GB2242589A

GB2242589A - Mobile radar

Info

Publication number: GB2242589A
Application number: GB8417789A
Authority: GB
Inventors: Phillip David Lane Williams
Original assignee: Decca Ltd
Current assignee: Decca Ltd
Priority date: 1984-07-12
Filing date: 1984-07-12
Publication date: 1991-10-02
Anticipated expiration: 2004-07-12
Also published as: GB2242589B

Abstract

Airborne radar, has a store 27 for radar returns received when the vehicle is at a first position. The track and speed of the vehicle are monitored 32 as it moves from the first to a second position and the radar returns received at the second position are spatially correlated with the stored returns to identify target positions by triangulation. The display uses different colours for increasing confidence that returns constitute a target. <IMAGE>

Description

MOBILE RADAR APPARATUS The present invention relates to radar apparatus for mounting on a mobile vehicle, especially an aircraft.

A problem with azimuth scanning radar mounted on aircraft is that the angular resolution of the radar system is limited by the relatively small antenna size which can be conveniently installed. For example, a radar beam width of 20 implies an azimuth spread of about 350 metres at a range of 10 kilometres. By comparison, range resolutions in pulse radar apparatus of the order of 10 to 15 metres is readily achievable using currently available simple magnetron transmitters operating with pulse lengths of the order of 100 nS.

A common use of airborne radar is for ground mapping or surveillance, eg over the sea for the identification of ships and topographical features such as the coast line. Compared to aircraft speeds, ships are very slow moving and can be regarded for many radar purposes as essentially stationary targets.

The present invention sets out a technique whereby the relatively good range resolution of typical airborne radar can be used to obtain substantial improvement in the spatial resolution of stationary or relatively slow moving targets detected by the radar.

According to the present invention, radar apparatus for mounting on a mobile vehicle comprises a radar transmitter and receiver and antenna means, arranged to have a predetermined range resolution from real time radar return signals, means for storing radar return signals received when the vehicle is at a first position, means monitoring the track and speed of the vehicle over the ground as it moves from said first position to a second position, means correlating radar return signals received when the vehicle is at the second position with the stored signals to identify the positions of radar targets by bistatic ranging, and means to display the identified target position.

Bistatic and multistatic radar is discussed in chapter 36 of the Radar Handbook, edited by M.I.Skolnik, published 1970. Indeed the earliest forms of radar were generally of the bistatic type using transmitter and receiver antenna separated by a distance comparable to the target distance. Target location in bistatic radar systems can depend on measuring the azimuth angle of the target from each of the two locations or the ranges from each of the two locations, or various other parameters. Multistatic radar systems employ three or more spatially separated sites and are useful in dealing with possible ambiguities in target location by triangulation techniques.

It will be appreciated, however, that the bistatic and multistatic techniques contemplated hitherto depend on provision of radar installations at two or more spatially separated sites, and are therefore not easily applicable to airborne radar.

In the present invention, the track and speed of aircraft itself is monitored as the aircraft moves from first to second positions, so that the aircraft's own movement can be used to define two spatially separated positions relative to the ground. By storing radar returns received when the aircraft is at the first position and correlating signals received when the aircraft is at the second position with the stored signals, a bistatic arrangement can be synthesised.

Targets detected by the radar system can be more accurately located from their ranges as detected by the apparatus at each of the two positions using the bistatic ranging technique. The invention is only applicable to targets which can be regarded as substantially stationary relative to the aircraft, ie Itoving by less than the spatial resolution of the radar in the time taken by the aircraft to travel from the first to the second positions.

Preferably, said means for storing are arranged to store also radar return signals received when the vehicle is at at least one intermediate position between the first and second positions, and said correlating means correlates also the stored return signals received at the or each intermediate position to identify the target positions by multistatic ranging. In this way some ambiguities resulting from bistatic techniques can be resolved and target positions can still be identified if a return signal at one of the positions has faded.

In an embodiment, said antenna means produces a radar beam of predetermined divergeance in azimuth which can be directed in at least selected different azimuth directions relative to and to the same side of the vehicle heading so that a single target can be illuminated by the beam when the vehicle is at successive said positions along its track, defined as the positions from which the different azimuth directions intersect at the target position. By using a defined azimuth beam, ambiguities arising from the bistatic and multistatic ranging techniques can be avoided.

