Patent GB-938956-A (GB)
Title: Doppler radar system
IPC: G01S13/60
Filed: 1960-04-08
Abstract: 938,956. Doppler radar. STANDARD TELEPHONES & CABLES Ltd. April 8, 1960 [April 9, 1959], No. 12554/60. Class 40 (7). In an airborne Doppler radar for determining the ground speed and drift angle of the aircraft a single narrow beam aerial for transmission and reception is scanned conically with the axis of the cone vertically downwards so that the frequency of the received signal is F+F v +F h  sin (2# ft+#) where F =frequency of transmitted signal; F v =Doppler shift due to the vertical velocity of the aircraft; F h =Doppler shift due to the horizontal velocity of the aircraft; f =rotation frequency of the conical scan; # = the phase of the sinusoidal frequency modulation component relative to the aerial scan frequency f, said phase # being a function of the drift angle. Means are described for deriving the drift angle, the horizontal and vertical velocities and the NS and EW components of the distance travelled. As shown in Fig. 5 the conical scan is produced by rotating an offset directional aerial 5 at the frequency f and the aerial rotation is also applied to one input of a differential gear 17, a D.C. velocity servomotor 15 being coupled to the other input of the differential 17. The received signal is converted in a receiver 10 to a corresponding I.F. signal F if +F v +F h  sin (2# ft+#) where F if  is the normal frequency of an I.F. oscillator 23 and the sinusoidal modulation component is extracted by a frequency discriminator 13 and applied to the servo 15 through a reversing switch 14 actuated by the output from the differential 17, the switch 14 thus giving a D.C. error voltage proportional to the phase difference between the outputs from the differential 17 and the discriminator 13. The servo 15 operates to reduce said phase difference to zero and the resultant angular position of its output shaft 16 is proportional to the phase difference # which is indicated on a dial 21 calibrated in drift angle. The oscillator 23 is frequency modulated sinusoidally at the frequency f by the output from a sine potentiometer 21a driven by the output shaft of the differential 17. The output from the potentiometer 21a is applied to the oscillator 23 through a potentiometer 22a and adder 27 and the frequency deviation of the modulated output from the oscillator 23 is automatically maintained equal to that of the output from the I.F. amplifier 11 by deriving the frequency difference of said outputs in a mixer 24 and applying said difference frequency signal through a frequency discriminator 25 and reversing switch 28, controlled by the output shaft of the differential 17 to a servomotor 30 which adjusts the potentiometer 22a; the magnitude of said adjustment is proportional to the horizontal velocity which is indicated on a dial 33. The mean frequency of the oscillator 23 is also automatically maintained equal to the mean frequency (F if +F v ) of the output from the I.F. amplifier 11 by applying an output from the discriminator 25 to the adder 27 through an integrator 26, the magnitude of the output from the integrator 26 as displayed on a meter 34 being indicative of the vertical velocity. A further output from the oscillator 23 is converted at 35 into a corresponding pulse train which is subtracted in a mixer 3b, Fig. 6, from a pulse train of recurrence frequency equal to the mean frequency (F if +F v ) of the oscillator 23. The resultant pulse Doppler frequency signal from the mixer 36 is applied to a gate 37 which applies the pulses with positive polarity to a reversible counter 39 when the aerial beam is scanning the 90 degrees sector centred on the N direction, Fig. 7 (not shown), and applies the pulses with negative polarity to the counter 39 when the beam is scanning the 90 degrees sector centred on the S direction so that the output from the counter 39 represents the distance travelled in the N-S direction. The E-W distance travelled is determined by a corresponding gate 38 and counter 40 and the resultant distance travelled is given by a computer 41.