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Aviation History
1962
1962 - 2144.PDF
FLIGHT International supplement, 20 September J 96 2 Air-Cushion Vehicles Marconi Doppler for ACVs IN ORDER to provide a very accurate measurement of its speed over land or water the Vickers VA-3 is to be fitted with Marconi Doppler equipment. Marconi can claim to have been the first electronics company to investigate the problem of measuring speed in an air-cushion vehicle. Their practical work dates back to the latter end of 1960 when a modified version of their portable electronic traffic analyser radar (PETA)—widely used by police forces —was installed in the SR.N1. This original installation has since been transferred to the SR.N2. Now, a completely new device, work ing on the same principle, has been developed and is soon to be installed in the VA-3. The problems of measuring speed in ACVs, Marconi remark, cannot be solved satisfactorily by conventional means. It is impossible to use a ground reference, as any form of pitot tube or log trailing in the water would create unacceptable drag. It would, in any case, be destroyed when the vehicle moved from water on to dry land. An aircraft-type airspeed indicator, using a pitot tube, gives rise to considerable errors, owing to the complex nature of the airflow round the craft and into the fan. Water Disturbance The equipment to be fitted in the VA-3 obtains its speed measurement from the Doppler shift in frequency of a narrow beam of radio energy focused on a point a few feet behind the vehicle, and in the area of water which has been disturbed by the air cushion as the vehicle passes. At this point the water is violently disturbed, but has not had time to be affected horizontally by the wind. The disturbance ensures that a good proportion of the radio energy transmitted is reflected back to the re ceiving aerial on the vehicle, but its proximity ensures that wind effects which could produce errors do not have time to build up. This type of speed measurement applies equally when the vehicle is passing over dry land, although protection from wind effects is not then important. The Marconi "Hovercraft speedmeter" consists basically of a low-power CW Doppler radar working in the 14,000Mc/s frequency band, which has been allocated for Hovercraft naviga tional devices. A pencil beam of radio energy is directed down from a 12in-diameter parabolic reflector mounted on the stern and pointing down at 45° to the surface over which the ACV is travelling. The beam is scattered by the disturbed water, or by the ground, and a small proportion returned to the aerial, where it is mixed with a sample of the trans mitted signal in a crystal diode. A three- port ferrite circulator, developed in the Marconi laboratories to operate at this high frequency, enables the same aerial to be used for both transmitting and receiving and prevents all but the required small amount of transmitted energy from the J-band klystron from entering the mixer, although the trans mitter is operating on CW. Vehicle Velocity The frequency of the energy returned to the aerial differs from the transmitted frequency by an amount which is pro portional to the velocity of the vehicle. This difference frequency, which lies in the audio range, appears directly as a beat frequency output from the crystal mixer. It is passed to an audio amplifier and is then fed into a tracking unit which identifies the centre of the Doppler frequency spectrum and produces a current proportional to this frequency. This current can then be applied to any meter with a full-scale deflection of 1mA and a linear calibration from 0-100kt. Where necessary, a number of meters may be connected in series. A small preset control on the tracker unit adjusts the d.c. output for the meter resistance and a test switch on the instru ment panel supplies a test signal to check the calibration of the whole equipment. This signal corresponds to 80kt, and the output of the tracker should be set to give this test reading on the meter in use. A socket on the tracker unit is pro vided for a test-box connection. This test box will monitor the entire system, including the klystron, and will provide additional test frequencies to check the linearity and calibration of the speed measurement. The aerial unit contains the aerial reflector, klystron transmitter, mixer crystal and audio amplifier. The reflec tor has an electrically matched glass- fibre window which is designed for mounting directly on to the hull and to form a watertight seal. The hull should therefore have a prepared housing with a 12in-diameter hole lying in a plane at approximately 45° to the horizontal to give the beam its correct angle of fire. This housing must also provide water proofing for the rear of the unit. Doppler aerial unit the box-like object) on SR.N2 The tracking unit contains the circuits which produce a clean Doppler signal and it is housed in a fully enclosed sheet metal box suitable for bulkhead or bench mounting and measuring overall about 8in X 8£in X 17£in. The power supply unit is housed in a similar enclosure to the tracker and derives stabilized voltages for the entire equipment from the nominal 28V Hovercraft supply. Supply voltages down to 23.5V are acceptable, and full protection against transient surges has been incorporated. The whole equipment has been de signed for a temperature range from 0' to 50° C and humidity specification H2. Tests so far have indicated that this new equipment will have an accuracy of better than 3 per cent, but, say Marconi, it is probable that exhaustive trials will prove the Doppler system accuracy to be considerably higher than this. DATA SUMMARY Type of transmission, CW; operating frequency, l4,000Mc/s; beam dimensions. 4C; angle of beam, 45° to horizontal in vertical fore-and-aft plane; transmitted power, 40mW; transmitter, English Electric K343 reflex klystron; aerial mounting, I2in-diam aperture in hull; speed range, 0-l00kt with ImA f.s.d. meter; accuracy, better than 3%; temperature range, 0-50°C; humidity specification, H2; supply voltage, nominal 28V (voltages down to 23.5 acceptable). Weights: Aerial unit, I4lb; tracking unit, I lib; power unit, I6^lb. Dimensions: Aerial unit, I6in diam, including mounting flange (depth overall, 7in); tracking unit, 8in y 8^in I7 in; power supply unit, 8in * 8^in I7i:n. New Doppler aerial unit, shortly to be fitted to the VA-3 62
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