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.