1. 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 km. Each satellite is equipped with an atomic clock so accurate that it keeps time to within three nanoseconds that’s sec, or three-billionths, of a seconds.
Calculate locations on Earth by determining distances from at least three GPS satellites.

2. 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.

3. 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.

4. 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

5. 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

6. 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

7. 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

8. 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)
± Meter
Typical accuracy with WAAS
± Meter