CA2001488A1 - Radar navigation system - Google Patents
Radar navigation systemInfo
- Publication number
- CA2001488A1 CA2001488A1 CA002001488A CA2001488A CA2001488A1 CA 2001488 A1 CA2001488 A1 CA 2001488A1 CA 002001488 A CA002001488 A CA 002001488A CA 2001488 A CA2001488 A CA 2001488A CA 2001488 A1 CA2001488 A1 CA 2001488A1
- Authority
- CA
- Canada
- Prior art keywords
- landing
- approach
- aircraft
- radar
- landing facility
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
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Classifications
-
- 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
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/88—Radar or analogous systems specially adapted for specific applications
- G01S13/91—Radar or analogous systems specially adapted for specific applications for traffic control
- G01S13/913—Radar or analogous systems specially adapted for specific applications for traffic control for landing purposes
-
- 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
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/88—Radar or analogous systems specially adapted for specific applications
- G01S13/93—Radar or analogous systems specially adapted for specific applications for anti-collision purposes
- G01S13/933—Radar or analogous systems specially adapted for specific applications for anti-collision purposes of aircraft or spacecraft
- G01S13/935—Radar or analogous systems specially adapted for specific applications for anti-collision purposes of aircraft or spacecraft for terrain-avoidance
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
- Y02A90/00—Technologies having an indirect contribution to adaptation to climate change
- Y02A90/10—Information and communication technologies [ICT] supporting adaptation to climate change, e.g. for weather forecasting or climate simulation
Landscapes
- Engineering & Computer Science (AREA)
- Radar, Positioning & Navigation (AREA)
- Remote Sensing (AREA)
- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Computer Networks & Wireless Communication (AREA)
- General Physics & Mathematics (AREA)
- Aviation & Aerospace Engineering (AREA)
- Radar Systems Or Details Thereof (AREA)
Abstract
ABSTRACT
An aircraft landing approach system in which the approach radar has accurate ranging and angle tracking facilities and a platform recognition facility which can identify an active transponder and identify a landing facility e.g. oil rig platform 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.
An aircraft landing approach system in which the approach radar has accurate ranging and angle tracking facilities and a platform recognition facility which can identify an active transponder and identify a landing facility e.g. oil rig platform 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
DFS13552 (~Ft Aircraft Landing Aperoach System This invention relates to an aircraft landing approach system employing radar detection of a land~ng 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 o11-rtgs.
A need ex1sts for an instrumented approach system for use in blind conditions in respect of smaller airports, airstrips or landing platforms (such as oil rigs, bu~ldings, etc.) for both fixed w1ng and helicopter aircraft. The approach may be a procedural approach which ls 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 invent~on is to provide a radar land~ng ald which is substantially self-contalned ~n the aircraft but at the same time can take advantilge of any landing aids that may be present on the platform or landing strip. The predom~nant weight of investment is therefore on behalf of the aircraft operator and may be negligible on the part of the landing facility authorlty.
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 capab~lity and ranging capabil~ty, and a landing facility recognition capability.
The azimuth angle tracking capability may be provided by a twin transmission beam e~ther simultaneous or sequential and compar1son 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 fac11ity returns selectively.
There may be 1ncluded 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 may comprise a plurality of pulses having a time relationship characteristic of the associated landing facil~ty. Alternatively, the pulse signal may comprise a plurality of pulses having a presence and absence pulse code relationship characteristic of the associated landing facility.
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: . . i , The active transponder preferably comprises a wide beam antenna exhiblting a wide beam in azimuth and receiving and transmitt1ng means adapted to receive a radar pulse s~gnal by way of the wide beam antenna and re-transmit by way of the wide beam antenna an ldentifying pulse signal characteristic of the associated landing facility. In this case, and for use with fixed wing atrcraft, the act1ve transponder may further comprise a static-split antenna providing azimuth and angle angle d~scrimination over angles which are small relative to those of the wide beam, the receiving and transmltt1ng means being adapted, in response to a radar pulse signal received by the statlc-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. ln the case of a helicopter landing facility on a sea based platform, the ser1es 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 insorporating manual override ~eans to control the landing once visual sighting is established.
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-. : -The means for initiating an offset course may be effective iin response to locat~on of the landing facility, or in response to acquisition of a predetermined range to go.
Alternatively, the means for initiating an offset course may be effect~ve to determlne, at the predetermined range to go, a course having a predetermlned angle offset from the landing facility.
~ here the static-split antenna is a four-horn antenna provid~ng angle dlscrimination in azimuth and elevation, the antenna being coupled to provide sum, azimuth difference, elevation difference and dummy output channels, monitoring means must be provided for monttoring power ~n the dummy ouput channel and disabllng the transponder in response to an increase in power indicative of a fault ccndition.
The landing facility recognitlon capabil~ty is preferably adapted to locate angular limits of a landing facility by means of a static split difference characterist~c, 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. 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 operat~ng an aircraft landing approach system employing an approach radar fitted to an aircraft, the approach radar having azimuth angle tracking capability, rang~ng capability and landing facility recognitlon capability, a landing ~acility ~s 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 mlss distance.
Alternatively, at a predetermined range to go, an offset angle is imposed on the aircraft course.
