Continuous Real-Time GPS Carrier Phase Based Frequency Transfer on Medium

We have studied a continuous real-time frequency transfer method on the SP-PTB baseline by using GPS carrier phase observations from geodetic GNSS receivers SP01 and PTBB, and post-processed two time periods in 2004 and 2007 with a continuous working filter under real-time conditions using integer phase ambiguity fixing. The results are a continuous time series of the relative time difference of the receiver clocks UTC(SP) and UTC(PTB), which was compared to results from TWSTFT between the two laboratories. We found that as long as the filter parameter modeling is well tuned and the necessary real-time orbit predictions are within a typical accuracy of a few 100 mm, the use of ambiguity resolution offers a correct estimation of the phase relations of the common phase measurements on this baseline. However, if errors in the orbit predictions are in the order of a few meters, which is generally difficult to detect in real-time, then a wrong resolution of the phase ambiguities can occur and thus create wrong phase and frequency estimates. Within the investigated time periods we have detected erroneous IGS orbit predictions in the order of 10 m, which caused a wrong estimation of phase relations. The paper quantifies the quality of current IGS orbit predictions and the impact of orbit errors on the ambiguity resolution for medium baselines. Continuous real-time frequency estimations are useful for system critical oscillator monitoring as for example in real-time interferometry applications, or as input for the calculation of real-time time scales.