Ambiguity Resolution with Multiband-Singleband Delay Differences

A new ambiguity resolution method for X/S-band VLBI group delays—estimated routinely by the correlation centers with fringe fitting program HOPS/fourfit—is introduced. The method is based primarily on multiband minus singleband delay differences (MBD-SBD). It exploits the fact that SBDs are unambiguous, whereas MBDs may contain ambiguity jumps. Their difference can therefore isolate the ambiguity structure on each baseline. However, due to differing effects on SBD and MBD, in conjunction with the implementation of phase calibration exclusively on legacy MBD, the precision of SBD reaches several nanoseconds and SBD cannot be utilized to anchor the MBD directly at each baseline, as demonstrated by Nothnagel et al. (2025, IVSGM2024 Proceedings). Therefore, median MBD-SBD differences are computed per baseline to obtain robust values unaffected by outliers, and these median values are combined in modified triangle closure relations to estimate consistent ambiguity corrections across the full station network. The approach was tested on all X/S CONT17-L1 sessions and demonstrated that ambiguities can be resolved automatically. We identified, that further development of the method is needed for challenging sessions with specific clock breaks. Our results show that MBD-SBD delay differences provide a highly effective, computationally efficient basis for autonomous ambiguity resolution and offer a promising path toward fully integrated pre-processing within VieVS.