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Adding Water Vapor Radiometer Data to GPS Carrier-Phase Time Transfer



C Hackman, Judah Levine


When analyzing GPS carrier-phase time transfer (GPSCPTT) data, it is often necessary to estimate the zenith troposphere delay (ZTD) at each site as a function of time. That is because the index of refraction of the troposphere varies rapidly. Both the ZTD values and the time-transfer values are estimated simultaneously from the GPSCPTT data. However, 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. Using ZTD values derived independently from water vapor radiometer (WVR) data allows for the exploration of this possibility. 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 based on WVR measurements were then incorporated into the processing and the time-transfer estimates recomputed. Introducing WVR-based estimates of a site¿s ZTD into the GPSCPTT data processing changes the ZTD values associated by the GPSCPTT estimation filter with that site. We find that this, in turn, causes the ZTD values estimated by the filter for the other sites to change. These changes in ZTD then cause the time-transfer estimates to change according to the equation {Δ}(CLK(A) – CLK(B)) = –K•[{Δ}ZTD(A) – {Δ}ZTD(B)]/c. In this experiment, K is ~ 1.5.
Proceedings Title
Annual Precise Time and Time Interval (PTTI) Mtg., Proc.
Conference Title
Precise Time and Time Interval Planning and Applications Meeting


carrier phase time transfer, frequency transfer, GPS carrier phase time transfer, GPSCPTT, time transfer, troposphere, tropospheric delay, water vapor radiometer, WVR


Hackman, C. and Levine, J. (2004), Adding Water Vapor Radiometer Data to GPS Carrier-Phase Time Transfer, Annual Precise Time and Time Interval (PTTI) Mtg., Proc., [online], (Accessed July 13, 2024)


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Created December 7, 2004, Updated June 2, 2021