precise GlobM Positioning System (GPS) geodesy improves the •ccur•cy and precision of the estimated quantities, and that the estimated gradients resemble real atmospheric moisture gradients observed with a water vapor radiometer (WVR).Using a low elevation angle cutoff, combined with a model of the atmospheric delay gradient as a random walk process leads to 19.5% and 15% average improvement in radial and horizontal site position repeatabihties, respectively, relative to a current state-of-the-art estimation strategy that does not model horizontal gradients and imposes high elevation angle cutoff. The agreement between estimated values of zenith wet delay from collocated GPS receivers and WVRs was improved by at least 25%. Merely lowering the elevation angle cutoff improves the repeatabihty of the radial component of the site's position vector but tends to degrade the repeatability of the horizontal components of the position vector if troposphere gradients are not properly modeled. The estimates of wet delay gradients from a collocated GPS receiver and a WVR at Onsala, Sweden, seem to be correlated over timescales as short as 15 min. The agreement in azimuth between the GPS-based and the WVRbased gradients was at the 10 ø level, for significant gradients. The GPS was found to under-estimate the magnitude of the gradients by about 60% relative to the WVR-based gradients. The abihty to sense atmospheric moisture gradients from a single GPS receiver increases the useful information content from networks of GPS receivers by providing additional spatial information for weather forecasting apphcations.
The International GNSS Service (IGS) has been producing the total troposphere zenith path delay (ZPD) product that is based on combined ZPD contributions from several IGS Analysis Centers (AC) since GPS week 890 in 1997.A new approach to the production of the IGS ZPD has been proposed that replaces the direct combination of diverse ZPD products with point positioning estimates using the IGS Combined Final orbit and clock products. The new product was formally adopted in 2007 after several years of concurrent production with the legacy product. We describe here the advantages of the new approach for the IGS ZPD product, which enhance the value of the new ZPD product for climate studies. We also address the impact the IGS adoption in November 2006 of new GPS antenna phase center standards has had on the new ZPD product. Finally we describe plans to further enhance the ZPD products.
The reduced dynamic GPS tracking technique has been applied for the first time as part of the GPS experiment on TOPEX/Poseidon. This technique employs local geometric position corrections to reduce orbit errors caused by the mismodeling of satellite forces. Results for a 29‐day interval in early 1993 are evaluated through postfit residuals and formal errors, comparison with GPS and laser/DORIS dynamic solutions, comparisons on 6‐hr overlaps of adjacent 30‐hr data arcs, altimetry closure and crossover analysis. Reduced dynamic orbits yield slightly better crossover agreement than other techniques and appear to be accurate in altitude to about 3 cm RMS.
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