GPS
The GPS (Global Positioning System ) navigation system employs a constellation of 24 satellites and ground support facilities to provide the three dimensional position, velocity and timing information to all the users worldwide 24 hours a day.
The GPS comprises of three segments, namely the space segment, control segment and user segment. All the three segments work in an integrated manner to ensure proper functioning of the system.
Space Segment
The space segment comprises of a 28 satellite constellation out of which 24 satellites are active satellites and the remaining four satellites are used as in-orbit spares. The satellites are placed in six orbital planes, with four satellites in each plane. The satellites orbit in circular medium Earth orbits (MEO) at an altitude of 20200 km, inclined at 55◦ to the equator. The orbital period of each satellite is around 12 hours (11 hours, 58 mins). The MEO orbit was chosen as a compromise between the LEO and GEO orbits. If the satellites are placed in LEO orbits, then a large number of satellites would be needed to obtain adequate coverage. The satellites are powered by solar energy. They have back-up batteries on board
to keep them running in the event of a solar eclipse. The satellites are kept in the correct path with the help of small rocket boosters, a process known as ‘station keeping’.
Control Segment
The control segment of the GPS system comprises a worldwide network of five monitor stations, four ground antenna stations and a master control station. Each of the monitor stations is provided with high fidelity GPS receivers and a Cesium oscillator to continously track all GPS satellites in view.
User Segment
The user segment includes all military and civil GPS receivers intended to provide position, velocity and time information. These receivers are either hand-held receivers or installed on aircraft, ships, tanks, submarines, cars and trucks. The basic function of these receivers is to detect, decode and process the GPS satellite signals. Some of the receivers have maps of the area stored in their memory. This makes the whole GPS system more user-friendly as it helps the receiver to navigate its way out.
GPS receivers comprise three functional blocks:
1.Radio frequency front end. The front end comprises one or more antennas to receive the GPS signal, filters and amplifiers to discriminate the wanted signal from noise and a downconverter to remove the carrier signal. Simple receivers process one GPS signal at a time using multiplexing techniques. Sophisticated receivers comprise multiple channels for processing the signals from various satellites simultaneously.
2.Digital signal processing block. It correlates the signals from satellites with signals stored in the receiver to identify the specific GPS satellite and to calculate pseudoranges.
3.Computing unit. This unit determines position, velocity and other data. The display format is also handled by the computing unit.
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