The Global Positioning System (GPS) has matured to achieve relative positioning accuracies of 0.1 ppm or better. Remaining error sources that limit this accuracy are: (a) the wet tropospheric path delay suffered by the GPS signal and (b) multipath reflections. The PIs will run a GPS baseline experiment in Colorado in order to study both these errors and methods of reducing them. They will investigate the effect of azimuthal variations of the wet troposphere on the determination of GPS baselines. In order to study this effect it is necessary to suppress multipath errors that might mask the tropospheric variations.
