Ionospheric corrections for VLA observations using Local GPS data

¹ Astronomy Department, University of Maryland, College Park, MD 20742, USA
² National Radio Astronomy Observatory (The National Radio Astronomy Observatory is operated by Associated Universities Inc., under cooperative agreement with the National Science Foundation.) , Socorro, NM 87801, USA
³ Code 7215, Remote Sensing Division, Naval Research Laboratory (Basic research in radio astronomy at the Naval Research Laboratory is supported by the Office of Naval Research.) , Washington, DC 20375-5351, USA

R. A. Perley

Received: 15 September 2000
Accepted: 27 November 2000

Abstract

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 , although one unexplained discrepancy remains. The interferometer phase shifts caused by large-scale ( km) ionospheric structures can be predicted by our model. However the phase shifts caused by smaller ( km) structures can be estimated only when the direction of observation lies within a few degrees of one of the GPS satellites.