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. .-.. . ~ ,, . ... ;, A number of different embodtments of the invention w~ll be considered, in various contexts: thus, in land based airstrips existing regulat10ns prescrlbe approach paths, glide slopes etc., particularly for fi~ed-wing aircraft, so that angle, including elevat~on, discr~mination becomes important. In such cases an angle responvie transponder on the ',airstr~p is valuable. In a sea-based s1tuatlon, e.g. a helicopter platform on an oil rig, identiflcation of the landing facility and azimuth determination of its position are the important factors. These requirements are accommodated in certain applications of the tnvention essent~ally 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 lnherently accurate angle discrimination properties or may be adapted from an exist~ng, e.g. weather, radar. While the former gives greater freedom of design parameters, the latter often has the advantage of existlng 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 accompany1ng drawings, of which:
F~gure 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 r~g 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) ~s a block diagram of the transponder transmitting and receiving equipment;
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Figures 8(b) and 8(c) are diagrams ~llustrating the coding of aircr~ft posltions in F1gure 8(a);
F1gure 8(d) is a diagram of the poss1ble return pulse traln resulting from an interrogating pulse shown in broken lines;
Figure 9 is a plan view of a helicopter approach path to a rig landing platform;
Figures lO(a) and (b) are diagrams of static-split difference character1st1cs showing the effect of a finite width 'target';
Figure 11 ~s a diagram of an omnidirectional antenna suitable for a rig landing platform;
Figures 12 and 13 are omnidirectional antenna arrangements sw1tchable for direction selection;
and Figure 14 is a plan view of a series of reflectors aligned with a required approach path and selectively angled to provide off-axis posltion encoding.
Referring to the drawings, Figure 1 shows a helicopter carrying an approach radar l mounted underneath and in a forward facing position. An oil rig 3 has a landing platform 5 and, in th~s embod~ment, a transponder 7 which 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, ~n more deta~l. A magnetron 9 controlled by a clock 11 and pulse modulator 13 is coupled to az~muth displaced elements of a dish antenna 15 by way of circulators 17 to produce a static spl~t 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 l9 by approximately a quarter beam width to g~ve an amplitude comparison system for azimuth angle determination. Sequential lobing or other accurate beam comparison system could be employed alternatively.
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Elevation angle tracking may well not be necessary slnce the operational circumstances are fairly 11miting and the a1rcraft pitch angle is 11kely to be small. Prov1sion may be made for measuring p1tch angle ln the aircraft and controlling elevation of the transm1t beams accord1ngly.
S1milarly altttude measurements are taken, e1ther by 1ndependent altimeter or by employ1ng the ranging fac11ity of the radar and a small portion of radar power directed downwards. In e1ther event elevat10n of the main beams 1s controlled s1mply to ensure 111um1nat10n of the landing fac111ty : no elevat~on angle measurements are made by the rece1ver.
A local oscillator 21 ls locked to the transmitter frequency to produce a constant I.F. output from m1xer 23. A frequency locking loop comprises offset filters 25 and 27, comparator 29 control voltage amplifier 31 and local oscillator 21. The resulting LØ 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 sw1tch 35 combines the received signal in one of two modes. For detection purposes it combines the two signals coherently hence giving a h1gher 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 d~splayed (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 clrcuit 47 where the separate target pulses are re-produced and compared to provlde 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 ~s controlled by this s~gnal, 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.
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In a non-angle-tracking mode the servo michanism 51 ls controllable by alternative inputs, ie an angle scan circuit 57 which ls responslve to range, and an overr1ding manual 1nput. In these circumstances the angle of the source equ1pment, 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 1n the dish drive servo 51. Aircraft angle motion can be fed in at 53 via the swttch waveform generator 55 causing a servo demand to cancel out that aircraft motion on the antenna 15. The inpwt signal to the dish servo gives an angle output essentially 1ndependent of aircraft angular movement away from the originally (or subsequently set) 11ne of sight, and can be used as an angle demand to the aircraft crew (or to 1ts autopilot) to achieve the desired course. Alternatively, the antenna pick-off output can be monitored and a~rcraft 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 slmple nature, e.g. an early/late gate operating on the nearest point of the rig with a further jittered gate to operate on the furthest po1nt of the rig, and hence allow an estlmate 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 sa~e gate. The 'early' gate output has an amplification x2 and the 'late' gate an ampl~fication 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.
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The far limit of the rig ls obta~ned by a 'jittered' gate 65 which 1s 'jittered' between two positions the w~dth of the gate apart. The outputs of the two gate posit1Ons are amplified different1ally and when a d1fference of a specif~ed threshold value e.g. 20dB exists between them the junct~on of the two positions lies on the far llmit of the rig return signal.
The range difference between these t~o positions thus gtves the rig dimenslon along the boreslght.
In an alternative rig range analysis system a block of contiguous range gates is employed. 40 range gates each of 30 metres will thus cover 1200 metres s~multaneously, as indicated tn 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 s~tuation. 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, with1n 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 ident1fying transponder or the transponder is out of action.
Figure 15 illustrates an alternative form of approach radar mentioned earlier. This ls 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. Interrogat~ng 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 produclng separation of the beacon signal in the upper path and the radar return in the lower path. Both signals are subjected to sensitivity-t~me-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 decod1ng.
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-10- , A processor 165 provides accurate azimuth angle determinatlon by samplillg the radar returns at br1ef intervals, to obtain values from a s~ngle 'target' at different points on the single beam as it scans. Range resolut~on is provided by selection, according to range, of the number of baslc 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 resolut10n. The grouping, or range collapsing, is performed by circuit 167, the reslutlng signals being integrated (169) and applled to the processor 165.
