GPS Results are Consistant with VLBI

John Gipson, Jim Ryan, Edwin Himwich, and Tom Clark

Abstract:	For nearly 20 years Very Long Baseline Interferometry (VLBI) has been a premier geodetic technique. In the early ’70s VLBI was used to measure transcontinental baselines to an accuracy of a few decimeters. By the late ’70s length accuracies under 2 cm were routine. In the ’80s VLBI grew into a mature science. VLBI measure- ments of absolute station position and long baselines are the most accurate of any competing technique. Over 125 station positions were measured using VLBI. The accu- racy of the best VLBI experiments is 5 parts in 10” -- 5 mm on 10,000 km baselines. VLBI routinely achieves ac- curacy of a few millimeters on a few hundred kilometer baselines. This great accuracy resulted in the direct con- firmation of continental drift, which had previously only been inferred from geologic data averaged over tens of thousands to millions of years. The uncertainty in the rate determinations is typically less than 1 mm/year. In the early and mid-go’s a series of regional VLBI cam- paigns were conducted in Alaska and southern California to measure crustal deformation. Although scientifically a success, these campaigns were discontinued for budget- ary reasons. Recently many of these sites have been re- visited by GPS receivers as part of NASA’s Dynamics of the Solid Earth Program (DOSE). The consistency of the GPS and VLBI measured station position is a critical sci- entific question. If the station positions measured by the two techniques are consistent, it is possible to combine measurements made at different epochs with different techniques to obtain information about station velocities. By examining a campaign in southern Alaska, we verify that the GPS and VLBI measured station positions are consistent at the 15 mm level. The International GPS Service for Geodynamics (IGS) co-ordinates the placement of GPS receivers and data ar- chiving on an international level. Presently there are 54 receivers in this network. Of these, more than 20 are co- located with VLBI sites, past or present. A comparison of the station coordinates of this global network found by the two techniques gives good agreement. The RMS dif- ference between the east and north coordinates is roughly 10 mm. The RMS difference between the local up coordi- nate is three times larger, at roughly 30 mm.
Published in:	Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994) September 20 - 23, 1994 Salt Palace Convention Center Salt Lake City, UT
Pages:	383 - 390
Cite this article:	Gipson, John, Ryan, Jim, Himwich, Edwin, Clark, Tom, "GPS Results are Consistant with VLBI," Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994), Salt Lake City, UT, September 1994, pp. 383-390.
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