GPS Block II satellite		GPS is a satellite based system för navigation and positioning that is built by the US Defence that also manages the system. The project started in 1973. IOC (Initial Operational Capability) was reached on 8 December 1993, i.e. the system was fully extended for all civilian applications. FOC (Full Operational Capability) was reached on 17 July 1995 , i.e. the system was fully extended for all military applications. GPS is currently used by more civilian users than military users.
	The GPS system is "free" to use, i.e. the users are not charged. GPS is a passive system with high accuracy and its design means that there are always at least 4 satellites available (elevation over 5 degrees) all around the Earth during 99.9% of time. 24 satellites are guaranteed by the US Defence and the specified lifetime of them is specified to 7.5-10 years. Most of the time there are more satellites, for example 29 satellites in April 2001. The system has coverage all over the world, round the clock, independent of the weather, in real-time. On May 1, the White House announced a decision to discontinue the intentional degradation of the GPS signals (Selective Availability) to the public beginning at midnight. Civilian users of GPS will be able to pinpoint locations up to ten times more accurately. The inclination, which concisely could be described as the latitude at which the satellites turn, is 55 degrees for GPS. This means that the satellites turn at the latitude of the southern tip of Bornholm. The orbital period of 11 hours 57.97 minutes means that the same satellite constellation, seen from the Earth, appears about 4 minutes earlier each day. The satellites are continuously controlled and monitored by the control segment. The task of the control segment is to monitor the satellite system, determine the orbit and clock parameters of the satellites, upload data to the satellites and, if required, move satellites. At the six monitor stations, mainly placed along the Equator, signals from the satellites above the horizon at each station respectively, are continuously registered. Data from the tracking stations are sent to the control centre in Colorado Springs, where the orbit parameters of the satellites and corrections for the satellite clocks are being calculated and predicted. The calculated data will then be uploaded to the satellites from one of the four supply stations. From the user's point of view GPS is a one-way system; the satellites send and the users receive. Between the control segment and the satellites there is a two-way communication. GPS uses the reference system WGS84 and the ellipsoid, with the same name, which belongs to it. The WGS84 ellipsoid is almost identical to the GRS80 ellipsoid.