The Global Positioning System: A Shared National Asset (1995)
Chapter: Racal Survey
The International GPS Service for Geodynamics
The International GPS Service for Geodynamics (IGS) is a network of more than 50 globally distributed GPS tracking sites that has been established by NASA and other organizations from various nations in order to support geodetic and geophysical research activities. 59 Rather than provide real-time differential corrections to users, the tracking sites are used to produce post-processed GPS orbits, or ephemerides, with an accuracy of 10 to 30 centimeters. Orbits are processed at the IGS central bureau at NASA's Jet Propulsion Laboratory in Pasadena, California, and at other sites within the United States and around the globe. These orbits are typically available on the Internet within a few days after they have been processed. 60
Private Sector DGPS Services
There are a number of private sector enterprises that now offer differential GPS services to the public at various levels of accuracy and at a wide range of prices. These systems use both space-based and land-based datalinks that are encrypted to provide access to only paying customers. Brief summaries of four of these services are provided below.61
Racal Survey
Racal Survey of Surrey England (U.K.) has developed a worldwide, space-based differential GPS service known as SkyFix for use in a number of surveying applications. The ground segment of the SkyFix system currently consists of over 25 reference stations around the globe that determine differential corrections that are sent to users via geostationary satellite. The four satellites currently in use are owned and operated by Inmarsat, and provide worldwide coverage except for the polar regions. Users access the differential corrections broadcast in L-band (1530-1545 MHz) using either an Inmarsat terminal or a specialized SkyFix terminal. Racal Survey advertises a positioning accuracy of 3 to 5 meters using this system.
