Analysis of the TXDOT RTN and OPUS-RS from the Geoid Slope Validation Survey of 2011: Case Study for Texas
Publication: Journal of Surveying Engineering
Volume 140, Issue 4
Abstract
As part of the Geoid Slope Validation Survey of 2011 (GSVS11), the global positioning system (GPS) was used in a variety of ways to determine the geodetic coordinates of nearly 400 passive geodetic control marks. The Texas DOT (TXDOT) real-time network (RTN) was one of the GPS positioning tools used. Both 6- and 180-s occupations were performed with the TXDOT RTN, as well as static 20-min occupations [processed using the National Geodetic Survey (NGS) Online Positioning User Service, Rapid-Static (OPUS-RS) tool], and occupations were adjusted as a network (using the NGS OPUS-Projects tool). Although the original 20-min occupations processed by OPUS-RS had an unforeseen systematic error (invalidating their use in this comparison), a direct comparison between OPUS-RS and the other techniques was still possible by subdividing the OPUS-Projects receiver independent exchange format (RINEX) files on 218 points into more than 30,000 individual 20-min segments. A comparison between all techniques, after accounting for various systematics such as antenna calibrations, different reference frames, and epochs, showed that the RTN had small biases, but, overall, its accuracy () was subcentimeter in longitude, less than 2 cm in latitude, and less than 3 cm in ellipsoid height. An investigation into outliers indicated that the RTN could occasionally have unexplained outliers as large as 10 cm, possibly, but not definitely dependent, on time of day or local environment. The analysis also showed that, for this in-the-field environment, the accuracy of OPUS-RS was somewhat better than expected (biases at the 0.1-cm level; SDs at 0.6 cm in latitude and longitude and 3.1 to 3.4 cm in height) but outliers () did occur in approximately 1.5% of all runs, the largest of which was approximately 1.5 m off in latitude and height, and 0.4 m in longitude. Part, but not all, of this could be explained by being near the coastline. Finally, a significant cautionary result came from this study—users of the NGS OPUS-RS software should make certain that their receivers are generating P2 data (pseudoranges on L2, but not those from L2C). Without that data, OPUS-RS will not function at its highest accuracy. Although this has always been a requirement of OPUS-RS and was historically not an issue, the authors discovered that some GPS receivers inadvertently turn P2 off when L2C is turned on.
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Acknowledgments
The authors thank all those who worked on the GSVS11 survey, but in particular the GPS crews who provided so much of this data. They especially acknowledge Dan Prouty, who performed the RTN survey, Kevin Jordan, who did the OPUS-RS occupations, and Cliff Middleton, who led the long-session GPS survey crews. They are also grateful to the NGS antenna calibration team (Andria Bilich, Charlie Geoghegan, and Clancy Lokken), who calibrated the individual Trimble R8 for this paper and provided insight into the problems of calibrating that particular antenna. Finally, the authors thank the Texas DOT for their cooperation in allowing NGS access to their private RTN during GSVS11.
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Published In
Journal of Surveying Engineering
Volume 140 • Issue 4 • November 2014
Copyright
© 2014 American Society of Civil Engineers.
History
Received: Mar 12, 2013
Accepted: Mar 24, 2014
Published online: Jun 4, 2014
Published in print: Nov 1, 2014
Discussion open until: Nov 4, 2014
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