Multiband delays that are estimated from visibility data using the program fourfit, which is distributed with the Haystack Observatory Postprocessing System (HOPS), do not have a unique ambiguity resolution. At TU Wien, we are looking into options to strengthen the Vienna VLBI and Satellite Software VieVS with an automatic ambiguity resolution module. In the course of these endeavors, we stumbled across the “mbd anchor” keyword of the HOPS fringe fitting program fourfit. This keyword controls how the decision is made as to which of the ambiguous multiband delays is reported. The option, which was introduced for VGOS observations, allows tying the multiband delays to the singleband delays. In order to find out whether we could use this option successfully also for legacy S/X fringe fitting, we performed an initial test. As it turned out, the precision of the S/X singleband delays is not sufficient if the singleband delays lie close to the middle between two multiband delay options.

https://ivscc.gsfc.nasa.gov/publications/gm2024/NASA-CP-20250002586.pdf -- https://ivscc.gsfc.nasa.gov/publications/gm2024/53_krasna_etal.pdf