VLBI measurement of the vector baseline between geodetic antennas at Kokee Park Geophysical Observatory, Hawaii

We measured the components of the 31-m-long vector between the two very-long-baseline interferometry (VLBI) antennas at the Kokee Park Geophysical Observatory (KPGO), Hawaii, with approximately 1 mm precision using phase delay observables from dedicated VLBI observations in 2016 and 2018. The two KPGO antennas are the 20 m legacy VLBI antenna and the 12 m VLBI Global Observing System (VGOS) antenna. Independent estimates of the vector between the two antennas were obtained by the National Geodetic Survey (NGS) using standard optical surveys in 2015 and 2018. The uncertainties of the latter survey were 0.3 and 0.7 mm in the horizontal and vertical components of the baseline, respectively. We applied corrections to the measured positions for the varying thermal deformation of the antennas on the different days of the VLBI and survey measurements, which can amount to 1 mm, bringing all results to a common reference temperature. The difference between the VLBI and survey results are 0.2 ± 0.4 mm, −1.3 ± 0.4 mm, and 0.8 ± 0.8 mm in the East, North, and Up topocentric components, respectively. We also estimate that the Up component of the baseline may suffer from systematic errors due to gravitational deformation and uncalibrated instrumental delay variations at the 20 m antenna that may reach ± 10 and −2 mm, respectively, resulting in an accuracy uncertainty on the order of 10 mm for the relative heights of the antennas. Furthermore, possible tilting of the 12 m antenna increases the uncertainties in the differences in the horizontal components to 1.0 mm. These results bring into focus the importance of (1) correcting to a common reference temperature the measurements of the reference points of all geodetic instruments within a site, (2) obtaining measurements of the gravitational deformation of all antennas, and (3) monitoring local motions of the geodetic instruments. These results have significant implications for the accuracy of global reference frames that require accurate local ties between geodetic instruments, such as the International Terrestrial Reference Frame (ITRF).

Four years of InSAR time series analysis reveals an unprecedent inventory of active DSGSD in the Western Alps

<p>Based on geomorphological criteria, large-scale slow gravitational deformation affecting entire mountain flank, often being referred as Deep-Seated Gravitational Slope Deformation (DSGSD), have been shown to affect most of the reliefs worldwide. For instance in the European Alps, these deformation patterns were identified in several areas such as the Aosta Valley (Martinotti et al., 2011) or the Mercantour massif (Jomard, 2006). DSGSD inventories based on visual interpretation of scarps and field mapping were then compiled (e.g. Crosta et al., 2013) revealing the widespread occurrence of DSGSD. However, many aspects of these large-scale gravitational processes remain unclear and in particular their present-day activity and temporal evolution remain largely unknown.</p><p>The present study aims at characterizing the spatial extent of DSGSD, and their velocity, at the scale of Western Alps through InSAR time series analysis using NSBAS processing chain (Doin et al., 2001). We used the whole SAR Sentinel-1 archive, between 2014 and 2018, with an acquisition every 6 days, on an ascending track. The processing was adapted to fit the specific conditions of the Alps (seasonal snow cover, strong local relief, vegetation and strong atmospheric heterogeneities). In particular we implemented a correction using the ERA 5 weather model and we used snow masks in winter allowing to select long temporal baseline interferograms with as little snow as possible. As we specifically aim to study deformation patterns at the scale of valley flanks, an average high-pass filter on moving subwindows has been applied to the interferograms prior to the implementation of time-serie inversions. This step strongly reduced the impact of residual atmospheric delays.</p><p>The resulting velocity map in the line of sight (LOS) of the satellite reveals ubiquitous gravitational deformation patterns over the whole Western Alps, with localized patches of moving slopes showing sharp discontinuities with stable surrounding areas. We used radar geometry and InSAR measurement quality factors as indicators to identify the most trusted areas and to extract an inventory of potential DSGSD with their spatial extent. Doing so, we identified more than two thousands slowly deforming areas characterized by LOS velocities from 4 to 20 mm/year. We then compared the geometries of our “InSAR-detected-deforming-slopes” with previously published DSGSD inventories. Good agreements were found for example in the Aosta valley where most of the deforming areas from our velocity map are falling into the DSGSD outlines of Crosta et al. (2013). Currently, we continue to investigate the potential of this large-scale velocity map for DSGSD understanding and we plan to use artificial intelligence to search for possible generic properties between the detected sites.</p>

On the impact of gravitational deformation on VLBI-derived parameters

<p><span><strong>On the impact of gravitational deformation on VLBI-derived parameters </strong></span></p><p>Sahar Shoushtari<sup>1,2</sup>, Susanne Glaser<sup>1</sup>, Kyriakos Balidakis<sup>1</sup>, Robert Heinkelmann<sup>1,2</sup>, James M. Anderson<sup>1,2</sup>, Harald Schuh<sup>1,2</sup></p><p>(1) GFZ German Research Centre for Geosciences, Section 1.1 Space Geodetic Techniques, Telegrafenberg, 14473 Potsdam, Germany</p><p>(2) Technische Universität Berlin, Chair of Satellite Geodesy, Strasse des 17. <span>Juni 135, 10623 Berlin, Germany</span></p><p>Very Long Baseline Interferometry (VLBI) is a highly accurate space geodetic technique that observes extragalactic radio sources to measure the time delay between arrival times of a plane wavefront at two distant radio telescopes. The gravitational deformation of the VLBI telescopes as a function of pointing direction, caused by gravitational forces acting on the massive telescope structures, mainly impacts the estimated station heights and can reach centimeter-level for large antennas. Thus far, this effect has not been considered in operational VLBI data analysis. In the next realization ITRF2020, it is envisaged to model this effect in an effort to reduce the persistent scale discrepancy in ITRF2014 between VLBI and Satellite Laser Ranging. Currently, there are models for only a minority of antennas available, six in total: Effelsberg, Gilcreek, Medicina, Noto, Onsala, and Yebes, which are provided by the International VLBI Service for Geodesy and Astrometry (IVS). In this study, the impact of the gravitational models on station positions, Earth orientation parameters and the network scale is assessed within VLBI data analysis. The standard 24-hours IVS-R1 and -R4 sessions are analyzed using the PORT (Potsdam Open-source Radio Interferometry Tool) software package. First results show that the gravitational models of the six antennas change the station heights by a few mm and the horizontal components by less than 1 mm (in case of Medicina).</p>

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 1 cm 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 $$\pm \,1\,\hbox {mm}$$±1mm. To combine the ray path variations, an illumination function was introduced as weighting function. The resulting total SPV is about $$- \,0.5$$-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 $$200\,\upmu \hbox {m}$$200μm ($$2\sigma $$2σ) and is derived by Monte Carlo simulations applied to the entire analysis process.