GB625034A - Improvements in and relating to pulse radar systems - Google Patents

Abstract

625,034. Radiolocation. ELLIOTT, W. S. May 31, 1946, No. 16563. [Class 40 (vii)] In a pulse radar system employing the Doppler effect to distinguish between fixed and moving targets and having a B-type, P.P.I. or like display, the received echo signals are mixed, either at radio-frequency or intermediate-frequency with a reference oscillation of a different frequency, but which bears a controlled phase-relationship with the transmitted signals, so that the striations produced across the indication displayed on the cathoderay tube due to the Doppler effect are inclined to the range direction to an extent dependent upon the magnitude and sign of the radial velocity of the reflecting object. Thus, Figs. 1d, 1e, 1f show the indications produced on a B-type display for an approaching, a stationary, and a receding target respectively, the receding target having a greater radial velocity than the approaching target. As shown in Fig. 2, the output of a 200 mc/s. oscillator 4 is modulated by pulse modulator 5 so that pulsed oscillations are applied through the unit 6 to the rotating aerial system. The pulsed oscillations are also mixed at 13 with a 187.5 mc/s. oscillation derived from the unit 16, and the 12.5 mc/s. pulses from the mixer 13 control the phase of the 12.5 mc/s. reference oscillation generated at 15 which may comprise apparatus as described in Specification 599,114 or in Specification 596,657. The unit 16 comprises a 200 kc/s. oscillator 17, the output of which undergoes successive frequency changes by mixing with the outputs from the fixed frequency oscillators 18, 19 and 20, so that a 187.5 mc/s. oscillation is applied to the mixer 13. The output of an oscillator 21 which is tunable in the range 100-200 kc/s. is similarly frequency changed so that the output on the line 35 comprises an oscillation which is variable over the range 187.4-187.5 mc/s. This signal is used as the local oscillation for the echo receiver and is applied through the switch 36 to the frequencychanger 9. The reference oscillation, which consequently differs from the intermediate frequency by a predetermined amount depending upon the tuning of the oscillator 21, is applied from 15 to the second detector 11. The switch 36 may be used to apply a 187.5 mc/s. signal from the line 28 to the frequencychanger for checking and testing purposes. In order to obtain striations of the desired form, it is necessary that the frequency-difference between the intermediate frequency and the reference oscillation be maintained at an integral multiple of the pulse-recurrence frequency. In practice, to achieve this the tuning of the oscillator 21 is periodically readjusted so as to maintain the display obtained from a stationary target as shown in Fig. 1e. A scale may be associated with the control of the oscillator 21 so that the radial velocity may be directly indicated on the scale when the striations are inclined at an angle of 45 degrees. In a further embodiment, Fig. 3 (not shown), a 12.5 mc/s. oscillation is produced by mixing signals at 11.9 mc/s. and 600 kc/s. derived from crystal-controlled oscillators. By frequencymultiplication, the 200 mc/s. transmission frequency and 187.5 mc/s. local-oscillator frequency are produced. The reference oscilla. tion, which is applied to the second detector, is generated by mixing the 11.9 mc/s. oscillation with one of a plurality of crystal-controlled oscillations so that a number of alternative different reference frequencies is provided. For centimetre-wave working the pulse signals may be modulated by a signal at approximately 200 mc/s.; the 200 mc/s. modulation is then extracted from the received echoes and used as described above. Alternatively, two centimetre-wave transmissions separated in frequency by approximately 200 mc/s. may be used. Specification 602,030 also is referred to.