Satellite Navigation

A variety of navigation systems have developed and evolved over the years. Their applicability depends on the desired accuracy, environment, cost, and to some extent historic association. Table 10.1 compares the accuracy of various types of navigation systems (source: Schanzer, 1995).

The Transit system, which uses the Doppler signature technique, comprises a constellation in a 1,100 km circular polar orbit. Each satellite transmits departure of its nominal frequency and orbital location continuously at about 150 and 400 MHz. The signals are used to estimate the Doppler shift and rate of Doppler change by comparing the measured and expected frequency. The information can be processed to estimate an observer's position. The system provides an accuracy of about 160 m, which is adequate for ship navigation but not for aircraft naviga. tion, due to uncertainties in position fixes introduced by Doppler components caused by aircraft motion. For example, a Doppler uncertainty of 1 m/s introduces an error of 1 km in a location fix. Moreover, the system is not available continuously users having to wait up to 2 hours for a satellite pass. Cicada, operated by the former Soviet Union, was based on the same principle.

These limitations were removed in the Navstar GPS and GLONASS navigation systems (see the next section).

The French Argos system is a single satellite active navigation system. It uses navigation transponders on NOAA satellites which operate in 805 km polar orbit. When requiring a fix, users transmit a signal which is relayed by the satellite to hub stations where the Doppler curve is extracted from the user signal. intersection of the Doppler 'sphere' with the Earth provides the location of the user. East West ambiguity is resolved by taking Earth rotation into consideration.