TABLE OF CONTENTS Current generation navigation systems (GPS and GLONASS) and those under development (e.g., GALILEO) determine the user terminal position through the time of arrival (TOA) ranging.
In general, this kind of ranging technique is based on the measurement of the time interval employed by a signal transmitted by an emitter (e.g., satellite, radio beacon) at a known location to arrive at the user receiver.
The TOA is defined by (2.1):
which is measured by the user receiver.
If the receiver knows the speed of the signal, it is able to determine the distance from the emitter simply by multiplying the TOA with the signal speed value. In the case of satellite navigation, electromagnetic signals, propagating at the speed of light (approximately 3 10 8m/s), are used; therefore, the fundamental equation of satellite navigation is [1]:
From (2.2) it is clear that the calculation of the true (i.e., geometric) distance between the satellite and the receiver can be obtained only through the measurement of the true TOA, which implies, as highlighted by (2.1), that the receiver has a precise knowledge of the time instant of arrival and the time instant of transmission of the satellite signal. The former can be achieved through direct reading of the receiver clock, whereas the latter is embedded in the signal (see Chapter 3 for more details), which is nominated navigation signal. To achieve the true difference between these time instants, the satellite and receiver clocks have to be synchronized to the...
