Global Positioning System (GPS)

The GPS is a system of positioning by satellite to give an accurate position anywhere on the planet within a hundred metres, by day or night. The GPS constellation consists of 24 satellites in 6 orbital planes with 4 satellites in each plane. The ascending nodes of the orbital planes are separated by 60 degrees and the planes are inclined 55 degrees. Each GPS satellite is in an approximately circular, semi-synchronous (20,200 km altitude) orbit.

The orbits of the GPS satellites are available by broadcast – superimposed on the GPS pseudorandom noise codes (PRN), or after post-processing to get precise ephemerides, they are available from organizations such as the Jet Propulsion Lab (JPL) or the International Geodetic Service (IGS) among others.

GPS provides highly accurate time, velocity and positional data as well as meeting the common radio positioning requirements of a broad spectrum of users. Depending on the mode of use and the equipment used, high precision measurements can be made in geodetic applications. It is therefore utilised in geodetic programmes to supplement and strengthen the databases which are used to build models of the Earth’s gravity fields, ocean tides, sea surface topography, orientation, global sea level and ocean circulation. It is especially suitable for high precision short baseline work. GPS is seen as the primary tool of geodesists due to the dramatic improvements in techniques and processing software, availability and economical access to GPS equipment, portability and the benefits of international collaboration.

GPS provides two levels of service, Standard Positioning Service and the Precise Positioning Service .

The Standard Positioning Service (SPS) is a positioning and timing service which will be available to all GPS users on a continuous, worldwide basis with no direct charge. SPS will be provided on the GPS L1 frequency which contains a coarse acquisition (C/A) code and a navigation data message. SPS provides a predictable positioning accuracy of 100 meters (95 percent) horizontally and 156 meters (95 percent) vertically and time transfer accuracy to UTC within 340 nanoseconds (95 percent).

The Precise Positioning Service (PPS) is a highly accurate military positioning, velocity and timing service which will be available on a continuous, worldwide basis to users authorized by the U.S. P(Y) code capable military user equipment provides a predictable positioning accuracy of at least 22 meters (95 percent) horizontally and 27.7 meters vertically and time transfer accuracy to UTC within 200 nanoseconds (95 percent). PPS will be the data transmitted on the GPS L1 and L2 frequencies. PPS was designed primarily for U.S. military use. It will be denied to unauthorized users by the use of cryptography. PPS will be made available to U.S. and military and U.S. Federal Government users. Limited, non-Federal Government, civil use of PPS, both domestic and foreign, will be considered upon request and authorized on a case-by-case basis, provided:

• It is in the U.S. national interest to do so.
• Specific GPS security requirements can be met by the applicant.
• A reasonable alternative to the use of PPS is not available.