Skyfix - A Professional DGPS Service Using Geostationary Communications Satellite

The positioning accuracy performance potential following differential correction of the Global Positioning System’s W’S) standard positioning service (SPS), is by now well documented and understood. The study of trials reports and an ever increasing volume of actual operational data serves to confirm that differential GPS (DGPS) can match the exacting positioning requirements specified by the offshore suvey industry, not only now, but also for all of the foreseeable future. Already a wide variety of the industry’s tasks involve the use of DGPS together with terrestrial systems and it is obvious that the degree of dependence put upon the DGPS system will increase markedly as the satellite constellation nears completion. Racal Survey have provided DGPS services primarily to marine users on a commercial basis since 1986, using a variety of terrestrial systems. Their latest innovative DGPS service, SkyFix, which uses Inmarsat satellites for correction broadcast, commenced operation in August 1990 and now has reference station networks covering North West Europe, the Middle East and the Far East, and coverage is expanding. This paper describes the SkyFix system, highlighting the advantages of using a communications satellite together with networked reference stations and arguing the need for extensive system monitoring and quality control facilities to guarantee continuing overall integrity in a professional DGPS environment. The paper also concentrates on the practical system performance through an extensive discussion of trials and operational data, including the effects of data age and reference user separation, no longer constrained by terrestial datalink range.