2004 Volume 82 Issue 1B Pages 301-314
In order to study small-scale water vapor variations over distances from a few km to 20 km, two campaign observations with a dense GPS network were carried out for 2.5 months in total at Tsukuba, Japan. For the observations 79 GPS antennas were installed at 75 sites within a 20 km by 20 km square area, at 1 to 3 km intervals. The PCV models provided by the US National Oceanic and Atmospheric Administration (NOAA) were applied to remove unmodeled phase center variation (PCV) specific to GPS antenna type. In addition, new PCV maps (MPS map) were constructed for all the antennas by stacking one-way postfit residuals over both campaign periods, to remove not only azimuth dependent PCV, but also the errors due to multipath effects.
After MPS maps were introduced into the analysis, strong elevation dependence as well as azimuth dependence of postfit phase residuals, almost disappeared for all the antennas. In addition, the time variations in postfit residuals which were common to all the GPS sites, were subtracted to remove satellite orbits and/or clock errors. This led to the accurate estimate of slant path delay (SPD), which enabled the SPD to be applied to tomography analyses of water vapor (Seko et al. 2003). The horizontal scale of SPD was estimated using correlation distributions. As a result, the horizontal scale of the zenith total delay, the gradient component, and the postfit residual may be roughly considered as 644±120 km, 62±23 km, and 2-3 km, respectively.
Improvement of the postfit residuals following the application of MPS maps also showed a positive impact on PWV estimation. Systematic biases of GPS derived PWV between different antenna types (Trimble and Ashtech) were reduced, resulting in a better agreement of GPS PWV, with RMS errors of 2.0 mm or less relative to PWV by rawinsonde or water vapor radiometer observations. The distribution of time-averaged PWV estimated at the 75 GPS sites showed a systematic pattern which has a negative correlation with the antenna height of each site.