Patent GB-1371844-A (GB)
Title: Squint mode radar system
IPC: G01S13/90
Filed: 1972-04-07
Abstract: 1371844 Pulse radar ROCKWELL INTERNATIONAL CORP 7 April 1972 16261/72 Heading H4D An airborne, ground mapping, radar system is operative on a look angle squinted in azimuth relative to the flight path and provides a signal whose bandwidth increases with range, bandwidth limiting means reducing the proportion of the bandwidth accepted by the system progressively with increasing range, the accepted proportion of the bandwidth corresponding to a substantially constant lined dimension on the ground transverse to the look angle direction. Figure 1 illustrates a representative spectral and spatial distribution of the echo return in an airborne squint-mode synthetic aerial pulsed radar used in ground mapping. Vertical plane 10 corresponds to the beam centre. Because of the azimuthally squinted angular orientation of the beam centre relative to the velocity vector of the system, and the different angles associated with the component returns from other directions within the beam width, it can be seen that the transverse linear resolution limit (i.e. normal to the plane 10) increases with range R, giving rise to an area registry problem involving a triangular overlap (Fig. 4, not shown). The system of the invention seeks to provide a uniform transverse linear resolution #l, about the Doppler-shifted frequency f d , in order to improve area registry and provide rectangular overlap (Figs. 5 and 6, not shown) by multiplying the bandwidth of the received echoes as a discrete function of the range from which they arise and passing the multiplied signals to a fixed bandwidth filter whose bandwidth corresponds to the predetermined transverse linear extent #l. The apparatus (Fig. 2) comprises a scan converter 15 which receives a video signal and a read-sweep generator 18 for sequentially scanning successive range bin portions of the stored range trace signals in order to recover a frequency-multiplied spectral content of the video envelope at successive range bin intervals. A fixed bandpass filter 19 has a bandpass #f d0  corresponding to the preselected transverse linear extent #l at an initial range R 0 . A multiplier 20 controlled by a ramp function generator 21 causes successively increased frequency multiplication of the spectral content of the video envelope for successive range bin portions, curves 23-25 (Fig. 3). Each of the frequencymultiplied bandpass spectra has a multipliedcentre frequency n 1 f d , n 2 f d , n 3 f d  which are translated, by means of a voltage controlled oscillator 30, coupled to the ramp function generator 21, and a single sideband modulator 31, to a common frequency n c f d  representing the centre frequency of filter 19, curves 33-35. The bandpass limits #f d  of filter 19 reject those portions of the frequency-translated spectra which represent returns from outside the beam centre region of interest for each range. An attenuator 22 having a control input connected to a source of a function u cos # where u is the aircraft velocity and # the azimuth look angle relative to the velocity vector compensatorily varies the translation frequency (n i -n c )f d  in response to variations in the factor u cos # in the actual Doppler shift of the echo return from the beam centre.