Satellite positioning systems (GNSS)
Global Navigation Satellite Systems (GNSS) is the generic term for space-based systems that transmit signals that can be used to provide three services: Position, Navigation, and Timing - known collectively as PNT. The best known and most popular of the GNSS is the US Global Positioning System (GPS), although the Russian GLONASS system is gaining in strength and other systems are being developed, most notably Galileo in Europe and Compass in China.
The systems all work in approximately the same way, so just GPS is described here for simplicity.
GPS can be split up into three areas; the ground, space and user segments.
The ground, or control, segment is used to upload data to the satellites, from ground based stations around the world to synchronize time across the constellation and to track the satellites to enable orbit and clock determination.
The space segment consists of the GPS satellites in six orbital planes, orbiting at around 20,000km above the surface of the earth, (medium-earth orbit). 24 satellites make a full constellation, although there are currently (January 2011) 32 in service, 2 of which have been declared unusable until further notice. The satellite's code is used to identify it in orbit (it should be noted that this is the fundamental difference between GPS and GLONASS which differentiates satellites by frequency channel).
A navigation message is transmitted from the satellite to the user and gives the satellite identifier together with information on satellite health, predicted range accuracy, ionospheric and clock correction coefficients as well as orbital ephemeris to allow the receiver to calculate the satellite position. The message also contains an almanac which gives status, location and identifier information for all satellites in the constellation.
The user segment consists of the receivers and associated antennas, used to receive and decode the signal to provide PNT information. GPS is a ranging system with three available carrier frequencies, all multiples of a fundamental frequency (Table 1).
Table 1: GNSS RF Carrier Frequencies
|L1 1575.42 MHz||L1 1602.00 MHz||E1 1575.42 MHz|
|L2 1227.6 MHz||L2 1246.00 MHz||E5A 1176.45 MHz|
|L5 1176.45 MHz||E5 1191.795 MHz|
|E5B 1207.14 MHz|
|E6 1278.75 MHz|
Each satellite is about the weight of a small car (around 1000 Kg) and say nearly 20m in length with it's solar panels extended. Each has a rubidium (atomic) clock accurate to 5 parts in 1011 controlled by more accurate ground based Cesium clocks. You would have to watch a Cesium clock for 100,000 years to see it gain or lose a second. By comparison, a quartz watch loses a second every 2 days.
The distance or range is derived primarily through measuring the time difference between the transmission from the satellite and reception at the receiver of a coded signal. This range is more properly known as the pseudorange since it is affected by a number of system unknowns including clock biases and propagation delays which must be solved for or estimated. The carrier phase of the signals can also be used to derive the range, providing for a more accurate position fix, but with inherent ambiguity. Ranges to at least four satellites are required to determine position and time (Timing applications can function with a single satellite in view, although for verification reasons, two are preferred).