Patent GB-1455871-A (GB)
Title: Side-looking coherent pulse radar systems
IPC: G01S13/90
Filed: 1973-04-27
Abstract: 1455871 Radar MARCONI CO Ltd 15 Jan 1973 [27 April 1973] 20054/73 Heading H4D In a side-looking airborne radar in which pulses of coherent radiation are transmitted laterally from the aircraft, and the resulting echo signals are demodulated in coherent detection means to produce a series of waveforms exhibiting doppler frequency modulation from one pulse repetition period to the next, the waveforms are fed to correlations processing means for correlation, within each of a plurality of range increments, with a reference correlation waveform having a centre frequency offset by half the pulse repetition frequency from that which would yield maximum correlation with echoes from objects not having a radial component of velocity, when on the axis of the radar, whereby maximum correlation is achieved with echoes from objects having such a radial component. The transmitted coherent pulses are produced by unblocking the output of an oscillator 31 under the control of a sync. pulse source 5. The received echo signals, after frequency conversion at 4, are coherently demodulated at 10 and 11 by the output of an oscillator 8 phase locked to the frequency converted output of oscillator 31. The two demodulation processes are made mutually in phase quadrature by a 90 degree phase shifter 9. The outputs of the detectors 10, 11 are time quantized at 12 from an initial time n#, with each quanta # being a half pulse width so that m samples are produced from each return i.e. at times:- n#, (n+1)#.... (n+m-1)#. Each sample then converted into digital form. When the object is in a region closely arranged around the axis of the transmission beam it is irradiated by I consecutive pulses, where l is very much less than the total number of pulses which irradiate the object whilst it is being swept by the entire beam. The m samples derived from each detector for the l repetition periods are stored in respective in-phase and in-quadrature phase shift-register stores 13, each having l rows for the successive returns and m columns for the m range quanta of each return. The l samples (P 1  ... P l1  in-phase samples, q 1  ... q l7  quadrature phase samples) in the stores for each range quantum are then multiplied by corresponding digitized samples (a 1  ... a l , b 1  ... b l ) of in-phase and quadrature phase linear frequency modulated reference correlation waveforms a and b having said offset centre frequency. The correlation processor 14 sums the various products to produce quadrature outputs U and V for each range quantum. These bipolar signals are converted to unipolar form by the process. /U/+/V/ or U<SP>2</SP> + V<SP>2</SP>, and the unipolar signals are fed to a display means 15. One method of producing said frequency offset, is to render alternate ones of the reference samples negative. The transmitted p.r.f. is ideally related to the velocity V A  of the aircraft by:-  Such relationship may be manually attained by the pilot or is preferably automatic. An additional correlation processor and indicator may be used, using a correlation reference waveform not offset by said ¢ p.r.f., and thereby displaying fixed target as well as moving ones. Fig. 4 (not shown). A single processor and display may be used Fig. 5, with moving targets being displayed when a switch 19 is in the position shown whereby alternate ones of the samples are rendered negative, and fixed targets being displayed when the switch is in the other position. If the aircraft is subject to a drift angle S, a doppler detector produces a corresponding signal U g  equivalent to a radial velocity Vg and a radial doppler shift which can be used to offset the frequency from oscillator 8, Fig. 6 (not shown) or to offset the correlation reference frequency. The correlation reference frequency may be constant but this gives poor resolution.