WO1990004795A1

WO1990004795A1 - Aircraft landing approach system

Info

Publication number: WO1990004795A1
Application number: PCT/GB1989/001275
Authority: WO
Inventors: Michael Arthur Jones
Original assignee: The Marconi Company Limited
Priority date: 1988-10-26
Filing date: 1989-10-26
Publication date: 1990-05-03
Also published as: EP0394425A1; DK276090D0; GB8924076D0; CA2001488A1; GB8825106D0; GB2224903A; DK276090A

Abstract

An aircraft landing approach system in which the approach radar (1) has accurate ranging and angle tracking facilities and a platform recognition facility which can identify an active transponder (7) and identify a landing facility e.g. oil rig platform (5) in the absence or failure of a transponder. For fixed wing aircraft an active transponder returns aircraft location information and for helicopter platform operation a passive reflector system may also return helicopter position information. In operation of the system a course is set, preferably from the point of platform recognition and location, which misses the platform and from which visual approach is effected.

Description

Aircraft Landing Approach System

This invention relates to an aircraft landing approach system employing radar detection of a landing strip or platform. While the invention finds application in both land based and sea based landing facilities it is particularly useful in operation with helicopter landing platforms at sea, for example on oil-rigs.

A need exists for an instrumented approach system for use in blind conditions in respect of smaller airports, airstrips or landing platforms (such as oil rigs, buildings, etc.) for both fixed wing and helicopter aircraft. The approach may be a procedural approach which is agreed between an aviation authority and an operation or a group of operators, or may be an approach which is as close as possible to the current I.L.S. or Instrumented Landing System at Category 1 or better. (The category indicates the proximity of the instrument/visual transition to touchdown, the higher the category the shorter the visual control period.)

In general, the current systems have all or most of their active and accurate equipment on the ground at the airstrip or landing platform. An object of-the invention is to provide a radar landing aid which is substantially self-contained in the aircraft but at the same tine can take advantage of any landing aids that may be present on the platform or landing strip. The predominant weight of investment is therefore on behalf of the aircraft operator and may be negligible on the part of the landing facility authority.

According to the present invention, an aircraft landing approach system comprises an approach radar system fitted to an aircraft, the approach radar system having azimuth angle tracking capability and ranging capability, and a landing facility recognition capability.

The azimuth angle tracking capability may be provided by a twin transmission beam either simultaneous or sequential and comparison of signals received in response to the two beams.

Alternatively, the azimuth angle tracking capability may be provided by a single transmission beam scanned in azimuth and comparison of signal returns received at periodic intervals within the beam width of said beam.

The approach radar may be a modified weather radar providing weather returns and landing facility returns selectively.

There may be included identification means associated with the landing facility, the identification means being adapted to transmit an identifying signal to the aircraft on interrogation by the approach radar system. The identification means may comprise an active transponder adapted to transmit a pulse signal characteristic of the associated landing facility on interrogation by the approach radar system. This pulse signal nay comprise a plurality of pulses having a time relationship characteristic of the associated landing facility. Alternatively, the pulse signal βay comprise a plurality of pulses having a presence and absence pulse code relationship characteristic of the associated landing facility. The active transponder preferably comprises a wide beam antenna exhibiting a wide beam in azimuth and receiving and transmitting means adapted to receive a radar pulse signal by way of the wide beam antenna and re-transmit by way of the wide beam antenna an identifying pulse signal characteristic of the associated landing facility. In this case, and for use with fixed wing aircraft, the active transponder may further comprise a static-split antenna providing azimuth and angle angle discrimination over angles which are small relative to those of the wide beam, the receiving and transmitting means being adapted, in response to a radar pulse signal received by the static-split antenna, to transmit a locating pulse signal indicative of the angle of the aircraft relative to the boresight of the static-split antenna and thus provide a standard glide slope for instrumented landing of fixed wing aircraft.

The identification means may comprise a series of reflectors having a linear distribution such as to produce a coded pulse train on reflection of a single interrogating pulse from the approach radar system, the coded pulse train being characteristic of the associated landing facility. There are then preferably included two reflectors which are directed so as to diverge from a common boresight which lies in a vertical plane containing a required landing approach path, the two reflectors having distinctive positions in line with the series of reflectors and providing an indication of approaching aircraft position relative to the common boresight. In the case of a helicopter landing facility on a sea based platform, the series of reflectors are preferably omnidirectional and the two reflectors directional.

