GB2305322A - Motion Compensation in Synthetic Aperture Radar - Google Patents

Abstract

Synthetic aperture radar apparatus comprises an airborne translational motion compensation unit 15 which receives the radar information and a signal B representing acceleration along the antenna bore-sight, and compensates for aircraft motion along the antenna bore-sight. Signals A, X representing aircraft accelerations in two mutually orthogonal directions one in the flight direction and both normal to the bore-sight are transmitted to the ground station hardware (Fig 4) for compensating for these components of residual motion. The airborne compensation for deviation along the bore-sight from the intended flight path preserves the Doppler frequency information from which target ground velocity may be derived.

Description

Motion Compensation in Synthetic Aperture Radar This invention relates to airborne synthetic aperture radar and in particular to the processing of the received radar data to compensate for the motion of the radar relative to the intended flight path.

In airborne synthetic aperture radar it is necessary to keep to a minimum the amount of processing apparatus on board the aircraft. For ground-mapping in real time, it is preferred to carry out the bulk of the signal processing at a ground station which communicates with the airborne radar by means of a data link. This arrangement poses a problem for the compensation of aircraft motion in the case where the velocity of moving targets is required.

The movement of the aircraft from the intended flight path causes a Doppler shift which is often much greater than the Doppler shift arising from target movement relative to ground. In order to reduce the bandwidth of the radar data for accommodation on a standard data link, it is necessary to compress the bandwidth (as described for example in our copending UK Patent Application No.

(Our Reference I/7230) of even date), but often the target velocity information would be lost if the data were not previously corrected for aircraft motion. This would seem to indicate that the aircraft motion processing has to be carried out on board the aircraft, whereas it would be desirable to carry out as much as possible of this motion compensation at the ground station.

Accordingly the invention provides an airborne synthetic aperture radar comprising means for detecting motion of the radar, along the antenna bore-sight and in at least one other direction to provide respective data signals, airborne motion compensating means responsive to the data signal representing bore-sight motion to apply an appropriate Doppler frequency correction to the received signal, and means for transmitting to a remote processing station the motion-compensated radar data and the or each other data signal representing motion in the other direction or directions for use in residual motion compensation by the remote processing station.

Preferably, the motion detection means provides data signals representing motion in two other directions, preferably in the direction along the flight path and in a radial direction orthogonal to the bore-sight.

It has been found that in many applications the antenna bore-sight lies at an angle fairly close to the horizontal, for example where the scanned swathe is at a range of from 54 to 66 km and the aircraft is flying at a height of 9 km, and the more serious Doppler errors occur as a result of aircraft motion along the bore-sight rather than that along the other two orthogonal directions. In accordance with the invention, therefore, the burden of motion compensation is shared between the airborne motion compensating means and the ground station motion compensating means, the former being responsible for the more serious correction and the latter for the residual correction.The correction for motion in the other direction or directions (the residual correction) does not have to be carried out before the bandwidth compression referred to above, and can therefore be left to the ground station to carry out.

In order that the invention may be better understood, one way in which it may be carried out will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a block diagram of the complete radar system including airborne hardware and ground station hardware; Figure 2a is a diagram in an elevation plane illustrating the position of the airborne hardware relative to the swathe of the ground being mapped; Figure 2b is a diagram illustrating three orthogonal directions for aircraft motion compensation; Figure 3 is a diagram in greater detail of part of the airborne hardware of Figure 1, illustrating translational motion compensation carried out on board the aircraft; and Figure 4 is a diagram of part of the target processor unit of the ground station hardware of Figure 1, illustrating the translational motion compensation carried out at the ground station.

With reference first to Figure 1, a side-looking synthetic aperture radar system comprises airborne hardware with a data link to ground station hardware, for the real-time data processing of stationary and moving target information obtained from mapping a swathe of the ground. In the numerical examples given, by way of example only, it is assumed that the aircraft containing the airborne hardware travels at a velocity of 150 metres per second and is required to scan a swathe comprising 4000 radial range gates, giving a radial range resolution of 3 metres. The maximum target acceleration is assumed to be one metre per second per second, and the range of radial target velocities is assumed to be from -15 to +15 metres per second. The pulse repetition frequency is 2000Hz so as to accommodate a maximum unambiguous target velocity of 15 metres per second.