Conveniently, the antenna is such that the radar beam can be directed only in selected said azimuth directions which are preset. The fixed beam directions may be produced by a fixed antenna system. For example the frequency sensitivity of a dispersive array can be used to generate two or more beams from the single array by selecting appropriate frequencies.

Alternatively, said radar beam is azimuth scanned by the antenna means, e.g. by mechanical rrovent with respect to the heading, and. selection means are provided to select only radar returns for storing and correlating which are received when the scanning beam is directed in said selected different azimuth directions, thereby presetting said directions. Such an arrangament may be useful when the apparatus of the present invention is provided by modification of an existing airborne radar which has an azimuth scanning antenna system.

Further details of an appropriate beam selection arrangement using an azimuth scanning radar are described in U.K. Patent No. 1589825.

Preferably, said means for storing comprises a frame store for storing digital signals representing a radar picture for display, with predetermined store locations corresponding to respective target positions in the ground plane,and is arranged to store radar return signals received in the more forward of the preset beam directions so as to build up a radar picture as the vehicle moves along track, and said correlating means is arranged, responsive to the monitoring means, to identify the store locations for the positions in the ground plane corresponding to currently received return signals in a more rearward preset beam direction, and to modify the digital contents of the identified locations so as to indicate those locations corresponding to the positions of target returns received at each of successive said positions of the mobile vehicle.

Conveniently again, uncorrelated returns received in one of the preset beam directions are stored in corresponding store locations so as to be distinguished from the modified contents of the store locations corresponding to returns which have correlated on successive positions of the vehicle, and said display means is arranged to display additionally said uncorrelated returns, with the correlated returns indicating the identified target positions being visually distinct therefrom. Said display means may include a multi-colour display and may be arranged so that the uncorrelated target returns are displayed in a first selected colour and the correlated returns indicating identified target positions in a second colour so as to be contrasted and more visually dominant than said first colour.Thus weak or fading signals e.g. from small targets which might not reappear at successive positions of the vehicle are still displayed relying on the beam width for angular resolution.

The apparatus of the invention can be regarded as performing area stationary target indication (as opposed to Area Moving Target Indication), from a mobile radar apparatus with full compensation for the movement of the apparatus. Provided the correlation of targets received at successive positions as the aircraft travels along its track is performed for aircraft positions which are sufficiently spaced apart along track, the relatively high range resolution of the radar apparatus will prevent those target returns from targets away from the bore sight of the radar beam from correlating at successive aircraft positions.

In a different embodiment of the invention, the antenna means produces an azimuth scanning beam and the apparatus includes a frame store for storing digital signals representing a radar picture for display, with predetermined store locations corresponding to respective target positions in the ground plane, a coordinate converter to convert the polar coordinates of radar video received in the current azimuth scan into Cartesian coordinates, a store address generator, responsive to vehicle track and speed indications fran said rtaiitoring means and said Cartesian coordinates of the currently received radar video, to generate address signals identifying locations in the frame store for the target positions in the ground plane corresponding to the currently received radar video, store control means to read out the existing value contained in each of the identified store locations, modify said value in response to any return signal in the current radar video for the corresponding target position to integrate digitally said returns on separate azimuth scans, and write the integrated value back into the location.

In this way, as will become apparent, the frame store may then be used to provide a T.V. type radar display in which newly detected targets just coming into detection range as the aircraft travels along track are displayed with the natural azimuth resolution of the radar system, but targets which have been within detection range whilst the aircraft has moved a substantial distance along track are displayed with improved resolution resulting from the integration procedure in the frame store. It will be appreciated that the range resolution of the radar system will result in only a central portion of the angular extent of a target indication integrating on successive azimuth scans as the aircraft moves along its track.

Preferably with this arrangenwt, said display means is arranged to provide visual indication of the degree of integration of the returns in each store location on separate azimuth scans.

The display means may include a multi-colour display.

