7 Conclusions

We have performed an experiment to evaluate the usefulness of local GPS data for the correction of ionospheric Faraday rotation and interferometric phase fluctuations in 327 MHz VLA data. Data from GPS receivers installed at the VLA site have been fitted to a simple ionospheric model, and this fitting process has been used to determine the receiver and transmitter offsets. The parameters derived from the model fits can be used to determine the interferometer phase and Faraday rotation caused by the ionosphere along any line of sight towards an astronomical source. We find that a single GPS receiver in the vicinity of the VLA site provides data that are sufficient to correct for Faraday effects to at 327 MHz and to predict the phase fluctuations caused by large-scale ($\geq 1000$ km) ionospheric structures. A comparison of our model to one using archived global data shows good agreement.

A grid of four GPS receivers was used to predict the phase fluctuations caused by small-scale ($\leq 100$ km) structures, but this prediction could be made only when the ray path from the radio source under observation and ray paths from one of the GPS satellites to the receivers punctured the ionosphere within the same isoplanatic patch - about four degrees for our experiment. A large grid of GPS receivers at 50 to 100 km spacings surrounding the site would be required to provide the large number of puncture points required to characterize isoplanatic patches throughout the whole sky and all small-scale ionospheric structures. Such a grid of receivers is impractical at the present time.

Acknowledgements

The GPS receivers for this experiment were provided by the Atmospheric Sciences Group of the Los Alamos National Laboratory and we especially wish to thank Mr. Jason Payne of that group who supervised their installation and operation.