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Aviation History
1957
1957 - 0941.PDF
2 July 1957 41 Marconi Doppler Navigators THREE YEARS IN SERVICE: RELEASED FOR CIVIL USE Above, the Marconi AD.2000 Doppler navigator with dials showing ground speed and drift and a veeder-counter presentation of ground miles travelled; on left, the AD.2000 aerial assembly, 3ft S'/jiin long, showing the four waveguides. The bifurcation, which is mechanically cycled to form a waveguide switch, can be seen in the centre. Maximum rotation, and therefore drift measure- ment, is 20 deg. DOPPLER navigators have been produced in quantity byMarconi since 1954 and have since then been in servicewith the R.A.F. and the air forces of several Commonwealth countries. Details of three military Dopplers produced by Marconihave now been released and serve to complete the picture given when details of the AD.2300 civil Doppler were published recently[see Flight for June 21, pages 882-3]. A Doppler navigator is a completely self-contained airborneequipment which provides a continuous indication of ground- speed and drift by measuring the frequency shift in radio signalstransmitted at an angle from an aircraft and reflected from the ground. This basic information is combined with magnetic head-ing indications derived from a gyro-magnetic compass system and then fed into a computer which can be arranged to give almost anykind of navigational information in any desired presentation. Marconi's first military equipment was the AD.2000, severalhundred of which have now been delivered to R.A.F. Bomber Command. The total weight of the equipment is some 240 lbincluding the aerial system, transmitter/receiver, tracking unit, indicator and the associated Ground Position Indicator Mk 4,which is the computer unit. The aerial unit, which is unpres- surized, consists of four parallel waveguides mounted horizontallyin a horn assembly. The waveguides are slotted in such a way that they can transmit four beams, 2.5 deg wide at the half-powerpoint, respectively forwards to left and right and rearwards to left and right. A Y-junction waveguide, mechanically cycled athalf-second intervals, feeds alternate pairs of these waveguides so that a forward beam is projected to one side and a rearward one tothe other at any given moment. The mean lateral deflection of the beams is 20 deg and all are directed at a fixed angle downwardrelative to the horizontal. Cam-operated switches on the driving shaft of the waveguide switch provide sense and timing pulses tothe remainder of the equipment. The transmitter/receiver unit, consisting of two interconnected, •pressurized containers, includes the transmitting magnetron, modu- lator circuits, receiver klystron, first I.F. amplifier and A.F.C.circuits. The second container houses the main H.T. and E.H.T. (8,000 V) circuits serving the magnetron. Pulses are fed to themagnetron in bursts of 40 milliseconds' duration repeated at half- second intervals in synchronism with the movement of the wave-guide switch in the aerial. In this pulse system peak power is 8 kW and mean power approximately 12 W. Operating frequencyis in the 8,750 to 8,850 Mc/s band. This part of the system produces the beam pattern from whichthe frequency shift is measured to assess the groundspeed. The measurement takes place in the tracking unit, housed in a pres-surized container, which includes the frequency-measuring circuits together with the second I .F. amplifier and A.G.C. circuits. Here, two discriminating channels compare the received Doppler fre-quency with that of a phonic velodyne. This device is a small, finely toothed wheel which rotates in close proximity to a coil unitwhich itself is energized by the Doppler frequency. An integrator circuit is applied to the wheel drive so that its rotational speedproduces a frequency which is the same as that of the Doppler frequency, the signals thus derived being used to produce thegroundspeed indication for the indicator and computer units. Two phonic wheels are in fact used to measure the frequency shift atthe forward and rearward edge of the 1\ deg projected beam. An average of these two readings is used because the intensity of thereflected signal varies over the width of the beam. If plotted on a graph this will show different slopes over sea and land. Becauseof this, some Dopplers have a selector switch for sea or land settings, but the AD.2000 does not require it. Drift indication is derived by automatically adjusting the align-ment of the aerial unit so that the frequency shift from the beam systems projected to each side is equal. For this purpose the aerialassembly is rotated on its vertical axis, a synchro providing an indication of the drift angle. In the AD.2000 the maximum drift The transmitter/receiver unit of the AD.2000 with the pressurized container covers removed. H.T. and 8,000 V E.H.T. components are on the left and the transmitter and receiver on the right.
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