GPS is used mostly on land, sea and airborne navigation, also in surveying, geophysical exploration, mapping and geodesy, vehicle location system, farming and transportation systems. Also for telecommunication infrastructure applications include network timing and enhanced 999 for mobile users. It is also important to global delivery of precise and common time to fixe and mobile users.
In the user segment we find GPS user equipment that are portable and fixed and are used by military and civilian for navigation, surveying and GIS. In the control segment are also two uses. One is ground facilities which are responsible for: 1) satellite tracking, 2) telemetry, 3) orbit and ephemeris computations, 4) up-linking of the computed data, 5) supervising the daily management of the space segment, 6) five ground control stations (Monitor Stations), 7) one Master Control Station. The second use is the Master Control Station that: 1) receive tracking data from the monitor stations, 2) calculates satellites ephemeris, 3) adjust satellite clocks, 4) maneuvers satellites if needed, 4) encrypts signals and 5) maintain GPS reference system (WGS84). The space segment likewise has also different uses like 1) constellation of 24 satellites, 2) in six orbital planes around the equator (60 degrees apart), 3) four satellite per orbit and 4) orbital planes inclined 55 degrees from the equator.
Characteristics of GPS Satellite: a) seven satellites are typically visible 10 degrees or more above the horizon, b) each satellite is about 2 to 3k lbs, c) satellites orbit the earth every 12 hours, d) time can be figured to within 100 nano secs.
There are several basic concept of GPS Satellites: 1) satellites are reference points to locations on earth (their locations are known), 2) a location of a point on earth is identified by "triangulation", 3) signals from the three satellites are used, 4) travel time of each signal is determined, 5) signals travel at the speed of light (distance = travel time x speed of light.
The triangulation equation has 3 variables and one is, where exactly are the satellites From orbital mechanics, the location of satellites are determined, an almanac of orbital information for all satellites are stored in each satellite and a ground control-stations continuously update location information of each satellite and transmit it to them (i.e. ephemeris). The second variable is: how long does it take for the radio signal to travel the distance. Under this equation are the functions of satellite like maintain an accurate time using on board atomic clocks, another is receive and store data transmitted by the control stations such as constellation almanac and individual ephimeris and still another is to transmit signal containing time and orbital information to the user receiver. The third variable is how far is the point from the satellite.
Pointers for survey:
Identify points to be surveyed, for example, stations. First group should contain a control station and each group should include at least one station from another group - pivoting station. All stations in a group should be observed during the same session