Abstract: The possiblity of direct estimation of the absolute value of precipitable water is studied by using two GPS receivers from two aspects, theory and practice. Using two GPS receivers at each end of a short baseline, high correlation is introduced between the ZND (zenith neutral delay) of ZWD (zenith wet delay) parameters of two stations when ZND or ZWD parameters are estimated, so correlation errors will contaminate the stability of these parameters deteminated by pure GPS technology. If we assume that the GPS satellite observation distribution in sky is continuous and uniform, we can derive the change of correlation coefficients with the length of baseline and cutoff angle of observation. these result is identical with the result from nratice.