GB2168812A - Aircraft landing aid - Google Patents

Abstract

An e.g. helicopter landing aid comprises a ground-based source of energy radiating audio-frequency signal-modulated carrier wave energy which is received by three or more receptors arranged on the aircraft in two cross-aligned pairs. For each pair a phase comparison of the two received and recovered signals indicates whether the source is closer to one receptor of the pair than the other or is equidistant therefrom and thus properly "aligned" therewith. When the source is aligned for both receptor pairs then the aircraft is vertically above the source. <IMAGE>

Description

GB 2 168 812 A 1

SPECIFICATION

Landing aids This invention concerns landing aids, and relates in 70 particular to the use of direction finding techniques as an aid in the landing of a VTOL aircraft such as a helicopter.

In the Specification as filed of our co-pending

Application for Letters Patent No. 84/26,942 (Publi- cation No) (I/6927/MAv), from which the present Application claims Priority, there is de scribed and claimed a direction finding arrange ment especially suitable for use in a process of finding the direction of a source of sound energy.

A copy of that Specification is annexed hereto. The arrangement is said to include:

at least two receiver elements spaced apart from each other with their respective received signals 20 (or signals derived therefrom) being fed to a time comparator via at least one variable delay device so that the two received signals arrive in synchron ism at said comparator; and means dependant on the value of the varied delay for determining the 25 direction of a remote source of waves giving rise to said received signals.

The present invention makes use of the essential principles disclosed in this earlier Specification to provide a landing aid, preferably sound operated, 30 for an aircraft - specifically, for a VTOL aircraft, such as a helicopter (or conceivably a machine like the Harrier Jump-jet), as will now be explained.

For some time it has been apparent that it would be highly advantageous to enable helicopters to 35 land, in the dark (or in generally poor-seeing con- 100 ditions) in confined spaces or at accurately defined positions. To provide such an ability it has clearly been necessary to utilise some sort of guidance systems, and various suggestions for guidance 40 systems employing electromagnetic radiation - generally of radar frequencies have been pro posed; all suffer from one or other of the disad vantages of easy detection, unsuitable range and expense. Moreover, because for security reasons the wavelengths used have generally been small - 110 in the centimetre and millimetre ranges - it has proved difficult unambiguously to compute com parative distances when the receptor aerials are spaced a reasonable distance apart (bearing in 50 mind their inherent sizes). The present invention 115 proposes a new system, specifically one where the radiated energy is in the form of short wave en ergy but carries a relatively long wave modulating signal, coupled with the application of interferome 55 tric techniques to the recovered signal. In a much 120 preferred embodiment the radiated energy is sound (preferably ultrasound sound at ultrasonic frequency; generally in the range 16KHz to 60KHz).

In this system a (ground-based) source of energy 60 (either an actual active generator, or a passive re- 125 flector) radiates signal-modulated carrier wave en ergy which is received by an array of (preferably four) individual (airborne) receptors mounted in cross-aligned pairs (usually fore-and-aft and side to-side), and for each pair a (phase) comparison of 130 the two received and recovered signals indicates whether the source is closer to one receptor of the pair than the other or is equidistant therefrom and thus properly "aligned" therewith, When the source is aligned for both receptor pairs then the array (and thus its carrier) is properly set up for an accurate approach.

In one aspect, therefore, this invention provides a landing aid for an aircraft, comprising:- 75 a) a source of radiatable energy, for positioning on the ground at or near the position where the aircraft is to land, the source being able to radiate energy modulated at audio frequency; b) at least three radiation receptors mounted on 80 the aircraft so as in effect to be in two crossaligned pairs with the two individual receptors of each pair being at a known mutual spacing, and so as to be in a generally horizontal plane and downwards-looking during a landing manoeuvre; 85 c) for each receptor, a signal amplification and demodulation channel; and d) means for combining the outputs of the two receptor/channels in each pair to give a resultant indicative of whether the source is nearer to one or 90 the other receptor, and thus of the aircraft's bear ing relative to the source.

The invention is a landing aid for an aircraft. It is a device that helps the pilot land the aircraft by providing him with information about his position 95 relative to a predetermined point on the ground, and it should, therefore, enable him to land in poor conditions. The aircraft may be any sort of airborne vehicle that can land either vertically or at a very steep angle, and the invention's aid is primarily for helicopters and fixed-wing Vertical Take-off and Landing (VTOL) aircraft such as the Harrier.

