We have conducted an experiment to evaluate the usefulness of ionospheric data produced using the Global Positioning System (GPS) for making Faraday rotation and interferometer phase corrections at the NRAO Very Large Array (VLA). Four GPS receivers were installed at the VLA site - one at the array center and one at the end of each arm. A simple ionospheric model consisting of a vertical TEC, a horizontal gradient, and the azimuth of that gradient was developed and fitted to the GPS Total Electron Content (TEC) data from each receiver. The model was then used to predict the TEC in the observing direction. Ionospheric Faraday rotation and phase gradients were then estimated and compared with VLA measurements taken at frequencies of 322 and 333 MHz. We find that we can normally make Faraday rotation corrections that are accurate to <~ 2degr , although one unexplained discrepancy remains. The interferometer phase shifts caused by large-scale (>~ 1000 km) ionospheric structures can be predicted by our model. However the phase shifts caused by smaller (<~ 100 km) structures can be estimated only when the direction of observation lies within a few degrees of one of the GPS satellites.