GB1589825A - Method of making a radar reconnaissance or survey record and apparatus therefor - Google Patents

Description

(54) A METHOD OF MAKING A RADAR RECONNAISSANCE OR SURVEY RECORD AND APPARATUS THEREFOR (71) We, DECCA LIMITED, a British Company, of Decca House, 9 Albert Embankment, London, SE1 7SW, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to making a radar reconnaissance or survey record from a moving vehicle. The invention also relates to apparatus for this purpose. It is well known to employ radar techniques for carrying out survey or reconnaissance oper ations. In a common technique, an airborne radar is arranged to provide ground returns which, when displayed, can give an indica tion of the nature of the terrain over which the aircraft flies.At sea the same system can be used for carrying out search operations for surface objects. Also, there is growing interest nowadays into the use of radar for locating and monitoring oil slicks.

The well known PPI type display may be employed to provide a picture of the radar ground returns in the aircraft during flight.

This is especially useful for "real time" search operations where substantially im mediate recognition of ground echoes is required so that suitable actions can be taken quickly.

However, it is also sometimes desirable to record the ground return signal data for subsequent play back and analysis. To this end it is, of course, possible to take succes sive photographs of the PPI display at intervals during the flight of the aircraft, but -this has various drawbacks. The installation of a camera to photograph the PPI display can make it difficult to use the display for normal visual monitoring. Thus, a second display would be required with its attendant expense and weight. Further, the photo graphs must be taken sufficiently often, or at short intervals of time, to ensure that substantially all the ground swathe along the aircraft's track covered by the radar apparatus is recorded. On the other hand, if the photographs are taken too often there is excessive overlapping between successive photographs which makes inefficient use of the record medium.

In order to provide a radar data record for subsequent analysis, it is known to provide sideways looking radar. This is a nonscanning radar system having an antenna substantially parallel with the main axis of the aircraft to direct a fixed narrow beam substantially perpendicular to the axis, and also the direction of motion, of the aircraft.

The radar apparatus employed is typically a pulse radar and the ground signal returns after each radar pulse are recorded on board the aircraft. It can be seen that as the aircraft moves along its path, successive radar pulses irradiate successive areas of ground extending from the aircraft substantially perpendicular from its heading. When the aircraft returns to its base, the record of all the signal returns during the flight can be played back to reproduce a radar map of the ground swathe covered by the aircraft.

Such sideways looking radar operates quite satisfactorily but it is relatively expensive to install and must be provided as a separate radar system to the aircraft's normal radar apparatus as used to provide information during the flight.

According to the present invention, a method of making a radar reconnaissance or survey record from a moving vehicle, comprises operating a pulse radar set mounted on the vehicle, the set having means for scanning in azimuth the direction of look of the set relative to the vehicle axis of motion, indicating the times when the instantaneous direction of look relative to the vehicle axis of motion is substantially equal to one or more of pre-determined said directions which are transverse relative to the axis of motion and recording radar return signals from at least one inter pulse period at each of the indicated times. The direction of look of a radar set is the direction relative to the apparatus from which it is most sensitive to received radar return signals. This direction of look is commonly scanned in azimuth by providing a rotatable antenna.Alternative ly azimuth scanning can be obtained from a fixed antenna of the form known as "phased array" in which the scanning is obtained by varying the relative phasing of the dipole elemcnts making up the array. It is quite common to provide a moving vehicle, especially an aircraft, with a radar apparatus which has means for scanning in azimuth.

Such sets may be employed as navigational aids or as weather warning systems. The method of the present invention may make use of such a standard type of radar.

By the method, radar return signals are recorded corresponding only to one or more predetermined directions of look of the set.

These predetermined directions are selected to be transverse relative to the axis of motion of the vehicle so that as the vehicle moves, the recorded return signals can provide a picture of targets sensed by the radar set adjacent the path of travel of the vehicle. It can be seen tbat the process of recording only return signals from these predetermined transverse directions of look corresponds to the operation of a sideways looking radar. Thus, the record so formed can be used subsequently to enable the targets pickcd up by the radar as the vehicle movcd along its path to be analyzed.

