Radio astronomical and ionospheric measurements interact in two ways. Radio astronomical data can be used to infer ionospheric structures, and ionospheric data can be used for the correction of radio data. The first of these processes has been relatively successful. Before the advent of sounding rockets and satellites, only celestial sources provided the required signals for trans-ionospheric propagation studies. These signals have been exploited by many researchers beginning with Hewish (1951). Many of these classic investigations were described by Lawrence et al. (1964). When artificially-produced signals and direct sampling became possible, the use of natural sources for investigating the ionosphere became less important. However, radio astronomical measurements are still useful for the study of certain phenomena such as acoustic gravity waves (Mercier 1986; Mercier et al. 1989; Kelder & Spoelstra 1987; Jacobson & Erickson 1992).
The second process, the correction of ionospheric effects upon radio astronomical measurements using ionospheric data, has been less successful. This is because the effects are often very large - hundreds of wavelengths of delay or several turns of Faraday rotation at meter wavelengths (Hagfors 1976; Thompson et al. 1986) - and ionospheric data of the required accuracy were simply not available. Often the available ionospheric data pertained only to the direction of a single satellite, or the data were obtained from ionosondes separated geographically by hundreds of kilometers. Using ionosonde data, Komesaroff (1960) was able to successfully correct 19.7 MHz source positions for ionospheric refraction. Spoelstra (1983) has presented a refraction correction procedure for WSRT observations employing both topside and bottom-side sounders. However, the use of sounder profiles is often rather tedious and cumbersome because the data must be obtained from several different organizations, resulting in limited application of this method.
Much more convenient and powerful techniques that employ only the radio astronomical data themselves involve closure phases and self-calibration. These techniques (as described by Thompson et al. 1986) are now widely used at wavelengths of a meter or less. They are particularly useful because they pertain to the direction of observation rather than to some other arbitrary direction in the sky. Unfortunately, these techniques are severely limited at low frequencies by poor signal-to-noise, due partly to very rapid phase variations which require short integrations, and partly to dilution of the target object's signal amongst the large number of background sources. Furthermore, self-calibration techniques cannot recover the absolute position of astronomical sources, nor correct for the rotation of the plane of polarization by Faraday rotation in the ionosphere.
The GPS (Global Positioning System, see Dixon 1992; Logsdon 1992; Hofmann-Wellenhof et al. 1993) now allows an observer to conveniently and continuously obtain ionospheric data of unprecedented accuracy with a relatively simple and inexpensive GPS receiver installed at the radio telescope site. With such a receiver, ionospheric parameters over the whole sky can be monitored for the local site 24 hours per day, which opens the possibility of making all-sky ionospheric corrections to both total phase and Faraday rotation. To test these possibilities, we have designed an experiment to evaluate the usefulness of GPS data for making such corrections for the VLA at frequencies near 327 MHz.
Conkright et al. (1997) have compared GPS-derived TEC data with TEC data obtained from the Faraday rotation of signals from geostationary satellites. They found good agreement between the TEC estimates obtained by these two methods. For their study they employed a very simple ionospheric model consisting of a thin, uniform, spherically-symmetrical shell at a height of 400 km.
Campbell (1999) has suggested the use of the United States Air Force Parameterized Ionospheric Model (PIM) for the correction of radio data. The PIM is a theoretical model of global ionospheric climatology. Campbell's approach may be useful, especially if locally determined data are incorporated into the global model. We have not attempted this procedure but present a method for making corrections using locally derived data only.
Ros et al. (2000) have employed GPS data for the correction of VLBI observations. They use only locally-derived data obtained from a GPS receiver located near the VLBI telescope and assume a "frozen" ionosphere that moves over the Earth following the Sun. They use the GPS-TEC measured at one longitude to estimate the TEC at the longitude where the line-of-sight to the radio source pierces the ionosphere. They accomplish this by assuming a frozen ionosphere and an appropriate time correction. Since the time corrections are small, the assumption of a frozen ionosphere should be valid. However, they also neglect North-South ionospheric gradients. This is puzzling because the North-South gradients at mid-latitudes are normally larger than the East-West gradients.
Chatterjee (1999) and Walker & Chatterjee (1999) have discussed the use of various world-wide and regional ionospheric models for the correction of Very Long Baseline Interferometer (VLBI) data.
Our initial observations were made between June and August, 1995. They involved the use of four GPS receivers installed at the VLA site along with simultaneous interferometric observations of radio source phases and Faraday rotation at 330 MHz. An ionospheric model was developed in order to predict the interferometer phase and the rotation of the position angle of polarized flux in the direction of the observed radio source from satellite data in other directions. It was found that this model could also be used to determine GPS receiver and transmitter offsets. This work is discussed in Sects. 2 through 6. Our 1995 data were not useful for determining the ionospheric rotation because the observed sources displayed low polarizations and little variation in rotation occurred during the rather short periods of observation that were available. By the time these intitial observations were reduced, the GPS receivers had been removed, so it was necessary to reinstall a GPS receiver (at the VLA's center) and to obtain rather long allocations of VLA observing time in order to produce useful data. Three successful observing sessions were eventually scheduled during 1997, 1998, and 1999, and their results are covered in Sect. 7.
Copyright ESO 2001