Patent GB-594346-A (GB)
Title: Improvements in methods of measuring distance by means of the reflection of radio waves, having particular application to radio altimeters
IPC: G01S13/34
Filed: 1944-05-06
Abstract: 594,346. Radiolocation. MITCHELL, J.H. May 6, 1944, No. 8602. [Class 40 (v)] [Also in Group XXXVI] In a radio distance-measuring apparatus the transmitted radiation is modulated in frequency or phase and a component at modulation frequency, derived from the beat oscillation produced by mixing reflected and directly transmitted waves, is compared in phase with the original modulating oscillation, the phase difference being a measure of the range of the reflecting object. In a radio altimeter, two modulating frequencies wl and w2 produced by generators O1, 02 controlled by a stable oscillator O, are fed to the modulator MO of the transmitter T to enable the apparatus to operate over two ranges providing fine and coarse indications. In the receiver circuit the mixer stage Mi is fed with the reflected signals, and also with the directly transmitted signals either by radiation or direct feed as shown. The mixer is followed by an amplifier A which may include A.G.C. and/or limiting stages and the output is fed to a discriminator circuit D which extracts the modulation frequency term. The receiver may. include a premixer amplifier, and the mixer, when direct connection to the transmitter is employed, may comprise a balances detector to reduce the effects of amplitude modulation. An intermediate frequency conversion may also be provided by an intermediate mixer 03, in the direct connection between transmitter and receiver, which is fed with a heterodyning frequency W3, the output from the receiver mixer being then centred about W3. Alternatively, the intermediate mixer may be inserted between the amplifier A and discriminator D, the heterodyning frequency W3 may be inserted between the amplifier A and discriminator D or the heterodyning frequency W3 may be injected into the mixer M1. The modulations frequencies W1 and W2 obtained from the discriminator are, for simplification of phase measurement, heterodyned to lower frequencies in mixers Mi4, Mi5. The frequencies W4, W5 injected into the mixers are produced by oscillations 04, 05 controlled by the stable oscillators O, and are so selected that the mixer outputs passed by the filters F1, F2 have frequencies W6, W7 equal to the frequencies produced by oscillators 06, 07 which are also controlled by oscillator O. The outputs of filters F1, F2 and oscillators 06, 07 are then fed to phase-difference measuring circuits PD1, PD2, the meters M1, M2 indicating height over different ranges. Any suitable type of phasemeter may be employed, such as a balanced demodulator, Fig. 2 (not shown), or a circuit in which short pulses generated by the voltages whose phases are to be compared are applied to the two halves of a " flip-flop " circuit, the difference in the anode currents in the two parts of the circuits being an indication of the phase difference. The two phasemeters may be calibrated for coarse and fine readings of height (say one 0-50,000 ft. in 500 feet steps, the other 0-500 ft. in 5 ft. steps) and may be combined so that their pointers co-operate with closely adjacent scales or the movements may be mounted coaxially. The phasemeters may be of conventional cathode-ray-tube type or alternatively the reference phase oscillation may produce a circular trace on the screen while pulses developed by the outputs from filters F1, F2 are employed to brighten the trace or so modify the deflection sensitivity that radial deflection is produced. The coarse and fine height indications may be provided means of a double beam c.r. tube or by rapid switching of the two sets of voltages to the tube. Specification 539,952, [Group XXXVI], is referred to.