AIDJEX Bulletin #40 - Polar Science Center - University of Washington

The translocation principle removes most of the errors external to the
receivers. Still remaining are instrumentation errors at each receiver due
to oscillator and receiver timing errors. For most of the main experiment,
the crystal oscillators used in our systems introduced no detectable errors
in our position measurements. However, because of inadequate oscillators,
translocation calibration tests before the main experiment produced few useful
data. During the first month, temperature fluctuations of a few tenths of a
degree Fahrenheit in 10 minutes at the oscillators produced frequency changes
of 1 part in lo1 O .
Such a frequency drift will cause fix errors of 5-10 m
[Denzler, 19701. Oscillators less sensitive to temperature were used after
the first month.
NavSat receivers used for navigation generally employ the simplification
of measuring Doppler counts with respect to time marks transmitted by the
satellite. Errors in decoding these time marks introduce 5-10 m position
errors. Receivers intended for more precise applications eliminate this
source of error by adding a stable internal time base to control or correct
Doppler counts. We used such a local clock, derived from the crystal oscillator,
to measure the errors in decoding satellite time marks and to correct
Doppler counts using the algorithm in Appendix A.
RECEIVER CONTROL
There were six NavSat satellites during the experiment, each completing
one orbit every 108 minutes and staying within range of high-latitude receivers
for about 18 minutes on each orbit (Fig. 4). Therefore, it commonly occurred
that more than one satellite was within range at the same time. Because our
objectives included the collection of data from as many passes as possible
while tracking the same passes at different camps, the standard practice of
tracking the first satellite within range for as long as possible was not
efficient enough for us. The data collection had to be more selective and
had to employ identical techniques at all camps.
The objective of our receiver control procedure was to obtain data from
the entire critical interval of as many passes as possible. The procedure
involved logical choices based on predictions of Doppler frequencies as a
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