Atmospheric Water Vapor as Noise and Signal for Global Positioning System Applications

Teresa M. Van Hove, Chris Alber, and James M. Johnson

Abstract:	High-accuracy geodetic work with the Global Positioning System (GPS) requires tropospheric signal delay correc- tions, especially for the vertical component of the GPS baseline. Scientists are interested in vertical coordinate information for crustal deformation studies. In addition there is growing interest in monitoring sea and ice levels for global climate studies. Water vapor in the atmosphere causes a highly variable signal delay ranging from 5 to 30 centimeters in the zenith direction. Errors in modeling the wet delay cause errors in determining baselines with GPS; The geometry of the GPS measurements causes errors in the vertical to be 3 times the length error. This is known as vertical dilution of precision (VDOP). The use of pointed water vapor radiometers to determine this wet delay was compared against computing a zenith offset from a stan- dard atmosphere modeled delay. A 2.5 month experiment was conducted in fall of 1992; with GPS receivers, WVRs and surface meteorological equipment operated at each end of a 50 km baseline. Results showed the pointed WVR data gave a 44% improvement in vertical precision. as determined from a weighted repeatability computation from the daily vertical solutions. Using the water vapor induced signal delay to measure the amount of precipitable water vapor in the atmosphere is an exciting new application of GPS. The distribution of water vapor has great intluence on atmospheric phenomena. Dif- ferential tropospheric zenith offsets computed from GPS measurements matched the difference in wet delays mea- sured by the radiometers at each site below the 1 cm rms level. Precipitable water vapor at one end of the baseline can be accurately determined from GPS measurements, surface pressure measurements at both sites. and WVR measurements at the other site. Comparison between pre- cipitable water vapor determined from GPS every half hour to that measured with the WVR showed rms devia- tion of less than 1 mm. Integrated precipitable water vapor measurements from GPS could be of great use to mete@ rologists.
Published in:	Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993) September 22 - 24, 1993 Salt Palace Convention Center Salt Lake City, UT
Pages:	797 - 804
Cite this article:	Van Hove, Teresa M., Alber, Chris, Johnson, James M., "Atmospheric Water Vapor as Noise and Signal for Global Positioning System Applications," Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993), Salt Lake City, UT, September 1993, pp. 797-804.
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