However, the method of the present invention has the advantage that it can be employed using a standard radar set alrcady provided on the vehicle, and further that the application of the method of the invention docs not interfere with the use of the radar apparatus for other purposes e.g. for providing a "real time" Pl'l display.

The method is especially useful when the vehicle is an aircraft and the pulse radar set mounted on the aircraft is arranged to look downwards from the aircraft so that the radar return signals include ground returns.

Then, the method of the invention can provide a reconnaissance or survey record of at least part of the ground sweep covered by the airerift's radar set during its flight.

The present invention further envisages a pulse radar apparatus for mounting on a vehicle, comprising a pulse radar set having means for scanning in azimuth the direction of look of the set rclative to the vehicle axis of motion, an azimuth angle detector for indicating the times when the instantaneous direction of look relative to the vehicle axis of motion is substantially equal to one or more predetermined said directions which are transverse relative to the axis of motion, and means responsive to said azimuth angle detector for recording radar signals from at least one inter pulse period at each of the indicated times.

The pulse radar set may be a standard set having a radar display for real-time viewing, as provided on the vehicle for, for example, navigational or weather warning purposes.

The azimuth angle detector and the recording means may be connected to the existing standard radar set without interfering with its normal real-time operation.

The present invention, accordingly, still further envisages a radar recording apparatus for use with a vehicle mounted pulse radar set which has means for scanning in azimuth the direction of look of the radar relative to the vehicle axis of motion, the recording apparatus including an azimuth angle detector for indicating the times when the instantaneous direction of look of the radar set relative to the vehicle axis of motion is substantially equal to one or more predetermined said directions which are transverse relative to the axis of motion, and means responsive to said azimuth angle detector for recording radar return signals from at least one inter pulse period at each of the indicated times.

In the method of the invention, the recording step may comprise sampling the radar return signals at a predetermined sampling rate and quantizing the samples in at least two levels to provide digitized radar video. writing into a digital store at said sampling rate the digitized video from the or each inter pulse period at each of the indicated times, subsequently reading out the stored digitized video from the store at a rate slower than the sampling rate in the time available between successive said indicated times, and recording the more slowly read out digitized video. In this way, the bandwidth of the return signals to be recorded can be reduced to render the recording process easier. In practice there is usually so much time available between successive said indicated times relative to the duration of each inter-pulse period that the digitized video may be read from the store at such a slow rate that it can be recorded on a magnetic tape recorder having a frequency response no greater than that necessary for recording music.

In either the pulse radar apparatus or the recording apparatus of the present invention, the recording means may comprise means for sampling the radar return signals at a predetermined sampling rate and quantitizing the samples in at least two levels to provide digitized radar video, a digital store, means for writing into the store at the sampling rate the digitized video from the or each inter pulse period at each of the indicated times, means for reading out the stored digitized video from the store at a rate slower than the sampling rate in the time available between successive said indicated times and a recorder for recording the more slowly read out digitized video. The recorder may be a magnetic tape recorder of the cassette type having a frequency response not extending above 20 kilohertz.

As mentioned previously, the pulse radar apparatus of the present invention includes means for scanning the direction of look of the radar. This scanning means may be operable to scan the direction of look through 360" in azimuth. In this case, the or each predetermined direction of look is preferably perpendicular to the axis of motion of the vehicle. Then the recorded radar returns correspond to the widest swathe perpendicular to the direction of motion of the vehicle. However, in some arrangements, the scanning means may be operable to scan the direction of look only in a sector extending less than 90" on either side of the axis of motion of the vehicle.

Then, preferably the or each predetermined direction of look is at the largest acute angle to the axis of motion on a respective side thereof within the sector. Once again, this enables the recorded return signals to correspond to the widest swathe available.

Examples of the present invention will now be described with reference to the accompanying drawings of which: Figure 1 is a schematic representation of the operation of the present invention and Figure 2 is a block diagram of a radar apparatus including recording apparatus for performing the present invention.

Figure 1 shows side-by-side two illustrations of how the present invention operates with different forms of radar apparatus.

