Doppler discrimination of aircraft targets

Claim 1

A Doppler radar capable of discriminating between different types of aircraft having rotating elements including circuitry for sensing the occurrence of increases in the Doppler signal amplitude of target returns resulting from increases in cross section of the rotating elements as they become orthogonal to the incident radar beam of said radar, said circuitry comprising, Doppler detector means for detecting the Doppler signal of said radar including a direct reference voltage indicative of the body of said aircraft and a spike frequency component indicative of the orthogonal rotating element of a particular aircraft type, bandpass filter means coupled to the Doppler detector, said bandpass filter means having a wide passband that blocks the direct voltage and passes the spike frequency component, a threshold circuit and a pulse stretcher connected in cascade to the output of said bandpass filter, said threshold circuit providing an output pulse upon the occurrence of said spike frequency above a set threshold, a microprocessor adapted to receive the output of said pulse stretcher, a clock connected to said microprocessor, and a plurality of indicators connected to the output of said microprocessor, each indicator being associated with a respective aircraft type, said microprocessor being adapted to count the number of stretched pulses occurring in a predetermined time period determined by the frequency of said clock and to operate an indicator if the frequency of said stretched pulses corresponds closely to the rotating element spike frequency of an aircraft represented by that indicator.