Examples of the present invention will now be described and explained in more detail with reference to the actpanying drawings in which Figure 1 is a diagrammatic illustration of how bistatic or multistatic ranging techniques can be employed to improve the resor lution of a radar system with limited angular resolution as the aircraft moves along track; Figure 2 is an enlarged view of part of Figure 1; Figure 3 is a block schematic diagram of airborne radar apparatus embodying the present invention and Figure 4 illustrates a typical display from apparatus as shewn in Figure 3.

Figure 1 illustrates a view in the ground plane of an aircraft moving along a track 10 in the direction of arrow 11, past a radar detectable target 12. As the aircraft moves along the track 10, it passes through successive positions 1, 2 and 3 as shown.

The aircraft is fitted with an azimuth scanning radar which has a finite beam width and substantially better range resolution so that the response on a Plan Position Indicator display of the target 12 as detected when the aircraft is at position 1 is such as illustrated by the arc 13. It can be seen that, as is quite normal for airborne radar apparatus, the range definition of the target is relatively good but the substantial beam width of the radar produces a relatively long response arc on the display. This gives rise to lack of spatial resolution on the display so that a cluster of targets may not be separately distinguishable, but appear on the display simply as a wide response arc.

When the aircraft has travelled to position 2 along track 10, the corresponding response on a PPI from the target is as shown by the dotted arc 14.

Similarly the response on the display when the aircraft has travelled to position 3 is that shown by the arc 15. It can be seen that, if the responses of the radar apparatus when in the positions 1, 2 and 3 along the track 10 are superimposed, ie corrected for the movement of the aircraft over the ground so as all to designate the same position in the ground plane, the arcs 13, 14 and 15 intersect to indicate the position of the target with much enhanced spatial resolution.

The apparatus embodying the present invention performs this function enabling the relatively good range resolution of the radar apparatus on the aircraft to be employed in determining the location of the target by bistatic or multistatic ranging techniques from successive positions of the aircraft.

Referring now to Figure 3, airborne radar apparatus comprises an antenna 20 arranged to produce an azimuth scanning radar beam. The direction of the beam relative to the heading of the aircraft (e) is derived in the usual way, eg from a shaft encoder or synchro converter, and fed on a line 21 to a PPI display unit 22. The antenna 20 is connected via a Transmit/Receive cell 23 with a radar transmitter/receiver 24 which supplies analogue radar video to the PPI display 22 on a line 25 for display in the usual way. The apparatus described so far can form part of a standard azimuth scanning airborne radar eg for navigation, surveillance or even weather warning purposes.

In the present arrangement, the analogue radar video is digitised in a video digitiser 26 for scan conversion and storing in a digital frame store 27.

The locations in the digital frame store 27 into which the digitised video is written are determined by a write address generator 28. The write address generator 28 receives the radar beam azimuth angle data (e) on a line 29 and also range data from the radar transmitter/ receiver 24 on a line 30. The range data may comprise simply trigger pulses synchronous with the transmitter pulses of the radar used for resetting a free running clock in the write address generator 28, whereby the output of the clock following each trigger pulse is indicative of the range of radar video received at that instant.

The angle data on line 29 and range data together comprise polar coordinates defining the position relative to the aircraft of any targets indicated by radar returns in the real time video. The write address generator 28 also receives data on a line 31 from a navigational aid unit 32 and defining the track and speed of the aircraft over the ground. The write address generator 28 converts the polar coordinates of the received radar video relative to the aircraft to Cartesian coordinates and employs these converted Cartesian coordinates together with the track and speed data from the navigational aid to generate store address data on a bus 33 identifying locations in the frame store 27 into which the digitised video can be written.

The write address generator 28 is arranged in response to the track and speed data to compensate for the movement of the aircraft over the ground so that digitised video received in successive selected azimuth scans of the radar and corresponding to reflections from targets at a fixed position in the ground plane is addressed to the same store location on each occasion.

Thus, the frame store 27 contains data defining a radar picture, with individual store locations corresponding to fixed target positions in the ground plane as the aircraft moves along past each position over the ground.

