12 Global Navigation Systems

Update

From a practical point of view, data from additional satellites can provide improved navigation performance. Also, political considerations have led to development and deployment of satellite navigation systems that are alternatives to GPS. When such systems are made interoperable with GPS, “GNSS” results (the Global Navigational Satellite System). Here we discuss briefly how relativistic effects are incorporated into these new systems.

GLONASS is a Russian system that is very similar to GPS. The satellites are at slightly lower altitudes, and orbit the earth 17 times while the GPS satellites orbit 16 times. Figure 2View Image shows that the factory frequency clock offset is slightly less than that for GPS. Although a full constellation of 24 satellites was originally envisioned, for many years no more than a dozen or so healthy satellites have been available.

GALILEO is a project of the European Space Agency, intended to put about 30 satellites carrying atomic clocks in orbit. In contrast to GPS which is free to users, the GALILEO system ultimately will be funded by user fees. Information released in 2006 by the GALILEO project [25] states that relativistic corrections will be the responsibility of the users (that is, the receivers). This means that GNSS devices capable of receiving both GPS and GALILEO signals will have to contain additional relativity software to process GALILEO signals. Since no “factory frequency offset” is applied to atomic clocks in the GALILEO satellites, relativity effects will cause satellite clock time to ramp away from TAI and will require large correction terms to be transmitted to users.

BEIDOU is a satellite navigation system being developed and deployed by the People’s Republic of China. In its early stages, there were three satellites capable of transponding timing signals between a master control station and receivers on the ground. Timed pulses are sent from the control station, to the satellites, and then to ground-based receivers, which sends them back through the satellites to the control station. With the timing information, and topographic information (the receivers have to be on earth’s surface), the receiver position can be computed and relayed back to the receiver. Since receivers must also transmit, they are bulky. The principal relativistic correction involved here is the Sagnac effect, which can amount to several hundred nanoseconds.

BEIDOU is intended to develop into a global satellite navigation system that is independent, yet interoperable with GALILEO. Very little information is currently available about the structure of this system.


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