Each of the two illustrations, referenced a and b, show how the invention can be used to provide a record of targets sensed by a radar set on a moving vehicle as the vehicle travels along its path. In each of a and b, the direction of travel of the vehicle is indicated by an arrow 10. In the illustration a, the vehicle is provided with a radar set having a completely rotating azimuthual scan providing 360" coverage. In such an example, it is preferred though not essential that the predetermined direction of look is perpendicular to the direction of motion of the vehicle. Then radar return signals in at least one inter pulse period are recorded when the radar apparatus is "looking" perpendicularly of the vehicle heading.In order to provide maximum coverage it is desirable that radar returns are recorded when the radar apparatus is looking in each direction on either side of the heading of the vehicle.

Thus, referring to Figure la, radar returns are recorded corresponding to radar range scans 11 and 12.

As the vehicle moves along its track, return signals are recorded from appropriate radar range scans every time the radar is looking perpendicularly either to the left or to the right of the direction of travel. Thus, the successive recorded scans correspond spatially to lines 13 and 14 shown in Figure 1. It can be seen therefore that as the vehicle moves along its path sufficient radar information is recorded to provide a record of all targets sensed by the radar as the vehicle passes.

Referring now to illustration b of Figure 1, there is shown a similar arrangement for a radar apparatus having a oscillating scan.

Normally such a scanning system would scan, looking forwards relative to the direction and motion of the vehicle, and scanning in azimuth approximately equally on either side of the vehicle heading so that a sector 15 is covered by each scan. In such an arrangement, it is desirable to detect when the apparatus is looking in transverse directions at the greatest acute angle to the heading of the vehicle and, preferably, on each side of the heading. If radar returns received during interpulse periods when the apparatus is looking in these directions are recorded, the spatial distribution of the recorded range scans have the "herring bone" appearance of the lines 18 and 19 shown in illustration b. Once again it can be seen that the arrangement provides a record of all targets sensed by the radar as the vehicle passes by.

Referring back to illustration a, it can be seen that the width of the swathe of the information recorded corresponds to the maximum range of the radar apparatus.

However, in the arrangement of illustration b, the recorded swathe extends perpendicularly of the heading of the vehicle by a distance RR Cos rl, where R is the maximum range of the radar set and n is the angle from which the perpendicular to the vehicle heading of the extremity of the scanned sector 15.

It will be appreciated that the radar apparatus looks in a perpendicular direction relative to the vehicle only once per complete antenna scan. Thus, for a rotating scan system as in illustration a, the radar apparatus looks perpendicularly to the left of the heading of the vehicle, i.e. along the line 11, every 1/k seconds, where k is the speed of rotation of the radar scanner in revolution per second. In 1/k seconds, the vehicle moves along its path a distance v/k meters where v is the speed of the vehicle in meters per second. Thus, in effect, the lines 13, or the lines 14, corresponding to successive recorded range scans in each particular direction, are a distance v/k apart. If the beamwidth in azimuth of the radar apparatus is extremely narrow, targets between adjacent recorded range scans can be missed in the recording process.However, in a practical arrangement the beamwidth of the radar will normally be sufficiently wide to ensure that all targets more than a relatively close distance away from the vehicle are indecd recorded.

For example, the present invention is especially convenient when used with air bornc radar equipment adapted to obtain radar returns from the ground to enable a radar map to be displayed. Such a radar typically has a beamwidth in azimuth at 6".

If the aircraft is flying at 100 metres per second and the radar scanncr rotates at 120 r.p.m., adjacent recorded scans in one particular direction are approximately 50 metres apart. With a beam width of 6", adjacent recorded beams overlap at a range of about 500 metres. At grcater ranges, a target will be sensed by two or more adjacent recorded beams.

