1 Stability of the Satellite Oscillator 
This was initially expected to be one of the difficult 
problems. The fact is that while the Doppler shift does determine 
the relative geometry of the satellite orbit and receiving station 
it requires a very accurate measure of the Doppler shift to obtain 
a good measure of this relative geometry. Although there is no 
simple relationship between an error in Doppler measurement and 
the corresponding position error, a crude rule-of-thumb relation- 
Ship, usually valid within an order of magnitude, is given by the 
statement that an error of 1 cps in Doppler gives an error of 1 mile 
in position if the transmitter frequency is 100 mcps. Since, in the 
TRANSIT system, any variation in transmitted frequency during the 
time of a pass will be erroneously ascribed to the Doppler effect, 
it is clear that a frequency drift of only 1 part in 10° during the 
time of a pass can result in a serious error in position determina- 
tion. It is true that many oscillators with stability better by 
orders of magnitude than 1 part in Lor had been built before the 
start of the TRANSIT program, but these were generally rather elab- 
orate devices including proportional heating ovens to control the 
crystal frequency and were not Suitable for use in small and 
Simple satellites. Actually, as can be seen from Fig. 1, the 
oscillator stability has not proved a very difficult problem. A 
rather elaborate temperature isolation of the crystal from external 
heat sources, taking advantage of the superb vacuum available in 
the operating environment, has kept the rate of change of frequency 
due to temperature change at the crystal within acceptable limits, 
and careful circuit design has minimized other sources of frequency 
change. Other techniques used include long burn-in of all crystals 
and selection of crystals on the basis of stability after this burn- 
in period. While techniques for still further improvement are under 
investigation, the presently achieved stabilities are good enough to 
meet the program objectives. 
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