Navigational Aids
Oboe AMES Type 9000
The text that follows is copied from "The Services Textbook of Radio, Volume 7, Radiolocation Techniques" by Brig. J. D. Haigh, O.B.E., M.A., M.I.E.E., Edited by the staff of "Wireless World", H.M.S.O., London, 1960. This volume was known to the British armed forces as Admiralty B.R.600(7), War Office 10224(7) and Air Ministry A.P.3214(7).
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Major system characteristics |
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System type |
Pulsed range tracking blind bombing system |
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Frequency band |
200 MHz (Mk.I) |
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Pulse repetition frequency |
133 pps |
|
Range measurement accuracy |
±17 Yards |
|
Speed measurement accuracy |
±1/2 M.P.H. |
Secondary radar principles were embodied in a precision bombing system used in the later stages of the last war known as Oboe. The principle is one which has applications to peacetime problems such as the precise location and control of an aircraft engaged in aerial survey.
The system is illustrated in Fig. l4.3.
Two ground stations are used which emit trains of pulses on the same carrier frequency but with different recurrence frequencies. The aircraft carries a responder which replies to the interrogations of both ground stations so that each knows the range to the aircraft.
In the bombing application one of the ground stations was known as the tracking station and the other as the releasing station. The aircraft is caused to fly on a course of constant radius about the tracking station the radius being such as to bring the aircraft over the target. When the aircraft arrives at the calculated point of bomb release, (that is to say, when it is at a precise range from the releasing station) this station sends a signal which automatically releases the bombs.
The train of pulses from the tracking station, which commonly has a recurrence frequency of 133 pulses per second is modulated by Morse signals to convey information to the aircraft. Thus if the aircraft is at too short range a series of dots is sent; if at too great a range, a series of dashes is sent. Additionally various letters in Morse code are sent to notify the pilot when he is within certain distances of the prescribed track, and other letters to denote certain distances from the release point.
The value of the secondary radar principle in this application is that not only does it ensure that the range of the system is only limited by the radar horizon but also, by coding the replies, it can be verified that the correct aircraft is being controlled.
The next extract is taken from "Radar - A Wartime Miracle", by C. Latham and A. Stobbs (Sutton Publishing Ltd., Phoenix Mill, Thrupp, Stroud, Gloucestershire, GL5 2BU, 1999, ISBN 0-7509-1114X) and presents a more thorough explanation of the Oboe principals.
OBOE
The following brief explanation of a highly sophisticated system includes, by kind permission of Marconi Radar Systems, extracts from their published articles written by the late Bruce Neale. His wartime RAF service as a radar mechanic (LAC to Warrant Officer) included work on GH, GHL, and Oboe before he became a distinguished postwar designer of radar equipment. He writes:
The inaccuracy of our all-weather bombing, especially at night, had been recognized for some time; something had to be done.
Oboe was the brainchild of Alec Reeves, the PCM (pulse code modulation) wizard of Standard Telephones and Cables Ltd, who along with Frank Jones of TRE, developed an idea from the back of an envelope to a potent instrument of war. The basic principle of Oboe is really very simple, it was the inspired engineering and operational concept that was the key to its success.
The range of a target, say the Krupps armament works in Essen, measured from two widely spaced ground stations in the UK, was derived from pre-war ordnance survey maps and aerial photographs [see sketch, top]. One ground station, code-named CAT, controlled the track of the marker aircraft (usually a Mosquito) by interrogating an on-board transponder.l The returned pulse was displayed on a cathode ray tube with a delayed, magnified time-base (one mile filled the whole 12 in screen). The precise range of the target from the CAT station was set up on the tube by a strobe marker.2
The Mosquito flies on a circular arc centred on the station at a radius equal to the target range, appearing stationary on the tube face, i.e. zero radial velocity.
The purpose of the CAT was to keep the returned pulse exactly in line with the target marker strobe by automatically signalling dots or dashes; dots if the Mosquito range was less than the target range, dashes if it was greater. The dots and dashes merged into a continuous or 'equi-signal' note when the Mosquito range
1 Transponder: a unit which, on receiving a specific radio pulse, transmits another in reply, usually on a slightly different frequency.
2 Strobe/strobe marker: portion(s) of a display expanded or specially marked to permit detailed examination.