Referring now to Figure 2, there is shown, schematically, a radar antenna 20, a transmittcr/rccciver 21 and a PPI display unit 22 connected together to provide a normal radar set in which the received signals can be viewed "in rcal time" on the PPI display. Received video signals are fed from the receiver to the display unit 22 by a line 23. Preferably, the receiver of the transmitter/receiver 21 includes a logarithmic amplificr of the kind well known in the radar art. Radar synchronisation pulses are also supplied to the display unit 22 from the unit 21 by a line 24. Further, information defining the angle of look of the antenna 20 at any time is supplied to the PPI unit 22 by a line 25.This information may be supplied by a synchro unit or by a digital angle encoder, both of which are well known in the art. As should be apparent from Figure 2, the units 2(), 21 and 22 may together form a standard radar set and the pcrformance of the invention in no way affects the normal operation of such a radar set. Indecd, one of the advantages of the present invention is that it may cmploy a standard and typically an existing radar set on the particular vehicle, typically an aircraft.

The video information from the receiver is also fed along a line 26 to a CFAR video processing unit 27. CFAR units are well known in the radar art and are operative to reduce the number of spurious target responscs in the received video, such as produced by clutter rcturns from rain. sca or land, to a substantially constant low level.

An especially desirable form of CFAR circuit which may be employed in the present case is one in which the circuit responds adaptively to the level of clutter prevailing in the video signals being re received. The CFAR circuit typically controls a threshold level which is applied to a gate so that received video signals which do not exceed the threshold are removed and only those exceeding the threshold level are passed on for display.

Although such a CFAR circuit is not essential in the apparatus of the present invention, it is desirable to ensure that the radar returns recorded in performing the present invention contain as little unwanted clutter as possible. Very conveniently, the CFAR circuit may be arranged also to provide relatively clutter free video signals directly to the PPI display unit 22. However, the display unit 22 should still be able to display, when desired, the "raw" video directly from the transmitter/receiver unit 21, especially when the radar apparatus is being used as weather radar.

The processed video from the CFAR unit 27 is fed to a sampler and quantizer 28 which samples the video at a regular sample rate and quantizes these samples in digital form.

The sample rate must be sufficiently high to sample the video signals properly and a suitable rate is about 1 megaherz for a pulse radar with a transmitted pulse width of two micro seconds. Although the sampler and quantizer 28 may quantize the video into several levels, i.e. producing a digitial word of two bits or more for each sample, it is quite convenient to sample only in two levels, i.e. a logic 0 or a logic 1 corresponding to the presence or absence of a received signal in each sample.

The digitized video from the sampler and quantizer 28 is fed to a 2-out-of-2 digital integrator 29. Digital integrators are well known in the radar signal processing art and are employed to compare the signal returns from successive range scans or periods between successive range scans. It will be appreciated that with normal radar apparatus, the beamwidth is such that any target is "illuminated" by several successive radar pulses as the radar antenna rotates. It is often very convenient to correlate successive sets of signal returns to eliminate certain forms of spurious radar interference.

In the present example, the simplest form of integration is used in which the received video in any one range scan is compared with the video received in the immediately preceding range scan. Only if there is a target indication in both the two compared range scans at the same range in this indication passed on for display. or in the present case for recording. The two-out-oftwo digital integrator is especially useful for eliminating the effects of interference from another radar operating on the same frequency.

Once again. the output of the two-out-oftwo digital integrator may be supplied directly to the PPI display unit 22 for displaying if desired.

The signals defining the instantaneous angular position of the radar antenna 20 are supplied also to an azimuth angle comparator 30. Radar sync pulses from the transmitter receiver unit 21 are also supplied to the comparator 30. The comparator 30 operates to compare the instantaneous angle of the radar antenna with the predetermined angle which has been previously selected. This predetermined angle corresponds, for a radar antenna which rotates through 3600, to the angles perpendicular to the heading of the vehicle on either side. When the angle comparator 30 detects that the radar antenna 20 is pointing in one of the predetermined directions, the comparator provides an enable pulse on a line 31 to a store write control unit 32.The store write control 32 also receives sync pulses on a line 33 and on receipt of the enable signal on the line 31, controls a digital store 34 to write in store the processed digital video signals from the integrator unit 29 in the interpulse period immediately following the next radar pulse.

Thus, when the angle comparator 30 detects that the radar antenna 20 is pointing in one of the selected directions, the digital store 34 is enabled to read in a complete set of processed digital video corresponding to a single range scan of the radar. The writing process is halted when a full range scan has been stored and no further writing takes place until the azimuth angle comparator 30 next detects that the antenna is in one of the selected positions.