The two major items of the landing aid of the invention are a source of radiatable energy and a number of receptors (together with the associated 105 electronics) for that energy. Naturally, the general nature of the source (and of the receptors) will depend upon the type of radiation involved. The radiation may, for example, be electromagnetic radiation at millimetre wavelengths (and thus in the GigaHertz frequency range), for this radiation is relatively short range (and so provides a secure system). Very preferably, however, the radiation is sonic radiation, specifically ultrasound radiation in the 16KHz to 40KHz range. The preferred sources and receptors are, therefore, sonic devices.

The source may be - and preferably is - an active source, that actually generates the energy it radiates. Alternatively, it may be a passive source, that merely reflects energy radiated at it from some active source (the latter might be carried by the aircraft itself, though that does rather advertise the aircraft's position, which could be undesirable).

In general the source may be of any type and size, though normally it will be small (so as to sit unobtrusively at or around ground level), cheap (because it may well be of prime use in conditions where it has to be regarded as expenclible) and - if active - of relatively low power output (too much power may attract too much attention, and the aid is intended mainly as a short range device). For a 2 GB 2 168 812 A sonic system a simple battery-powered electronic package driving a piezoelectric speaker is probably quite acceptable.

Though the source can be an omni-directional 5 device, radiating energy in all directions, nevertheless it is preferably somewhat directionally selective, and radiates only along the general line of a chosen direction (which, in use, will commonly be vertical). A tight beam, however, is not required; 10 what is wanted is a beam that is divergent enough so that it is broad, and easy for the aircraft to find, at 100 metres (330 ft) or so. A conical beam with an apex solid angle of about -rrl2 radians seems quite satisfactory.

The preferred source is an ultrasound source, and the preferred receptors are therefore naturally ultrasound receptors. This use of ultrasound is a very much preferred feature of the invention, for it enables the broadcase radiation of a signal that is 20 inaudible (by definition), not easily detected (unlike electromagnetic radiation, even millimetre radiation, which can all too readily be picked up by many sorts of available equipment), and with a range that is short but not too short (modern tech25 riiques, especially interferometic techniques, as explained later, can enable a weakish signal to be picked up 50 to 100 metres - about 160 to 330 feet - from the source). Ultrasound is sound of ultrasonic frequency that is, above the range of hu- 30 man hearing (which, except in very young children, 95 rarely extends beyond 16KHz, and usually not beyond (12KHz). It is convenient, then, to think of ultrasound being sound of a frequency of 16KHz and above, possible reaching as high a value as 35 100KHz (though more commonly ultrasound is thought of as being in the 16KHz to 40KHz range).

The source is one capable of transmitting modulated energy - that is, it transmits a carrier wave at high frequencies that is modulated by a second 40 signal of much lower - audio - frequency. The reason for using a modulated signal is, as explained further hereinafter, to do with the physical dimensions of the receptor arrangement and, when using interferometric techniques, the desirability of 45 avoiding ambiguous situations. Put succinctly, it is necessary to have the two receptors in each pair spaced apart a distance less than a wavelength of the signal to be detected - which is not really possible if the signal itself is either a millimetre radio 50 wave or is sound at ultrasound frequencies (with a minimum wavelength of about 1.5cm - 20KHz at 3 X 104 cm/sec). The solution used by the invention is to employ the higher frequency energy as a carrier, and modulate it with a much lower frequency 55 signal - 100 Hz, say, so that the spacing of the two receptors need only be less than 300cm (3 x 104/ 100). 3 metres is quite practicable! The modulation, which in a noisy environment is preferably a frequency modulation (though in general amplitude 60 modulation is not impossible) conveniently pro- vides a Simple Harmonic Motion, or Sine wave, signal (on the carrier) having a frequency of from 1 OOHz to 50OHz (in the low end of the audio range), particularly around 15OHz.

65 Mounted on the aircraft are a number of recep- 130 tors appropriate to the nature of the radiated energy. When using an ultrasound source they are ultrasound receptors - essentially microphones capable of detecting ultrasound energy and convert- 70 ing it (transducing it) to electrical energy. The precise nature of these receptors is not important, though they should preferably be small, cheap and sensitive. Typical ultrasound sensors are piezoelectric devices or "electret" devices. The receptors 75 may be mounted on the aircraft in any convenient way, and no more need to be said about that here, save to point out that it may well be advantageous for one (or both) receptor of a pair to be adjustably mounted such that, over a limited range, it can be 80 moved towards and away from the other, thus varying the mutual spacing. This is -referred to again hereinafter.

