will, even in an extreme case, be reduced to values far below the limit values pre-scribed by the ICA0 for so-called category ~II landings (+5 mA on the runway).
Fig. 2 also shows that w;th phase shifts corresponding to 18 and 163 of the 30-Hz fundamental frequency and obstacles on the 150-Hz s;de of the runway centerline, the curve goes through zero less steeply than at the other zeros. At these points, deviations from the phase-sh;ft setting, as may be caused, for example, by slight out~of-sync condit;ons and variat;ons of the modulator toLerances, resuLt in a smaller ;ncrease of DDM distortions than at points where the curve crosses the zero line very steeply ~e.g., p30 = 90)~ For ob-stacles on the 90-Hz side, minimally steep zero crossings are observed at 32 and 150 (not shown in the figure).
A phase shift corresponding to one of the above angles of the fundamental frequency, which represent distortion zeros, el;minates the need for many of the convent;onal, generally expensive or otherw;se d;sadvantageous measures for distort;on suppression, and offers a number of add;-tionaL advantages:
For exampLe, a reduction in transmitter power to in-crease the capture ratio or a reduction of the dif-ference in depth of modulation (DDM) for the clearance signal can be dispensed with. Even an increase in the DDM m;nimum ~alue for the clearance signal is possible without increasing the risk of intolerable DDM dis-tortions of the course signal along the runway.
.
, ~97~
~o --The design of the transmitting antennas need not be adapted to the terrain. Since a higher transmitting power of the clearance s;gnal is possible, a greater range of the clearance signal at interference min;ma result;ng from the reflections in the far field is achieved. DDM
distortions (DDM dips) are also reduced in the far field of the clearance-s;gnal transmitter through the possible ;ncrease in the DDM of the clearance s;gnal.
S;nce strong interference sources (large obstacles re-flect;ng the clearance s;gnal) are seldom located on both sides of the runway centerline, optimum ;nterference suppression can be achieved by sett;ng a phase difference wh;ch corresponds, for example, to a 18 phase shift of the 30-Hz fundamental frequency in case of ;nterferences ;ncident from the 150-Hz s;de, and to a -150 phase sh;ft of the 30-Hz fundamental frequency in case of interferences ;ncident from the 90-Hz side. -50 and -163 phase sh;fts of the 30-Hz fundamental frequency for inter-ferences from the 150-Hz s;de and -32 phase shifts for interferences from the 90-Hz side are also effect;ve for suppressing interference in the area of the runway, but they present problems in the transition region be-t~een course signal and clearance signal which make them less suitable than the phase shifts specified above.
If, ;n except;onal cases, strong interference sources should be located on both s;des of the runway centerl;ne, a phase difference correspond;ng to a -90 phase sh;ft o~ the 30-Hz fundamental frequency can prov;de non-opt;-mum, but suff;c;ent interference suppression which is equally well effectiue for reflections from both , ~7g7~
interference sources.
The observed phase-shift values of the common fundamental frequency, which provide DDM interference minima (zeros ;n F;g. 2), apply for the case where exact synchronism exists between the two modulation frequencies~ Even slight inaccuracies ;n synchron;zat;on may change the opt;mum phase-shift values by a few degrees upwards or downwards. Another factor which slightly influences the pos;tions of the opt;mum phase-sh;ft values ;s the ;ntens;ty of the ;nc;dent ;nterference s;g-nal.
Preferably, the transmitting apparatus according to the invent;on ;s so des;gned that besides a sett;ng without phase sh;ft, prepared phase sh;fts are selectable which correspond to phase shifts of the common 30-Hz funda-mental frequency of 18, 90, and 150.
Claims (5)
1. A two-frequency transmitting apparatus for an instru-ment landing system defining an approach path for land-ing aircraft, comprising two transmitters (S1, S2) which operate with a small difference in their carrier frequencies and by which radio-frequency energy ampli-tude-modulated with two different, synchronized audio-frequency signals (90 Hz, 150 Hz) fed to the trans-mitters out of phase is radiated via an antenna array (LA) in a direction opposite to the direction of approach into two angu-lar ranges of different widths located respectively on opposite sides of a plane containing the approach path, such that the field strength of one (S1) of the transmitters (S1, S2) predominates in the narrower angular range, while the field strength of the other transmitter (S2) predominates in the other, wider angular range, and that the signals from the two transmitters, modulated with the indi-vidual audio-frequency signals, are received on both sides of the plane with different depths of modulation depending on the direction of radiation and decreasing in the direction of the plane, with the depth of modu-lation of the first audio frequency (90 Hz) predominating on one side of the plane, and the depth of modulation of the other audio frequency (150 Hz) predominating on the other side of the plane, c h a r a c t e r i z e d i n that the phase difference between the equal-frequency audio-frequency signals used to respectively modulate the sig-nals (K, R) radiated by the transmitters is different for the two audio frequencies and corresponds to a prede-termined phase shift (?30) of a fundamental frequency (30 Hz) which is common to the two different audio fre-quencies (90 Hz, 150 Hz).