The enable signal from the angle comparator and the sync signal are supplied also to a store read control unit 35. The store read contrl unit operates to start reading the stored information from the store 34 after the writing process is terminated. The read control strobes the information out of the store at a very much slower rate than the writing in process. It will be understood that the information is written into the store at the sampling rate of the sampler and quantizer 28. On the other hand, the information may be read out of the store much more slowly since it is only necessary to have finished reading the information by the time the angle comparator 30 next detects that the antenna 20 is in one of the predetermined positions.

In a typical arrangement, where the antenna rotates at 120 r.p.m. and the azimuth angle comparator 30 detects when the antenna is looking perpendicularly in either direction relative to the vehicle heading, there is nearly 1/4 second in which to read out the stored information from the store 34. By comparison, if 50 kilometers range of video is stored, this information is written into the store in 333 micro seconds.

Thus the information stored at 1 megahertz may be read out at 1.3 kilohertz.

The data read out of the store 34 is supplied by a line 36 to a taper recorder unit 37 by which the information is recorded on magnetic tape. It will be apparent that with two level digital video, the bit rate of the information to be recorded is approximately 1.3 kilohertz. This relatively low data rate can readily be recorded by many commercially available types of tape recorder specifically those designed for recording and playing back music. Thus, in an advantageous embodiment of the present invention the tape recorder 37 has a frequency response not exceeding 20 kilohertz. Conveniently, the recorder 37 is of the "cassette" type so that the recorded tape can be removed and reloaded into the machine very quickly and easily.

As the azimuth angle comparator 30 successively detects that the antenna 21 is pointing in each of the selected directions, the tape recorder 37 records successive bunches of video signals corresponding to the video received when the antenna is pointing in these directions. The bandwidth of the recorded video is very much lower than that of the actually received video.

It will be appreciated that the recorded information is to be played back to recreate a "radar map" of the terrain which the aircraft, for example, has passed over during its flight. It is desirable therefore to provide also a record of the movement of the aircraft. This record should be correlated with the successively recorded range scans so that the target indications in the recorded information can be geographically positioned during the subsequent analysis.

For this purpose, a navigational aid 38, which may already be provided in the aircraft, is connected to the tape recorder 37 so that information regarding the position of the aircraft at successive intervals of time is recorded by the recorder 37 simultaneously with the radar information. This may be done either by interrupting the recording of the radar information momentarily at intervals to record the geographical position information, or by employing two separate tracks of the tape recorder, using one for the radar information and the other for navigational information. Apart from the positional information it is also desirable to record the heading of the aircraft at any moment and preferably of its track over the ground.

Also information regarding the height of the aircraft may be recorded.

The recorded information may be played back for analysis on an oscilloscope which has a time base across the diameter of its screen corresponding to the time taken to record a single range scan, i.e. nearly quarter of a second. The intensity of the oscilloscope beam is then modulated in accordance with the digital video played back from the tape record. A system of lenses may be used to focus the oscillosope screen onto photographic paper to form a permanent record. Automatic apparatus can be devised which also responds to the recorded navigational information so that the radar map information recorded can be superimposed directly on an actual map.

It will be appreciated that airborne radar apparatus used for ground mapping will, automatically, provide an indication of the height of the aircraft since the range of the first received ground responses corresponds to this height. Thus, the apparatus and method of the present invention when used in an aircraft automatically provides a record of the height of the aircraft. Furthermorc, it is possble when playing back this record automatically to detect the altitude line, corresponding to the beginning of receiving ground returns in each range scan, to correct the distortion in the ground return signals rcsulting from the difference between truc horizontal ground range and the slant range from the aircraft actually detected by the airborne radar.

In a further arrangement, the information being recorded by the recorder 37 is also transmitted from the aircraft over a narrow band radio link, e.g. a speech channel, to a shore or ship station or another aircraft.

Then, a display of the information may be obtained almost immediately if required.