The invention requires there to be at least three receptors, and that these be mounted so as in ef- 85 fect to be in two cross-aligned pairs. If there are three only receptors then the cross-aligned pair condition can be met by mounting them at the two free ends and the "elbow" of a L, the "elbow" receptor being common to each pair (one pair is the 90 two receptors at one free end and at the elbow, the other is those two at the other free end and at the eblow). However, such an arrangement necessi tates somewhat more complex signal processing techniques, and, though it may overall increase the electronics, it is more convenient to employ four receptors arranged in two quite separate pairs. In cidentally, more than four receptors can be utilized, but this seems unnecessarily complicated, and is not presently preferred.

100 The receptors are in two cross-aligned pairs, by which is meant that the two straight lines forming and extending through the two receptors of each pair cross each other. The crossing angle may be any convenient, but is desirably at least 60', and is 105 advantageously 90' (i.e., the pairs are othogonal). Preferably one such pair is aligned fore-and-aft of the aircraft, while the other is aligned left-to-right. Where there are four receptors these may be arranged either in symmetrical ly-position ed pairs 110 that actually cross, like an X (as though they were disposed at the four corners of a square) or in pairs that are cross-aligned but don't actually cross (as though they were disposed at four sequential corners of a regular octagon, or three were dis- 115 posed at the corners of an equilateral triangle, with the fourth. spaced further out from one corner along the line of a perpendicular therefrom). The actual cross (X) is generally simplest and most convenient, but the L or T configuration has some 120 advantages, particularly in that, provided theaircraft's attitude is known, its height can be deduced from the two sets of signals (or, indeed, vice versa; if its height is known then its attitude can be deduced). Height measurement is mentioned further hereinafter.

The two individual receptors of each pair are at a known mutual spacing (the two spacings, for the two pairs, need not be the same). The purpose of this spacing is to allow there to be compared the signal arriving at one receptor (having travelled a GB 2 168 812 A 3 certain distance) with the signal arriving at the other (having travelled another certain distance), using the results of this comparison (conveniently a phase difference, as discussed further herein- 5 after) to indicate whether the one or other receptor is nearer the source. Naturally, the mutual spacing must therefore be known, and be sufficient to give reasonably accurate results. Moreover, in order to avoid the possibility of ambiguous results (also 10 discussed further hereinafter) the spacing is very desirably less than one wavelength of the signal carried by the carrier wave. For reasons which will become apparent hereinafter, it is very much preferred that the spacing be exactly one quarter wavelength - thus, corresponding to a phase shift of ir/2 radians (90').

Where the receptors are adjustably mounted it follows that the mutual spacing of the two receptors in each pair can be changed to provide the de- 20 sired/optimum spacing, in terms of wavelength/ phase difference, if the frequency of the signal is changed. In this way different aircraft could be made sensitive - could be "tuned" - to different signal sources, at different but neighbouring 25 places, and land quite safely all together.

The receptors are mounted on the aircraft so that, during a landing manoeuvre, they are in a generally horizontal plane and looking downwards. The terms "generally horizontal" and "down- wards" are to be interpreted broadly, for in reality the aircraft may, during the landing, adjust its attitude over quite a wide range as it seeks to control the line of its descent. Both pitch (fore-and-aft tilt) and roll (side-to-side tilt) may vary as much as 30' 35 either side of the true horizontal. Nevertheless, the principle is clear - and, with a craft making a steady and truly vertical descent the receptors are very preferably actually in a horizontal phase, and do look vertically downwards.

Naturally, the mounting position of the receptors 105 can be anywhere that is convenient, but it will usually be both possible and satisfactory to mount them on the belly of the aircraft.

In the landing aid of the invention there is, for 45 each receptor, a signal amplification and demodu- 110 lation channel. It is, in fact, not impossible for there to be a single such channel, somehow shared by the outputs from all the receptors (pos sibly on a Time Division Multiplex system, for ex- 50 ample), but most conveniently there is actually a separate channel for each receptor, each channel containing its own, distinct, amplification and demodulation electronics.