As the aircraft travels along track past a target detected by the radar, digitised video returns from the target are received in successive azimuth scans for storage in the same store location. In fact, if two azimuth scans of the radar are considered when the aircraft is at two respective positions sufficiently spaced apart along track, then only those central portions of the response arcs from the target at the two aircraft positions which overlap,(ie at the crossover of the arcs 13 and 15 in Figure 1), will be stored in the same locations in the frame store 27.

The present apparatus operates to integrate the digital returns received for storage in the same store location on successive azimuth scans of the radar, so that those locations for which returns arise in azimuth scans extending over a substantial length of track of the aircraft travel are clearly indicated in the frame store by high integration values. The apparatus for performing the integration in the digital frame store is illustrated in Figure 3 by an "AND" gate 34 which is shown as adding the existing value contained in a particu-lar addressed store location as supplied on a data output bus 35 to the currently received digital return from the digitiser 26 and supplying the sum on an input bus 36 for storing back in the addressed location.It may be appreciated that the "AND" gate 34 illustrated in Figure 3 is essentially a schematic representation of any form of digital integrator which may be incorporated in this apparatus.

Normally, it will not be desirable to integrate video returns over every successive azimuth scan. The integration is desired to extend over two and preferably more azimuth scans taken when the aircraft is at respective positions along track which are substantially spaced apart by a total distance comparable to the range of the target. With an antenna system providing one azimuth scan per second, there would be about a hundred azimuth scans during the time the aircraft took to travel 10 kilometres along track. Only a fraction of this number of scans need be used in the integration process to achieve the spatial resolution improvement for substantially stationary targets. Thus, a controller 37 is provided to enable store reading and writing cycles only during selected azimuth scans as determined by selection signals on a line 38.In this way, radar video is integrated only on selected azimuth scans taken at, say, 10 kilometre intervals as the aircraft travels along track.

It can be seen therefore that the digital frame store 27 contains data defining a complete radar display.

Targets which have been detected by the radar in azimuth scans whilst the aircraft has travelled a substantial distance along the track are integrated to provide enhanced spatial resolution. It can be seen that the address generator 28 operates so that the addressed store locations corresponding to the position on the ground plane of the aircraft progressively move through the frame store in correspondence with the movement of the aircraft along track. The store and the address generator are arranged so that the store always contains the latest radar video for a distance in front of the present position of the aircraft corresponding to the maximum range of the radar apparatus.The store may be arranged to be oversized in the sense of having space to contain not only the video from targets the maximum range behind the aircraft but also historical data corresponding to the received video further back along the aircraft track. The spatial resolution of the targets detected by the apparatus will depend on the degree of integration which has been possible since these targets were first detected. Thus targets only just coming into detection range in front of the aircraft are detected with the initial resolution of the radar apparatus. As the aircraft flies past these targets the spatial integration procedure enables the range resolution of the radar to provide improved spatial resolution. Once targets fall behind the maximum range of the radar apparatus, the display no longer changes and is simply an historical record.

The contents of the digital frame store 27 is used to generate a T.V.-type display on a T.V. monitor 40. Data from the store 27 is read out under the control of a read address generator 41 which generates the addresses of the store locations from which data is to be read for use in generating the T.V. display.

Data read out from the store on line 42 is fed to a digital to analoque converter 43 for conversion to T.V. video signals supplied to the monitor 40 on a line 44. The data reading process and digital to analogue conversion, along with the scanning of the T.V. monitor 40 is controlled in synchronism by signals from a T.V.

sync generator 45.

The read address generator 41 is essentially arranged to read out the data in locations in the digital store corresponding to lines right across the resultant T.V. display. However, the start point for the first line of a new frame of the T.V. display 40 is controlled by track and speed signals from the navigational aid 32, fed to the read address generator 41, so that the position of the radar carrying aircraft on the resultant display always appears at the same position on the display screen.

The resultant display on the T.V. monitor 40 may be such as illustrated in Figure 4. The position of the radar carrying aircraft is indicated at an origin point 50 and the maximum range of the radar is indicated by the circle 51 centred on the origin 50.

The track of the aircraft up to its present position is illustrated by the vertical line 52. It is assumed in this example that the aircraft has been flying straight and level.

