A Real-Time GPS-Based Differential Positioning System

Hydrographers, surveyors, and geophysicists, are among the many groups who would benefit from precise position information either in real-time or with very little delay. The most promising technology for meeting this need is the Global Positioning System (GPS). Used in the differential mode with a communication link between a base station and a user, GPS should be capable of providing real-time relative positions over distances of hundreds of kilometres with accuracies approaching tens of centimetres. We have begun the development of a prototype of such a system at the University of New Brunswick. The system consists of two Ashtech XII Model L receivers with two Macintosh microcomputers handling receiver communications and decoding, data processing, and data transmission tasks. The system is capable of different levels of positioning accuracy depending on the application. In the differential pseudorange correction mode, the transmitting station will broadcast pseudorange corrections which can be applied at remote sites to obtain real-time position accuracies at the 5 metre level. For high precision kinematic positioning and site stability monitoring, the transmitting station will broadcast pseudorange and carrier phase observations. The format of these broadcast messages conforms to those proposed by Radio Technical Commission for Marine Services Special Committee 104 (RTCM SC-104) although a new message type has been added for high precision applications. System design and processing methodology will be discussed and test results presented.