The resultlng lnformation, is then displayed ln 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 S, 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 crit~cal. Variants of this transponder may also be used for land based helicopter platforms. The transponder, shown in Figure 5, comprises a wide beam local1ser antenna 70 for receiving an interrogat~ng 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 ~n azimuth and 6 in elevation, the beam being upwardly directed at 3.
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Central ~n the localiser beam are the 4 beams of the stat~c split antenna having an overall beam w~dth of 10 1n azimuth and 2~ in elevation and again has a (gl1de~ slope of 3. The static split antenna 71 is purely a rece~wing antenna, the resulttng location informat~on belng transmitted by the local1ser antenna 70.
Referring now to F~gure 8(a), this shows, diagrammat1callyt the c1rcuitry and function of the transponder of F1gure 5. An alrcraft carrying the approach radar of Figure 2 and ly~ng ~ust (say) within the localiser beam 73 will transmit an interrogating pulse which will be recelved by the localiser. The radar frequency is 1n the X band, typically 9-10 6Hz. The pulse is received by antenna 70, filter 75, circulator 77 and mixer 79 where it 1s mixed with a local oscillator (81) signal of frequency fo~l50 MHz, fO
being the radar frequency. The mixer 79 thus produces an IF at 150 MHz wh~ch, after transmission by a blanktng gate 83 is passed to an array of delay lines. A first of these delay 11nes, 85, has a delay of dl/2, ie half ~f a basic unit delay. The pulse thus delayed ~s applied to a detector 87 and a monostab1e 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 d1sabled by way of gates 91. The first received pulse is further delayed a half unit by delay l~ne 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' ~s 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 d6 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 delay stages.
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As thus described, the ID pulse code is returned to the approaching a1rcraft once the alrcraft is within the localiser beam. ~hen the aircraft comes closer to the approach path, and in partitular w1thin 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 receiYed power levels between localiser and static split beam ~;
are recognised wlthin 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 resultlng signals are applied to hardware logic 103 and threshold circuits 105. Llmits 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 beam 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 reg~on around the boresight, ie on the correct guide slope, l~es in all four regions 2, 3, 4 and 5. These angular regions, distinguished by the sum and difference signals and the threshold circu1ts 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 to the top left it will produce pulses 3 and S, top right 2 and 5, bottom r~ght 2 and 4, and bottom left 3 and 4.
The pulses are produced by gates 122, 123, 124 and 125 and delay lines d2, d3, d4 and d5 according to the aboYe 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 .. . . ~, ,, . .~. . . . .
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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 ob~ective of the land1ng system.
Clearly other pulse codes are possible and the number of delay lines can be ad~usted accord1ngly. 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 btnary 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 llne 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.
Cons~dering 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.
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An appropr1ate angle scan to cover uncerta1nty 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 1ntervals 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 stepp1ng (or equivalently continously scann1ng) these gates over the uncerta1nty 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 3~ 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 10~ 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, te. 6 sec for a 60-deg scan. As there exists the poss1bility always of encountering the rig as the helicopter closes, provided no obstacle avoidance is needed at detect1On, ie. the aircraft flies at safe height,in 12 sec at 75 m/s helicopter speed, the helicopter w111 have travelled less than 1 Km towards the rig before the rig must again be encountered, ie. only ~ of the range uncertainty set in th1s 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 ~s increased and/or scan rate can be appropriately 1ncreased. Similarly, if the number of pulse echoes desired for each alrcraft is reduced from 100 to, say, 50 then the scan rate can be doubled etc.
All target conta~ts or only those of rig size may be displayed to the crew.
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The radar ident1fles a rig autonomously by lts length being greater than a certa1n 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. ~n addition, the approach radar wlll 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 1~ null at 18 Km range and approximdtely 3 at 9 Km range. This is ind~cated in Figures lO(a), showing the difference characterlstic for a polnt source, and lO(b) for an extended source. Hence if the antenna ls ~ittered after detection about the target by a small angle the angular extent of the null can be measured and used as an extra ldentlfication means.
If there is an ID transponder on the r~g 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 determ~ned 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 10nm 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 ~ 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 9 in azimuth between the radar and the rig. Assuming the helicopter has an incidence ~0.
In general in this example, lt is assumed that either the helicopter can measure the incidence in azimuth and pass lt to the approach radar ~here it is processed or that the angle of incidence ~s small - it ~s possible also to allow large uncompensated incidence angles by increasing the radar angle of , . ~ . .
. :
look. The likely case is that the helicopter would approach the r~g into the wind and develop only a small angle of ~ncidence. The approach radar will have tolerances which glve a total azimuth angle result of O+SO + radar errors. The radar ts ~x~ accurate and to ensure that the above 10 turnaway, in general X turnaway, is achieved, X+2x turnaway is used, approximately, resulting in an extra 2xC/D at Dnm. 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 ~easured 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 form 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 whilst still maintaining its velocity vector pointing away from the rig when close to the rig. Using a normal to approach path reference, a suitable steering angle i~
then becomes E J - O - tan~1 rD - Ssin~ + G = R
LScosJ ~3 where S = M2+N2 and J = tan~1 (N/M) M is distance on XX axis N is distance on YY axis S is total distance to go J is radar look angle O is velocity ~ector angle wrt XX
where E, F and 6 together determine the turnaway achieved and helicopter velocity angle at the nearest approach point D from the rig and at polnts beforehand. Suitable values have been found to be E = 2 to 4, F = 2 and G = O.
