Assimilation of GPS Radio Occultation Observations with a Near Real-Time GPS PPP-Inferred Water Vapor System

J. Shi, Y. Gao

Abstract:	GPS radio occultation (RO) technique is a method for remote sensing of the Earth’s atmosphere using a GPS receiver on board of a Low Earth Orbit (LEO) satellite. RO is able to offer much higher vertical resolution than the ground-based atmospheric sensing techniques. Currently several GPS radio occultation missions such as CHAllenging Minisatellite Payload (CHAMP), Gravity Recovery and Climate Experiment (GRACE), and Constellation Observing System for Meteorology Ionosphere & Climate (COSMIC) are in operation for research and climate forecasting purposes. A number of GPS RO products, including raw GPS onboard data, bending angle, refractivity, temperature, pressure and water vapor profiles, are made available by various organizations. A near real-time water vapor system has been developed at The University of Calgary which employs GPS precise point positioning (PPP) technique and observation data from a Canadian geodetic GPS network. Currently the system can process multiple GPS stations equipped with temperature, pressure and relative humidity recording instruments concurrently. Based on raw observations from the GPS tracking stations at 1 Hz, real-time JPL precise orbit and clock correction data and met data available every 5 minutes, this system is able to produce perceptible water vapor (PWV) estimates at 1 Hz with accuracy of 2 mm which is consistent to the International GNSS service (IGS) troposphere products. Although some efforts in assimilating GPS RO observations with direct ground meteorological measurements have been conducted, the resultant temporal resolutions are still lower than this near realtime GPS PPP-inferred system. This has led to the consideration of assimilating GPS RO observations with this near real-time GPS PPP-inferred water vapor system to improve the spatial resolution of a water vapor sensing system. The two data sources are basically complementary since the GPS RO observations can provide atmospheric profiles with higher vertical resolution at a global scale while the near real-time GPS PPP-inferred water vapor system can produce PWV estimates with higher horizontal and temporal resolution. As a result, the vertical, horizontal and temporal resolutions as well as precision can all be improved by proper assimilation of GPS RO observations with the ground based near real-time water vapor system. In this paper, a dataset from a nation-wide geodetic GPS network in Canada over a selected period and collocated COSMIC RO events are assimilated. The assimilation result has demonstrated an improvement over regions with assimilated PWV samples. Some conclusions and recommendations for further improvement are also summarized.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	2584 - 2590
Cite this article:	Shi, J., Gao, Y., "Assimilation of GPS Radio Occultation Observations with a Near Real-Time GPS PPP-Inferred Water Vapor System," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 2584-2590.
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