For use with a helicopter landing facility on a sea based platform, the approach radar system preferably incorporates range responsive means for initiating an offset course effective to steer the helicopter within a predetermined miss distance of the platform, the helicopter incorporating manual override means to control the landing once visual sighting is established. The means for initiating an offset course may be effecti e iin response to location of the landing facility, or in response to acquisition of a predetermined range to go.

Alternatively, the means for initiating an offset course nay be effective to determine, at the predetermined range to go, a course having a predetermined angle offset from the landing facility.

Where the static-split antenna is a four-horn antenna providing angle discrimination in azimuth and elevation, the antenna being coupled to provide sum, azimuth difference, elevation difference and dumπ\y output channels, monitoring means must be provided for monitoring power in the dumηy ouput channel and disabling the transponder in response to an increase in power indicative of a fault condition.

The landing facility recognition capability is preferably adapted to locate angular limits of a landing facility by means of a static split difference characteristic, the angular extent of a boresight null, when the boresight fs aligned with said landing facility, in conjunction with the range of the landing facility, providing an indication of the angular extent of the landing facility. Means are preferably included for imposing an angular jitter on the boresight to determine the limits of the null.

According to another aspect of the invention, in a method of operating an aircraft landing approach system employing an approach radar, fitted to an aircraft, the approach radar having azimuth angle tracking capability, ranging capability and landing facility recognition capability, landing facility is identified and located, and a course is set to make a predetermined miss of the landing facility. The miss distance may be predetermined and the course set to a point at this miss distance. Alternat vely, at a predetermined range to go, an offset angle is imposed on the aircraft course. A number of different embodiments of the invention will be considered, in various contexts: thus, in land based airstrips existing regulations prescribe approach paths, glide slopes etc., particularly for fixed-wing aircraft, so that angle, including elevation, discrimination becomes important. In such cases an angle responyie transponder on the airstrip is valuable. In a sea-based situation, e.g. a helicopter platform on an oil rig, identification of the landing facility and azimuth determination of its position are the important factors. These requirements are accommodated in certain applications of the Invention essentially by the helicopter approach radar alone or with minimal transponder equipment. The approach radar carried by the aircraft may, in additon, be custom made with inherently accurate angle discrimination properties or may be adapted from an existing, e.g. weather, radar. While the former gives greater freedom of design parameters, the latter often has the advantage of existing accommodation within the aircraft, and particularly within the nose of the aircraft.

Several arrangements of these radar assisted landing approach systems will now be described, by way of example, with reference to the accompanying drawings, of which:

Figure 1 is a diagram of a helicopter approaching a landing platform at sea;

Figure 2 is a block diagram of a helicopter mounted approach radar;

Figures 3 and 4 are displays of rig returns in relation to limit gate determinations;

Figure 5 is a diagram of an active transponder for use on a landing strip or platform;

Figures 6 and 7 are plan and elevation diagrams of the Figure 5 transponder beam characteristics;

Figure 8(a) is a block diagram of the transponder transmitting and receiving equipment; Fi gures 8(b) and 8(c) are diagrams illustrating the coding of aircraft positi ons in Figure 8(a) ;

Figure 8(d) is a diagram of the possible return pulse train resulting from an interrogating pulse shown in broken lines;

Figure 9 is a plan view of a hel icopter approach path to a ri g landing platform;

Figures 10(a) and (b) are diagrams of stati c-split di fference characteristics showing the effect of a finite width 'target* ;

Figure 11 is a diagram of an omnidirectional antenna suitable for rig landing platform;

Figures 12 and 13 are omnidirectional antenna arrangements switchable for di rection selection; and Figure 14 is a plan view of a series of reflectors aligned with a requi red approach path and selecti vely angled to provide off -axis position encoding.

Referring to the drawings , Figure 1 shows a helicopter carrying an approach radar 1 mounted underneath and in a forward facing position. An oi rig 3 has a landing platform 5 and, in this embodiment, a t iransponder 7 whic may be active, as illustrated by Figure 7, or passive, merely comprising reflectors.