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F1gure 9 also shows an alternatlve course in whlch the approach path ls dlrected to the mlss polnt off the rig lm~,~,edlately the rlg is ~denttf1ed, le from 10nm out. Thls option may ~ell be preferable stnce no change in the course is required. ~'ith adequate S/N at the point where '~
approach to the r~g 1s begun, th~s method allows a pilot to steer (1f manual) or watch (if the steer1ng slgnals are coupled to the hellcopter autopllot) a constant bearlng course rather than one ln whlch the bearlng has to be altered.
As mentioned above, for an oll rlg or landlng platform at sea only the wlde beam locallser ls needed on the rig or platform but an approach from any direction may be desired.
F~gures 11, 12 and 13 show alternative embodiments of transponder localiser antenna from that employed in Figure 8, partlcularly for use wlth rigs. Flgure 11 shows a fixed 4-horn antenna pro~iding omnld~rectlonal coverage. If these antennas 115 are comblned statlcally, continuous omnidirectional coverage is obtalned but at a relatively low gain and requiring careful design at the four beam overlaps to avoid interference nulls. It needs sw~tching to achieve a relatively h~gh gain and an easy deslgn.
Figure 12 shows a sim11ar arrangement but ln whlch the elements are switchable 1n 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.
Flgure 13 shows a further omnidirectional antenna but havlng four elements 129 selectable (131) one at a tlme according to slgnal source direction. The alrcraft is in radio contact wlth the landlng controller and the latter can switch to the antenna covering the approach.
Flgure 14 illustrates a form of passive transponder for use on rigs. In this embodiment a serles of reflectors (133-139) are arranged spaced apart in a l~ne 140 lndicating the required approach path. Four reflect~rs are shown, the first and last (133 and 135) being omnidirectional ,. ~ ...................... .. ,. - ~ .
: : ~: . . .. - . . . --: :, - .~- . : : . .
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1n azimuth and spaced so as to produce a relative delay of an 1nterrogat1ng pulse such as to ldentify the r~g. The other two reflectors 137, 139 are dlrectional and are directed so as to d1verge 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 distingu1shed. The divergence is such that an aircraft on the correct path w111 miss both 'beams' but will detect a reflection from one or the other if it ls off track to left or right. The resulting pulse trains are shown: P is the interrogating pulse, Pi the ID
pulses, Pl the pulse indicatlng '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 dum~y channel providing no useful information. The power in this channel is normally very 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 dumnw channel increases significantly. Hence the power in this channel is 00nitored and a rise is used to d~sable the transponder. As the aircraft closes it 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 w~ll be prepared to fall back on its own autonomous techniques described above for detecting and ident~fying the rig.
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A need ex1sts for an instrumented approach system for use in blind conditions in respect of smaller airports, airstrips or landing platforms (such as oil rigs, bu~ldings, etc.) for both fixed w1ng and helicopter aircraft. The approach may be a procedural approach which ls 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 invent~on is to provide a radar land~ng ald which is substantially self-contalned ~n the aircraft but at the same time can take advantilge of any landing aids that may be present on the platform or landing strip. The predom~nant weight of investment is therefore on behalf of the aircraft operator and may be negligible on the part of the landing facility authorlty.
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 capab~lity and ranging capabil~ty, and a landing facility recognition capability.
The azimuth angle tracking capability may be provided by a twin transmission beam e~ther simultaneous or sequential and compar1son 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 fac11ity returns selectively.
There may be 1ncluded 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 may comprise a plurality of pulses having a time relationship characteristic of the associated landing facil~ty. Alternatively, the pulse signal may comprise a plurality of pulses having a presence and absence pulse code relationship characteristic of the associated landing facility.
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: . . i , The active transponder preferably comprises a wide beam antenna exhiblting a wide beam in azimuth and receiving and transmitt1ng means adapted to receive a radar pulse s~gnal by way of the wide beam antenna and re-transmit by way of the wide beam antenna an ldentifying pulse signal characteristic of the associated landing facility. In this case, and for use with fixed wing atrcraft, the act1ve transponder may further comprise a static-split antenna providing azimuth and angle angle d~scrimination over angles which are small relative to those of the wide beam, the receiving and transmltt1ng means being adapted, in response to a radar pulse signal received by the statlc-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. ln the case of a helicopter landing facility on a sea based platform, the ser1es 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 insorporating manual override ~eans to control the landing once visual sighting is established.
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-. : -The means for initiating an offset course may be effective iin response to locat~on of the landing facility, or in response to acquisition of a predetermined range to go.
Alternatively, the means for initiating an offset course may be effect~ve to determlne, at the predetermined range to go, a course having a predetermlned angle offset from the landing facility.
~ here the static-split antenna is a four-horn antenna provid~ng angle dlscrimination in azimuth and elevation, the antenna being coupled to provide sum, azimuth difference, elevation difference and dummy output channels, monitoring means must be provided for monttoring power ~n the dummy ouput channel and disabllng the transponder in response to an increase in power indicative of a fault ccndition.
The landing facility recognitlon capabil~ty is preferably adapted to locate angular limits of a landing facility by means of a static split difference characterist~c, 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. 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 operat~ng an aircraft landing approach system employing an approach radar fitted to an aircraft, the approach radar having azimuth angle tracking capability, rang~ng capability and landing facility recognitlon capability, a landing ~acility ~s 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 mlss distance.
Alternatively, at a predetermined range to go, an offset angle is imposed on the aircraft course.
