Comparative Study Between the Accuracy of Stand-Alone GPS, Maritime DGPS and ESTB

In the nineties, GPS receivers proliferated throughout the World, providing all-weather, continuous, global positioning to users with an accuracy of 100m or better. In order to improve the accuracy and the integrity of GPS, several differential techniques were developed. In fact, the transmission of differential corrections to the GPS signals allows elimination of most of the errors of GPS and improves significantly the integrity of the service. While GPS does not provide instantaneous integrity, the use of differential stations - which monitor permanently the signals of the visible satellites - enables the timely warning of any malfunction or failure in less than 10 seconds. Probably the most important means of delivering differential corrections to users have been through the old Non-Directional Beacons (technique generally referred to as maritime DGPS) and through geostationary satellites (technique generally referred to as space-based augmentation or wide area augmentation). During the last decade, maritime DGPS stations were installed by all European countries with Atlantic coastlines, as well as by Italy, Turkey, Canada, US, Bermuda, Panama, Brazil, Argentina, Australia, New Zealand, China (including Hong Kong), Taiwan, Japan, Korea, Singapore, Thailand, Malaysia, India, Bangladesh and some of the Gulf states. Portugal has recently joined to these services, having installed 2 maritime DGPS stations, which are located at Cape Carvoeiro and Sagres. In terms of satellite based augmentations, the most important systems are the Wide Area Augmentation System (WAAS) - developed at the United States; the European Geostationary Navigation Overlay Service (EGNOS) - developed in Europe; and the Multi-function Satellite-based Augmentation System (MSAS) - developed in Japan. Although EGNOS is not scheduled to be fully operational until 2004, the EGNOS System Test Bed (ESTB), a simplified version of EGNOS available within Europe, began transmitting signals, via an INMARSAT satellite, to potential users in February 2000. Differential systems provide important services to users, but their accuracy advantage, when compared to stand-alone GPS, has decreased significantly due to the discontinuation of Selective Availability (SA), in the 1st of May 2000, which led to an improvement in stand-alone GPS accuracy. Presently, the main benefit of augmentation services is the integrity, which gives users an extra-assurance that the position is correct. Nevertheless, the extra accuracy afforded by differential services (like maritime DGPS and EGNOS) is still extremely important in some maritime applications (such as dredging, hydrographic surveying, buoy positioning, safety of life applications and, particularly, some phases of pilotage waters navigation, including harbour manoeuvres), in some aeronautical applications (namely en-route navigation, landing and air traffic control) and also in land applications (namely positive train control and inland waterways navigation). Therefore, the authors present a comparative study between the accuracy of stand-alone GPS, maritime DGPS and ESTB. The authors evaluate the standard positioning accuracy and discuss the potential benefits gained from the two afore-mentioned augmentation services.

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