GPS smoothed P3 code for time transfer

In this paper, international time transfer using GPS smoothed P3 code is proposed and tested. The traditional GPS common-view technique uses single channel C/A code for computation; therefore, the ionospheric delay can not be eliminated but only be reduced by an ionosphere model. Recently, GPS P3 code is used in time transfer for TAI computation to overcome this significant error; however, this observation still has large variations when the satellites are at low elevation angles. To acquire more stable time comparison results, we adopt some carrier smoothing techniques to reduce this noise. Two smoothing techniques based on different considerations are tested in our paper. The first method uses the concept of averaging; in other word, the mean difference of P3 and L3 measurements is regarded as the cycle ambiguity and L3 is used to substitute for P3 code for the further computation. To ensure the estimate ambiguity is acceptable, the span of data should be as long as possible after detecting and isolating cycle slips and outliers. In the second technique, the concept of filtering is adopted for data smoothing. The weighting average of outputs from two filters, forward filter and backward filter based on forward and backward directions of measurements, is used for time comparison. The IGS precise ephemerides are also used to reduce the satellite orbital errors in order to obtain better time comparison results. For verifying our programs, we processed some GPS data for international time comparison between laboratories and the corresponding P3 code and carrier phase time transfer results are also shown for comparison. The test data show that using smoothed P3 code can reduce the uncertainty of GPS time comparison and the output data interval can be shortened since the noise level caused by the code measurements is lowered.