Experience from S/X-VGOS mixed-mode observations

25th European VLBI Group for Geodesy and Astronomy Working Meeting, 14-18 March 2021
Year 2020 was the culmination of a few earlier years of meticulous preparatory work for obtaining observations simultaneously with the legacy S/X and the VGOS networks as if they were a single, seamless network. The main goal of these so-called mixed-mode sessions is to determine the positions of the new VGOS stations in the terrestrial reference frame defined by the long-lived, stable legacy stations, thus effectively tying both networks together. Three mixed-mode sessions had been executed prior to 2020, each session adding one new small-step to the previous session to ensure technology development and advancement. Indeed, the first session involved a single VGOS station, Westford, participating in a standard R&D S/X observing network in tag-along mode in 2016. The second and third sessions involved the participation of two (add GGAO) and three (add KPGO) VGOS stations, respectively, in 2018, hence expanding from a station to a VGOS baseline and then a triangle, or closure. An important component of this progressive approach has been to identify, and where possible, solve software challenges and limitations, to then move forward with knowledge transfer to the IVS community. An ambitious plan was then launched in 2020 when, building on the success of the earlier sessions, a series of three mixed-mode sessions were run whereby the existing entire VGOS network of 8 stations joined in R&D-type 8-station S/X sessions, thus synthesizing a 16-station mixed-mode network. The main goal of these augmented sessions was to strengthen the network ties for the upcoming ITRF2020 realization. This presentation will focus on the goals of the original vision for the sessions, how the goals morphed to what became the reality, and the impact on the processing and software of these modifications; a lessons-learned perspective. We will conclude with what may a best way forward for 2021