GB2185869A - Doppler technique of synthetic aperture radar motion compensation - Google Patents

Abstract

A method of compensating for the motion of an antenna in a synthetic aperture radar system laterally of the direction of extent of the aperture comprises the steps of (1) deriving at each of a plurality of sampling positions along the aperture a pair of radar returns which are successive in time (2) comparing the phases of the pair of returns derived at each sampling position, and (3) shifting the phases of the returns in dependence upon the comparisons. Apparatus for carrying out the method is also disclosed. <IMAGE>

Description

SPECIFICATION Doppler technique of synthetic aperture radar motion compensation The present invention relates to a synthetic aperture radar (SAR) system. In particular it relates to the compensation of phase errors due to lateral deviation of the antenna of the SAR system from its nominal path of motion.

A synthetic aperture radar system comprises a coherent pulsed radar apparatus mounted on an aircraft to look transversely of the aircraft as it flies along a straight flight path. The system further comprises a signal processor which focuses, using a correlation process, returns from a (nominal) point target received at a series of spaced apart positions called a "synthetic aperture" along the flight path. The effect of this is to produce a higher azimuth resolution than is achievable with a real aperture. A real scene effectively comprises a multiplicity of point targets and the SAR system scans the scene as the aircraft flies past it to produce a map of the scene.

Ideally, the aircraft flies exactly along a straight path. However, in practice the aircraft deviates from the path and can be controlled only to maintain it within a fixed range centred on the path.

The production of a high azimuth resolution map requires that any lateral deviation from the straight path be measured to a high degree of accuracy. The lateral position of the phase centre of the SAR antenna must be known to within a fraction of a wavelength over the time required to form the synthetic aperture.

It has been proposed to measure the lateral deviation using an inertial navigation system.

However, the accuracy required may approach the limit feasible with present technology, and furthermore a suitable inertial navigation system is very expensive.

According to one aspect of the invention, there is provided a method of compensating for motion of an antenna in a synthetic aperture radar system laterally of the direction of extent of the synthetic aperture comprising the steps of 1) deriving at each of a plurality of sampling positions along the aperture a pair of radar returns which are successive in time 2) comparing the phases of the pair of returns, derived at each sampling position, and 3) shifting the phases of the returns in dependence upon the comparisons.

According to another aspect of the invention there is provided a radar apparatus comprising an aerial, means for deriving from the aerial, whilst the aerial moves along a straight path, at each of a plurality of positions along the path a pair of returns which are successive in time but related to substantially the same position on the path, coherent receiver means for deriving a signal representing the phase of derived returns, means for comparing the phases of the portions relating to the same band of ranges, of the said signal, derived from a corresponding said pair of returns, and a phase shifter for shifting the phase of the pulses of the video pulse train in dependence upon the comparisons.

In an example of the invention, the aerial is a displaced phase centre aerial (DPC).

For a better understanding of the present invention, reference will now be made by way of example to the accompanying drawing in which Figure 1 is a block diagram of a synthetic aperture radar system in accordance with the invention, Figure 2 shows an example of a waveguide aerial arrangement usable as a DPC aerial, and Figure 3 shows the operational effect of the aerial arrangement of Fig. 2.

In order to achieve a resolution of a few metres, an I band SAR system must synthesize an aperture several hundreds of metres in length at long range (say 50 km) and the lateral motion of the antenna must be known to an accuracy of a fraction of a wavelength, i.e. several millimetres. The system comprises an aerial which transmits radar pulses and receives corresponding returns at each of a plurality of spaced apart sampling portions along the aperture. The radar system of Fig. 1 infers the lateral antenna motion from the radar returns.

The SAR system comprises a displaced phase centre (DPC) aerial which is arranged to transmit successive ones of a pair of pulses and receive their returns from the same sampling position in the aperture (ignoring lateral motion) despite movement of the aerial along the aperture.

Given that the radar pulse repetition interval (p.r.i.) is r and that the lateral motion (up or down the radar beam) during the p.r.i. is d, then the phase of scatterers on the ground will appear to change by d=4zd/A=4zvT/A where v is the mean lateral velocity over the pri. A radar operating out to ranges of 100 km would probably have r-lms to avoid range ambiguities. If the aircraft speed was V=200 m/s and it kept a steady heading to + 1", say, then v < 4 m/s. Consequently, at I-band, d < 1.6 rads.

