GB1487740A - Side-looking coherent pulse radar systems - Google Patents

Abstract

1487740 Sideways looking radar MARCONI CO Ltd 25 Nov 1974 [12 Feb 1974] 6305/74 Heading H4D In a side-ways looking, preferably airborne radar, in which pulses of coherent radiation are transmitted laterally from the carrying body and the resulting echoes are demodulated in coherent detection means to produce a pair of mutual phase quadrature waveforms exhibiting Doppler frequency modulation from one pulse repetition period to the next, which are converted into digital form as described U.K. Application 20054/73, a predetermined number of pairs of so produced samples corresponding to a predetermined number of range increments being stored along with a fixed number of previously received samples, and the pairs of samples so stored are separately correlated with at least two reference frequency modulated digitized waveforms having their centre frequencies mutually offset by less than the system p.r.f. The correlated signals from each separate correlation processor are converted to a unipolar signal by having their phase quadrature components summed to detect the moduli thereof in respective indicators, to produce output positive unipolar signals, means being provided to subtract from these signals, parameters dependent on the carrying body velocity relative to, e.g. the earth, and means being provided for comparing the signals so derived from each of the correlation processors to obtain parameters defining the target velocity relative to earth. In the embodiment described, four correlation processors 141-144 have the centre frequency of their swept correlation reference frequency mutually displaced by D kHz which is less than the p.r.f., so that a target is seen by, e.g., processor 141 t seconds before processor 142, t being dependent on the centre frequency difference D kHz, the radar wavelength, the slant target range and the carrying body constant velocity C. The relative position of the target on consecutive processors at t second intervals allows detection of the target velocity relative to the carrying body in terms of the linear displacement of the two unipolar signals, e.g. on two adjacent indicators 151, 152, whereon the target radial velocity is the distance Vrt, and the target velocity parallel to the carrying body displacement is the distance Ct+2Vat where C is the carrying body velocity, all relative to earth. Comparator 16 receives the signals from, e.g. indicators 151, 152 and subtracts the parameter Ct in the above expression to obtain the target velocity components relative to earth which are displayed on indicator 17. Alternatively two photographs at t second intervals of each indicator 151, 152 may be taken whereupon stationary targets will be superimposed when the photographs are superimposed and moving targets will be transposed, measurement of which transposition distance gives the target velocity components. A plurality of correlation processors are used to improve detection of moving target representative signals.