Scheduling of Twin Telescopes and the Impact on Troposphere and UT1 Estimation

Recently, several VGOS twin telescopes in\ua0Europe were completed.We examine the use of VGOS\ua0twin telescopes by a new scheduling approach. This approach\ua0is based on integer linear programming and creates\ua0uniform distributed observations over time. Several\ua0VLBI intensive sessions are rescheduled involving\ua0the VGOS twin telescopes and the impact on the troposphere\ua0and UT1 estimation is investigated

Small scale atmospheric variations sensed with very short baseline interferometry (VSBI) and microwave radiometry

We have compared differential zenith wet delays, estimated between the 20 m telescope and the twin telescopes at Onsala, with linear horizontal gradients from a water vapour radiometer (WVR). The east and north gradients from the WVR are projected on to the baseline between the telescopes. The formal errors of the estimated differential zenith delays are comparable to the size of the estimated values.We obtain correlation coefficients for specific 24 h experiments in the range from 0 to 0.2, and the overall correlation is 0.1. Although the correlations are low, we use simulations to verify that they are in the expected range

The Current and Future Performance of VGOS

In this work we investigate the performance\ua0of the 24-hour VGOS sessions observed in 2019–2021.\ua0We look at the station positions and the Earth Orientation\ua0parameters (EOP), and we compare them with\ua0the results from the legacy S/X VLBI sessions as well\ua0as with simulations. We find that the station position\ua0repeatabilities obtained from the VGOS sessions are\ua0significantly better than what is obtained from the\ua0legacy S/X VLBI sessions. However, the EOP from the\ua0VGOS sessions are less accurate than those from the\ua0legacy S/X sessions, a consequence of the low number\ua0and poor global coverage of the currently operational\ua0VGOS stations

VLBI and GPS inter- and intra-technique combinations on the observation level for evaluation of TRF and EOP

We study the effects of combination on the observation level (COL) of different space-geodetic techniques and of networks of the same technique and present the corresponding improvement for the determination of station positions and earth orientation parameters. Data from the continuous geodetic very long baseline interferometry (VLBI) campaign CONT17 are used in a batch least-squares (LSQ) estimator. This campaign includes 15 days of observations with two legacy S/X networks, namely Legacy-1 (L1) and Legacy-2 (L2). For this study the VLBI L1 network is used as the base and reference solution. Data from the L1 network are combined first with data from co-located Global Positioning System (GPS) stations by estimating common tropospheric parameters. The derived station positions repeatabilities of the VLBI and GPS networks are evaluated with respect to single-technique solutions. In terms of precision, we find a 25% improvement for the vertical repeatability of the L1 network, and a 10% improvement for the horizontal one. The GPS network also benefits by 20% and 10% in the horizontal and vertical components, respectively. Furthermore, a combined solution using data of the L1 and L2 network is performed by estimating common earth orientation parameters. The combined L1&GPS and L1&L2 solutions are compared to the reference solution by investigating UT1 and polar motion estimates. UT1 is evaluated in terms of mean bias and formal errors with respect to the International Earth Rotation Service (IERS) C04 products which were used as a\ua0priori values. The L1&GPS solution has the lowest formal error and mean bias for UT1 with a 30% improvement. The weighted root mean square (WRMS) and weighted mean offset (WMO) differences between the obtained polar motion estimates and the ones derived by the International GNSS Service (IGS) are also compared. We find that the L1&GPS solution gives the lowest WRMS and WMO, exhibiting an average 40% improvement with respect to the reference solution. The presented results highlight the potential of COL for ongoing transition to multi-space geodetic analysis, e.g., Global Navigation Satellite Systems (GNSS) with the next-generation VLBI system

Short-baseline interferometry local-tie experiments at the Onsala Space Observatory

