Relations of the Wet Term Excess Path Delays for the Troposphere and Altitude of GPS Observation

Abstract

The wet component of excess path delays in the troposphere were studied with regards to geodetic baseline measurements using the Global Positioning Systems (GPS). The wet delays at the zenith obtained by the software (SONAP, provided by Western Geophysical Division, Western Atlas International, Inc.) for the simultaneous estimation of the geodetic parameters of stations in the highlands were found to be significantly larger than those for the sites near sea level. The apparent discrepancy could not be resolved in this investigation and further study is needed. We evaluated the wet delays at the zenith τ_z^w taking into account the characteristics of distributions of meteorological parameters near the surface layer over central Japan. A modified formula for τ_z^w to that of Saastamoinen (1972) was derived as, τ_z^w=191e1*/T1* (cm), where e1 =1.3e1, with e1 being the water vapor pressure (mb) on the ground surface and T1 being the extrapolated temperature (K) on the ground surface from the constant-lapse rate distribution prevailing in the dominant region of the troposphere. The procedures to infer the wet-term excess path delays at the zenith on the highlands are proposed, and the resultant correction coefficient versus the altitude is presented herein.