The present invention involves observing the apparent change in the phase of the returns from a particular set of range cells.

If the in-phase I and quadrature 0 returns from the particular range cell on the nth pulse are InS On, respectively, then with the DPC aerial, 6=tan-'(O,/l,) -tan-l(O,/l,) Equation -1 where Q,l, and Q212 are produced at the same sampling position along the aperture (ignoring lateral motion). If the inter-pulse phase shifts were known to be no more than about 0.5 rads, a simpler estimate would be J(l102-l201)/R Equation 2 where R is the resultant of 11,01 or 12,Q2 R2=l21+021l22+022 Clutter-to-Noise Ratio C/N In this context "clutter" refers to desired signals from scatterers on the ground used to observe the lateral motion.If the C/N power ratio is p, the r.m.s. error in a will be about V2/p rads. However by averaging values of oD observed in N range cells, the r.m.s. error in a becomes V2/pN rads.

Thus it is advantageous to observe a swathe of ground of such width as gives a desired C/N.

However, it should be borne in mind that as range increases the signal-to-noise ratio of the returns will decrease and so it may be necessary to observe a swathe at substantially less than maximum mapping range.

Stable Antenna Beam In this system, there is a fundamental ambiguity beteween Doppler frequencies induced by beam-pointing errors and lateral velocities.

Insofar as the requirement is simply for the aerial to remain locked to inertial space, there is no need for absolute angular measurement. A massive and properly-balanced aerial would tend to provide this characteristic naturally, although, in practice, it will probably be necessary to provide a servo loop referenced to a strapped-down gyro or similar.

Antenna Height In order to map out to large ranges eg 100 km without too much shadowing occurring, the aircraft will probably have to fly at a height of about 15 km. In such a case it may be necessary to compensate for movement of the aircraft away from the ideal path both laterally (horizontally) and in an orthogonal (vertical) direction.

The exemplary radar system of Fig. 1 comprises a DPC aerial 11 which is, and operates, as shown in Figs. 2 and 3.

Referring now to Fig. 2, the aerial is capable of both transmitting and receiving radar pulses and comprises three adjacent sections of waveguide 21, 22 and 23, sections 21 and 23 being of equal lengths. The waveguide in this example is mounted along the fuselage of an aircraft.

The sections of waveguide have a series of radiating slots formed therein, the relative disposition of which is arranged to form the directive pattern of the aerial. The radiating slots are arranged such that waveguide elements 21 and 22 provide in combination a directive pattern having a main lobe displaced to the left of the centre of the total aerial length, and such that waveguide elements 2 and 3 provide in combination a directive pattern identical in magnitude and phase but having a main lobe displaced to the right of the centre of the total aerial length.

At any time either elements 21 and 22, or elements 22 and 25 are-arranged to both transmit and receive pulses. When the aerial operates in the transmitting mode, microwave power for transmission is applied on an input line 24 to a microwave coupler 25. Coupler 25 applies microwave power from a transmitter TX to the waveguide element 2 and also to a microwave switch 26. The switch 26 in the aerial feed lines is controlled by electrical pulses from a control circuit 27 and is arranged to direct microwave power to either of the microwave aerial elements 21 or 22. When the aerial operates in the receiving mode, received signals from either of the elements 21 or 23 are selected by switch 26 and combined with the received output from element 22 to provide an output signal on line 24 indicative of received signals from elements 21 and 22 or 22 and 23 depending on the setting of switch 26.

In operation, if the aircraft is flying in the direction shown in Fig. 3, and when the aerial is in the position A, switch 26 is set so that elements 22 and 23 transmit a pulse and receive a corresponding return, while at a time T later when the aircraft has moved forward by the length of the equal length elements 21 and 22, elements 22 and 23 are selected so as to derive another radar return in a congruent position to the first return. The phase of these returns may be compared and any discrete change therein for a particular range cell is indicative of a moving target. In this example, the required time T between pulses is given by z=l/s where s is the velocity component of the aircraft in the direction shown.

