Tropospheric correction in VLBI phase-referencing using GPS data

Comparing the tropospheric zenith delays derived from VLBI and GPS data at VLBA stations collocated with GPS antenna, the systematic biases and standard deviations of the difference are both found to be at the level of a sub-centimeter. Based on this agreement, we used GPS data to correct the tropospheric effects in VLBI phase-referencing, resulting in close peak-to-noise ratios of images after tropospheric correction using GPS and VLBI data.