Figure 2 shows the approach radar, of the above custom made type, in more detail. A magnetron 9 controlled by a clock 11 and pulse modulator 13 is coupled to azimuth displaced elements of a dish antenna 15 by way of circulators 17 to produce a static split system of simultaneous twin beams. The antenna elements are so disposed as to produce individual radiation characteristics, beams, which are offset from the dish boresight 19 by approximately a quarter beam width to give an amplitude comparison system f azimuth angle determination. Sequential lobing or other accurate beam comparison system could be employed alternatively. Elevation angle tracking may well not be necessary since the operational circumstances are fairly limiting and the aircraft pitch angle is likely to be small. Provision may be made for measuring pitch angle in the aircraft and controlling elevation of the transmit beams accordingly. Similarly altitude measurements are taken, either by independent altimeter or by employing the ranging facility of the radar and a small portion of radar power directed downwards. In either event elevation of the main beams is controlled simply to ensure illumination of the landing facility : no elevation angle measurements are made by the receiver.

A local oscillator 21 is locked to the transmitter frequency to produce a constant I.F. output from mixer 23. A frequency locking loop comprises offset filters 25 and 27, comparator 29 control voltage amplifier 31 and local oscillator 21. The resulting L.O. frequency is mixed (33) with the outputs of circulators 17 to produce respective I.F. received pulse signals. The mixers 33 are associated with T.R. cells to protect the receiver. A switch 35 combines the received signal in one of two modes. For detection purposes it combines the two signals coherently hence giving a higher gain, single beam antenna, and in a tracking mode it time multiplexes the two signals into a common receiver channel.

The I.F. signal is detected (37) and applied to a range gate assembly 39. This includes a range error detection circuit which controls a loop 41 to keep the range gate (time) centred on the received pulse. The range so determined is then displayed (42).

An integrator 43 and AGC amplifier 45 maintain a normalised signal level.

The resulting range tracked pulse is applied to a de-multiplex and comparator circuit 47 where the separate target pulses are re-produced and compared to provide an error signal which is integrated (49) to produce, in the angle tracking mode, the target angle off helicopter axis as an output. The dish 15 is controlled by this signal, by way of a servo circuit 51, to track the target (rig or other landing platform) ie, to align the antenna boresight with the rig. In a non-angTe-tracking mode the servo ichanism 51 is controllable by alternative inputs, ie an angle scan circuit 57 which is responsive to range, and an overriding manual input. In these circumstances the angle of the source equipment, ie the rig, with respect to the mechanical axis of the radar, which is usually that of the aircraft, is taken from the antenna pick-off's in the dish dr ve servo 51. Aircraft angle motion can be fed in at 53 via the switch waveform generator 55 causing a servo demand to cancel out that aircraft motion on the antenna 15. The input signal to the dish servo gives an angle output essentially independent of aircraft angular movement away from the originally (or subsequently set) line of sight, and can be used as an angle demand to the aircraft crew (or to its autopilot) to achieve the desired course. Alternatively, the antenna pick-off output can be monitored and aircraft motion combined with it to obtain the correct angle demands outside the radar.

Range tracking, effected by the range gate assembly 39 and tracking loop 41, can be of a simple nature, e.g. an earl /late gate operating on the nearest point of the rig with a further jittered gate to operate on the furthest point of the rig, and hence allow an estimate of the rig length along the radar boresight.

Figure 3 shows the rig return 59 (amplitude against range), and the range gates determining the range limits. At the near limit of range, early/late gates 61/63 which overlap in time are supplied with the return signal either by provision of separate gates or alternate time designations of the same gate. The 'early' gate output has an amplification x2 and the 'late' gate an amplification of xl (relatively). The two gates each cover a range of 30 metres, the two overlapping by half. It may be seen that when the rig return just falls within the whole of the late gate (as shown) there will be equal outputs from the two amplifiers. One or other output will, predominate as the rig limit falls earlier or later than as shown. The outputs are compared and the error integrated to control the timing of the early/late gates to lock on to the condition shown. The range so determined is presented on the display. The far limit of the rig is obtained by a 'jittered' gate 65 which is 'jittered' between two positions the width of the gate apart. The outputs of the two gate positions are amplified differentially and when a difference of a specified threshold value e.g. 20dB exists between them the junction of the two positions lies on the far limit of the rig return signal.

The range difference between these two positions thus gives the rig dimension along the boresight.

