Correcting radio astronomy interferometry observations for ionospheric refraction

Radio astronomical observations with local interferometers and very long baseline interferometers (VLBI) are affected by atmospheric (i.e., tropospheric and ionospheric) refraction. A correction procedure for ionospheric refraction for radio astronomy interferometry is reviewed. This correction is applied to observations made with the Westerbork Synthesis Radio Telescope (WSRT). The method used for WSRT observations can also be used to correct VLBI observations for ionospheric refraction. If networks of ionosonde and satellite observing stations operate during the astronomical VLBI observations, pathlength corrections ranging from several wavelengths to a fraction of a wavelength are possible, provided the geographical distribution of these stations is similar to the VLBI network. The possibilities of this procedure are compared with the results derived by e.g., dual frequency VLBI observations.