The nature of each signal amplification and de- 55 modulation channel - of the means therein whereby the signal is amplified and demodulated may be any that is convenient (and appropriate to the modulation technique in use). No more need be said about that here, other than to comment that each channel will advantageously include a tuned filter, a signal detector and a modulating-frequency amplifier, and that, as in most modern-day electronic equipment, these will be solid state devices.

65 The landing aid of the invention includes means 130 for combining the outputs of the two receptors/ channels in each pair to give a resultant indicative of whether the source is nearer to one or the other receptor of the pair. The principle involved is es- 70 sentially quite simple: because of the spacing of the two receptors in either pair the signal will in general arrive at one slightly later (or earlier, depending on the relative directions involved) then at the other. Obviously, if the signal arrives at one re- 75 ceptor first then that receptor must be closer to the source that the other - which (apart from the special case when the aircraft is very low over the source) can only mean that the aircraft is off to one "side" of where it should be, and for a successful 80 pin-point landing should be manoeuvered in a direction from the receptor further from the source of the receptor nearer to the source.

There are a number of ways in which the signal arrival time delay can be determined. It is possible, 85 for instance, to employ a source that emits a signal consisting of a series of individually identifiable pulses, and thus literally to time the period between receiving a particular pulse at one receptor and receiving the same pulse at the other. How- 90 ever, the periods involved are small and not easy to measure, and the problem of identifying individual pulses is considerable. Accordingly, a much preferred arrangement involves the source emitting a simple sine wave signal, and the two recep- 95 tor outputs being combined interferometrically to give a resultant that in effect describes the phase difference between the signal when received at one receptor and the same signal when received at the other receptor.

There are some difficulties associated with using phase difference to measure signal delays, not least of which is that arising from the repetitive na ture of the signal. Any part of the signal is identical to a different part separated from the first by 360' (2 rr radians) and with delays of this order (result ing from the mutual spacing of the two receptors being greater than the signal wavelength) the com bination results become ambiguous. The matter may perhaps be clearer from the following discus sion.

Assuming that the carrier wave is arriving at the receptor pair as a parallel beam (which is not strictly true, especially at very low heights) and that the pair is exactly horizontal, then with the 115 pair directly over the source the path length to each receptor is identical and there is no phase difference between the received signals. However, if the pair is to one side of the source, such that the ultrasound arrives along a line at an angle 0 to the 120 vertical, then the path length difference is dependent upon the pair spacing and upon the sine of the angle 0. Specifically, if the spacing corresponds to a phase difference of xir radians, then the delay corresponds to a phase delay of xiT sin 0 radians.

125 Clearly, as the displacement of the pair to one side or the other increases, so the delay increases. Indeed, the delay varies between x7r sin 90 = x7r radians (when the signal at one receptor lags behind that at the other by an amount corresponding to a phase difference of xiT radians) and x7r sin -90' = 4 GB 2 168 812 A -x7r (when the signal at the one receptor leads that at the other by x7r radians) - thus, from Xrr to -x7r. Because signal values (for a simple harmonic motion of sine wave form) repeat themselves 5 every wavelength - every iT 2 phase change - and because the values of such a signal wave are mirrored every half wavelength - every 7r phase change - a degree of ambiguity occurs if the delay is too long (an 7r phase delay value is exactly the 10 same as an nrr+2,rr value, and so on). Although this can be allowed for, it is highly desirable to avoid this ambiguity altogether, which may only be accomplished simply by narrowing the delay range down to from 7r/2 to -7r2. Very preferably, then, 15 the mutual spacing of the receptors in each pair is equal to a phase delay of 7r/2.

It should be noted that if the mutual spacing is equivalent to a -Tr/2 phase delay then the wavelength of the signal must be such that a iT/2 phase 20 shift - one quarter of a wavelength - is equivalent to a reasonable distance. As explained above, high frequency signals with short wavelengths necessitate impossibly small mutual spacings, but low audio frequency signals (as used to modulate a high 25 frequency carrier wave) have wavelengths such that a quarter of a wavelength is several tens of centimetres, which is quite feasible.

