We will provide an overview of the trop combination activities to-date, describe the structure and content of the new combination reports, and provide usage statistics. We will also present results from statistical analysis of the long-term time series of zenith path delay.

Zenith neutral delay (ZND) estimates derived from ground-based GPS receivers exhibit variations at harmonics of the solar day. The aim of this work is to characterize the semidiurnal (S2) variation and determine its probable origin.

Data from 100 GPS sites are compared with surface pressure easurements to reveal close agreement between the estimated ZND S2 variation and the S2 surface pressure tide. Error analysis suggests that the S2 variation in ZND estimates is not due primarily to orbit, solid earth, or Earth orientation modeling errors. Atmospheric loading and mapping function errors are each expected to contribute less than 11% to the estimated ZND S2 amplitude. Local incongruities reflect the influence of water vapor or sitedependent errors.

Site dependent carrier phase multipath and elevation dependent antenna phase center variations of permanent GPS stations are investigated using the least squares residuals from the GPS data analysis.

First, a method is presented to compute un-differences least squares residuals and its covariance matrix in case of single or double differenced processing. Residual stacking code has been developed in Matlab to estimate site dependent multipath, antenna phase center variations, and elevation standard deviation of the receivers, using the un-differenced residuals. The multipath maps are updated on a daily basis based on data from the previous days, and then used to correct the GPS observations of the next day.

Investigations have been carried out for several IGS and EUREF sites. Very significant site and antenna related effects were found, which after correction, improved the GPS observations considerably. Also, for different receiver types, different elevation dependent standard deviations were found. Back-substitution of the multipath corrections into the GPS processing led to a small improvement in estimates of position, in particular the height component, and tropospheric zenith delays. Furthermore, residual stacking proofed to be a very useful tool in detecting and diagnosing problems in the processing and with the stations.