Investigations into the Estimation of Tropospheric Delay and Wet Refractivity Using GPS Measurements

The principal error source in the GPS technology is a delay experienced by the GPS signal in propagating through the electrically neutral atmosphere, usually referred to as a tropospheric delay. This delay is normally calculated in the zenith direction, and is referred to as a zenith tropospheric delay. The delay consists of a zenith hydrostatic delay, which can be modeled accurately using surface barometric measurements, and a zenith wet delay, which cannot be modeled from surface barometric measurements and depends on atmospheric water vapor. In this research presented here, the approach for the production of zenith wet delays from GPS observations is demonstrated. Slant wet delays are recovered for each epoch to all satellites in view assuming that the atmosphere is inhomogeneous. The RMS errors in slant wet delay recovery are found to be about 2-3 cm, through validation of wet delays estimates compared with water vapor radiometer (WVR) " truth " data. Experiments are conducted to determine the optimal processing parameters for estimation of tropospheric delay parameters such as elevation cutoff angle, batch processing interval, and baseline length. Observations of the slant wet delay can be used to model the vertical and horizontal structure of water vapor over a local area. These techniques are based on a tomographic approach using the slant wet delays as input observables, where a 4-D model of the wet refractivity may be derived. Extensive simulations are performed for various vertical resolutions, elevation cutoff angles, and reference station vertical geometries to determine the sensitivity and accuracy of the tomographic solution for a given network. The tomographic technique is tested with real GPS data from the Southern California Integrated GPS Network (SCIGN) to define the wet refractivity in a local network. Results indicate that slant wet delays may be derived from the estimated wet refractivity fields with accuracies of 2-3 cm. iv ACKNOWLEDGEMENTS