Global Positioning System (GPS) receivers, water vapor radiometers (WVRs), and surface meteorological equipment were operated at both ends of a 50-km baseline in Colorado to measure the precipitable water vapor (PWV) and wet delay in the line-of-sight to GPR satellites. Using high precision orbits, WVR-measured and GPS-inferred PWV differences between the two sites usually agreed to better than 1 mm. Using less precise on-line broadcast orbits increased the discrepancy by 30%. Data simulations show that GPS measurements can provide mm-level separate PWV estimates for the two sites, as opposed to just their difference, if baselines exceed 500 km and the highest accuracy GPS orbits are used. ¿ American Geophysical Union 1993