Global Positioning
    System
 The System
 The Position Solution
 Hardware
 Ionospheric And
    Atmospheric Delays
 The Signals
 Dynamic Positioning
 Static Positioning
 Coordinate Systems
 Conclusions

The GPS network of satellites is owned and operated by the US Navy, and made available for civilian use in a less precise mode than that available for the military. When fully configured, the constellation will consist of approximately 24 satellites, some 18 of which should be operational at any time (but this is still under debate in the US). They are in a geocentric orbit around the planet at an altitude of 26,000km+.The orbit path is elliptical, with 4 satellites in each of six orbital planes and a period of orbit of 12 hours. The height of the orbit means that they are not affected to a great extent by the Earth's gravitational field. The gravitational effect of the sun and the moon on the GPS orbits are modelled as systematic errors. The satellites transmit positioning codes which are then received by GPS signal receivers.

The secret to the operation of the GPS network is the use of atomic clocks to measure time. The two time systems of the vehicles generally use caesium clocks accurate to around 1 part in 10^-12 per day, and are based on the international standard for time (being the duration of 9,192,631,770 periods of radiation corresponding to the transition between two energy levels of Caesium 133). The times of these clocks are monitored by ground stations and corrected as necessary. They are kept within 1msec of the Universal Coordinated Time (UTC). The time base of the GPS is so well monitored and accurate that it has become the standard for the transfer of time. The time is even corrected for relativity effects caused by the differing gravity in the orbit and the eccentricity of the orbit. The receivers also contain accurate clocks, luckily not the expensive atomic clocks used by the satellites. However we need to compensate for the errors that accumulate in clocks used by GPS receivers.

It is possible to determine relative positions on the surface of the earth to within centimetres from the use of GPS. The determination of absolute datum used to express location and including the US Navy's intentional corruption of the satellite ephemeris (known as selective availability). It can be around 20-30m on a good day, but up to 150+ whenever the US Navy decides.

The reference books on GPS surveying mention three concepts of the ground segment, the space segment and the user segment. It is within the ground segment that the time corrections are generated and broadcast to the satellites. The main ground control station is located at Colorado Springs in the USA, and there are three transmission stations that track the satellites and relay data to the ground control. The position of the satellites (the ephemeris, same term as used for position of the sun and other stars), the health of the satellites, the clock errors and the ionospheric corrections are determined at the control station and broadcast to the satellites. This continual interface between the ground segment and the space segment (the satellites, obviously) contributes greatly to the success of the system for accurate navigation and positioning. We the users are the user segment, which is where the processing occurs, where the dynamic positioning takes place, and where the ingenuity of civilian minds defeats the US Navy's attempts to degrade our precision.