Patent GB-1424391-A (GB)
Title: Pulsed doppler radar systems
IPC: G01S13/58
Filed: 1973-01-29
Abstract: 1424391 Pulse radar SIEMENS AG 29 Jan 1973 [28 Jan 1972] 4356/73 Heading H4D A pulse Doppler radar which includes at least one Doppler filter for fixed echo suppression and a discriminator for measuring the Doppler frequency of a specific moving target, includes at least one digital filter having a two-part passband which initially together cover the Doppler filter pass-band and further includes control means which in response to the detection of a possible moving by the discriminator, reduces the subsidiary pass-bands to cover the subsidiary pass-band which had the larger output, this being repeated over a number of steps, to accurately determine the target Doppler frequency. In a radar having a rotating antenna 1 or a phased array antenna, coherently detected echoes are converted into digital signals and fed to a switching device S which feeds them sequentially during a transmitter pulse period to range channels Kl-Kn. Each channel has a wide-band digital Doppler filter D1-Dn whose band-pass is set by control device 9. Thresholds W1-Wn have active levels just above normal noise level and thus suspect targets are identified to control device 9. This then starts the Doppler frequency measuring cycle. If the suspect target is in channel K1, switch S2 is closed thus connecting channel K2 to the same input as K1. The band-passes of filters D1, D2 are adjusted by control 9 such that D1 occupies the lower half and D2 the upper half of the total Doppler pass-band. The filter outputs are connected to discriminator B which signals to control 9 which has the greater output. Control 9 then adjusts the pass-bands such that they cover the relevant half of the original pass-band. This procedure is repeated until the accuracy is as great as is needed. It may be necessary for the control device 9 to increase the length of the transmitter pulses or the dwell time of the attenna while measuring the Doppler frequency in order to complete the measurement. When the transmitter pulses are coded and pulse compression is effected at the receiver the number of pulses in the compression may be reduced when measuring the Doppler frequency. Since in general many fewer suspect targets will be detected than there are range channels, correspondingly fewer discriminators are provided and control 9 may utilize as the second filter any channel which does not contain a suspect target. If no range channel blocking may be allowed at all, the input to the Doppler filter in the suspect channel is connected to the discriminator input via a separate digital filter. This may also be used if only one range channel is present. Alternatively the digital filter in each channel may be switched successively to the two half pass-bands and the outputs therefrom stored before comparison. The output from the threshold W1 is detected, integrated and passed to thresholds V1-Vn which have high active levels. Any signal overshooting this level indicates a real target which is signalled to control 9 and indicator 8, e.g. PP1. Having determined the Doppler frequency the velocity of the moving target may be displayed separately on indicator 10 or associated with the echo on display 8. The digital filter (Fig. 3, not shown) comprises a recursive filter including delays and amplifiers. Variation of the amplifier gains and delay times is used to set the filter band-pass.