Real-Time Differential GPS with 20 to 50 cm Accuracies

L. Harold Spradley

Abstract:	Since late 1989, GPS Technology has operated an integrated network of GPS base stations on a permanent basis, providing all-in-view tracking for use in geodetic surveys, for photogrammetry, and for construction of GIS databases. A number of these surveys involve use of moving GPS antennas, with accuracy requirements in the range of 10 to 30 centimeters over large geographical areas, such that the separation of GPS reference stations are typically 150 to 250 kilometers apart. In early 1991, tests were conducted wherein recorded raw measurements from base stations and mobile receivers were reprocessed in a mode of real-time simulation, with results which indicated that fractional-meter accuracies could be achieved in real time. Following these simulations, a series of trials were conducted in July and August, 1991, with actual real-time DGPS operations using a variety of communication links, station spacings, and time latencies of the corrections being applied. Pre-surveyed points, with coordinated developed by post-processing and NGS control stations were used as "truth values" for comparisons of accuracy of the real-time results. At the time of these July-August, 1991 trials, Selective Availability (S/A) was inactive, and predictably, the results were outstanding (CEPSO of 20 to 30 cm). After the re-establishment of S/A in November, 1991, real-time accuracies declined somewhat, depending upon the data link, latency of corrections, and methods for look- ahead prediction of range dither. For most cases and GDOP tested, real-time accuracy results can still be maintained in the 40 to 60 cm range with off-the-shelf communication links and 6 to 8 second update intervals. The paper details a number of data examples, and explores the matrix of accuracy vs. data message bandwidth, DOP, update frequency, and station separation.
Published in:	Proceedings of the 1993 National Technical Meeting of The Institute of Navigation January 20 - 22, 1993 Parc 55 Hotel San Francisco, CA
Pages:	409 - 414
Cite this article:	Spradley, L. Harold, "Real-Time Differential GPS with 20 to 50 cm Accuracies," Proceedings of the 1993 National Technical Meeting of The Institute of Navigation, San Francisco, CA, January 1993, pp. 409-414.
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