Adding Water Vapor Radiometer Data to Gps Carrier-phase Time Transfer

The analysis of GPS carrier-phase time transfer (GPSCPTT) data often requires that the zenith troposphere delay (ZTD) be estimated at each site as a function of time. This is because the index of refraction of the troposphere varies rapidly. Both the ZTD values and the timetransfer values are estimated simultaneously from the GPSCPTT data. This complicates the estimation of the desired time-transfer values, because, at a given site, the time difference of the receiver clock is correlated to the ZTD. Thus, it is desirable to avoid estimating the ZTD from the GPSCPTT data if possible. This concept can be explored by using ZTD values derived from water vapor radiometer (WVR) measurements. In this experiment, GPSCPTT data were obtained for three stations, each of which was also equipped with a WVR. A control experiment was performed in which the GPSCPTT data were processed in the conventional manner, i.e., the time-transfer values were estimated from the GPS data, as were the ZTD values for each site. Estimates of ZTD derived from WVR measurements were then incorporated into the processing and the time-transfer estimates recomputed. Introducing WVR-based estimates of a site’s ZTD into GPSCPTT data processing changes the ZTD values associated by the GPSCPTT estimation filter with that site. We found that this changed the values that the filter estimated for the ZTDs of the other sites. These ZTD changes then changed the time-transfer estimates according to the equation ∆[CLK(A) – CLK(B)] = -K·[∆ZTD(A) – ∆ZTD(B)]/c. In this experiment, K was found to be approximately equal to 1.5.