Patent GB-975177-A (GB)
Title: Improvements in or relating to doppler radar apparatus
Inventor: PHYLIP-JONES GWILYM
IPC: G01S13/58
Filed: 1961-09-26
Abstract: 975,177. Doppler radar. GENERAL ELECTRIC CO. Ltd. Sept. 25, 1962 [Sept. 26, 1961], No. 34543/61. Heading H4D. Radar apparatus suitable for use in conditions giving rise to clutter signals, comprises a continuous wave transmitter and a receiver at the same location, information in respect of the target range and velocity being obtained by comparison of the transmitted and received signal frequencies, the transmitted signal having a periodic non-sinusoidal frequency modulation such that in each period of modulation the transmitted signal includes a velocity determining portion and a range determining portion, the receiver including mixing means to provide a beat frequency signal by comparison of the transmitted and received signals, any clutter signals in the beat frequency signal constituting an unstable frequency band component, a continuous wave oscillator arranged to provide a signal modulated in accordance with predicted clutter signals, mixing means whereby the modulated signal from the oscillator is arranged to be mixed with the heat frequency signal to provide an output signal in which frequency variation of the clutter signal band due to the modulation of the transmitted signal is suppressed, filter means to remove the clutter signal band from the output signal and further mixing means to remove from the filtered signal the modulation due to the modulation of the continuous wave oscillator signal, the resulting signal comprising stable signals representative of discrete objects having velocity components other than such as to produce clutter. In the arrangement shown a signal at a frequency X mc/s. is generated by oscillator 4 and frequency modulated under the control of waveform generator 7. The resulting signal, which is amplified and passed to the transmitting aerial 3, via circulator 8, is frequency modulated in four steps, each step occupying 15 m/secs., as determined by clock pulse generator 6, and comprising a first step linearly increasing in frequency to (X+0.15) mc/s., a second step at a constant frequency of (X+ 0.15) mc/s., a third step linearly decreasing in frequency to X mc/s. and a fourth step at a constant frequency of X mc/s. The second and fourth steps are used for Doppler speed determination, whilst the first and third steps are used for distance determination. The echo is received by aerial 3 and passed via the circulator 8 to a mixer 13. The frequency modulated output of oscillator 4, is also fed to a mixer 15, where it is raised in frequency by means of local oscillator 14, by 40 mc/s. The resulting signal is applied to the second input of mixer 13, to produce an output having a basic frequency of 40 mc/s., frequency modulated in accordance with the targets distance and relative speed. The signal is then heterodyned down to one with a basic frequency of 2 mc/s., by means of local oscillators 14 and 18 and mixers 17 and 19, and is passed to mixer 25. The output of mixer 17 contains a clutter frequency band centred on a constantly changing frequency. The purpose of the invention is to make this central frequency constant such that the clutter band can be filtered out. The arrangement as described is for use in an aircraft with an aerial scanning in azimuth. The ground &c. will therefore produce a clutter signal dependant on the aircraft's ground speed and the aerial direction, and also on the frequency of the transmitted signal. A local oscillator 26 is thus frequency modulated from a basic frequency of 500 kc/s. in accordance with signals from a ground speed detector 27, an aerial direction detector 28, and transmission frequency detector 29, controlled by clock pulse generator 6, to produce a signal having predicted clutter frequency modulation. The basic frequency is changed to 1450 kc/s. in mixer 30, by the signals from oscillator 18, and the resulting signal is mixed with the output from mixer 17, in mixer 25, to produce a signal having a clutter frequency band constantly centred around 550 kc/s. The clutter frequency band is removed by a band stop filter 31 centred on 550 kc/s. and the remaining signal is passed to a mixer 32 where the modulation of the target distance and speed information by the signal from mixer 30 is removed by a signal from mixer 33. The corrected information is then passed to the latter parts of the receiver 35. Any breakthrough of transmitted power into the receiver via the circulator is counteracted by deliberately reflecting some transmitted power in antiphase by means of adjustable stub lines 20 and 21. This method of breakthrough counteraction forms the subject-matter of Specification 916,486.