In an alternative rig range analysis system a block of contiguous range gates 1s employed. 40 range gates each of 30 metres will thus cover 1200 metres simultaneously, as indicated in Figure 4.

In this case, the near and far points of the rig return signal 67 are arranged symmetrically about the centre gate of the block by means of an a.g.c. controlled threshold comparison and the range tracking loop operates to maintain this situation. Range to the nearest point of the rig and the length of the rig are then simultaneously and continuously available, whilst the antenna is looking at the rig, within the range gate block.

It will be appreciated that the above rig structure analyses are particularly valuable in a case in which the rig has no identifying transponder or the transponder is out of action.

Figure 15 illustrates an alternative form of approach radar mentioned earlier. This is based on a weather radar which would normally be housed in the nose of the aircraft.

An antenna 143 is scanned continuously under the control of a scanner circuit 145. Interrogating pulses are transmitted in the usual manner by magnetron 147, modulator 149, circulator 151 and antenna 143. The return signal, including radar returns at the transmitter frequency and beacon (ie transponder) returns at a shifted frequency, are passed to a mixer 153 producing separation of the beacon signal in the upper path and the radar return in the lower path. Both signals are subjected to sensitivity-time-controls 155 followed by matched filters 157 and detectors 159. The beacon signal, consisting of a coded pulse series, is then converted to parallel format 161 and applied to a processor 163 for decoding. A processor 165 provides accurate azimuth angle determination by sampling the radar returns at brief intervals, to obtain values from a single 'target' at different points on the single beam as 1t scans. Range, resolution is provided by selection, according to range, of the number of basic range gate units, each of say 20 metres, that are grouped together for processing. The greater the range the greater the grouping and the coarser the resolution. The grouping, or range collapsing, is performed by circuit 167, the resluting signals being integrated (169) and applied to the processor 165.

The resulting information, is then displayed in various forms under manual control.

It will be appreciated that the essential feature of this particular approach radar is the ability to define an azimuth course accurately and in particular to follow a couse to a specified miss point off the rig as illustrated in Figure 9.

Figures 5, 6, 7 and 8 illustrate an identifying transponder for use in conjunction with the approach radar of Figure 2, and particularly for use in the case of land based airstrips for fixed wing aircraft where elevationon approach is critical. Variants of this transponder may also be used for land based helicopter platforms. The transponder, shown in Figure 5, comprises a wide beam localiser antenna 70 for receiving an interrogating radar pulse and returning an identifying pulse, and a narrow beam, 4-horn feed, static split antenna 71 to define an approach path and locate an approaching aircraft relative to this path. The dual antenna assembly, having a common boresight 72, is mounted on a pedestal at a height of approximately 1 metre and is energised by a remote power source.

Figures 6 and 7 show the transponder in plan and elevation, fitted to an airstrip. The localiser beam 73 has a beam width of 100° in azimuth and 6° in elevation, the beam being upwardly directed at 3°. Central in the localiser beam are the 4 beams of the static split antenna having an overall beam width of 10° in azimuth and 2i° in elevation and again has a (glide) slope of 3°. The static split antenna 71 is purely a receiving antenna, the resulting location information being transmitted by the localiser antenna 70.

Referring now to Figure 8(a), this shows, diagrammatlcally, the circuitry and function of the transponder of Figure 5. An aircraft carrying the approach radar of Figure 2 and lying just (say) within the localiser beam 73 will transmit an interrogating pulse which will be received by the localiser. The radar frequency is in the X band, typically 9-10 GHz. The pulse is received by antenna 70, filter 75, circulator 77 and mixer 79 where it is mixed with a local oscillator (81) signal of frequency f_o-150 M4z, f₀ being the radar frequency. The mixer 79 thus produces an IF at 150 MHz which, after transmission by a blanking gate 83 is passed to an array of. delay lines. A first of these delay lines, 85, has a delay of d_j/2, ie half of a basic unit delay. The pulse thus delayed is applied to a detector 87 and a monostable 89 which disables the gate 83 and effectively disables the receiver for a period of 7 delay units. Reception via the static split antenna is also disabled by way of gates 91. The first received pulse is further delayed a half unit by delay line 93 to give a total delay of one unit, and re transmitted by way of mixer 95, amplifier 97, circulator 77 and antenna 70. This pulse 'echo' is used by the approach radar for range assessment as explained above, the total echo time being the transmission time plus the standard unit delay.