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. .-.. . ~ ,, . ... ;, A number of different embodtments of the invention w~ll be considered, in various contexts: thus, in land based airstrips existing regulat10ns prescrlbe approach paths, glide slopes etc., particularly for fi~ed-wing aircraft, so that angle, including elevat~on, discr~mination becomes important. In such cases an angle responvie transponder on the ',airstr~p is valuable. In a sea-based s1tuatlon, e.g. a helicopter platform on an oil rig, identiflcation of the landing facility and azimuth determination of its position are the important factors. These requirements are accommodated in certain applications of the tnvention essent~ally 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 lnherently accurate angle discrimination properties or may be adapted from an exist~ng, e.g. weather, radar. While the former gives greater freedom of design parameters, the latter often has the advantage of existlng 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 accompany1ng drawings, of which:
F~gure 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 r~g 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) ~s a block diagram of the transponder transmitting and receiving equipment;
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Figures 8(b) and 8(c) are diagrams ~llustrating the coding of aircr~ft posltions in F1gure 8(a);
F1gure 8(d) is a diagram of the poss1ble return pulse traln resulting from an interrogating pulse shown in broken lines;
Figure 9 is a plan view of a helicopter approach path to a rig landing platform;
Figures lO(a) and (b) are diagrams of static-split difference character1st1cs showing the effect of a finite width 'target';
Figure 11 ~s a diagram of an omnidirectional antenna suitable for a rig landing platform;
Figures 12 and 13 are omnidirectional antenna arrangements sw1tchable for direction selection;
and Figure 14 is a plan view of a series of reflectors aligned with a required approach path and selectively angled to provide off-axis posltion encoding.
Referring to the drawings, Figure 1 shows a helicopter carrying an approach radar l mounted underneath and in a forward facing position. An oil rig 3 has a landing platform 5 and, in th~s embod~ment, a transponder 7 which 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, ~n more deta~l. A magnetron 9 controlled by a clock 11 and pulse modulator 13 is coupled to az~muth displaced elements of a dish antenna 15 by way of circulators 17 to produce a static spl~t 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 l9 by approximately a quarter beam width to g~ve an amplitude comparison system for azimuth angle determination. Sequential lobing or other accurate beam comparison system could be employed alternatively.
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Elevation angle tracking may well not be necessary slnce the operational circumstances are fairly 11miting and the a1rcraft pitch angle is 11kely to be small. Prov1sion may be made for measuring p1tch angle ln the aircraft and controlling elevation of the transm1t beams accord1ngly.
S1milarly altttude measurements are taken, e1ther by 1ndependent altimeter or by employ1ng the ranging fac11ity of the radar and a small portion of radar power directed downwards. In e1ther event elevat10n of the main beams 1s controlled s1mply to ensure 111um1nat10n of the landing fac111ty : no elevat~on angle measurements are made by the rece1ver.
A local oscillator 21 ls locked to the transmitter frequency to produce a constant I.F. output from m1xer 23. A frequency locking loop comprises offset filters 25 and 27, comparator 29 control voltage amplifier 31 and local oscillator 21. The resulting LØ 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 sw1tch 35 combines the received signal in one of two modes. For detection purposes it combines the two signals coherently hence giving a h1gher 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 d~splayed (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 clrcuit 47 where the separate target pulses are re-produced and compared to provlde 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 ~s controlled by this s~gnal, 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.
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In a non-angle-tracking mode the servo michanism 51 ls controllable by alternative inputs, ie an angle scan circuit 57 which ls responslve to range, and an overr1ding manual 1nput. In these circumstances the angle of the source equ1pment, 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 1n the dish drive servo 51. Aircraft angle motion can be fed in at 53 via the swttch waveform generator 55 causing a servo demand to cancel out that aircraft motion on the antenna 15. The inpwt signal to the dish servo gives an angle output essentially 1ndependent of aircraft angular movement away from the originally (or subsequently set) 11ne of sight, and can be used as an angle demand to the aircraft crew (or to 1ts autopilot) to achieve the desired course. Alternatively, the antenna pick-off output can be monitored and a~rcraft 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 slmple nature, e.g. an early/late gate operating on the nearest point of the rig with a further jittered gate to operate on the furthest po1nt of the rig, and hence allow an estlmate 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 sa~e gate. The 'early' gate output has an amplification x2 and the 'late' gate an ampl~fication 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.
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The far limit of the rig ls obta~ned by a 'jittered' gate 65 which 1s 'jittered' between two positions the w~dth of the gate apart. The outputs of the two gate posit1Ons are amplified different1ally and when a d1fference of a specif~ed threshold value e.g. 20dB exists between them the junct~on of the two positions lies on the far llmit of the rig return signal.
The range difference between these t~o positions thus gtves the rig dimenslon along the boreslght.
In an alternative rig range analysis system a block of contiguous range gates is employed. 40 range gates each of 30 metres will thus cover 1200 metres s~multaneously, as indicated tn 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 s~tuation. 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, with1n 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 ident1fying transponder or the transponder is out of action.
Figure 15 illustrates an alternative form of approach radar mentioned earlier. This ls 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. Interrogat~ng 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 produclng separation of the beacon signal in the upper path and the radar return in the lower path. Both signals are subjected to sensitivity-t~me-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 decod1ng.
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-10- , A processor 165 provides accurate azimuth angle determinatlon by samplillg the radar returns at br1ef intervals, to obtain values from a s~ngle 'target' at different points on the single beam as it scans. Range resolut~on is provided by selection, according to range, of the number of baslc 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 resolut10n. The grouping, or range collapsing, is performed by circuit 167, the reslutlng signals being integrated (169) and applled to the processor 165.
