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Gee-H - AMES 100

Major system characteristics

System type

Hyperbolic navigation system

Frequency band

20 - 80 Mc/s

Cycle repetition frequency

100 c/s

The text that follows in this section is taken 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 pages 250 - 252. 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).


Twin Range Systems

In the preceding paragraphs we have seen that systems based on the range difference principle become inaccurate in proportion to the square of the range; to balance this drawback they h~wc the great advantage of not being subject to saturation effects. However, for purposes such as blind bombing at great range a much more accurate system is required and this was achieved by adopting the twin range or 'H' principle. The position of the aircraft is determined by measuring its range from two ground beacons. The accuracy of measurement of range depends simply on the accuracy of pulse aligning and on the accuracy of the calibrator and is independent of range. The accuracy of fix, however, will depend on the angle at which the two range vectors intersect, being greatest when they intersect at right angles. It will therefore follow that with a fixed distance between the ground beacons the accuracy of fix will de-crease with range. It can be shown that this decrease in accuracy depends in a linear manner on the range whereas in hyperbolic systems, as we have seen, the accuracy decreases according to the square of the range. As an example, if the ground beacons be 100 miles apart the fix will be accurate to within about 50 yards at ranges of 100 miles, 100 yards at 200 miles and 150 yards at 300 miles. For a longer baseline the range at which a given accuracy is obtained will increase in proportion.

The method of range measurement is for the aircraft to emit a series of pulses which cause ground beacons to transmit a corresponding series of pulses. The delay between the emission of a pulse and the reception of the transponded pulse from the ground, making due allowance for the delay which occurs between the interrogation of the beacon and the transmission of its response, is directly proportional to the range of the aircraft from the beacon. The first H systems used the standard Gee indicator and the time delays were measured using the Gee calibrator. For this reason the system became known as G.H.

The transmitters in the aircraft operated in the 20 - 80 Mc /s band and used a pulse recurrence frequency in the neighbourhood of 100 c /s. In order that an aircraft might be able to identify the responses to its own transmissions its pulse recurrence frequency was 'jittered' automatically. As the time-base of the indicator unit is controlled by the pulse recurrence generator, only responses with the same 'jitter' in their frequency of recurrence will appear stationary on the screen. It will be appreciated that the beacons may be interrogated by many aircraft at one time and some such device as that just described was necessary to prevent confusion.

The time taken by the beacon to receive a pulse, send out the response arid return to the receiving condition, was about 100 microseconds. With a pulse recurrence frequency of 100 c/s a beacon would be busy for 10,000 microseconds in any one second dealing with the enquiries of one aircraft. It would therefore have 990,000 microseconds free in each second in which to respond to other aircraft, giving a theoretical maximum handling capacity of 100 aircraft. In practice of course the aircraft cannot be expected so to phase their pulses as to make the best possible use of the beacons and a handling capacity of 70 to 80 was all that was normally realized.

For blind bombing the aircraft was caused to fly on a course of constant range from one of the beacons, which passed over the target. The release point is reached when the range from the second beacon becomes equal to a previously calculated value. This is illustrated in Fig. 17.9.

This system is the inverse of the Oboe system which was described in chapter 14.

Shoran

G.H. suffered to some degree from the war-time need to make the maximum use of existing Gee equipment which was not designed for this purpose. To obtain greater accuracy a higher radio frequency would preferably have been chosen which would enable more steep-fronted pulses to be generated whose positions could therefore be measured with greater precision. This in fact was what was done in America in the Shoran system. Radio frequencies in the 300 Mc/s region were used and the two beacons were on different frequencies to facilitate their identification. To give a longer time-base, and therefore more scope for accurate pulse position measurement, a circular time-base with radial deflection was used. With these refinements a range accuracy of the order of 20 yards was obtained at 200 miles. 


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Updated 04/05/2002

Constructed by Dick Barrett

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ęCopyright 2000 - 2002 Dick Barrett

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