The Evaluation of the Accuracy andMeasurement of Latency ofInternet-Based DGPS and RTK-GPSPositioning

摘要

The differential GPS techniques, such as DGPS andrnRTK-GPS, are indispensable when highly accurate servicernis needed, although the accuracy of GPS positioning hasrnbeen improved substantially by turning off the SelectivernAvailability which was the largest source of error for GPS receivers before. For the DGPS and RTK-GPS positioningrnsystem, how to transmit the correction data is veryrnimportant. We use Internet to transmit correction data.rnInternet has lots of merits such as convenience, globalrncovering and so on. However, one of the criticalrncharacteristics of Internet is the unexpected transferrnlatency, which will have a great effect to the DGPS andrnRTK-GPS positioning accuracy. This paper describes therntransfer latency of GPS correction data via Internet and thernresults of Internet-based DGPS and RTK-GPS positioning.rnWe measure the transfer latency via Internet of GPSrncorrection data. TCP protocol and a client-serverrnrequest-answer model are used. We propose a model tornmeasure the data transfer time corresponding to thisrnreliable connection-oriented protocol. In this model,rnthe client computer, which assumes the datarntransmissions to the rover, is connected to Internet via arn56 kbps modem and public telephone line in thernpertinent experiments. The server computer is assignedrnwith a global IP address. The software employed in thernexperiments is written in the C programming languagernand uses the Winsock API functions.rnThe precision of the measured results is 10ms. Thernlength of DGPS correction data to be transmitted is 752rnbits, and that of RTK-GPS is 3664 bits. The averagerntransfer latency, including the time lag caused byrnmodem and telephone line, is about 385 ms; the averagerntransfer latency of RTK-GPS correction data is aboutrn502 ms.