The resultlng lnformation, is then displayed ln 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 S, 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 crit~cal. Variants of this transponder may also be used for land based helicopter platforms. The transponder, shown in Figure 5, comprises a wide beam local1ser antenna 70 for receiving an interrogat~ng 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 ~n azimuth and 6 in elevation, the beam being upwardly directed at 3.
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Central ~n the localiser beam are the 4 beams of the stat~c split antenna having an overall beam w~dth of 10 1n azimuth and 2~ in elevation and again has a (gl1de~ slope of 3. The static split antenna 71 is purely a rece~wing antenna, the resulttng location informat~on belng transmitted by the local1ser antenna 70.
Referring now to F~gure 8(a), this shows, diagrammat1callyt the c1rcuitry and function of the transponder of F1gure 5. An alrcraft carrying the approach radar of Figure 2 and ly~ng ~ust (say) within the localiser beam 73 will transmit an interrogating pulse which will be recelved by the localiser. The radar frequency is 1n the X band, typically 9-10 6Hz. The pulse is received by antenna 70, filter 75, circulator 77 and mixer 79 where it 1s mixed with a local oscillator (81) signal of frequency fo~l50 MHz, fO
being the radar frequency. The mixer 79 thus produces an IF at 150 MHz wh~ch, after transmission by a blanktng gate 83 is passed to an array of delay lines. A first of these delay 11nes, 85, has a delay of dl/2, ie half ~f a basic unit delay. The pulse thus delayed ~s applied to a detector 87 and a monostab1e 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 d1sabled by way of gates 91. The first received pulse is further delayed a half unit by delay l~ne 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' ~s 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 d6 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 delay stages.
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As thus described, the ID pulse code is returned to the approaching a1rcraft once the alrcraft is within the localiser beam. ~hen the aircraft comes closer to the approach path, and in partitular w1thin 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 receiYed power levels between localiser and static split beam ~;
are recognised wlthin 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 resultlng signals are applied to hardware logic 103 and threshold circuits 105. Llmits 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 beam 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 reg~on around the boresight, ie on the correct guide slope, l~es in all four regions 2, 3, 4 and 5. These angular regions, distinguished by the sum and difference signals and the threshold circu1ts 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 to the top left it will produce pulses 3 and S, top right 2 and 5, bottom r~ght 2 and 4, and bottom left 3 and 4.
The pulses are produced by gates 122, 123, 124 and 125 and delay lines d2, d3, d4 and d5 according to the aboYe 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 .. . . ~, ,, . .~. . . . .
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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 ob~ective of the land1ng system.
Clearly other pulse codes are possible and the number of delay lines can be ad~usted accord1ngly. 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 btnary 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 llne 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.
Cons~dering 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 appropr1ate angle scan to cover uncerta1nty 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 1ntervals 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 stepp1ng (or equivalently continously scann1ng) these gates over the uncerta1nty 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 3~ 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 10~ 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, te. 6 sec for a 60-deg scan. As there exists the poss1bility always of encountering the rig as the helicopter closes, provided no obstacle avoidance is needed at detect1On, ie. the aircraft flies at safe height,in 12 sec at 75 m/s helicopter speed, the helicopter w111 have travelled less than 1 Km towards the rig before the rig must again be encountered, ie. only ~ of the range uncertainty set in th1s 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 ~s increased and/or scan rate can be appropriately 1ncreased. Similarly, if the number of pulse echoes desired for each alrcraft is reduced from 100 to, say, 50 then the scan rate can be doubled etc.
All target conta~ts or only those of rig size may be displayed to the crew.
. . . - - - -; : . -?
. .
, . ~ .
The radar ident1fles a rig autonomously by lts length being greater than a certa1n 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. ~n addition, the approach radar wlll 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 1~ null at 18 Km range and approximdtely 3 at 9 Km range. This is ind~cated in Figures lO(a), showing the difference characterlstic for a polnt source, and lO(b) for an extended source. Hence if the antenna ls ~ittered after detection about the target by a small angle the angular extent of the null can be measured and used as an extra ldentlfication means.
If there is an ID transponder on the r~g 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 determ~ned 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 10nm 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 ~ 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 9 in azimuth between the radar and the rig. Assuming the helicopter has an incidence ~0.
In general in this example, lt is assumed that either the helicopter can measure the incidence in azimuth and pass lt to the approach radar ~here it is processed or that the angle of incidence ~s small - it ~s possible also to allow large uncompensated incidence angles by increasing the radar angle of , . ~ . .
. :
look. The likely case is that the helicopter would approach the r~g into the wind and develop only a small angle of ~ncidence. The approach radar will have tolerances which glve a total azimuth angle result of O+SO + radar errors. The radar ts ~x~ accurate and to ensure that the above 10 turnaway, in general X turnaway, is achieved, X+2x turnaway is used, approximately, resulting in an extra 2xC/D at Dnm. 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 ~easured 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 form 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 whilst still maintaining its velocity vector pointing away from the rig when close to the rig. Using a normal to approach path reference, a suitable steering angle i~
then becomes E J - O - tan~1 rD - Ssin~ + G = R
LScosJ ~3 where S = M2+N2 and J = tan~1 (N/M) M is distance on XX axis N is distance on YY axis S is total distance to go J is radar look angle O is velocity ~ector angle wrt XX
where E, F and 6 together determine the turnaway achieved and helicopter velocity angle at the nearest approach point D from the rig and at polnts beforehand. Suitable values have been found to be E = 2 to 4, F = 2 and G = O.
