The Evaluation of the Accuracy and Measurement of Latency of Internet-Based DGPS and RTK-GPS Positioning

Zhixing Liu, Akio Yasuda and Chunming Fan

Abstract:	The differential GPS techniques, such as DGPS and RTK-GPS, are indispensable when highly accurate service is needed, although the accuracy of GPS positioning has been improved substantially by turning off the Selective Availability which was the largest source of error for GPS receivers before. For the DGPS and RTK-GPS positioning system, how to transmit the correction data is very important. We use Internet to transmit correction data. Internet has lots of merits such as convenience, global covering and so on. However, one of the critical characteristics of Internet is the unexpected transfer latency, which will have a great effect to the DGPS and RTK-GPS positioning accuracy. This paper describes the transfer latency of GPS correction data via Internet and the results of Internet-based DGPS and RTK-GPS positioning. We measure the transfer latency via Internet of GPS correction data. TCP protocol and a client-server request-answer model are used. We propose a model to measure the data transfer time corresponding to this reliable connection-oriented protocol. In this model, the client computer, which assumes the data transmissions to the rover, is connected to Internet via a 56 kbps modem and public telephone line in the pertinent experiments. The server computer is assigned with a global IP address. The software employed in the experiments is written in the C programming language and uses the Winsock API functions. The precision of the measured results is 10ms. The length of DGPS correction data to be transmitted is 752 bits, and that of RTK-GPS is 3664 bits. The average transfer latency, including the time lag caused by modem and telephone line, is about 385 ms; the average transfer latency of RTK-GPS correction data is about 502 ms. In order to exclude the errors caused by other elements, which have effects to positioning accuracy, two RT-2 receivers are used for the reference and the rover with zero base line. The twice drms of Internet- based DGPS positioning is within one meter and that of Internet-based RTK-GPS positioning is within one centimeter. They are of the same order of accuracy compared to the positioning results of DGPS/RTK-GPS which use cable to transmit the differential GPS corrections.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	415 - 422
Cite this article:	Liu, Zhixing, Yasuda, Akio, Fan, Chunming, "The Evaluation of the Accuracy and Measurement of Latency of Internet-Based DGPS and RTK-GPS Positioning," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 415-422.
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