GB1343120A - Focusing control of synthetic aperture processing for side- looking radar - Google Patents

Abstract

1343120 Pulse radar NORTH AMERICAN ROCKWELL CORP 23 Dec 1970 61254/70 Heading H4D An airborne, ground-mapping, radar system having a side-looking synthetic antenna operates with doppler focusing of the receiver output. Due to the change in direction of a target relative to the radar, owing to aircraft movement, the doppler shift associated with the target will change, as will the time-phase of the doppler shift, due to changes in the slant-range. Thus, to correlate the doppler returns from a given target, means are provided to compensate dopplerfrequency and time-phase changes. The compensation means embodies an IF output which has a spectral distribution whose centroid differs from a preselected frequency by an amount corresponding to the doppler shift of the on-boresight returns received by antenna 18. Accordingly, a closed-loop doppler tracking system is provided, which tracks the centroid of the clutter frequencies. The closed-loop system incorporates a doppler frequency tracking means 25 coupled to the output of a mixer 10 through IF amplifier 13 and also coupled to a coherent source 15 of the preselected IF frequency, f IF , for generating an auxiliary local oscillator injection signal having a frequency #f dc indicative of the frequency difference between the inputs applied to the tracker 25. Coherent detection of the clutter-reference IF output from amplifier 13 is accomplished by means of a phase-sensitive detector 14 whose reference input 29 is coupled to the source 15. Interposed between source 15 and input 29 is a phase-shifter 30 for compensatory shifting the phase of the reference signal applied to input 29. A control input of the phase-shifter 30 is coupled to the output of a doppler replica generator 34. The compensation is progressively varied over a preselected number of pulse repetition intervals. Coherence between signals from a terrain target, for a given range bin, permits integration of the signals, over the doppler processing period, for achieving azimuth resolution using an azimuth cross-correlator.