Global Positioning System

Techniques to improve GPS accuracy

The accuracy of GPS can be improved in a number of ways:

• Using a network of fixed ground based reference stations. These stations broadcast the difference between the measured satellite pseudoranges and actual (internally computed) pseudoranges, and receiver stations may correct their pseudoranges by the same amount. This method is called Differential GPS or DGPS. DGPS was especially useful when GPS was still degraded (via the Selective Availability described below), since DGPS could nevertheless provide 5-10 meter accuracy. The DGPS network has been mainly developed by the Finnish and Swedish maritime administrations in order to improve safety in the archipelago between the two countries.
• Exploitation of DGPS for Guidance Enhancement (EDGE) is an effort to integrate DGPS into precision guided munitions such as the Joint Direct Attack Munition (JDAM).
• The Wide-Area Augmentation System (WAAS). This uses a series of ground reference stations to calculate GPS correction messages, which are uploaded to a series of additional satellites in geosynchronous orbit for transmission to GPS receivers, including information on ionospheric delays, individual satellite clock drift, and suchlike. Although only a few WAAS satellites are currently available as of 2004, it is hoped that eventually WAAS will provide sufficient reliability and accuracy that it can be used for critical applications such as GPS-based instrument approaches in aviation (landing an airplane in conditions of little or no visibility). The current WAAS system only works for North America (where the reference stations are located), and due to the satellite location the system is only generally usable in the eastern and western coastal regions. However, variants of the WAAS system are being developed in Europe (EGNOS, the Euro Geostationary Navigation Overlay Service), and Japan (MSAS, the Multi-Functional Satellite Augmentation System), which are virtually identical to WAAS.
• A Local-Area Augmentation System (LAAS). This is similar to WAAS, in that similar correction data are used. But in this case, the correction data are transmitted from a local source, typically at an airport or another location where accurate positioning is needed. These correction data are typically useful for only about a thirty to fifty kilometer radius around the transmitter.
• Wide Area GPS Enhancement (WAGE) is an attempt to improve GPS accuracy by providing more accurate satellite clock and ephemeris (orbital) data to specially-equipped receivers.
• Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning sytem. In this approach, accurate determinination of range signal can be resolved to an accuracy of less than 10 centimeters. This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests - possibly with processing in real-time (real-time kinematic positioning, RTK).

Continue to GPS applications.

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