Antti Lehtinen Doppler Positioning with GPS - Matematiikan laitos

the receiver needed to collect measurement data over several minutes. The actual
computation of the receiver position was based on the Doppler curve of a satellite
pass. Doppler curves of satellites are flatter than in Figure 2.2. This is because
they travel alongcircular orbits rather than straight lines. The position of the
receiver was determined by curve fitting. The latitude and longitude estimates
were optimised such that the computed Doppler curve fitted to the measured
data. The computational details can be found in [Laurila, p. 474]. The accuracy
of the Transit system was about half a kilometre.
In order to compute the Doppler curves, one also needs to know the position
and velocity vectors of the satellite. Thus, the Transit satellites transmitted
binary data by phase modulation. This data is called ephemeris. One was able
to compute the satellite position and velocity from the ephemeris. The Transit
satellites transmitted continuous wave signals on two distinct frequencies, namely
150 MHz and 400 MHz. The purpose of this was to eliminate the first order
ionospheric refraction and receiver frequency measurement errors. The accuracy
of the Transit system was about 100 meters for a stationary receiver, but was
severely worse for a receiver in unknown motion [Laurila, p. 476]. According
to [Parkinson & Spilker, p. 4], the Transit system is still operational, but new
satellites are not beinglaunched.
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