A GPS-based method to model the plasma effects in VLBI observations

Global Positioning System (GPS) satellites broadcast at frequencies of 1,57 5.42 MHz (L1) and of 1,227.60 MHz (L2). The dispersive property of the iono sphere can be used to combine independent measurements at the two frequenci es to estimate the total electron content (TEC) between a GPS receiver site and a broadcasting satellite. Such measurements, made at sites near to Ver y Long Baseline Interferometry (VLBI) sites, can be used to estimate the io nospheric contribution to VLBI observables. For our 1991.9 astrometric VLBI experiment in which we obtained group-delay observations in the 8.4 and 2. 3 GHz bands simultaneously, we found that the GPS and VLBI determinations o f the ionosphere delays agreed with root-mean-square differences below 0.15 ns for intercontinental baselines and 0.10 ns for continental ones. We als o successfully applied the GPS-based procedure to reduce the ionospheric ef fect in phase delays used for high precision differenced astrometry at 8.4 GHz for this same experiment.