For the sake of illustration, a coast line 53 is shown in Figure 4 and a series of spaced substantially fixed small targets 54, 55, 56, 57 and 58. The target 54 and the portion of coast line 59 at the upper end of the display is illustrated with the typical spatial resolution of an airborne radar apparatus. As can be seen the small target is well resolved in range but shows a substantial response arc corresponding to the significant beam width of the radar system.

Similarly the coast line is fuzzy and indefinite. For the targets such as 55 and 56, and the nearby coast line portion, which have been in detectable range of the aircraft over a substantial distance of travel along its track, the displayed spatial resolution is significantly improved due to the spatial integration procedure described above. Once the targets and coast line have fallen behind the maximum detection range of the radar apparatus, such as targets 57 and 58 and the adjacent portions of coast line, no further integration can take place but the display can continue to show these historic responses until they "roll off" the bottom end of the display.

All target returns stored are displayed on the T.V. display 40. Weak or fading returns from small or distant targets may not appear in sufficient selected azimuth scans, as the vehicle travels along track to integrate significantly. However even these target returns can be displayed preferably at a lower level than highly integrated returns. For such weak and fading returns angular resolution is dependent on the azimuth beam width. However, the angular resolution of the radar with a particular beam width is better for weak targets than strong targets from which returns can be detected further from the bore sight.

The display 40 is preferably a colour display so that, for example, weakly integrated returns are displayed in low level blue, moderately integrated targets in red and highly integrated targets in yellow at a higher level. In the example where selected scans are stored to provide only say three "looks" at each target as the aircraft flies past, targets appearing in one look only may be in blue, those in two looks in red and those in all three in yellow.

Data from the digital store 27 can be permanently recorded e.g. on magnetic tape so as to provide a complete history of the responses received over desired lengths of aircraft track. Then any portion of the radar picture over the aircraft track can be displayed from the recording as desired.

Further details of a convenient technique for recording a radar picture are described in the aforementioned Patent No. 1589825.

Claims

CLAIMS:

1. Radar apparatus for mounting on a mobile vehicle, comprising a radar transmitter and receiver, and antenna means, arranged to have a predetermined range resolution from real time radar return signals, means for storing radar return signals received when the vehicle is at a first position, means monitoring the track and speed of the vehicle over the ground as it moves from said first position to a second position, means correlating radar return signals received when the vehicle is at the second position with the stored signals to identify the positions of radar targets by bistatic ranging, and means to display the identified target positions.

2. Radar apparatus as claimed in Claim 1 wherein said means for storing is arranged to store also radar return signals received when the vehicle is at at least one intermediate position between the first and second positions, and said correlating means correlates also the stored return signals received at the or each intermediate position to identify the target positions by multistatic ranging.

3. Radar apparatus as claimed in Claim 1 or, Claim 2 wherein said antenna means produces a radar beam of predetermined divergeance in azimuth which can be directed in at least selected different azimuth directions relative to and to the same side of the vehicle beading so that a single target can be illuminated by the beam when the vehicle is at successive said positions along its track, defined as the positions from which the different azimuth directions intersect at the target position.

4. Radar apparatus as claimed in Claim 3, wherein the radar beam can be directed only in selected said azimuth directions which are preset.

5. Radar apparatus as claimed in Claim 3, where said radar beam is azimuth scanned by the antenna means and selection means are provided to select only radar returns for storing and correlating which are received when the scanning beam is directed in said selected different azimuth directions, thereby presetting said directions.

6. Radar apparatus as claimed in Claim 4 or Claim 5 wherein said means for storing comprises a frame store for storing digital signals representing a radar picture for display, with predetermined store locations corresponding to respective target positions in the ground plane, and is arranged to store radar return signals received in the more forward of the preset beam directions so as to build up a radar picture as the vehicle moves along track, and said correlating means is arranged, responsive to the monitoring means, to identify the store locations for the positions in the ground plane corresponding to currently received return signals in a more rearward preset beam direction, and to modify the digital contents of the identified locations so as to indicate those locations corresponding to the positions of target returns received at each of successive said positions of the mobile vehicle.

