Observation strategies for current and future geodetic very long baseline interferometry

Geodetic Very Long Baseline Interferometry (VLBI) is an essential technique for space geodesy. It is uniquely capable of simultaneously observing all Earth Orientation Parameters (EOP) and directly giving access to the Earth’s rotation angle (related to Universal Time, UT1). The EOP provide the link between the terrestrial and celestial reference frame. The latter is defined by positions of extra-galactic radio sources observed with geodetic VLBI whereas the terrestrial frame is realised through station positions and velocities. Ongoing phenomena such as the sea-level rise caused by global warming have magnitudes in the millimetre per year range. An accurate global reference frame is therefore crucial for reliably measuring these changes. The Global Geodetic Observing System (GGOS) and its VLBI component, the VLBI Global Observing System (VGOS), are designed to meet these challenges. The transition to the VGOS era brings challenges for all aspects of geodetic VLBI: telescope design, receiver development, recording, data transfer, correlation, observation planning, and analysis. The transition to VGOS involves gradually phasing out the legacy dual- frequency S/X telescopes, while delivering all IVS geodetic products and ensuring the continuity of the time series of geodetic parameters. The VGOS targets include continuous observations and delivery of initial geodetic products in less than 24 hours. This will require a fully automated VLBI analysis chain to make results available in near-real time.This thesis aims at contributing to the improvement of current geodetic VLBI products and supporting the transition from the legacy S/X systems to observations in the VGOS era. Broadband VGOS observations necessitate upgrades for the receiver chain and the data recording devices. The Onsala Space Observatory (OSO) operated its analogue and a new digital back-end in parallel for almost two years. We present the results from a comparison, in which the new system was found to have no biases w.r.t. the old setup. We also investigate ways to improve the current IVS Intensive sessions. This involves using approaches that have relevance for the upcoming VGOS observations. We present fully automated analysis of INT1 sessions between 2001 and 2015 to investigate different analysis strategies and the impact of mapping functions, the use of auxiliary data, and lack of recent a priori EOP on the UT1-UTC accuracy. Up-to-date a priori polar motion was recognized as a key factor for the accuracy of UT1 estimates. Results from implementation and testing of fully automated robust L1-norm based ambiguity estimation are presented. We find that the L1-norm outperforms least-squares for ambiguity estimation. Lastly, optimal locations for a third station in tag-along mode for INT sessions are determined. We conclude that UT1-UTC WRMS can be reduced to 61 % (INT1) and 67 % (INT2) of the WRMS without the tag-along station. The UT1-UTC was improved significantly even without optimised schedules

Onsala Space Observatory – IVS Network Station Activities during 2017—2018

During 2017 and 2018 we participated in 88 legacy S/X sessions with the Onsala 20 m telescope. Additionally, we observed a number of VGOS test sessions with one or both of the Onsala twin telescopes

Onsala Space Observatory – IVS Network Station Activities during 2015–2016

During 2015 and 2016 we participated in 98 IVS sessions. Additionally, we observed a small number of experimental sessions

Current Status of the EU-VGOS Project

The EU-VGOS project began in 2018 with\ua0the aim of using the VGOS infrastructure in Europe\ua0to investigate methods for VGOS data processing. The\ua0project is now structured into Working Groups dealing\ua0with operations (stations), e-transfer, correlation and\ua0post-processing, and analysis. This is a report on the\ua0status of the project

Geodetic analysis for the Very Long Baseline Interferometry Global Observing System

Very Long Baseline Interferometry (VLBI) is an essential technique forspace-geodesy. It realizes the International Celestial Reference Frame(ICRF) and provides a link between the Earth- and space-fixed coordinatesystems by directly observing all Earth Orientation Parameters (EOP)simultaneously. In particular, it is the only technique available that candirectly measure UT1-UTC and nutation. This of special importance tosatellite-based techniques, which need regular input from VLBIobservations to account for drifts in their derived UT1-UTC estimates.Currently, daily UT1-UTC estimates from VLBI are provided by 1-hourIntensive sessions with three regular baseline configurations, whichprovide UT1-UTC with an appropriate accuracy of 20 μs. Increased UT1-UTC accuracy is given by bi-weekly 24-hour Rapid turnaround sessions forEOP determination, which employ a network of at least 8 stations.However, the typical delay for the results obtained from these sessions isclose to the specified upper limit of 15 days.The VLBI Global Observing System (VGOS) is the upcoming VLBIcomponent of the Global Geodetic Observing System (GGOS) of theInternational Association of Geodesy (IAG). It represents a completeredesign of the current VLBI system to meet the requirements for a systemcapable of observing phenomena with a magnitude of a few millimetres.For VGOS the main goals are a global accuracy of 1 mm for positionsand 1 mm/y for velocities and continuous monitoring of EOP and stationpositions. Major effort in hardware and software across the whole signalchain are needed to accomplish these goals. This includes investments in,to name a few, new telescopes, front- and backends, recording systems,correlation, and data analysis. Most of the related systems need to beautomated to ensure reliable continuous operations. In this thesis theaspects of geodetic VLBI data analysis related to the transition to VGOSare investigated through two practical cases.The VGOS requirements necessitate upgrades in the stationhardware. In 2011 Onsala Space Observatory installed a digital backend(Digital Base-Band Converter (DBBC) system) alongside the operationalanalogue Mark IV system. The effect of this hardware change on the VLBIobservables and estimated geodeticparameters is investigated through analysing a series of sessions recordedin parallel on both the old and the new systems.Automated near-real time VLBI analysis is studied using the Intensivesessions on the Kokee—Wettzell baseline. The impacts in terms ofavailability of a priori data for the analysis are investigated to determinethe most crucial factors for high-accuracy UT1-UTC production