2. A two-frequency transmitting apparatus as claimed in claim 1, characterized in that the predetermined phase shift of the common fundamental frequency is so chosen that the influence of the transmitter sig-nal modulated with the out-of-phase audio-frequency signals on the difference in depth of modulation, measured for the two audio frequencies, of the trans-mitter signal modulated with the audio-frequency sig-nals left in their phase position is a minimum.
3. A two-frequency transmitting apparatus as claimed in claim 1 or 2, characterized in that the prede-termined phase shift of the common fundamental frequency is one of ?90° or ?150° for reducing interference sig-nals incident from the side on which the modulation of the first audio frequency (90 Hz) predominates and one of ?18°, ?90° or ?130° for reducing interference signals incident from the side on which the second audio fre-quency (150 Hz) predominates.
4. A two-frequency transmitting apparatus as claimed in claim 3, characterized in that the predetermined phase shift of the common fundamental frequency is one of ?18° or ?150°.
5. A two-frequency transmitting apparatus as claimed in any one of the preceding claims, characterized in that in the transmitters, several phase shifts of the first and second audio frequencies corresponding in pairs to a predetermined phase shift of the common fundamental frequency (30 Hz) are preset and selectable.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE4220101A DE4220101A1 (en) | 1992-06-19 | 1992-06-19 | Two frequency transmitter combination for instrument landing system - uses differential phase shift of common audio frequency modulations of course and clearance signals |
| DEP4220101.2 | 1992-06-19 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2097976A1 true CA2097976A1 (en) | 1993-12-20 |
Family
ID=6461385
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002097976A Abandoned CA2097976A1 (en) | 1992-06-19 | 1993-06-08 | Two-frequency transmitting apparatus with tone-modulation phasing for an instrument landing system |
Country Status (4)
| Country | Link |
|---|---|
| CA (1) | CA2097976A1 (en) |
| DE (1) | DE4220101A1 (en) |
| FR (1) | FR2692994B1 (en) |
| NO (1) | NO302001B1 (en) |
Families Citing this family (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE4206327C2 (en) * | 1992-02-29 | 2000-10-19 | Sel Alcatel Ag | Dual frequency transmitter with audio frequency modulation phasing for an instrument landing system |
| DE19722913A1 (en) * | 1997-05-31 | 1998-12-03 | Alsthom Cge Alcatel | Glide path transmitter for the ILS instrument landing system |
| DE19757044A1 (en) * | 1997-12-20 | 1999-06-24 | Cit Alcatel | Landing course monitoring method for airplane automatic landing system |
| JP3427790B2 (en) * | 1999-08-02 | 2003-07-22 | 日本電気株式会社 | Farfield monitor device |
| IL292954B2 (en) * | 2019-11-15 | 2024-10-01 | Thales Usa Inc | Antenna and glide path array for small footprint aircraft precision approach and landing system |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1062551A (en) * | 1963-04-26 | 1967-03-22 | Standard Telephones Cables Ltd | Radio guidance systems |
| US3887922A (en) * | 1974-01-23 | 1975-06-03 | Int Standard Electric Corp | Instrument landing system with microwave course pattern transmission |
| GB1469507A (en) * | 1974-05-10 | 1977-04-06 | Plessey Co Ltd | Aircraft navigation systems |
| DE4206327C2 (en) * | 1992-02-29 | 2000-10-19 | Sel Alcatel Ag | Dual frequency transmitter with audio frequency modulation phasing for an instrument landing system |
-
1992
- 1992-06-19 DE DE4220101A patent/DE4220101A1/en not_active Ceased
-
1993
- 1993-06-08 CA CA002097976A patent/CA2097976A1/en not_active Abandoned
- 1993-06-11 NO NO932143A patent/NO302001B1/en not_active IP Right Cessation
- 1993-06-15 FR FR9307183A patent/FR2692994B1/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| NO302001B1 (en) | 1998-01-05 |
| FR2692994A1 (en) | 1993-12-31 |
| NO932143L (en) | 1993-12-20 |
| DE4220101A1 (en) | 1993-12-23 |
| FR2692994B1 (en) | 1995-09-08 |
| NO932143D0 (en) | 1993-06-11 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Discontinued |