WET DELAY VARIABILITY CALCULATED FROM RADIOMETRIC MEASUREMENTS AND ITS ROLE IN SPACE GEODETIC PARAMETER-ESTIMATION

The ''wet delay,'' the excess radio path length due to atmospheric wat er vapor, has been derived from 71 days of microwave radiometer measur ements at the Onsala Space Observatory, Onsala, Sweden. The temporal a nd spatial variability in the wet delay was analyzed. When we estimate d daily ''variance rates,'' the parameter characterizing a random walk process, values in the range 3.1 x 10(-9) to 1.1 x 10(-7) m(2)/s were found for the timescales 10-20 min. We estimated horizontal gradients in the wet delay and found that the temporal variability of the gradi ent components changed significantly from day to day. The variations i n the gradients were also found to be significantly larger if data acq uired only at relatively high elevation angles were used in the calcul ation. The effect of the wet delay variations on Global Positioning Sy stem (GPS) geodetic estimates was analyzed by performing Monte Carlo s imulations. We used a Kalman filter with parameters for geodetic GPS d ata processing, first modeling the atmosphere as a horizontally homoge neous random walk process in time. In this case the estimated wet dela y was found to be more sensitive to a detuning of the Kalman filter th an the vertical component estimates. The RMS errors in the wet delay e stimates increased from 2.2 to 3.6 mm when the atmospheric variance ra te changed from 1.0 x 10(-8) to 1.0 x 10(-7) m(2)/s and when the filte r parameter was set to 1.0 x 10(-8) m(2)/s. When simulated wet delay g radients were added to the data, it was seen that if gradients are not estimated by the Kalman filter on days with large gradient variabilit y, the scatter introduced by the gradients can dominate the other mode led error sources.