A stabilised platform 10 onboard the aircraft supports the radar antenna 12 and three translational motion sensors 13, which are accelerometers sensitive to motion in three orthogonal directions A, B and X. As shown in Figure 2b, the direction B is that along the antenna bore-sight BS, the direction A is that radial direction perpendicular to the bore-sight, and the direction X is that perpendicular to A and B. A rotational stabilisation unit 11 stabilises the platform 10. A radio frequency transmitter/receiver 14 supplies received target information to a translational motion compensation unit 15, which provides motion-compensated data to a signal pre-processing unit 16.Briefly, the acceleration B of the stabilised platform 10 along the bore-sight away from the intended flight path is fed to the translational motion compensation unit 15, so as to apply a Doppler correction to the target information. The accelerations X, A in the other two orthogonal directions are used in the ground station processing of the target information, and are sent directly to a data link transmitter 17 for transmission to ground.

The signal pre-processing unit 16, as described in greater detail in our copending U.K. Patent Application No. (Our Ref: I/7230) of even date, divides the total unambiguous Doppler spectrum into a plurality of contiguous frequency bands, and superimposes these bands so as to provide a corresponding reduction in frequency bandwidth of the output signal. The output signal is sent to the data link transmitter 17, for transmission from an antenna 18.

The ground station data link receiver 20 has an antenna 19 for receiving the transmitted data from the aircraft and provides target information signals to a target processor unit 21 and to a digital recorder 22.

The output of the target processor unit 21 is stored in the digital recorder 22 and is also sent to a display store 23 and thence to a display device 24. The ground station hardware, described in greater detail below with reference to Figure 4, produces a stationary target and map data display and also provides a superimposed indication of the positions of moving targets together with their motion parameters. However, where only stationary targets or moving targets are of interest, the ground station hardware could be adapted accordingly.

For the purpose of illustration, it is assumed that the maximum radial acceleration of the aircraft away from its intended flight path is lg (10 metres per second per second) and that there is a maximum radial velocity from the intended flight path of 26 metres per second. The flight direction is defined as that direction normal to the antenna bore-sight BS (Figure 2a). It is assumed further that the aircraft flight control system (not shown) is capable of maintaining an intended flight direction to within 1 km maximum deviation over the period during which continuous mapping of the ground is required.

Assuming that the aircraft is attempting to maintain 150 metres per second forward velocity, the peak forward velocity deviation is assumed to be +/- 30 metres per second, and a maximum figure for acceleration in the forward direction8 whilst attempting constant velocity flight, is taken as 5 metres per second per second. As shown in Figure 2a, the aircraft and the radar R are at a height of 9km above ground, and mapping is carried out at slant ranges for 54 km to 66 km. In this example, the 66 km vector direction will contain a component of 0.38 metres per second, and the 54 km vector direction will contain a component of -0.38 metres per second, for a. 26 metre per second velocity perpendicular to the antenna bore-sight BS.These velocities are clearly substantially smaller than the radial velocities arising from motion of the aircraft along the antenna bore-sight.

With reference to Figure 3, signals from the accelerometers are converted into digital form in an analogue-to-digital converter 131, and the bore-sight data signal B only is sent to the translational motion compensation unit 15. The other data signals A, X, representative of aircraft acceleration in the corresponding directions, are sent to the data link transmitter 17 for use in ground station processing.

Within the translational motion compensation unit 15, an integrator 151 converts the bore-sight acceleration data signal into a signal representative of ,the instantaneous mid-swathe velocity, and networks (not shown) may be provided to minimise switch-on transient effects within the integrator 151. The velocity data are then used to control a Doppler frequency generator 153, which generates a variable instantaneous frequency proportional to the mid-swathe radial velocity. This is mixed with the intermediate frequency received radar data in a simple (I and Q) analogue cancellation mixer 154 whose output is at base band frequency and is supplied to an analogue-to-digital converter and shift register store 158. The base-band cancelled signals are then measured, by a Doppler offset error detector 155, at mid-swathe for cancellation error, as described in our U.K.Patent Application No. 8428699 (Publication No.

The errors derived from this method are used as feed back corrections to improve the cancellation accuracy; they are summed in a summer 152 with the output of the integrator 151.