WHAT WE CLAIM IS: 1. A method of making a radar recon naissance or survey record from a moving vehicle, comprising operating a pulse radar set mounted on the vehicle, the set having means for scanning in azimuth the direction of look of the set relative to the vehicle axis of motion, indicating the times when the instantaneous direction of look relative to the vehicle axis of motion is substantially equal to one or more of predetermined said directions which are transverse relative to the axis of motion and recording radar return signals from at least one inter pulse period at each of the indicated times.

2. A method as claimed in claim 1 wherein the recording stcp comprises sampling the radar return signals at a predetermined sampling rate and quantizing the samples in at least two levels to provide digitized radar video, writing into a digital store at said sampling rate the digitized video from the or each inter pulse period at each of the indicated times, subsequently reading out the stored digitized video from the store at a rate slower than the sampling rate in the time available between successive said indicated times, and recording the more slowly read out digitized video.

3. Pulse radar apparatus for mounting on a vehicle, comprising a pulse radar set having means for scanning in azimuth the direction of look of the set relative to the vehicle axis of motion, an azimuth angle detector for indicating the times when the instantaneous direction of look relative to the vehicle axis of motion is substantially equal to one or more predetermined said directions which are transverse relative to the axis of motion, and means responsive to said azimuth angle detector for recording radar return signals from at least one inter pulse period at each of the indicated times.

4. Pulse radar apparatus as claimed in claim 1 wherein the pulse radar set is a standard set as provided on the vehicle, having a radar display for real-time viewing, and the azimuth angle detector and the recording means are connected to the standard radar set without interfering with its normal real-time operation.

5. Pulse radar apparatus as claimed in either of claims 3 or 4, wherein the recording means comprises means for sampling the radar return signals at a predetermined sampling rate and quantizing the samples in at least two levels to provide digitized radar video, a digital store, means for writing into the store at the sampling rate the digitized video from the or each inter pulse period at each of the indicated times, means for reading out the stored digitized video from the store at a rate slower than the sampling rate in the time available between successive said indicated times and a recorder for recording the more slowly read out digitized video.

6. Pulse radar apparatus as claimed in claim 5, wherein the rate of reading out the stored digitized video is so slow that the read out video has a bandwidth not exceeding 20kHz.

7. Pulse radar apparatus as claimed in claim 6, wherein the recorder is a magnetic tape recorder having a frequency response not extending above 20 kHz.

8. Pulse radar apparatus as claimed in any of claims 3 to 7 wherein the scanning means is operable to scan the direction of look through 360 in azimuth and the or each predetermined direction of look is perpendicular to the axis of motion of the vehicle.

9. Pulse radar apparatus as claimed in any of claims 3 to 7 wherein the scanning means is operable to scan the direction of look only in a sector extending less than 90C on either side of the axis of motion of the vehicle and the or each predetermined direction of look is at the largest acute angle to the axis of motion on a respective side thereof within the sector.

10. Radar recording apparatus for use with a vehicle mounted pulse radar set which has means for scanning in azimuth the direction of look of the radar relative to the vehicle axis of motion, the recording appar

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   back from the tape record. A system of lenses may be used to focus the oscillosope screen onto photographic paper to form a permanent record. Automatic apparatus can be devised which also responds to the recorded navigational information so that the radar map information recorded can be superimposed directly on an actual map.

   It will be appreciated that airborne radar apparatus used for ground mapping will, automatically, provide an indication of the height of the aircraft since the range of the first received ground responses corresponds to this height. Thus, the apparatus and method of the present invention when used in an aircraft automatically provides a record of the height of the aircraft. Furthermorc, it is possble when playing back this record automatically to detect the altitude line, corresponding to the beginning of receiving ground returns in each range scan, to correct the distortion in the ground return signals rcsulting from the difference between truc horizontal ground range and the slant range from the aircraft actually detected by the airborne radar.

   In a further arrangement, the information being recorded by the recorder 37 is also transmitted from the aircraft over a narrow band radio link, e.g. a speech channel, to a shore or ship station or another aircraft.

   Then, a display of the information may be obtained almost immediately if required.

