ACCURACY OF ABSOLUTE PRECIPITABLE WATER-VAPOR ESTIMATES FROM GPS OBSERVATIONS

We present GPS, radiosonde and microwave radiometer (MWR) estimates of precipitable water vapor (PW) at Cape Grim, Tasmania, during November and December 1995. The rms differences between GPS and radiosonde, MW R and radiosonde and GPS and MWR estimates of PW were 1.5 mm, 1.3 mm a nd 1.4 mm, respectively, whilst the biases between the three systems w ere similar to 0.2 mm. However, there are occasions when the amount of PW was underestimated by GPS whilst at other times was over-estimated by MWR. The average overlap error of the GPS estimates of PW between adjacent daily solutions is related to the orbit overlap error and we removed a 2 mm bias introduced using International GPS Service orbits by estimating more accurate global orbits. The discrepancies of up to 3-4 mm between the MWR and GPS systems are not caused by rain, wavegui de losses, varying waveguide temperature, detector non-linearity or in accurate estimates of the mean radiating temperature of the atmosphere . However, small differences between mapping functions at low elevatio ns can produce biases comparable with the bias between the two systems . Consequently, we suspect that the biases arise because the mapping f unctions do not represent the localized atmospheric conditions at Cape Grim. The most accurate GPS estimates are achieved when the GPS analy sis contains station separations of more than 2000 km, an elevation cu toff angle of 12 degrees is used and the CFA2.2 wet mapping function i s used to map the wet delay at any angle to the delay in the zenith.