We present results from observation, correlation and analysis of interferometric measurements between the three geodetic very long baseline interferometry (VLBI) stations at the Onsala Space Observatory. In total, 25 sessions were observed in 2019 and 2020, most of them 24 h long, all using X band only. These involved the legacy VLBI station ONSALA60 and the Onsala twin telescopes, ONSA13NE and ONSA13SW, two broadband stations for the next-generation geodetic VLBI global observing system (VGOS). We used two analysis packages: nu Solve to pre-process the data and solve ambiguities, and ASCOT to solve for station positions, including modelling gravitational deformation of the radio telescopes and other significant effects. We obtained weighted root mean square post-fit residuals for each session on the order of 10-15 ps using group-delays and 2-5 ps using phase-delays. The best performance was achieved on the (rather short) baseline between the VGOS stations. As the main result of this work, we determined the coordinates of the Onsala twin telescopes in VTRF2020b with sub-millimetre precision. This new set of coordinates should be used from now on for scheduling, correlation, as a priori for data analyses, and for comparison with classical local-tie techniques. Finally, we find that positions estimated from phase-delays are offset similar to+3 mm in the up-component with respect to group-delays. Additional modelling of (elevation dependent) effects may contribute to the future understanding of this offset

Obtaining Local-Tie Vectors from Short-Baseline Interferometry

With the VLBI Global Observing System (VGOS) being the next step in the development of geodetic VLBI, it is necessary to connect the new VGOS network to the existing legacy S/X telescopes. At the Onsala Space Observatory (OSO), this is being done by short-baseline interferometry between the VGOS Onsala twin telescopes ONSA13SW and ONSA13NE and the legacy antenna ONSALA60.The main aim of these sessions, referred to as ONTIE, is to obtain local-tie vectors between these three OSO telescopes that all take part in regular geodetic VLBI observations. Each ONTIE session is about 24 h long, during which all three telescopes observe simultaneously the same sources at X-band. A total of 37 ONTIE sessions have been observed since April 2019. In November 2021, the ONTIE sessions were for the first time observed with alternative observation frequency setups in order to mitigate the influence of known RFI. Additionally, scheduling was done — also for the first time — with VieSched++ instead of sked.Interesting findings of the ONTIE sessions include unexpected offsets in the results of group and phase delays, jumps in the coordinates of the twin telescopes, and apparent yearly trends that might be an artifact of unmodeled thermal expansion of the telescopes that is left in the data.Future ONTIE sessions are envisioned to happen on a regular basis and could, as a by-product, also serve as quasar flux-monitoring sessions by investigation of the recorded system temperatures during observation.This paper summarizes the current status and results of the ONTIE sessions

Gravitational deformation of ring-focus antennas for VGOS: first investigations at the Onsala twin telescopes project

The receiving properties of radio telescopes used in geodetic and astrometric very long baseline interferometry (VLBI) depend on the surface quality and stability of the main reflector. Deformations of the main reflector as well as changes in the sub-reflector position affect the geometrical ray path length significantly. The deformation pattern and its impact on the VLBI results of conventional radio telescopes have been studied by several research groups using holography, laser tracker, close-range photogrammetry and laser scanner methods. Signal path variations (SPV) of up to 1cm were reported, which cause, when unaccounted for, systematic biases of the estimated vertical positions of the radio telescopes in the geodetic VLBI analysis and potentially even affect the estimated scale of derived global geodetic reference frames. As a result of the realization of the VLBI 2010 agenda, the geodetic VLBI network is currently extended by several new radio telescopes, which are of a more compact and stiffer design and are able to move faster than conventional radio telescopes. These new telescopes will form the backbone of the next generation geodetic VLBI system, often referred to as VGOS (VLBI Global Observing System). In this investigation, for the first time the deformation pattern of this new generation of radio telescopes for VGOS is studied. ONSA13NE, one of the Onsala twin telescopes at the Onsala Space Observatory, was observed in several elevation angles using close-range photogrammetry. In general, these methods require a crane for preparing the reflector as well as for the data collection. To reduce the observation time and the technical effort during the measurement process, an unmanned aircraft system (UAS) was used for the first time. Using this system, the measurement campaign per elevation angle took less than 30 min. The collected data were used to model the geometrical ray path and its variations. Depending on the distance from the optical axis, the ray path length varies in a range of about \ub1 1 mm. To combine the ray path variations, an illumination function was introduced as weighting function. The resulting total SPV is about − 0.5 mm. A simple elevation-dependent SPV model is presented that can easily be used and implemented in VLBI data analysis software packages to correct for gravitational deformation in VGOS radio telescopes. The uncertainty is almost 200 μm (2σ ) and is derived by Monte Carlo simulations applied to the entire analysis process

