Patent GB-592749-A (GB)
Title: Improvement in radio altimeters and speed indicators
IPC: G01S13/58
Filed: 1944-02-22
Abstract: 592,749. Radiolocation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Feb. 22, 1944, No. 3322. Convention date, Aug. 29, 1942. [Class 40 (v)] In a radar system of the frequencymodulated type or of the constant frequency type in which relative movement of the apparatus and reflecting object, is, detected by the Doppler effect, the radio frequency carrier is transmitted in the form of pulses. The pulse length is longer than the time required for signals to be reflected back to the system, and is preferably at least equal to the duration of one cycle of modulation, while the pulse repetition frequency is synchronized to the modulation frequency if frequency modulation that is non-linear is employed. The use of pulses enables the transmitted power to be increased and also enables the receiver to be blocked during the intervals between pulses. The altimeter of Fig. 1 comprises an oscillator 5 coupled to an antenna 6 which radiates a signal which is frequency-modulated by electrodes 10, 11 and diaphragm 7 which is vibrated by a coil 8 fed from an audio frequency oscillator 9. The receiver includes a balanced detector 1 comprising valves 16, 17 fed in push-pull with reflected signals from antenna 18 and in parallel with signal direct from the transmitter over line 19. Amplitude modulation is cancelled out,in the push-pull output transformer 31, the beat frequency between transmitted and reflected waves being fed through amplifiers 21, 22 to frequency indicator 23. Oscillator 5 is provided with a grid bias which normally prevents oscillation and the radiation of pulses is controlled, by a relaxation or similar oscillator 36 which is locked to oscillator 9 and produces pulses in fixed time relation to the modulating signal from oscillator 9. The pulses are fed to a relay coil 37 and closure of contacts 38, 41 applies a positive voltage to the grids of the valves in oscillator 5 to cause the transmission of the radio frequency pulse. Simultaneously contacts 39, 42 remove a shortcircuiting condenser 35 from the input to the audio amplifier 21 and enable the beat frequency to be passed to the frequency indicator 23. The transmitting periods are short compared with the intervals between pulses so that the oscillator 5 may be driven to a much greater output than the continuous rating of the valves and also during the inter-pulse periods no noise or undesired signals are fed to the frequency meter to produce false readings. Indicator 23 should have sufficient damping and the pulse recurrence frequency should be sufficiently high to cause a steady reading to be obtained. In a modified oscillator, Fig. 3 (not shown), a condenser-resistance combination is inserted in the grid circuit to'cause automatic pulsing and a considerable simplification is thus obtained if the receiver is not blocked between pulses. The apparatus of Fig. 1 may be utilized to measure the Doppler effect, for example to determine the ground speed of an aeroplane carrying the equipment by disconnecting the frequency-modulating oscillator 9 and radiating pulses of constant frequency carrier wave. Specification 317,068, [Class 40 (v)], is referred to.