Observing UT1-UTC with VGOS

We present first results of UT1-UTC determinations using the VLBI Global Observing System (VGOS). During December 2019 through February 2020 a series of 1~hour long observing sessions were performed using the VGOS stations at Ishioka in Japan and the Onsala twin telescopes in Sweden. The data of this VGOS-B series were correlated, post-correlation processed, and analysed at the Onsala Space Observatory. The derived UT1-UTC results were compared to corresponding results from standard legacy S/X Intensive sessions (INT1/INT2), as well to the final values of the International Earth Rotation and Reference Frame Service (IERS), provided in IERS Bulletin~B. The VGOS-B series achieve 3-4 times lower formal uncertainties for the UT1-UTC results than standard legacy S/X INT series. Furthermore, the root mean square (RMS) agreement with respect to the IERS Bulletin~B is 30-40 % better for the VGOS-B results than for the INT1/INT2 results.


Introduction
Geodetic Very Long Baseline Interferometry (VLBI) (Sovers et al. 1998) is to date the only space geodetic technique that can observe all earth orientation parameters (EOP). This is primarily due to that VLBI directly observes natural radio sources in the International Celestial Reference Frame (ICRF, Charlot et al. 2020) with radio telescopes on the surface of the earth, i.e. in the terrestrial reference frame (ITRF, Altamimi et al. 2016). The EOP are the transformation parameters between these two frames. The EOP that is most difficult to predict due to rapidly varying geophysical excitation is the daily rotation of the earth. It is usually reported as difference between UT1 and Universal Time Coordinated (UTC), and a precise and accurate monitoring of this parameter is of great importance for satellite navigation systems and satellite orbit determination (Bradley et al. 2016).
The International VLBI Service for Geodesy and Astrometry (IVS) (Nothnagel et al. 2017) therefore organises dedicated regular observation sessions to determine UT1-UTC, so that the International Earth Rotation and Reference Frames Service (IERS) can produce precise, accurate and reliable data series of 2 UT1-UTC with low latency that scientific users and society at large can use. These IVS session are the so-called IVS Intensive sessions (INT), which have been observed routinely since decades with the legacy S/X VLBI system that the IVS organises. The INT series make use of long east-west oriented baselines due to their high sensitivity for UT1. The two main series are INT1, usually observed on weekdays on the baseline between Kokee (Hawaii, US) and Wettzell (Germany), and INT2, usually observed on weekends on the baseline between Wettzell (Germany) and Ishioka (Japan). Before Ishioka was involved in INT2, instead the station Tsukuba (Japan) was part of this series. There is also a third INT series, INT3, which observes on Monday mornings usually with a three-station network. During the years, a number of variations were seen in terms of which stations were involved, mainly due to replacing stations for times of station outages due to e.g. maintenance.
While the standard INT are observed with the IVS legacy S/X system, during the last years new stations for the next generation VLBI system called the VLBI Global Observing System (VGOS) have been constructed. VGOS makes use of very fast slewing telescopes with broadband receiving systems covering four frequency ranges, and dual polarization (H/V) capability (Petrachenko et al. 2009;Niell et al. 2018).
Ishioka (Wakasugi et al. 2019) is one of these stations. Furthremore, it can exchange the receiving system, i.e. Ishioka can be used for some months of the year as legacy S/X station and for other months of the year as VGOS station. Another example of VGOS stations are the Onsala twin telescopes (OTT, Haas et al. 2019), which are the currently only operational VGOS twin telescopes. Both Ishioka and the OTT are participating routinely to the IVS VGOS operations series (VO). In discussions with the IVS coordinating center in late 2019, the idea came up to start test observations in order to make use of VGOS for Intensive sessions and thus explicitly UT1-UTC determination. Using the Onsala twin telescopes in this so-called VGOS-B series allows simultaneous UT1-UTC determination with two parallel long east-west baselines connecting to Ishioka.

Scheduling and observing the VGOS-B sessions
For the period December 2019 through February 2020 in total 12 VGOS-B sessions were scheduled. The scheduling was done with the software VieSched++ (Schartner and Böhm 2019) involving the VGOS stations ISHIOKA (Is) in Japan and ONSA13NE (Oe) and ONSA13SW (Ow) in Sweden. The schedules were prepared to be simultaneous to standard INT1 observations. During scheduling, special emphasis was given to include observations at the corners of the mutually visible sky since these observations are known to provide the most impact on the precision of the derived UT1-UTC results (Uunila et al. 2012;Gipson and Baver 2015). To achieve this, a special scheduling algorithm was used. The schedule starts by observing a source at one of the two corners of the mutually visible sky, followed by scans selected using standard geodetic scheduling optimization. Every ten minutes, the algorithm forces a scan of a source located in one of the corners, alternating between the two possibilities. Therefore, it is ensured that observations at the corners of the mutually visible sky are well represented in the schedule.
Due to the modern and fast-slewing VGOS telescopes, the number of scheduled observations per baseline was more than 50 during the 1 hour long sessions. This is significantly more than the usually 20-25 observations during 1 hour long INT1 and INT2 sessions with legacy S/X stations. We note that the VGOS-B sessions analysed in this paper were scheduled assuming standard VO-session recording overheads, such as buffer-flush times needed for Mark6-recording systems, which limit the number of scans per unit time. Future VGOS-B sessions will be further optimised for the Is-Oe/Ow systems, which do not have these overheads, and can therefore schedule ∼ 100 observations per baseline per hour, fully utilising the fast slewing speed. From simulations with VieSched++ the formal uncertainties of the UT1-UTC estimates are 4-5 µs for the VGOS-B sessions, while they are a factor of 2-3 worse for the legacy S/X intensive schedules.
Unfortunately, during four of the twelve VGOS-B sessions technical problems occurred at Ishioka, so that the observed raw data were not complete, see the comments in the table.

Correlation and post-correlation analysis
The observed raw data of the Ishioka station were transferred electronically to the VLBI correlator at . This included forming the pseudo-stokes I polarisation product, from the recorded H and V data, to account for parallactic-angle differences. Both H and V are required for this process, and therefore the single-polarisation Is-data (see Table 1) were omitted from post-processing. The short (75 m) baseline Oe-Ow suffered from disturbing local radio frequency interference (RFI). The full VGOS band-A, and a few channels in other VGOS-bands, were therefore

Data analysis and results
The geodetic data analysis was performed with the ASCoT software (Artz et al. 2016 A standard analysis approach for INT sessions was used: • All station coordinates were kept fixed on their a priori values, i.e. the IVS VTRF2019d (BKG 2019) values. For the OTT we used the corresponding VTRF2019d coordinates that were determined through dedicated short-baseline interferometry campaigns (Varenius and Haas 2020).
• The radio source positions were kept fixed on their ICRF3 (Charlot et al. 2020) values.
• We fixed the clock for one of the stations as reference, while estimating 2nd order clock polynomials for the other stations involved.
• We estimated one zenith wet delay parameter for each of the stations but atmospheric gradients were not estimated.
• The UT1-UTC parameter was estimated.
First, we investigated the agreement of the results of the two parallel baselines Is-Oe and Is-Ow by analyzing the two baselines individually and together. The short baseline Oe-Ow was excluded from any analysis due to the previously mentioned RFI problems. Figure