Site-Dependent Effects in High-Accuracy Applications of GNSS

Licentiate thesis, 2006

Global Navigation Satellite Systems, GNSS are used in many applications demanding millimetre-level accuracy in positioning. Such applications include monitoring of crustal movements. The GNSS technique also facilitates estimates of the tropospheric water vapour content, an important parameter in numerical weather predictions and climate research. The accuracy of estimated parameters are however degraded by a number of error sources related to the satellite system and the ground-based receivers. During 2006 the International GNSS Service, IGS will implement absolute antenna calibration for both satellite antennas and the different antenna types used at the ground-based stations. This calibration will improve the accuracy as antenna type related errors are mitigated. However, unmodelled error sources still exists since a antenna couples electromagnetically with its surrounding environment. The absolute antenna calibrations, to be implemented by the IGS, will however not compensate for site-dependent effects. To further improve the accuracy, the site-dependent effects and their dependency on the direction of the observation need to be identified and removed in the GNSS data analysis. In this thesis the site-dependent error sources have been studied for the stations in the Swedish permanent GNSS network, SWEPOS. Strong similarities in terms of site-dependent effects were found. Differences in the site characteristics, caused by multipath and different antenna surroundings imply that an individual calibration of each station is needed. We have developed and evaluated two different methods for calibration of site-dependent effects. The site-dependent error sources are a complex problem and further research is needed especially in the light of the international agreed implementation of absolute antenna calibrations.