The original pulse output of gate 83 is also passed to a delay line dg providing 6 units of delay prior to transmission via antenna 70. The two pulses thus transmitted provide the transponder identification, the delay between them being unique. More complex ID codes can of course be provided by further dela stages. As thus described, the ID pulse code is returned to the approaching aircraft once the aircraft is within the localiser beam. When the aircraft comes closer to the approach path, and in particular within the static aplit beam, its interrogating pulse will be received by the antenna 71. Confirmation of the location within the main static split beam is given when the relative received power levels between localiser and static split beam are recognised within a suitable tolerance. This prevents acceptance on a sidelobe of the radar transmission. The pulse signal received by the antenna 71 is summed and differenced in azimuth and elevation in known manner to produce the three channels S, Da and De. These are detected (101) and applied to the gates 91 which are not yet closed. AGC amplifiers normalise the signals to the sum reference level and the resulting signals are applied to hardware logic 103 and threshold circuits 105. Limits of angle error are set by these threshold circuits to determine the locations shown in Figures 8(b) and (c) which are, diagrammatically, end views of the static split bea 71, centred on the boresight 72.

Angular region 2 extends from the extreme right of the beam to the threshold 107; region 3 extends from the extreme left to the threshold 109; region 4 extends up to threshold 111; and region 5 extends down to threshold 113. Thus the region around the boresight, ie on the correct guide slope, lies in all four regions 2, 3, 4 and 5. These angular regions, distinguished by the sum and difference signals and the threshold circuits are encoded by return pulses delayed by a corresponding number of unit delays, ie 2, 3, 4 and 5, the pulses occurring between the ranging return pulse 1 and the ID pulse 6, as shown in Figure 8(d). If the aircraft lies t the top left 1t will produce pulses 3 and 5, top right 2 and 5, bottom right 2 and 4, and bottom left 3 and 4.

The pulses are produced by gates 122, 123, 124 and 125 and delay . lines d₂, 3» d^ and dg according to the above coding, thus a 'reflected' pulse train consisting of pulses 1, 3, 5 and 6 indicates a requirement to dive and turn right, and so on. An absence of all pulses 2, 3, 4 and 5 indicates the aircraft is not in the glide beam at all and the presence of all pulses 2, 3, 4 and 5 indicates the aircraft is on the correct glide slope, the objective of the landing system.

Clearly other pulse codes are possible and the number of delay lines can be adjusted accordingly. In practice the pulse code would preferaly be long enough to give a normal looking analog signal to the operator or the aircraft autopilot via standard binary coding and an ADC, ie the retransmitted pulses would be coded to represent a number of degrees off boresight in standard binary forms. This is an example using time delay ID of the landing platform. The return angle indication pulses would be interposed in a similar way if pule code ID of the landing platform was used.

In the case of a rig at sea having a platform for helicopter use, the above accurate elevation control is not in general necessary. Consequently, that part of Figure 8(a) outside the broken line 74 is not reqauired and only identification employing the localiser antenna 70 is performed. The rig and platform location is determined by the helicopter approach radar.

Considering now the progress of a helicopter approaching a rig at sea, reference is made to Figure 9. Helicopters operating in the North Sea are fitted with area navigation and weather radars and altimeters. Hence the helicopter 61 in Figure 9 can steer by its area navigation system and its weather radar at a controlled altitude until the point A, typically 12 nautical miles 'to go' at which the approach radar can usefully be switched on.

It will be assumed initially that the rig has no effective ID transponder.

