The U.S. government is committed to provide GPS to civilian community with performance levels specified in the GPS Standard Positioning Service Performance Standard.
The Global Positioning System consists of space based positioning, navigation and timing available for free use to any peaceful civil, company or scientist worldwide.
The U.S. is committed to meet and exceed the minimum level of service performance specified in SPS PS.
GPS operational capabilities began in 1993, and since then the GPS performance has continuously met and exceeded with the current performance level (2007) of horizontal accuracy of 3 meters and better, and horizontal accuracy of 5 meters or better in 95% of time.
The Navstar Global Positioning System or as in SPS performance standard document referred as GPS is space radio navigation system owned by the United States Government and operated by the United States Air Force offers positioning, navigation and timing services worldwide since it’s launch in 1978.
Any civil or military user with GPS receiver can determine accurate time and location in any weather, day or night, anywhere in the world. GPS became global utility whose multi-use services are integral to U.S. and the global security, economic growth, transportation, and worldwide economic infrastructure.
The GPS accuracy users get can depend on many factors , like atmospheric effects, sky blockage and the quality and type of receiver used to receive data from the satellites. The FFA data shows that with high quality receivers you can get better accuracy than 3.5 meter horizontal.
The accuracy for the civilian GPS (SPS) and military GPS (PPS) is same except that the military GPS works on two frequency that enables performing ionospheric correction technique that reduces radio degradation caused by earth atmosphere.
You can get even more accurate measurements of GPS by using GPS in combination with augmentation systems that enable real time positioning to within few centimeters, and post mission measurement at millimeter level.
Augmentation system is any system that aids GPS by providing accuracy, integrity, availability, and timing. There are five declared augmentation systems :
- Nationwide differential GPS system (NDGPS) – Is ground – based augmentation system that provides increased accuracy to all users in U.S.
- Wide Area Augmentation System (WAAS) – Is satellite – based augmentation system operated by Federal Aviation Administration (FFA) and supports aircraft navigation across north America
- Continuously Operating Reference Stations – Is network of stations managed by National Oceanic and Atmospheric Administration that archives and distributes GPS data for precise positioning.
- Global Differential GPS – Is high accuracy augmentation system developed by NASA for aiding GPS positioning, timing and determination requirements of NASA science missions.
- International GNSS Service – Is network of 350 GPS monitoring stations from 200 contributing organizations in 80 countries to provide quality data for global navigation in support of earth science research, education and other benefit to the society.
GPS accuracy vary, depending on the location, signal strength from satellites in orbit, other available networks and GPS aid technologies, atmospheric effects, sky blockage, type of GPS system and the quality of the receiver.