It will be apparent that the aerial is effectively stationary for the two pulses. A series of such pairs of returns may be taken along the flight path to provide an indication of moving targets in a region being mapped.

As known to those skilled in the art, forms of DPC aerial are known, other than the example described above.

The returns from the DPC aerial are fed to a coherent receiver 12 which is, for example as described with reference to Fig. 4.5 in Section 4.1 (p 117) of Introduction to Radar Systems by M.l. Skolnik (McGraw-Hill), to produce an i.f. signal.

The i.f. signal is fed from a phase sensitive detector PSD included in the receiver to a range gate 13 which is actuated by a range gate control 14 operating in timed relationship to the switch control 27 to produce signals I and Q representing the in-phase and quadrature phase components of returns from particular ranges.

Referring to Fig. 2 successive ones of a pair of pulses are associated with positions A and B of the aerial. Hereinafter, the phase components associated with position A are referred to as 11, Q1, and these associated with B are referred to as 1202. The components 11,Q1 are followed in time by 12,02 and are applied to respective channels 14 and 15 by a switching arrangement 16 controlled by the switch control 27. The channel 14 includes a delay 17 whereby the components I1Q. and 12Q2 are applied simultaneously to a circuit 18 which estimates os according to, equation 1 or 2 above.An averager 29 averages the values of a obtained over the range swathe and applies the average value a to a phase shifter 10.

The phase shifter 20 receives the i.f. signal from the receiver via a delay 21 and applies to that signal the phase shifts.

Thus the output of the phase shifter is a signal which is compensated for motion of the antenna laterally of the synthetic aperture. This compensated train is applied to an SAR processor 22 which may be conventional. Alternatively, the processor 22 may be a processor as described in our copending patent application of even date 79 (PO 20760) entitled "Phase Estimation Technique of SAR Motion Compensation". This processor, in order to operate, requires to be initially fed with a video pulse train which is compensated for lateral motion over the first N samples of a synthetic aperture N+1 samples in length. Thereafter, apart from occasional up-dates it compensates for lateral motion in the manner described in the copending application without the need to be provided with compensated values. Such an arrangement is advantageous over the use of the SAR processor disclosed in our copending application PQ 20760 (79 ) in the system of Fig. 1 where the antenna beam is not sufficiently stable in yaw over more than one synthetic aperture.

In the foregoing example, the invention has been described in relation to the use of a DPC aerial for providing pairs of returns successive in time but related to the same position in the aperture. However, in some circumstances where less precision is acceptable the DPC aerial could be replaced by a conventional aerial. With an appropriate pulse repetition frequency successive returns, aithough not precisely related to the same position in the aperture, would be sufficiently closely related to the same position to provide substantially the same information as would be obtained using a DPC aerial.

Claims (6)

1. A method of compensating for motion of an antenna in a synthetic aperture radar system laterally of the direction of extent of the synthetic aperture comprising the steps of 1) deriving at each of a plurality of sampling positions along the aperture a pair of radar returns which are successive in time, 2) comparing the phases of the pair of returns derived at each sampling position, and 3) shifting the phases of the returns in dependence upon the comparisons.

2. A radar apparatus comprising an aerial, means for deriving from the aerial, whilst the aerial moves along a straight path, at each of a plurality of positions along the path a pair of returns which are successive in time but related to substantially the same position on the path, coherent receiver means for deriving a signal representing the phases of derived returns, means for comparing the phases of the portions relating to the same band of ranges of the said signal derived from a corresponding said pair of returns, and means for shifting the phases of the said portions of the said signal in dependence upon the comparisons.

3. Apparatus according to claim 2, wherein the aerial and the deriving means comprise a displaced phase centre aerial system.

4. Apparatus according to claim 2 or 3, further comprising means for averaging the comparisons derived from the said portions relating to bands of ranges within a predetermined range swathe, the shifting means being responsive to the averaged comparisons.

5. A method of compensating for motion of an antenna in a synthetic aperture radar system laterally of the direction of extent of the synthetic aperture substantially as hereinbefore described.

6. A radar apparatus substantially as hereinbefore described with reference to Figs. 1 and 2 of the drawings.