This rrl2 separation, however, is not the end of the matter, for a considerable ambiguity can still 30 arise where the means for combining the two signals is unable to differentiate between signal combinations that are mirror images one of the other. Such an inability is common in conventional signal comparators (that "work" by adding or subtracting the values of the two receptor outputs). Thus, if, say, the two outputs are exactly in phase - which, as the system has so far been described, should beam that the aircraft is right above the source and the aircraft then drifts slightly to one side so 40 that the output lags behind the other so that the amplitude of one output drops slightly compared with the other, than the sum of the two will also drop slightly. Unfortunately, if the aircraft drifts slightly to the other side then while the output from the other receptor now lags slightly behind that of the one, the sum of the two amplitudes still drops slightly in exactly the same way as when the aircraft drifted to the one side. This problem, too, can be overcome - and quite easily - by the simple 50 expedient of placing in the amplification/demodulation channel of one receptor of each pair a phase delay device such that at the combining means the pair's receptor outputs' amplitudes behave differently depending on whether one or other lags in 55 phase. Any delay in useful, but most preferably the 120 delay isTr/2 radians (90'); this results in the com bining means output being zero for maximum lag to one side, a maximum value for maximum lag to the other side, and a "middle" value for no lag (the 60 source is then directly beneath the receptor pair). Very preferably, therefore, the- landing aid of the invention, when using a sine-wave- modulated carrier wave, includes in one of the two signal amplification and demodulation- channels for each 65 receptor pair means whereby the extracted modu- lating signal from the receptor is delayed in time (and thus phase) with regard to that of the other - and the delay is most conveniently that corre sponding to a phase delay of rr/2 radians.

70 The delay means may be of any convenient type - typical varieties use conventional inductancelca pacitor delay networks, shift registers or change coupled devices -but a preferred type is a charge coupled analogue shift register (a "bucket brigade" 75 device).

The resultant from combining the outputs of the two receptors/channels in each pair is indicative of whether the source is nearer one or the other receptor, and thus of the aircraft's bearing relative to 80 the source. This result may, of course, be used to drive some form of automatic flying/landing equipment, but in general it will most conveniently be fed to a standard blind-flying instrument of the. type used to show an aircraft's glide slope and 85 track - thus, two crossed needles one of which moves left or right to indicate the aircraft is right or left of the required track, the other of which moves up or down to indicate the aircraft is below (behind) or above (in front of) the required tr ack 90 (descent path). When the needles cross centrally, the aircraft is descending correctly; otherwise, the pilot steers the aircraft towards the needle crossing point.

The basic system described so far is adequate, 95 but may be improved in some respect. Thus, if the aircraft's attitude is incorrect - if it rolls or pitches significantly - the delay between the receptors in the relevant pair will change even though the descent path is correct. This can be dealt with by in- 100 corporating in each channel of each pair a variable delay, determining the attitude of the aircraft, and deriving some value indicative thereof, and then using this attitude value to adjust - either autornaflcally or manually - the value of the relevant chan- 105 nel delay to counteract the out-of-attitude-induced delay change.

As has been pointed out hereinbefore, where the crossing receptor pairs are not symmetrically disposed it may be possible to use the received signal 110 data to determine the height of the aircraft. For example, if the pairs have a T configuration (say, with the pair defining the vertical stroke of the T positioned fore-and-aft of the aircraft) and each pair's channels' outputs are summed without the 115 delay in one channel, then, assuming the correct aircraft attitude, the resultant two signals (one-for each pair) will be out of phase by an amount (inversely) dependent on the aircraft's height. It is preferable, however, to have the receptor pairs in a symmetrical fore-and-aft, side- by-side crossing arrangement - in which case height information can still be attained if an extra receptor (associated with the necessary electronics) be mounted some distance away - near the tail, say, of the aircraft - 125 its output being compared with that of one of the basic receptors; the basic receptors' outputs are used to guide the aircraft down, while the comparison with the extra receptors's output provides height data. It should be observed, however, that 130 the "height" data obtained will be correct only if GB 2 168 812 A 5 the aircraft's attitude is right, so that if the latter is not so then it may be desirable to inhibit height data presentation and/or use.

Various embodiments of the invention are now 5 described, though by way of illustration only, with reference to the accompanying drawings in which:Figure 1 is an artistic perspective view of a helicopter making a landing using the aid of the invention; and Figure 2 is a schematic diagram of the receptors, amplification/demodulation channels, combining means and display means used in the invention.

The artistic view of Figure 1 shows a helicopter (10) landing with the help of a landing aid of the 15 invention. On the ground at the landing site is a small transmitter (11) of audio-modulated ultrasound, radiating sound upwardly over a fairly wide angle. The helicopter has, mounted in a crossing relationship on its skids, four ultrasound receptors 20 (12fal and r) arranged in two crossing pairs - 12f and 12a are the fore-and-aft pair, while 12/ and 12r are the left-right pair. As the helicopter enters the "cone" of radiated ultrasound so the four receptors 12 pick up the sound, each feeding a signal to the 25 remainder of the equipment (not shown in Figure 1).