, ..- . ~ . . . . .
:~ . - .: . . ;
F1gure 9 also shows an alternatlve course in whlch the approach path ls dlrected to the mlss polnt off the rig lm~,~,edlately the rlg is ~denttf1ed, le from 10nm out. Thls option may ~ell be preferable stnce no change in the course is required. ~'ith adequate S/N at the point where '~
approach to the r~g 1s begun, th~s method allows a pilot to steer (1f manual) or watch (if the steer1ng slgnals are coupled to the hellcopter autopllot) a constant bearlng course rather than one ln whlch the bearlng has to be altered.
As mentioned above, for an oll rlg or landlng platform at sea only the wlde beam locallser ls needed on the rig or platform but an approach from any direction may be desired.
F~gures 11, 12 and 13 show alternative embodiments of transponder localiser antenna from that employed in Figure 8, partlcularly for use wlth rigs. Flgure 11 shows a fixed 4-horn antenna pro~iding omnld~rectlonal coverage. If these antennas 115 are comblned statlcally, continuous omnidirectional coverage is obtalned but at a relatively low gain and requiring careful design at the four beam overlaps to avoid interference nulls. It needs sw~tching to achieve a relatively h~gh gain and an easy deslgn.
Figure 12 shows a sim11ar arrangement but ln whlch the elements are switchable 1n 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.
Flgure 13 shows a further omnidirectional antenna but havlng four elements 129 selectable (131) one at a tlme according to slgnal source direction. The alrcraft is in radio contact wlth the landlng controller and the latter can switch to the antenna covering the approach.
Flgure 14 illustrates a form of passive transponder for use on rigs. In this embodiment a serles of reflectors (133-139) are arranged spaced apart in a l~ne 140 lndicating the required approach path. Four reflect~rs are shown, the first and last (133 and 135) being omnidirectional ,. ~ ...................... .. ,. - ~ .
: : ~: . . .. - . . . --: :, - .~- . : : . .
,~. , I
1n azimuth and spaced so as to produce a relative delay of an 1nterrogat1ng pulse such as to ldentify the r~g. The other two reflectors 137, 139 are dlrectional and are directed so as to d1verge 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 distingu1shed. The divergence is such that an aircraft on the correct path w111 miss both 'beams' but will detect a reflection from one or the other if it ls off track to left or right. The resulting pulse trains are shown: P is the interrogating pulse, Pi the ID
pulses, Pl the pulse indicatlng '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 dum~y channel providing no useful information. The power in this channel is normally very 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 dumnw channel increases significantly. Hence the power in this channel is 00nitored and a rise is used to d~sable the transponder. As the aircraft closes it 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 w~ll be prepared to fall back on its own autonomous techniques described above for detecting and ident~fying the rig.
- , ~ - .1 .. :
. . . . ~ ~ . .
: . ~ . . . , . . . :
. : . - :.
Claims (30)
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 active transponder 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 said wide beam antenna and re-transmit by way of said wide beam antenna an identifying pulse signal characteristic of the associated landing facility.
10. A system according to Claim 9 for fixed wing aircraft, wherein said active transponder further comprises a static-split antenna providing azimuth and elevation angle discrimination over angles which are small relative to those of said wide beam, said receiving and transmitting means being adapted, in response to a radar pulse signal received by said static-split antenna, to transmit a locating pulse signal indicative of the angle of said aircraft relative to the boresight of said static-split antenna and thus provide a standard glide slope for instrumental landing of fixed wing aircraft.
11. A system according to Claim 10, wherein said transmitting means is adapted to transmit said locating pulse signal by way of said wide 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 said approach radar system, said coded pulse train being characteristic of the associated landing facility.
13. A system according to Claim 12, further including 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, 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 lin 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 switchable 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.
27. A aircraft landing approach system including an airborne approach radar substantially as hereinbefore described with reference to Figure 2 or Figure 15 of the accompanying drawings.
28. A fixed wing aircraft landing approach system including a landing facility having an active identification transponder substantially as hereinbefore described with reference to Figure 8 of the accompanying drawings.
29. A helicopter landing approach system including a sea based landing facility having a passive identification transponder substantially as hereinbefore described with reference to Figure 14 of the accompanying drawings.
