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Xia et al. Satell Navig (2021) 2:8 Page 8 of 19

Fig. 7 Sky plots of the observed satellites on a BDS B1, b GPS L1, c Galileo E5a and d GLONASS G1 for two hours

obtain a stationary time series, the single diferencing Figure 11 shows the phase minus code combinations
is made for the combination in the time domain. Te and their TD values on BDS B1, GPS L1, Galileo E5a and
Time-Diferenced (TD) phase minus code combination GLONASS G1 frequencies.
(de Bakker et al. 2009) eliminates the ambiguity term Te satellites with longer observation time are selected
and greatly reduces the infuence of ionospheric delay, for illustration. Note that the smartphone interrupted
hardware bias and multipath efect. Terefore, the data recording at the 4230th second. We can see that due
residuals refect pseudorange noise, even though the to the infuence of duty cycling the phase minus code
noise is amplifed by the diferencing process. Here we combinations of BDS, GPS and Galileo satellites all drift
normalized the standard deviation of the diferenced to diferent degrees, which indicates the increasing diver-
combination to the undiferenced level. gence between carrier phase and pseudorange observa-
tions within an observation episode until a large cycle