Weighting and Smoothing of Data in GPS Common View Time Transfer

摘要

It is now possible to compare a clock with UTC(NBS) anywhere in common view of a GPS satellite with Boulder, Colorado at the full level of accuracy and stability of the NBS atomic time scale for integration times of about four days and longer via the NBS Global Time Service. This availability includes Japan, Europe, and the entire United States. The service includes a dial-up service for current estimates of the user's clock performance and a monthly report with improved estimates after the Tact. We discuss here the method by which the common view time transfer values in the monthly reports are computed. Measurements are taken using a satellite in common view of NBS and a second location. These measurements are repeated each sidereal day so that the geometry at measurement time remains fixed. The data are carefully examined for bad points, and these are removed by interpolation or extrapolation. A measurement geometry which repeats each sidereal day defines a time series. The measurement noise of each time series is determined using a decomposition of variance or N-corner hat technique which in turn defines weights used to compute a weighted average. Finally, a Kalman smoothed estimate of time and fractional frequency offset is computed for each time series separately using the measurement noise estimates, and these are also combined with the weights to define optimal estimates of time and frequency offsets. Using this technique we have been able to transfer time with time stability of less than 10 ns, time accuracies of the or order of 10 ns, and a frequency stability of 1 part in 10' and better for measurement times of four days and longer. In addition to using this method for the above-mentioned Global Time Service, it is used in computing the data sent to the BIH for the generation of UTC and TAI.