30. A helicopter landing approach system including an airborne approach radar having means for determining an offset course lying within a predetermined miss distance of the landing facility substantially as hereinbefore described with reference to Figure 9 of the accompanying drawings.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB8825106.1 | 1988-10-26 | ||
GB888825106A GB8825106D0 (en) | 1988-10-26 | 1988-10-26 | Radar navigation system |
Publications (1)
Publication Number | Publication Date |
---|---|
CA2001488A1 true CA2001488A1 (en) | 1990-04-26 |
Family
ID=10645852
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
CA002001488A Abandoned CA2001488A1 (en) | 1988-10-26 | 1989-10-25 | Radar navigation system |
Country Status (5)
Country | Link |
---|---|
EP (1) | EP0394425A1 (en) |
CA (1) | CA2001488A1 (en) |
DK (1) | DK276090A (en) |
GB (2) | GB8825106D0 (en) |
WO (1) | WO1990004795A1 (en) |
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
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US11292615B2 (en) * | 2018-08-31 | 2022-04-05 | Airbus Operations Gmbh | Deformation sensing system |
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US5364716A (en) * | 1991-09-27 | 1994-11-15 | Fujitsu Limited | Pattern exposing method using phase shift and mask used therefor |
US7136012B2 (en) | 2003-04-01 | 2006-11-14 | Lockheed Martin Corporation | Approach radar with array antenna having rows and columns skewed relative to the horizontal |
RU2520872C2 (en) * | 2012-07-10 | 2014-06-27 | Открытое акционерное общество "Раменское приборостроительное конструкторское бюро" (ОАО "РПКБ") | Complex system for controlling aircraft trajectory during approach landing |
RU2496131C1 (en) * | 2012-07-10 | 2013-10-20 | Открытое акционерное общество "Раменское приборостроительное конструкторское бюро" (ОАО "РПКБ") | Method of aircraft control in landing approach |
US9646506B2 (en) * | 2015-09-30 | 2017-05-09 | Honeywell International Inc. | Methods and apparatus for managing a premature descent envelope during descent of an aircraft |
US9745078B2 (en) * | 2016-02-01 | 2017-08-29 | Honeywell International Inc. | Systems and methods of precision landing for offshore helicopter operations using spatial analysis |
US10578733B2 (en) | 2016-02-05 | 2020-03-03 | Honeywell International Inc. | Low-power X band beacon transponder |
US9836064B2 (en) | 2016-03-02 | 2017-12-05 | The Boeing Company | Aircraft landing systems and methods |
RU2620587C1 (en) * | 2016-04-27 | 2017-05-29 | Акционерное общество "Лётно-исследовательский институт имени М.М. Громова" | Method of determining the coordinates of an aircraft relative to the flight strip |
US10684365B2 (en) | 2017-08-22 | 2020-06-16 | Honeywell International Inc. | Determining a location of a runway based on radar signals |
RU2668597C1 (en) * | 2017-11-30 | 2018-10-02 | Андрей Викторович Тельный | Method of troubleshooting and failures of aircraft measurement parameters of movement and satellite navigation systems of moving objects |
US20210390865A1 (en) * | 2018-11-01 | 2021-12-16 | Bae Systems Plc | Signal transmitting device |
CN112046770B (en) * | 2020-08-20 | 2022-07-08 | 中国南方电网有限责任公司超高压输电公司检修试验中心 | Helicopter plug-in device and installation method thereof |
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BE481269A (en) * | 1942-03-23 | |||
US3243804A (en) * | 1963-07-26 | 1966-03-29 | Jr Ira D Smith | Four horn sequential lobing radar |
US3355733A (en) * | 1964-10-19 | 1967-11-28 | Bell Aerospace Corp | Designated area instrument landing system |
GB1092821A (en) * | 1965-02-02 | 1967-11-29 | Richard Thomas Cella | Instrument landing system |
GB1375221A (en) * | 1970-04-13 | 1974-11-27 | ||
US3739385A (en) * | 1970-07-15 | 1973-06-12 | Texas Instruments Inc | Mechanically swept radar antenna for use with an aircraft landing monitor system |
US3716855A (en) * | 1970-07-15 | 1973-02-13 | Texas Instruments Inc | Glideslope position detection system for use with an independent aircraft landing monitor |
US3754252A (en) * | 1971-10-06 | 1973-08-21 | Itt | Adaptive array retrodirective landing control responser |
US3765019A (en) * | 1972-03-20 | 1973-10-09 | United Aircraft Corp | Passive radar glide slope orientation indication |
SU695586A3 (en) * | 1973-12-14 | 1979-10-30 | Томсон-Цсф (Фирма) | Instrumental aeronavigation system |
US3922674A (en) * | 1974-01-24 | 1975-11-25 | Raytheon Co | Transponder for use in a radio frequency communication system |
US3934250A (en) * | 1974-06-07 | 1976-01-20 | The United States Of America As Represented By The Secretary Of The Navy | Helicopter blind landing and hover system |
FR2445534A1 (en) * | 1978-12-29 | 1980-07-25 | Thomson Csf | AIR-TO-SOLAR RADAR TELEMETRY DEVICE FOR AIR-BASED SHOOTING SYSTEM AND SHOOTING SYSTEM HAVING SUCH A DEVICE |
DE3644478A1 (en) * | 1986-12-24 | 1988-07-07 | Licentia Gmbh | SYSTEM FOR LANDING AID FOR AIRCRAFT WITH OWN ON-BOARD RADAR |
-
1988
- 1988-10-26 GB GB888825106A patent/GB8825106D0/en active Pending
-
1989
- 1989-10-25 CA CA002001488A patent/CA2001488A1/en not_active Abandoned
- 1989-10-26 GB GB8924076A patent/GB2224903A/en not_active Withdrawn
- 1989-10-26 WO PCT/GB1989/001275 patent/WO1990004795A1/en not_active Application Discontinuation
- 1989-10-26 EP EP89912268A patent/EP0394425A1/en not_active Withdrawn
-
1990
- 1990-11-20 DK DK276090A patent/DK276090A/en unknown
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US11292615B2 (en) * | 2018-08-31 | 2022-04-05 | Airbus Operations Gmbh | Deformation sensing system |
Also Published As
Publication number | Publication date |
---|---|
GB2224903A (en) | 1990-05-16 |
GB8825106D0 (en) | 1988-11-30 |
EP0394425A1 (en) | 1990-10-31 |
DK276090D0 (en) | 1990-11-20 |
DK276090A (en) | 1990-11-20 |
GB8924076D0 (en) | 1989-12-13 |
WO1990004795A1 (en) | 1990-05-03 |
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