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How does NASA know where a spacecraft is in
deep space? The spacecraft's precise
range, velocity and angular position are determined with the aid of highly
stable frequency standards. The range
is determined from the propagation time of microwave radiation between an
antenna on Earth and the spacecraft.
The velocity is determined from the "doppler," i.e., by
comparing the phase of the incoming carrier signal with that of a reference
signal generated from the ground station frequency standard. The angular position is determined by very
long baseline interferometry (VLBI) in which widely separated stations (in
California, Spain and Australia) simultaneously receive signals from the
spacecraft. Differences between times
of arrival coupled with knowledge of the baseline vectors joining the station
antennas provide direct geometric determination of the angles between the
baseline vectors and the direction to the spacecraft. Hydrogen masers (see chapter 6) provide the
best stability (~10-15) for the propagation times of interest,
which typically range from minutes to hours.
VLBI is also used for high resolution angular measurements in
radioastronomy.
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J. S. Border &
E. R. Kursinski, "Deep Space Tracking and Frequency Standards,"
Proc. 45th Ann. Symp. on Frequency Control,
pp. 594-607, 1991, IEEE Cat. No. 91CH2965-2.
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R. F. C. Vessot,
"Applications of Highly Stable Oscillators to Scientific
Measurements," Proc. of the IEEE, pp. 1040-1053, 1991.
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W. K. Klemperer,
“Long-Baseline Radio Interferometry with Independent Frequency Standards,”
Proc. of the IEEE, vol. 60, pp. 602-609, 1972.
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