On activating the radar the angle and range controls disable the angle and range tracking loops. An appropriate angle scan to cover uncertainty in the expected 'angle-off of the rig is implemented by the angle control. A minimum of three range gates are placed contiguously to cover 90m or can be spaced with intervals such that the smallest rig to be approached is bound to be detected by one of them (e.g.3 gates spread over 180m should ensure that all rigs with a 45m minimum dimension are detected). The range uncertainty between aircraft and rig is covered by stepping (or equivalently continously scanning) these gates over the uncertainty before the antenna beam angle changes and slowly enough to allow an adequate number of Tx pulse echoes to be received e.g. approx. 20. Hence for a PRF of 5 KHz, and an estimated range uncertainty of 3i Km, then the gates must remain at each step of 4ms and 20 range steps, each of 180m are needed i.e. 80ms. For an 8° beam, the maximum usable scan rate would be 100 deg/sec. If 100 Tx pulse echoes are desired for each assessment, ie at each range step, and only the middle half of the beam is utilised to maximise antenna gain, this scan rate would be reduced to 10 deg/sec, ie. 6 sec for a 60-deg scan. As there exists the possibility always of encountering the rig as the helicopter closes, provided no obstacle avoidance is needed at detection, ie. the aircraft flies at safe height,in 12 sec at 75 m/s helicopter speed, the helicopter will have travelled less than 1 Km towards the rig before the rig must again be encountered, ie. only £ of the range uncertainty set in this example's worst case. Hence the exact estimation of the extent of the range uncertainty is not critical, only that the minimum range set should not be such that the rig is inside this range at the start.

If the range gate block alternative is used, then the number of steps needed by the block is reduced (perhaps to one) and speed of detection is increased and/or scan rate can be appropriately increased. Similarly, if the number of pulse echoes desired for each aircraft is reduced from 100 to, say, 50 then the scan rate can be doubled etc.

All target contacts or only those of rig size may be displayed to the crew. The radar identifies a rig autonomously by its length being greater than a certain value programmed in, either permanently, for each flight, or by the crew knowing roughly the direction of approach, and the length of the rig in this direction. In addition, the approach radar will show a size related null on boresight due to finite across-boresight "width" of the rig, e.g. a 450m "wide" rig would show approximately a 1J° null at 18 Km range and approximately 3° at 9 Km range. This is indicated in- Figures 10(a), showing the difference characteristic for a point source, and 10(b) for an extended source. Hence if the antenna 1s jittered after detection about the target by a small angle the angular extent of the null can be measured and used as an extra identification means.

If there is an ID transponder on the rig the approach radar will identify it and lock on to it as described above. Having identified the required rig by one method or another, and determined its bearing, the radar angle measurement of the required rig is then used to steer the helicopter directly towards the rig, ie. a steering angle of the error angle between the rig as measured by the radar and an estimate of the helicopter velocity vector. This section of the course is shown in Figure 9 from B at typically lOn to C at l±nm. As the helicopter closes on the rig, the radar S/N will rise and its angle error decrease, allowing more accurate angle (and range) estimation.

At C the approach radar instructs a 10° turn away from the rig to prevent collision with it. This course can be maintained or the course can be controlled from the radar information until 0.5nm to go, at D. At this point the approach radar indicates to the operator that he must transfer to visual for landing. There will at this point be an angle of θ in azimuth between the radar and the rig. Assuming the helicopter has an incidence Sθ. In general in this example, it is assumed that either the helicopter can measure the incidence in azimuth and pass it to the approach radar where it is processed or that the angle of incidence is small - it is possible also to allow large uncompensated incidence angles by increasing the radar angle of look. The likely case 1s that the helicopter would approach the rig into the wind and develop only a small angle of incidence. The approach radar will have tolerances which give a total azimuth angle result of Θ+SΘ + radar errors. The radar is ±x° accurate and to ensure that the above 10° turnaway, in general X° turnaway, is achieved, X+2x° turnaway is used, approximately, resulting 1n an extra 2xC/_D at Dn . The course can be controlled to ensure no collision with the rig before visual sighting by other courses, for example, indicating angle to the helicopter, such that the point of minimum radar approach distance Dnm is steered towards, as measured normal to the line between rig and helicopter, or as measured normal to the approach path as remembered by a gyro or other reference prior to turnaway. At ranges shorter than Cnm the object is to control the helicopter to an estimated point either left or right of the rig (see Figure 9) this point being on the normal (YY) to that of the initial approach (XX) and a distance D from the rig. This f «orm of control can use angle and range measurements from the radar continually, to determine the approach point D from the rig and compute an angle such that the helicopter approaches this point whi lst stil l maintaining its velocity vector pointing away from the ri g when close to the rig. Using a normal to approach path reference, a suitable steering angle then becomes

E(J - 0 - tan ,^'-1 FD - SsinJ + G = R

ScosJ where S = F^+N² and J = tan^"1 (N/M)

M is distance on XX axis

N is di stance on YY axis

S is total distance to go

J is radar look angle

0 is velocity vector angle wrt XX where E, F and G together determine the turnaway achieved and helicopter velocity angle at the nearest approach point D from the rig and at points beforehand. Suitable values have been found to be E = 2 to 4, F = 2 and G = 0. Figure 9 also shows an alternative course in which the approach path is directed to the miss point off the rig immediately the rig is identified, 1e from lOn out. This option may well be preferable since no change in the course is required. With adequate S/N at the point where approach to the rig is begun, this method allows a pilot to steer (if manual) or watch (1f the steering signals are coupled to the helicopter autopilot) a constant bearing course rather than one in which the bearing has to be altered.

