  Global Positioning System (GPS) satellites broadcast at frequencies of
1,575.42 MHz (L1) and of 1,227.60 MHz (L2). The dispersive property of the
ionosphere can be used to combine independent measurements at the two
frequencies to estimate the total electron content (TEC) between a GPS receiver
site and a broadcasting satellite. Such measurements, made at sites near to
Very Long Baseline Interferometry (VLBI) sites, can be used to estimate the
ionospheric 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 of the
ionosphere delays agreed with root-mean-square differences below 0.15 ns for
intercontinental baselines and 0.10 ns for continental ones. We also
successfully applied the GPS-based procedure to reduce the ionospheric effect
in phase delays used for high precision differenced astrometry at 8.4 GHz for
this same experiment.
