GPS NAVSTAR (Navigation System with Timing And Ranging), known as GPS (Global Positioning System), launched by U.S. Department of Defense for military use, with eleven satellites in orbit (1978 – 1985). The system consists of a constellation of 24 satellites. Nowadays, about 30 active satellites orbit the earth in a distance of 20200 km. Each satellite is equipped with an atomic clock so accurate that it keeps time to within three nanoseconds that’s 0.000000003sec, or three-billionths, of a seconds. Calculate locations on Earth by determining distances from at least three GPS satellites.

When a civilian airplane of the Korean Airline (Flight 007) was shot down by Soviet fighters after it had gone lost over Soviet territory. GPS moved from pure military effort to public. It was decided to allow the civilian use of the GPS system.

Selective Availability (SA) Temporal deactivation of SA (Selective Availability) during the Gulf war. In this period civil receivers were used as not enough military receivers were available. On 1st July 1991 SA was activated again. 1995, 17th July:Full constellation of 24 GPS satellites in orbit. Full Operational Capability (FOC) is announced. 2000, 2nd May: SA phased out as a result of the 1996 executive order by U.S. President Clinton. Civilians could now achieve 10 – 15 meters accuracy, compared to the previously available 100 meters accuracy.

GPS Positioning Services Standard Positioning Service (SPS) Civil users worldwide use the SPS without charge or restrictions. Most receivers are capable of receiving and using the SPS signal. The SPS accuracy is intentionally degraded by the DOD by the use of Selective Availability. 100 meter horizontal accuracy 156 meter vertical accuracy 340 nanoseconds time accuracy

GPS Positioning Services Precise Positioning Service (PPS) Authorized users with specially equipped receivers use the Precise Positioning System. U.S. and Allied military, certain U. S. Government agencies, and selected civil users specifically approved by the U. S. Government, can use the PPS. PPS Predictable Accuracy 22 meter Horizontal accuracy 27.7 meter vertical accuracy 200 nanosecond time (UTC) accuracy

Selective Availability (SA) Multipath Atmospheric Effects GPS Error Sources Selective Availability (SA) Multipath Atmospheric Effects Weather Conditions Slope of Area Satellite Geometry Clock Inaccuracies Rounding Errors

Ionospheric effects ± 5 meters Shifts in the satellite orbits ± 2.5 meter Clock errors of the satellites' clocks ± 2 meter Multipath effect ± 1 meter Tropospheric effects ± 0.5 meter Calculation- und rounding errors

Overview of Typical Accuracies Accuracy of GPS system with SA activated ± 100 Meter Typical accuracy with SA deactivated ± 15 Meter Typical accuracy of differential GPS (DGPS) ± 3 - 5 Meter Typical accuracy with WAAS ± 1 - 3 Meter