As mentioned above, for an oil rig or landing platform at sea only the wide beam localiser is needed on the rig or platform but an approach from any direction may be desired.

Figures 11, 12 and 13 show alternative embodiments of transponder localiser antenna from that employed in Figure 8, particularly for use with rigs. Figure 11 shows a fixed 4-horn antenna providing omnidirectional coverage. If these antennas 115 are combined statically, continuous omnidirectional coverage is obtained but at a relatively low gain and requiring careful design at the four beam overlaps to avoid interference nulls. It needs switching to achieve a relatively high gain and an easy design.

Figure 12 shows a similar arrangement but in which the elements are switchable in pairs (117, 119) according to the direction of the source. A comparator 121 compares the signal level of the two pairs and operates a switch 127 to select the dominant pair.

Figure 13 shows a further omnidirectional antenna but having four elements 129 selectable (131) one at a time according to signal source direction. The aircraft is in radio contact with the landing controller and the latter can switch to the antenna covering the approach.

Figure 14 illustrates a form of passive transponder for use on rigs. In this embodiment a series of reflectors (133-139) are arranged spaced apart in a line 140 indicating the required approach path. Four reflectors are shown, the first and last (133 and 135) being omnidirectional in azimuth and spaced so as to produce a relative delay of an interrogating pulse such as to identify the rig. The other two reflectors 137, 139 are directional and are directed so as to diverge from the approach path 140 one to left and the other to the right. They are also spaced apart so as to enable their reflected pulses to be distinguished. The divergence is such that an aircraft on the correct path will miss both 'beams' but will detect a reflection from one or the other if 1t is off track to left or right. The resulting pulse trains are shown: P is the interrogating pulse, Pi the ID pulses, PI the pulse indicating 'left of centre' and Pr indicating 'tight of centre'.

The active transponder of Figure 8 has provision for a safety feature in the event that its beam patterns are distorted by external reflecting obstacles or internal failure of one or more components. The static split antenna provides outputs which are summed and differenced in known manner to provide a sum or reference channel, an azimuth difference channel and an elevation difference channel. A fourth channel normally results from this process which is a dumrηy channel providing no useful information. The power in this channel is normally ery small compared to that in the sum channel and is applied to a dummy load. If the boresight of the beam distorts in any fashion then the power in this dummy channel increases significantly. Hence the power in this channel is monitored and a rise is used to disable the transponder. As the aircraft closes 1t may itself cause some rise and, if so, then the tolerance on the rise can be appropriately increased on command from the aircraft as the range to go decreases. The ability of the monitor to self test for external or internal beam distortion still remain and expensive routine calibration Is then avoided. The approach radar will thus receive no echo and the system is fail-safe since the approach radar will be prepared to fall back on its own autonomous techniques described above for detecting and identifying the rig.

Claims

1. An aircraft landing approach system comprising an approach radar system fitted to an aircraft, said approach radar system having azimuth angle tracking capability and ranging capability, and a landing facility recognition capability.

2. A system according to Claim 1, wherein said azimuth angle tracking capability is provided by a twin transmission beam either simultaneous or sequential and comparison of signals received in response to the two beams.

3. A system according to Claim 1, wherein said azimuth angle tracking capability is provided by a single transmission beam scanned in azimuth and comparison of signal returns received at periodic intervals within the beam width of said beam.

4. A system according to Claim 3, wherein said approach radar is a modified weather radar providing weather returns and landing facility returns selectively.

5. A system according to any preceding claim, including identification means associated with said landing facility, said identification means being adapted to transmit an identifying signal to said aircraft on interrogation by said approach radar system.

6. A system according to Claim 5, wherein said identification means comprises an active transponder adapted to transmit a pulse signal characteristic of the associated landing facility on interrogation by said approach radar system.