   WHAT WE CLAIM IS: 1. A method of making a radar recon naissance or survey record from a moving vehicle, comprising operating a pulse radar set mounted on the vehicle, the set having means for scanning in azimuth the direction of look of the set relative to the vehicle axis of motion, indicating the times when the instantaneous direction of look relative to the vehicle axis of motion is substantially equal to one or more of predetermined said directions which are transverse relative to the axis of motion and recording radar return signals from at least one inter pulse period at each of the indicated times.
2. 2. A method as claimed in claim 1 wherein the recording stcp comprises sampling the radar return signals at a predetermined sampling rate and quantizing the samples in at least two levels to provide digitized radar video, writing into a digital store at said sampling rate the digitized video from the or each inter pulse period at each of the indicated times, subsequently reading out the stored digitized video from the store at a rate slower than the sampling rate in the time available between successive said indicated times, and recording the more slowly read out digitized video.
3. 3. Pulse radar apparatus for mounting on a vehicle, comprising a pulse radar set having means for scanning in azimuth the direction of look of the set relative to the vehicle axis of motion, an azimuth angle detector for indicating the times when the instantaneous direction of look relative to the vehicle axis of motion is substantially equal to one or more predetermined said directions which are transverse relative to the axis of motion, and means responsive to said azimuth angle detector for recording radar return signals from at least one inter pulse period at each of the indicated times.
4. 4. Pulse radar apparatus as claimed in claim 1 wherein the pulse radar set is a standard set as provided on the vehicle, having a radar display for real-time viewing, and the azimuth angle detector and the recording means are connected to the standard radar set without interfering with its normal real-time operation.
5. 5. Pulse radar apparatus as claimed in either of claims 3 or 4, wherein the recording means comprises means for sampling the radar return signals at a predetermined sampling rate and quantizing the samples in at least two levels to provide digitized radar video, a digital store, means for writing into the store at the sampling rate the digitized video from the or each inter pulse period at each of the indicated times, means for reading out the stored digitized video from the store at a rate slower than the sampling rate in the time available between successive said indicated times and a recorder for recording the more slowly read out digitized video.
6. 6. Pulse radar apparatus as claimed in claim 5, wherein the rate of reading out the stored digitized video is so slow that the read out video has a bandwidth not exceeding 20kHz.
7. 7. Pulse radar apparatus as claimed in claim 6, wherein the recorder is a magnetic tape recorder having a frequency response not extending above 20 kHz.
8. 8. Pulse radar apparatus as claimed in any of claims 3 to 7 wherein the scanning means is operable to scan the direction of look through 360 in azimuth and the or each predetermined direction of look is perpendicular to the axis of motion of the vehicle.
9. 9. Pulse radar apparatus as claimed in any of claims 3 to 7 wherein the scanning means is operable to scan the direction of look only in a sector extending less than 90C on either side of the axis of motion of the vehicle and the or each predetermined direction of look is at the largest acute angle to the axis of motion on a respective side thereof within the sector.
10. 10. Radar recording apparatus for use with a vehicle mounted pulse radar set which has means for scanning in azimuth the direction of look of the radar relative to the vehicle axis of motion, the recording appar

    atus including an azimuth angle detector of look of the radar set relative to the vehicle axis of motion is substantially equal to one or more predetermined said directions which are transverse relative to the axis of motion, and means responsive to said azimuth angle detector for recording radar return signals from at least one inter pulse period at each of the indicated times.
11. 11. Radar recording apparatus as claimed in claim 10, wherein the recording means comprises means for sampling the radar return signals at a predetermined sampling rate and quantizing the samples in at least two levels to provide digitized radar video, a digital store, means for writing into the store at the sampling rate the digitized video from the or each inter pulse period at each of the indicated times, means for reading out the stored digitized video from the store at a rate slower than the sampling rate in the time available between successive said indicated times and a recorder for recording the more slowly read out digitized video.
12. 12. A method of making a radar reconnaissance or survey record substantially as hereinbefore described with reference to the accompanying drawings.
13. 13. Pulse radar apparatus substantially as hereinbefore described with reference to the accompanying drawings.