The bore-sight velocity at mid-swathe is integrated once again, in a integrator 156, to give the radial range deviation, and this output is used to control the generation of a block of 4,000 range gates across the 12 km swathe, by means of a range-gate generator 157. The ranging delays are varied effectively to provide lines of constant range on the ground which are parallel to the intended flight path. Each time the aircraft deviates by an extra increment of 3 metres (one range gate) slant range from the intended path, one of the end gates of the 4,000 range block is dropped, and an additional gate appears at the other end. In this manner, the mapping swathe is kept at approximately constant range from the aircraft, and all targets are illuminated for the maximum period irrespective of aircraft path deviation.The block of 4,000 range gates at 50 MHz clock rate are used to drive the analogue-to-digital converter 158, the rate being chosen to match the transmitted pulse width of 20 nanoseconds (equivalent to 3 metres radial resolution); 4,000 samples are clocked into the shift register store 158, taking a total time of 80 microseconds. Since the pulse repeat period is assumed to be 500 microseconds, the range increments can be clocked out of the shift registers of the store 158 and through the remainder of the airborne processing hardware at less than 10 MHz, allowing the use of standard available digital hardware in those succeeding stages. The 10MHz clock is provided by the range-gate generator 157.

The motion-compensated signals from the translational motion compensation unit 15 are fed to the signal preprocessing unit 16, and transmitted to ground. Before this data is processed for azimuthal compression in accordance with synthetic aperture radar techniques, it is processed to correct for the residual aircraft motion, by means of the apparatus shown in Figure 4. The X accelerometer data are integrated in an integrator 40 to provide instantaneous forward velocity (relative to the intended forward velocity), and this is combined with lowfrequency corrections derived from the radar signal data, in a "correct forward velocity" unit 47, to give an accurate measure of forward velocity for later signal integration.The A accelerometer data are also integrated, in an integrator 41, and the resulting measure of cross-radial aircraft velocity is used, in block 42, to compute phase corrections at all swathe ranges. As the aircraft path deviates from the intended track, the phase data will build up within the phase store (within block 42) and would eventually exceed the unambiguous sampled data rate if it were allowed to continue. Therefore the phase store is reset to zero at convenient intervals such as 0.25 second. The phase computation output is fed to phase cancellation mixers 43, 44 in both the clutter and moving target channels, in order to remove the residual aircraft motion components.The cancellation mixers 43, 44 provide outputs in the form of clutter data and moving target data to the further signal processing stages of the target processor 21 of Figure 1, as described in our copending patent application (Reference I/7230) referred to above. The cancelled output signals of the clutter channel are used for Doppler offset error and slope error measurements (blocks 45 and 46). The measurement of Doppler slope is carried out in accordance with the method described in our patent application no.

2148651 A, and the Doppler offset measurement is carried out in accordance with the method described in our UK Patent Application No. 8428699 (Publication No.

referred to above. Further, the measured errors are fed along line 48 to the offset computation block 42, and are used as low-frequency aircraft motion corrections.

The clutter and moving target outputs thus contain no unwanted components of aircraft non-linear motion, and a wide bandwidth measure of forward velocity is available to enable accurate phase-focussing in the integrations carried out in subsequent signal processing stages.

Claims (7)

1. An airborne synthetic aperture radar comprising means for detecting motion of the radar along the antenna boresight and in at least one other direction to provide respective data signals, airborne motion compensating means responsive to the data signal representing boresight motion to apply an appropriate Doppler frequency correction to the received signal, and means for transmitting to a remote processing station the motioncompensated radar data and the or each other data signal representing motion in the other direction or directions for use in residual motion compensation by the remote processing station.

2. A radar according to Claim 1, wherein the motion detection means provides data signals representing motion in two other directions.

3. A radar according to Claim 2, wherein the two other directions are in the direction along the flight path and in a radial direction orthogonal to the bore-sight.

4. A radar according to any preceding claim, wherein the motion detecting means comprise accelerometers for producing the said data signals, and the airborne motion compensating means comprises an integrator for determining the bore- sight velocity of the radar.

5. A radar according to any preceding claim, comprising a range gate generator for gating the bore-sight-motioncompensated received signal to provide digital radar returns'data for each of several radial range gates over a detected swathe8 and integrating means responsive to the said Doppler frequency correction to delay the range gate generator to compensate for low-frequency variations in the distance between the radar and the near edge of the swathe being detected.

6. An airborne synthetic aperture radar substantially as described herein with reference to the accompanying drawings.

7. A radar comprising airborne hardware and ground station hardware substantially as described herein with reference to the accompanying drawings.