The landing aid equipment carried by the helicopter is, as shown in Figure 2, the four receptors 12, an amplification/demodulation channel (as 30 13falr) for each receptor, two u/2 signal delay means (as 14a, 14/: one in one channel of each channel pair), two phase comparators (15ta, 151r: one for each channel pair), and a display device (16). The output from each receptor 12 is passed to the relevant channel 13 where it is subjected successively to a filter- preamplifier (as 17), a detector/ demodulator (as 18) and a post- amplifier (as 19). Thence the signal from one channel of the pair (13a or 13e) is passed through the 7r/2 delay (14fa, 40 141v) and combined with the undelayed signal from the other channel of the pair (13f or 13v) in the appropriate comparator 15. The outputs of the two comparators are in turn passed to a conventional crossed-needle flight path indicator 16, where they control (as normal) the position of the indicator needles, so providing the pilot with a visual indication of where he is relative to the correct descent path.

Claims (14)

50 CLAIMS

1. A landing aid for an aircraft, comprising:- a) a source of radiatable energy, for positioning on the ground at or near the position where the 55 aircraft is to land, the source being able to radiate energy modulated at audio frequency; b) at least three radiation receptors mounted on the aircraft so as in effect to be in two crossaligned pairs with the two individual receptors of 60 each pair being at a known mutual spacing, and so as to be in a generally horizontal plane and downwards-looking during a landing manoeuvre; c) for each receptor, a signal amplification and demodulation channel; and 65 d) means for combining the outputs of the two receptors/channels in each pair to give a resultant indicative of whether the source is nearer to one or the other receptor, and thus of the aircraft's bear ing relative to the source.

2. An aid as claimed in Claim 1, wherein the source of radiatable energy and the receptors for that energy are ultra-sonic devices.

3. An aid as claimed in either of the preceding Claims, wherein the source is an active source, that 75 actually generates the energy it radiates.

4. An aid as claimed in any of the preceding Claims, wherein the source is directionally selective, and radiates a vertical conical beam.

5. An aid as claimed in any of the preceding 80 Claims, wherein the modulation applied to the energy transmitted by the source is a frequency modulation, ard provides a Sine wave signal (on the carrier) having a frequency of from 10OHz to 50OHz.

6. An aid as claimed in any of the preceding 85 Claims, wherein at least one receptor of each pair is adjustably mounted such that, over a limited range, it can be moved towards and away from the other, thus varying the mutual spacing.

7. An aid as claimed in any of the preceding 90 Claims, wherein there are four receptors arranged in two separate pairs which actually cross, like an X

8. An aid as claimed in any of the preceding Claims, wherein the receptors are in two cross- 95 aligned pairs the crossing angle of which is 90.

9. An aid as claimed in Claim 8, wherein one such pair is aligned foreand-aft of the aircraft, while the other is aligned left-to-right.

10. An aid as claimed in any of the preceding 100 Claims, wherein the mutual spacing of the two in- dividual receptors of each pair is exactly one aurter wavelength - thus, corresponding to a phase shift of rr/2 radians (90o) - of the signal carried by the carrier wave.

105

11. An aid as claimed in any of the preceding Claims, wherein, to give a resultant indicative of whether the source is nearer to one or the other receptor of a pair, the signal arrival time delay is determined by an arrangement in which the source 110 emits a simple sine wave signal, and the two receptor outputs are combined interferometrically to give a resultant that in effect describes the phase difference between the signal when received at one receptor and the same signal when received at 115 the other receptor.

12. An aid as claimed in Claim 11, wherein, to avoid any ambiguity arising where the means for combining the two signals is unable to differentiate between signal combinations that are mirror im- 120 ages one of the other, there is included in the amplification/clemodulation channel of one receptor of each pair a phase delay device such that at the combining means the pair's receptor outputs' amplitudes behave differently depending on whether 125 one or the other lags in phase.

13. An aid as claimed in Claim 12, wherein the delay is rr/2 radians.

1 6 GB 2 168 812 A 6

14. A landing aid as claimed in any of the preceding Claims and substantially as described hereinbefore.

Printed in the UK for HMSO, D8818935, 5,186, 7102. Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.