GB1408016A - Pulse-operated radar receivers - Google Patents

Abstract

1408016 Pulse radar SIEMENS AG 21 Dec 1972 [23 Dec 1971] 59277/72 Heading H4D A pulse radar receiver includes in its video stage a moving-echo filter followed by a variable cut-off digital low-pass filter whose cut-off frequency is progressively reduced during the target observation time. The Specification states that in a radar of this type the optimum frequency of the low-pass filter is inversely proportional to the observation time. Therefore as the observation time increases without detection of the target the filter cut-off is reduced, thus nominally increasing the signal-to-noise ratio, If the filter is followed by a threshold detector the threshold can be simultaneously lowered to provide a constant false alarm rate, but ensuring a more rapid positive targetno target decision if sequential detection is used. In a radar using a phased array antenna 1 coherently detected pulses are amplitude coded and subjected to fixed target cancellation in moving echo filter 11. The unipolar pulses due to moving targets are fed to a digital low-pass filter TP comprising two identical cascaded stages F1, F2. The filter parameters are determined by the gains of A1, A2, D1, D2 which are controlled by a store or computer 25. This also controls the switching of the antenna array 1 and sets the maximum observation times in any particular direction; these may be preprogrammed or derived from a tracking sequence. The filter output is passed to a threshold 23 which has upper and lower limits OS, US. These limits are lowered and raised respectively during the target observation time. Crossing of either limit by the filter output terminates the observation and produces a target/no target signal for indicator 24. On the decision being made store or computer 25 progresses to the next antenna direction. The variation of the cut-off frequency may be begun or ended by a portion of constant frequency or the variation may be progressive throughout.