Patent GB-1455833-A (GB)
Title: Pulsed doppler radar systems
IPC: G01S13/53
Filed: 1973-11-16
Abstract: 1455833 Pulse doppler radar SIEMENS AG 16 Nov 1973 [17 Nov 1972] 53258/73 Heading H4D The digitised Doppler signals are first passed through a Doppler filter 9 (Fig. 1) having a pass band wide enough to accept all possible Doppler frequencies and are then digitally mixed with a series of stepped frequency local oscillations in a digital mixer 10 whereby whatever the Doppler frequency, one of the local frequencies enables the product of the mixing to pass through a much narrower bandwidth bandpass filter 11. The production and use of the local frequencies is on a sequential basis whereby, assuming that only a limited number of moving targets are detected and that most of the time period between transmitted pulses is not occupied by returns from moving targets, the sequential use of these local frequencies can take place during return free intervals. After passage through the second filter 11 the digital signal is converted back to analogue form for detection and storage at 13 and threshold detection 13a. The Doppler signals are analysed in terms of range by means of n range gate channels K1, K2 ...Kn, each channel being enabled in sequence by a set of input gates 8 and a set of output gates 14. The local digital oscillations are produced in mixer 10 under the control of a circuit 16, the signals from output gate 14 are collectively fed to a display 15. When it is detected that only, for example, two range channels e.g. K5 and K15 contain a moving target, control means 16 may accordingly extend the processing time allocated to each of these targets into the other no target range gate times. Instead of equipping each of the range channels K1 to Kn with separate items 9, 10, 11, 12, 13, 13a certain of these items may be shared by all the channels. Thus the digital oscillator included in the digital mixer 10 may be used for supplying local oscillations for all the range channels. Alternatively control device 16 upon detecting those range channels in which a moving target has been detected may cause a digital mixer 10 to be only supplied for said detected channels. The digital mixer comprises a storage and repetition device enabling the mixing of each signal within the respective range channel to be successively mixed with each of the local frequencies the mixing being carried out in a digital multiplying stage. The control circuit 16 comprises Fig. 4 (not shown) a counter receiving the combined output of the output range switches 14 to establish which range channel carries a target and logic circuitry deciding on the basis of the number of so occupied range channels the timing of the production by the oscillator in the digital mixer 10 of the various local frequencies and the processing time which may be allocated to the detecting target. It is also possible by analysis of which local frequency has been necessary to allow the target signal through the second narrow bandwidth filter to accurately determine the speed of the target. This may be carried out Fig. 5 (not shown) by switching the output of the digital multipliers in a digital mixer 10 to M channels where M is the number of local frequencies sequentially generated in synchronism with the generation of said frequencies. Each of the M channels therefore carries signals corresponding to a specific speed. The outputs of these channels are synchronously sampled for passage to the display 15.