GPS signal measurements provide observations of the total electron content (TEC) which are applicable for the correction of the ionospheric excess delays in case of VLBI measurements made at a single frequency band. The directions of GPS satellites from a station are, however, usually different from those of radio stars observed by VLBI. Hence the TEC obtained by GPS measurements must be mapped to other directions. A practical model has been developed to perform this mapping with sufficient accuracy. In the model, trajectories of GPS measurements are represented by UT and latitude coordinate of intersection points of sight with the ionospheric shell. A TEC distribution on the UT-latitude plane is first estimated from GPS measurements, then TEC is mapped to radio star direction. Instrumental offsets of each GPS satellites must be taken into account in the estimation. However, on a short baseline (<2000 km) these offsets tend to be cancel out because of a similarity in trajectories on the UT-latitude planes at both ends of baseline. Evaluation of model by means of comparison with the dual-band VLBI shows that the model can apply to the ionospheric correction of a single band VLBI with an accuracy of about 0.2 ns (0.1 ns in case elevation angle larger than 30 deg) in terms of X band excess delay. The model has been applied to an actual VLBI conducted on the Kashima-Minamidaitojima baseline. Consequently we have been able to detect the motion of Minamidaitojima reflecting the Philippine sea plate motion (Fig.1).
