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Gee AMES Type 7000

then, 1000 uSec later, by a pulse from C. After a further 1000 uSec the sequence repeats thus taking 4000 uSec in all corresponding to 250 complete cycles per second. The sequence is shown in Fig. 17.5. The basic pulse recurrence frequency of the master station is 500 per second.

The essential equipment in the aircraft consists, in addition to a suitable receiver, of a cathode-ray tube with a two-sweep time-base and suitable calibrating arrangements to enable the time differences to be measured precisely. The two-sweep time-base is used so that the AB and AC pulse pairs may be separately displayed. In practice both sweeps are duplicated and arrangements are made to invert the pulses from the slave stations so that the appearance of the cathode-ray tube might be as shown in Fig. 17.6. It is not proposed to described the "drill" for lining up the pulses and measuring the time differences. For this, the equipment handbook must be consulted.

To enable the time differences to be measured with accuracy an oscillator is provided in the aircraft which produces calibration marks on the two time-bases. The oscillator has a frequency of 150 kc/s and provides 1 5 kc/s calibration marks on the normal display and 150 kc /s calibration marks on the expanded display which is used when making accurate measurements.

It is clear that the precision of position finding will depend entirely on the accuracy of the 150 kc /s oscillator. It is not very practicable, and certainly would be very expensive to carry a high precision oscillator in every aircraft and an ingenious solution was found to the problem in the following manner. One high precision 150 kc/s oscillator is maintained at the Master (A) station. The pulse repetition frequency is obtained by subdivision of this 150 kc/s oscillator. In the aircraft an oscillator with fairly short period stability is provided which is capable of slight frequency adjustment. This oscillator, in addition to providing the calibration marks also, by subdivision, is made to control the time-bases of the display. If the time-bases are generated at exactly the pulse repetition frequency the pulses will appear stationary on the cathode-ray tube screen, but if the locally generated 150 kc/s oscillation is of incorrect frequency the pulses will be seen to drift either to right or left depending on whether the oscillator is fast or slow. If the operator adjusts the oscillator so that the pulses remain stationary it must mean that the frequency is exactly 150 kc /s and measurements made in the air will have the same accuracy as they would have had if a high grade precision oscillator had been carried. The time for ten cycles of this 150 kc/s oscillation, 66 2/3 uSsec was called a Gee unit and corresponded to a range difference of 12.4 miles.

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Updated 11/03/2002

Constructed by Dick Barrett

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ęCopyright 2000 - 2002 Dick Barrett

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