Radiometry performance of the VGOS receivers of the Onsala twin telescopes

With the introduction of the VLBI Global Observing System (VGOS) the parallel use of the VGOS receiver asradiometer in order to estimate the wet propagation delay was recognised as a future possibility. That is whenobservations can be carried out at higher frequencies, closer to the water vapour emission line at 22.2 GHz.An advantage of having the radiometer in the VLBI telescope, compared to the use of a stand-alone WaterVapour Radiometer (WVR), is that the radiometer will observe the same atmospheric volume that is causing thesignal propagation delay.We have assessed this method using simulations and arrived at the following two important conclusions:(1) the receiver’s measurements of the sky brightness temperature is likely to be the main error source, rather thanthe algorithm error introduced when calculating the wet delay from the observed sky brightness temperatures;(2) the method requires an extension of the frequency range of the receiver well beyond 14 GHz in order toincrease the sensitivity for water vapour. The radiometric measurements shall be made within a couple of GHzfrom the emission line at 22.2 GHz.In spite of the fact that the present VGOS receivers observe at too low frequencies we find it meaningful toassess the radiometric stability of these receivers at the higher end of the frequency band. We have used one ofthe Onsala Twin Telescopes for this purpose, which is able to observe both polarizations in the frequency band15.36–15.58 GHz. The system temperature has been observed at different elevation angles in order to separatethe atmospheric sky brightness temperature and the receiver noise temperature. The observations are carried outduring different atmospheric conditions and the estimated sky temperatures are compared to the observationsdone with one of our stand-alone WVRs. By using one-frequency algorithms we may also, during cloud-freeconditions, compare the wet propagation delays using 20.7 GHz observations from the stand-alone WVR and15 GHz observations from the VGOS receiver

Observing UT1‑UTC with VGOS

We present first results for the determination of UT1-UTC using the VLBI Global Observing System (VGOS). During\ua0December 2019 through February 2020, a series of 1 h long observing sessions were performed using the VGOS stations\ua0at Ishioka in Japan and the Onsala twin telescopes in Sweden. These VGOS-B sessions were observed simultaneously\ua0to standard legacy S/X-band Intensive sessions. The VGOS-B data were correlated, post-correlation processed,\ua0and analysed at the Onsala Space Observatory. The derived UT1-UTC results were compared to corresponding results\ua0from standard legacy S/X-band Intensive sessions (INT1/INT2), as well as to the final values of the International Earth\ua0Rotation and Reference Frame Service (IERS), provided in IERS Bulletin B. The VGOS-B series achieves 3–4 times lower\ua0formal uncertainties for the UT1-UTC results than standard legacy S/X-band INT series. The RMS agreement w.r.t. to\ua0IERS Bulletin B is slightly better for the VGOS-B results than for the simultaneously observed legacy S/X-band INT1\ua0results, and the VGOS-B results have a small bias only with the smallest remaining standard deviation

Broad band flux-density monitoring of radio sources with the Onsala twin telescopes

The Onsala twin telescopes (OTT) are two 13 m telescopes located at the Onsala Space Observatory in Sweden. With dual linear polarized broad-band (3-14 GHz) receivers, they are part of the next generation Very Long Baseline Interferometry (VLBI) Global Observing System (VGOS) for geodesy and astrometry. In addition to purely geodetic data products, VGOS will regularly produce full-polarisation images of hundreds of radio sources. These rich monitoring data will be valuable for both astronomy and geodesy. In this pilot study we aimed at monitoring ten bright radio sources to search for flares or similar activity, and at testing the instrument calibration on long (months) and short (hours) time scales. Using the OTT as a standalone instrument, we observed and analysed 91 short (<30 min) sessions spanning seven months. We monitored seven potentially variable radio sources (0059+581, 0552+398, 1144+402, 1156+295, 1617+229, 3C418, OJ287) and three reference calibrators (3C147, 3C286, 3C295). We used the Common Astronomy Software Applications (CASA) package to fringe-fit, bandpass-correct and scale the data to obtain flux densities in the four standard VGOS bands: 3.0-3.5 GHz (band 1), 5.2-5.7 GHz (band 2), 6.3-6.8 GHz (band 3), and 10.2-10.7 GHz (band 4). We obtained simultaneous multi-frequency light curves for ten radio sources. A bright multi-frequency flare is observed in the radio source 0059+581. OJ287 and 1156+295 show significant long-term variability in their light curves. After correcting for instrumental biases, we determine the empirical flux density uncertainty as similar to 5 %