7. A system according to Claim 6, wherein said pulse signal comprises a plurality of pulses having a time relationship characteristic of the associated landing facility.

8. A system according to Claim 6, wherein said pulse signal comprises a plurality of pulses having a presence and absence pulse code relationship characteristic of the associated landing facility.

9. A system according to any of Claims 6, 7 and 8, wherein said acti ve transponder comprises a wi de beam antenna exhibiting a wide beam in azimuth and recei ving and transmitting means adapted to recei ve a radar pulse signal by way of sai d wi de beam antenna and re-transmit by way of said wide beam antenna an identifying pul se si gnal characteristic of the associated landing faci lity.

10. A system according to Claim 9 for fixed wing aircraft, wherein said acti ve transponder further comprises a static-split antenna provi ding azimuth and elevation angle discrimination over angles which are small relati ve to those of said wi de beam, said recei ving and transmitting means being adapted, in response to a radar pul se si gnal recei ved by said static-split antenna, to transmit a l ocating pulse signal indicative of the angle of said ai rcraft relative to the boresight of said stati c-split antenna and thus provi de a standard glide slope for instrumental landing of fixed wing ai rcraft.

11. A system according to Claim 10, wherein said transmitting means is _. adapted to transmit said locating pulse signal by way of said wi de beam antenna.

12. A system according to Claim 5, wherein said identification means comprises a series of reflectors having a linear distribution such as to produce a coded pulse train on reflection of a single interrogating pulse from sai d approach radar system, said coded pulse train being characteristic of the associated landing faci lity.

13. A system according to Claim 12, further including two reflectors which are directed so as to diverge from a common boresight which lies 1n a vertical plane containing a required landing approach path, said two reflectors having distinctive positions in line with said series of reflectors and providing an indication of approaching aircraft position relative to said common boresight.

14. A system according to Claim 12 or Claim 13, for use with a helicopter landing facility on a sea based platform, said series of reflectors being omni-directional and said two reflectors being directional.

15. A helicopter landing approach system according to any of Claims 1 to 5 and 12 to 14, for use with a landing facility on a sea based platform, wherein said approach radar system incorporates range responsive means for initiating an offset course effective to steer the helicopter within a predetermined miss distance of the platform, the helicopter incorporating manual override means to control the landing once visual sighting is established.

16. A system according to Claim 15, wherein said means for initiating an offset course is effective iin response to location of said landing facility.

17. A system according to Claim 15, wherein said means for initiating an offset course is effective in response to acquisition of a predetermined range to go.

18. A system according to Claim 17, wherein said means for initiating an offset course is effective to determine, at said predetermined range to go, a course having a predetermined angle offset from said landing facility.

19. A system according to Claim 9, for use with a landing facility on a sea based platform wherein said wide beam antenna is omni-directional in azimuth.

20. A system according to Claim 9, for use with a landing facility on a sea based platform, wherein said wide beam antenna has a plurality of branch elements together covering 360° in azimuth, said branch elements being swltchable into operation in dependence upon the direction of an interrogating aircraft.

21. A system according to Claim 10, wherein said static-split antenna is a four-horn antenna providing angle discrimination in azimuth and elevation, the antenna being coupled to provide sum, azimuth difference, elevation difference and dummy output channels, and wherein monitoring means are provided for monitoring power in said dummy ouput channel and disabling said transponder in response to an increase in power indicative of a fault condition.

22. A system according to Claim 1, wherein said landing facility recognition capability is adapted to locate angular limits of a landing facility by means of a static split difference characteristic, the angular extent of a boresight null, when the boresight is aligned with said landing facility, in conjunction with the range of the landing facility, providing an indication of the angular extent of the landing facility.

23. A system according to Claim 16 including means for imposing an angular jitter on the boresight to determine the limits of said null.

24. A method of operating an aircraft landing approach system employing an approach radar fitted to an aircraft, the approach radar having azimuth angle tracking capability, ranging capability and landing facility recognition capability, in which a landing facility is identified and located, and a course is set to make a predetermined miss of the landing facility.

25. A method according to Claim 24, in which the miss distance is predetermined and said course is set to a point at this miss distance.

26. A method according to Claim 24, in which, at a predetermined range to go, an offset angle is imposed on the aircraft course.