GB1459444A - Moving target indicators using synthetic aperture processing techniques - Google Patents

Abstract

1459444 Radar EMI Ltd 11 Feb 1974 [24 Jan 1973] 3687/73 Heading H4D A sideways looking synthetic aperture radar has its video signal produced by a displaced phase centre aerial and processed by a single delay line and MT1 comparison circuits to eliminate doppler signals from fixed (relative to ground) objects, and the remaining doppler video signals from moving objects are processed by one or more correlator means having staggered centre frequencies and total bandwidth equal to the pulse repetition frequency fs of the transmitted pulses. The invention relies on the principle that whatever the frequency of the main component of the spectrum of the doppler signal of the moving object, the fold over of the components about the multiples of the pulse repetition frequency fs will always give a component within the bandwidth of the correlator means. If the speed of the moving object gives rise to a doppler frequency f D when it is on the axis of the radar beam and the centre frequency of the reference frequency sweep in the correlation steps is not f D , then the correlation will give an incorrect along track position from the object. A plurality (e.g. 4) of narrow bandwidth correlators having centre frequencies fk 1 = -#fs, fk 2 = -#fs, fk 3 = +#f s ,fk 4 = +#fs (a, Fig. 3), are therefore used, having a total bandwidth within Œfs/2, such that, with the doppler signal falling within one of the correlator bandwidths, the position error, although still present, will be small. For a main doppler component f D =2(Vt/#) the doppler signal spectrum is shown at b, Fig. 3. This is weighted by the effect c, Fig. 3, of the MT1 canceller, such that the signals produced by the correlators have relative amplitudes given by d, Fig. 3. A doppler signal can give consecutive output from two correlators, and to combine these into one signal, the output of the correlators can be combined via delays. Fig. 4 shows a complete radar, wherein the output of the MT1 canceller, is divided into in-phase and in-quadrature components for respective processing by the four correlators. The output of the correlators are combined by squaring and adding to give the outputs S1, S2, S3, S4. These may be combined (Fig. 5, not shown) by adding or amplitude discriminated before combination by respective threshold stages (Fig. 6, not shown). The radar beam need not be at 90 degrees to the aircraft flight axis. If the p.r.p. of the transmitted pulses is varied in dependence on the aircraft's speed, such that the aircraft travels a constant distance between pulses, the number of p.r.p's over which it is necessary to perform the correlation is constant. The variation of p.r.p. varies the MT1 canceller blind speed.