Page 170 - Intro to Space Sciences Spacecraft Applications

Satellite Navigation 157

Figure 7-1. Doppler positioning. Successive position iterations may be
achieved from a single pass of a single Transit satellite.

slant range change precisely and to know the change in receiver position
over the observation time. Multiple fixes can be combined to provide posi-
tion, track, and speed for subsequent iterations resulting in increased accu-
racy. If a single satellite is used, a dead reckoning (DR) plot is used
between times the satellite is below the horizon to give an estimated posi-
tion, track, and speed for the next satellite encounter.

Errors and Accuracies

The theoretical accuracy of the Doppler ranging method is associated
with the wavelength of the frequency used for the navigation signal. For
instance, a 400 MHz signal (currently used by Transit) corresponds to a
0.75 meter wavelength and a similar theoretical accuracy (for a single
fix). Unfortunately, many sources of errors exist which affect the accura-
cy of satellite navigation systems.

Refraction Errors. There are two sources of refraction errors as the satel-
lite signal propagates between the satellite and the receiver. The first is
introduced by the ionosphere which changes the path of propagation of
the signal as described in Chapter 5. This change introduces a shift in the
frequency of the signal which would affect the determination of the slant
range change between the satellite and receiver. In order to compensate
for this shift, navigation satellites transmit over two different frequencies
and compare the Doppler measurements made at each to reduce the error
introduced by the ionosphere.