Automated analysis of Kokee–Wettzell Intensive VLBI sessions—algorithms, results, and recommendations

The time-dependent variations in the rotation and orientation of the Earth are represented by a set of Earth Orientation Parameters (EOP). Currently, Very Long Baseline Interferometry (VLBI) is the only technique able to measure all EOP simultaneously and to provide direct observation of universal time, usually expressed as UT1-UTC. To produce estimates for UT1-UTC on a daily basis, 1-h VLBI experiments involving two or three stations are organised by the International VLBI Service for Geodesy and Astrometry (IVS), the IVS Intensive (INT) series. There is an ongoing effort to minimise the turn-around time for the INT sessions in order to achieve near real-time and high quality UT1-UTC estimates. As a step further towards true fully automated real-time analysis of UT1-UTC, we carry out an extensive investigation with INT sessions on the Kokee–Wettzell baseline. Our analysis starts with the first versions of the observational files in S- and X-band and includes an automatic group delay ambiguity resolution and ionospheric calibration. Several different analysis strategies are investigated. In particular, we focus on the impact of external information, such as meteorological and cable delay data provided in the station log-files, and a priori EOP information. The latter is studied by extensive Monte Carlo simulations.Our main findings are that it is easily possible to analyse the INT sessions in a fully automated mode to provide UT1-UTC with very low latency. The information found in the station log-files is important for the accuracy of the UT1-UTC results, provided that the data in the station log-files are reliable. Furthermore, to guarantee UT1-UTC with an accuracy of less than 20 μs, it is necessary to use predicted a priori polar motion data in the analysis that are not older than 12 h

Automated ambiguity estimation for VLBI Intensive sessions using L1-norm

Very Long Baseline Interferometry (VLBI) is a space-geodetic technique that is uniquely capable of direct observation of the angle of the Earth\u27s rotation about the Celestial Intermediate Pole (CIP) axis, namely UT1. The daily estimates of the difference between UT1 and Coordinated Universal Time (UTC) provided by the 1-h long VLBI Intensive sessions are essential in providing timely UT1 estimates for satellite navigation systems and orbit determination. In order to produce timely UT1 estimates, efforts have been made to completely automate the analysis of VLBI Intensive sessions. This involves the automatic processing of X- and S-band group delays. These data contain an unknown number of integer ambiguities in the observed group delays. They are introduced as a side-effect of the bandwidth synthesis technique, which is used to combine correlator results from the narrow channels that span the individual bands. In an automated analysis with the c5++ software the standard approach in resolving the ambiguities is to perform a simplified parameter estimation using a least-squares adjustment (L2-norm minimisation). We implement L1-norm as an alternative estimation method in c5++. The implemented method is used to automatically estimate the ambiguities in VLBI Intensive sessions on the Kokee–Wettzell baseline. The results are compared to an analysis set-up where the ambiguity estimation is computed using the L2-norm. For both methods three different weighting strategies for the ambiguity estimation are assessed. The results show that the L1-norm is better at automatically resolving the ambiguities than the L2-norm. The use of the L1-norm leads to a significantly higher number of good quality UT1-UTC estimates with each of the three weighting strategies. The increase in the number of sessions is approximately 5% for each weighting strategy. This is accompanied by smaller post-fit residuals in the final UT1-UTC estimation step

Robust Ambiguity Estimation for an Automated Analysis of the Intensive Sessions

Very Long Baseline Interferometry (VLBI) is a unique space-geodetictechnique that can directly access the Earth\u27s phase of rotation, namely UT1.The daily estimates of the difference between UT1 and Coordinated UniversalTime (UTC) are computed from 1-hour long VLBI Intensive sessions. Thesesessions are essential in providing timely UT1 estimates for satellitenavigation systems. To produce timely UT1 estimates, efforts have been made tocompletely automate the analysis of VLBI Intensive sessions. This requiresautomated processing of X- and S-band group delays. These data often contain anunknown number of integer ambiguities in the observed group delays. In anautomated analysis with the c5++ software the standard approach in resolvingthe ambiguities is to perform a simplified parameter estimation using aleast-squares adjustment (L2-norm minimisation). We implement the robustL1-norm with an alternative estimation method in c5++. The implemented methodis used to automatically estimate the ambiguities in VLBI Intensive sessions onthe Kokee-Wettzell baseline. The results are compared to an analysis setupwhere the ambiguity estimation is computed using the L2-norm. Additionally, weinvestigate three alternative weighting strategies for the ambiguityestimation. The results show that in automated analysis the L1-norm resolvesambiguities better than the L2-norm. The use of the L1-norm leads to asignificantly higher number of good quality UT1-UTC estimates with each of thethree weighting strategies