GLONASS ambiguity resolution (AR) between inhomogeneous stations requires correction of inter-frequency phase biases (IFPBs) (a “station” here is an integral ensemble of a receiver, an antenna, ﬁrmware, etc.). It has been elucidated that IFPBs as a linear function of channel numbers are not physical in nature, but actually originate in differential code-phase biases (DCPBs). Although IFPBs have been prevalently recognized, an unanswered question is whether IFPBs and DCPBs are equivalent in enabling GLONASS AR. Besides, general strategies for the DCPB estimation across a large network of heterogeneous stations are still under investigation within the GNSS community, such as whether one DCPB per receiver type (rather than individual stations) sufﬁces, as tentatively suggested by the IGS (International GNSS Service), and what accuracy we are able to and ought to achieve for DCPB products. We thus pay attention to the hardware induced DCPBs which are thought to be less of a factor, but are responsible to the station and observable speciﬁc features of DCPBs, rather than the DSP induced counterparts. Finally, we suggest that (1) the rigorous DCPB model should be implemented instead of the classic, but inaccurate IFPB model; (2) DCPBs of sub-ns accuracy can be achieved over a large network by efﬁciently resolving ionosphere-free ambiguities; (3) DCPBs should be estimated and applied on account of their station and observable speciﬁc properties, especially for ambiguities of short wavelengths.
Fig.1 DCPBs (ns) on the L1, L2 and ionosphere-free combination observables for the ten ultra-short baselines over the 212 days of 2015. “Ref. L0 DCPBs” are computed using Eq. 14 based on the DCPB estimates from L1/L2 baseline processing. “Est. L0 DCPBs” are directly estimated using undifferenced ionosphere-free observables and ionosphere-free AR in a network solution. “L0 DCPB differences” are the discrepancies between “Ref. L0 DCPBs” and “Est. L0 DCPBs”, and the RMS statistics are plotted beside the site codes. Note that to facilitate inter-panel comparison, we shift all DCPB values to the range of 0–25 ns, but keep unaltered the magnitude differences between the L1, L2, reference L0, and estimated L0 DCPBs within each panel. Most of the large DCPB jumps are caused by changes of receivers or antennas, or upgrade of ﬁrmware, etc. This figure shows that it is expected that the L0 DCPBs derived from the 200-station network solution in Europe have an accuracy of better than 2ns approximately (3σ).
1. Geng J, Zhao Q, Shi C, Liu J (2017) A review on the inter-frequency biases of GLONASS carrier-phase data. J. Geod., 91(3):329-340, doi:10.1007/s00190-016-0967-9
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