WIDE AREA DIFFERENTIAL GPS FIELD-STUDY

Inmarsat commissioned the field study to be carried out by Signal Comp uting Ltd. in association with the Royal Greenwich Observatory. The st udy aimed to identify the commercial suitability of Wide Area Differen tial GPS (WADGPS) corrections. The corrections provided to users will be relayed via Inmarsat-3 geostationary satellites and are required to be valid over the footprint of an entire Inmarsat ocean region (appro ximately one third of the Earth's surface). The study has been conduct ed within the Inmarsat Atlantic Ocean Region East. Trials took place o ver a five-month period to achieve a representative set of data. Six w idely-distributed collection sites were used to ensure that the maximu m duration of satellite data were received and that a representative m odel for atmospheric effects was obtained. Analysis is restricted to t he use Of GPS carrier and phase data obtained using the C/A code trans mitted by Block II satellites only. This paper presents the analysis o f data collected during a four-day trial period. The ability to determ ine the Gps satellite ephemeris (independent of GPS broadcast ephemeri s), Klobuchar model parameters tailored to the oceanic region, and sat ellite clock corrections using Gps data obtained from this limited dep loyment of collection sites is addressed. The primary degradation effe cts on stand-alone Gps positioning of Selective Availability (SA) are corrected using WADGPS, resulting in a significant improvement in posi tion accuracy.