7. Radar apparatus as claimed in Claim 6 wherein uncorrelated returns received in one of the preset beam directions are stored in corresponding store locations so as to be distinguished from the modified contents of store locations corresponding to returns which have correlated on successive positions of the vehicle, and said display means is arranged to display additionally said uncorrelated returns, with the correlated returns indicating the identified positions being visually distinct therefrom.

8. Radar apparatus as claimed in Claim 7 wherein said display means includes a multi-colour display and is arranged so that the uncorrelated target returns are displayed in a first selected colour and the corre--lated returns indicating identified target positions in a second colour so as to be contrasted and more visually dominant than said first colour.

9. Radar apparatus as claimed in Claim 3 wherein the antenna means produces an azimuth scanning beam and the apparatus includes a frame store for storing digital signals representing a radar picture for display, with predetermined store locations corresponding to respective target positions in the ground plane, a coordinate converter to convert the polar coordinates of radar video received in the current azimuth scan into Cartesian coordinates, a store- address generator, responsive to vehicle track and speed indication from said monitoring means and said Cartesian coordinates of the currently received radar video, to generate address signals identifying locations in the frame store for the target positions in the gound plane corresponding to the currently received radar video, store control means to read out the existing value contained in each of the identified store locations, modify said value in response to any return signal in the current radar video for the corresponding target position to integrate digitally said returns on separate azimuth scans, and write the integrated value back into the location.

10. Radar apparatus as claimed in Claim 9 wherein said display means is arranged to provide visual indication of the degree of integration of the returns in each store location on separate azimuth scans.

11. Radar apparatus as claimed in Claim 10 wherein said display means includes a multi-colour display.

12. Radar apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

Amendments to the chimps have been fled as foKows.

1. Radar apparatus for mounting on a mobile vehicle, comprising a radar transmitter and receiver, and antenna means, arranged to have a predetermined range resolution from real time radar return signals, means for storing radar return signals, including target range data, received when the vehicle is at a first position, means monitoring the track and speed of the vehicle over the ground as it moves from said first position to a second position, means correlating target range data of radar return signals received when the vehicle is at the second position with the stored target range data to identify the positions of radar targets by range triangulation, and means to j,.

display the identified target positions.

2. Radar apparatus as claimed in Claim 1 wherein said means for storing is arranged to store also radar return signals, including target range data, received when the vehicle is at at least one intermediate position between the first and second poSitions, and said correlating means correlates also the stored target range data received at the or each intermediate position to identify the target positions.

3. Radar apparatus as claimed in Claim 1 or, Claim 2 wherein said antenna means produces a radar beam of predetermined divergeance in azimuth which can be directed in at least selected different azimuth directions relative to and to the same side of the vehicle heading so that a single target can be illuminated by the beam when the vehicle is at successive said positions along its track, defined as the positions from which the different azimuth directions intersect at the target position.

7. Radar apparatus as claimed in Claim 6 wherein uncorrelated returns received in one of the preset beam directions are store6 in corresponding store locations so as to be distinguished from the modified contents of store locations corresponding to returns which have correlated on successive positions of the vehicle, and said display means is arranged to display additionally said uncorrelated returns, with the correlated returns indicating the identified positions being visually distinct therefrom.

8. Radar apparatus as claimed in Claim 7 wherein said display means includes a multi-colour display and is arranged so that the uncorrelated target returns are displayed in a first selected colour and the corre-lated returns indicating identified target positions in a second colour soars to be contrasted and more visually dominant than said first colour.

9. Radar apparatus as claimed in Claim 3 wherein the antenna means produces an azimuth scanning beam and the apparatus includes a frame store for storing digital signals representing a radar picture for display, with predetermined store locations corresponding to respective target positions in the ground plane, a coordinate converter to convert the polar coordinates of radar video received in the current azimuth scan into Cartesian coordinates, a store address generator, responsive to vehicle track and speed indication from said monitoring means and said Cartesian coordinates of the currently received radar video, to generate address signals identifying locations in the frame store for the target positions in the gound plane corresponding to the currently received radar video, store control means to read out the existing value contained in each of the identified store locations, modify said value in response to any return signal in the current radar video for the corresponding target position to integrate digitally said returns on separate azimuth scans, and write the integrated value back into the location.