GPS METEOROLOGY - DIRECT ESTIMATION OF THE ABSOLUTE VALUE OF PRECIPITABLE WATER

A simple approach to estimating vertically integrated atmospheric wate r vapor, or precipitable water, from Global Positioning System (GPS) r adio signals collected by a regional network of ground-based geodetic CPS receivers is illustrated and validated. Standard space geodetic me thods are used to estimate the zenith delay caused by the neutral atmo sphere, and surface pressure measurements are used to compute the hydr ostatic (or ''dry'') component of this delay. The zenith hydrostatic d elay is subtracted from the zenith neutral delay to determine the zeni th wet delay, which is then transformed into an estimate of precipitab le water. By incorporating a few remote global tracking stations (and thus long baselines) into the geodetic analysis of a regional GPS netw ork, it is possible to resolve the absolute (not merely the relative) value of the zenith neutral delay at each station in the augmented net work. This approach eliminates any need for external comparisons with water vapor radiometer observations and delivers a pure GPS solution f or precipitable water. Since the neutral delay is decomposed into its hydrostatic and wet components after the geodetic inversion, the geode tic analysis is not complicated by the act that some GPS stations are equipped with barometers and some are not. This approach is taken to r educe observations collected in the held experiment GPS/STORM and reco ver precipitable water with an rms error of 1.0-1.5 mm.