Global Positioning System

Dominant electronic position and time reference system

Uses multiple satellites
Small antenna

Resolution options

Regular: SeS

Differential GPS (DGPS)

Differential GPS adds precision to basic GPS by complementing the satellite-generated data with data on the position of known ground stations. Receivers then correct their satellite-derived information with the differences transmitted by the ground stations,uses a network of fixed ground based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions. 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.

The term can refer both to the generalized technique, as well as specific implementations using it. It is often used to refer specifically to systems that re-broadcast the corrections from ground-based transmitters of shorter range. For instance, the United States Coast Guard runs one such system in the US and Canada on the longwave radio frequencies between 285 kHz and 325 kHz. These frequencies are commonly used for marine radio, and are broadcast near major waterways and harbors. Australia runs a similar service for land and air navigation, broadcasting their signal on commercial AM radio stations.

The European Geostationary Navigation Overlay Service, Japan's Multi-Functional Satellite Augmentation System, Canada's CDGPS and the commercial VERIPOS, StarFire and OmniSTAR systems are all Satellite Based Augmentation Systems

These signals are broadcast on marine longwave frequencies, which could be received on existing radiotelephones and fed into suitably equipped GPS receivers.

maritime service provides 10 meter (2 dRMS) navigation accuracy, integrity alarms for GPS and DGPS out-of-tolerance conditions within 10 seconds of detection, availability of 99.7% per month, coastal coverage to the continental United States, the Great Lakes, Puerto Rico/US Virgin Islands, and selected portions of Alaska and Hawaii.

Antenna Requirements