GPS METEOROLOGY - REDUCING SYSTEMATIC-ERRORS IN GEODETIC ESTIMATES FOR ZENITH DELAY

Differences between long term precipitable water (PW) time series deri ved from radiosondes, microwave water vapor radiometers, and GPS stati ons reveal offsets that are often as much as 1-2 mm PW. All three tech niques are thought to suffer from systematic errors of order 1 mm PW. Standard GPS processing algorithms are known to be sensitive to the ch oice of elevation cutoff angle at this level. We present a processing protocol that is shown to reduce elevation angle dependence in geodeti c estimates of zenith delay and, hence, reduce the systematic errors i n derived precipitable water. This protocol includes use of the Niell zenith delay mapping functions and International GPS Service phase cen ter models.