Improved modelling of high precision wide area differential GPS
Wide Area Differential GPS (WADGPS) aims at overcoming the main drawback of conventional DGPS, namely the limited range over which the differential corrections are valid, due to the rapid decorrelation of the error sources with increasing distance from the reference station to the user. In the WADGPS approach, error sources are generated for users over a large portion of the earth, such as a communication satellite's entire footprint, by separating and modelling the main sources of error in GPS measurements. This has the effect of eliminating the limit imposed on the validity of the corrections by the geographical or atmospheric environment. A main advantage of WADGPS is the fact that far fewer reference stations are needed to cover the same area, compared to conventional DGPS. Interest in WADGPS has grown during the last few years, the use of WADGPS both for positioning accuracy requirements and the provision of integrity are being explored. Several research establishments have come up with their own WADGPS methodology and algorithms, all sharing a basic principle, namely the requirement that different error sources which affect GPS navigation are dealt with separately, and their spatial and temporal properties are investigated and modelled individually. Research was carried out to develop an advanced method and the corresponding algorithms, which could provide a high precision WADGPS positioning service. The service would be suitable for single or dual frequency users, and could be introduced with very few reference stations. The two main components of the service are a precise, near real-time orbit determination of the GPS satellites, and an accurate estimation and modelling of ionospheric and tropospheric effects. Results from real data indicate that user position accuracy of the order of 2 m and 3 m (RMS) in plan and height components respectively, were achieved over user-reference distances ranging from 2,000 to 3,500 km.