Observations of Artificial Radio Sources within the Framework of Geodetic Very Long Baseline Interferometry

Very long baseline interferometry (VLBI) is a mature and fascinating technique with unique and indisputable applications in radio astronomy, planetary sciences, and space geodesy. The latter discipline is a field of science facilitating our understanding of various global-scale phenomena connected to Earth dynamics. Space geodesy provides, in the microwave regime, accurate and long-term stable celestial and terrestrial reference frames, to which those environmental changes can be properly referenced and their spatio-temporal variability can be subsequently accurately investigated. In order to attain better knowledge on complex, and yet subtle, geodynamical phenomena of scientific and economic importance, there is a need for an improved global geodetic infrastructure and enhanced quality of space-geodetic measurements. The common effort of the geodetic community known as the Global Geodetic Observing System (GGOS) shall address that need and provide the highest possible accuracy of geodetic products and reference frames as well as the high consistency across space-geodetic techniques. The ambitious goals of GGOS necessitate appropriate changes to be made also in the area of geodetic/astrometric VLBI, realized at preset in the form of the VLBI Global Observing System (VGOS), a next-generation system aiming to meet the requirements of GGOS and deliver geodetic products with an unprecedented quality. In order to make VGOS succeed, the key components of this complex system need to be refined, including also new observing concepts and scheduling strategies, in order to fully exploit the enhanced performance that this system can bring. Thanks to its characteristics, VGOS creates also a great opportunity for extending the current VLBI research with new applications, for the benefit of the scientific community and society at large.The subject of this thesis concerns observations of artificial radio sources within the framework of geodetic VLBI, in connection to both the current VLBI system and VGOS. This includes information on the combination of observations of natural radio sources and satellite/lunar objects as well as benefits and challenges related to the observing strategy and the technical feasibility of the presented concept. The thesis is based mostly on extensive simulation studies concerning objects on the Moon and geodetic Earth-orbiting satellites, but it also includes an analysis of VLBI observations of the lunar lander performed during dedicated experiments and with a global network of radio telescopes. The information content of this thesis may be treated as a further step towards global observations of artificial radio sources with VLBI in the VGOS era and stimulate new observing concepts for space geodesy

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

Geodetic VLBI with an artificial radio source on the Moon: a simulation study

We perform extensive simulations in order to assess the accuracy with which the position of a radio transmitter on the surface of the Moon can be determined by geodetic VLBI. We study how the quality and quantity of geodetic VLBI observations influence these position estimates and investigate how observations of such near-field objects affect classical geodetic parameters like VLBI station coordinates and Earth rotation parameters. Our studies are based on today\u27s global geodetic VLBI schedules as well as on those designed for the next-generation geodetic VLBI system. We use Monte Carlo simulations including realistic stochastic models of troposphere, station clocks, and observational noise. Our results indicate that it is possible to position a radio transmitter on the Moon using today\u27s geodetic VLBI with a two-dimensional horizontal accuracy of better than one meter. Moreover, we show that the next-generation geodetic VLBI has the potential to improve the two-dimensional accuracy to better than 5 cm. Thus, our results lay the base for novel observing concepts to improve both lunar research and geodetic VLBI

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

Position determination of the Chang’e 3 lander with geodetic VLBI

We present results from the analysis of observations of the Chang’e 3 lander using geodetic Very Long Baseline Interferometry. The applied processing strategy as well as the limiting factors to our approach is discussed. We highlight the current precision of such observations and the accuracy of the estimated lunar-based parameters, i.e., the lunar lander’s Moon-fixed coordinates. Our result for the position of the lander is 44.1219 3 ∘ N , -19.51159∘E and -2637.3 m, with horizontal position uncertainties on the lunar surface of 8.9 m and 4.5 m in latitude and longitude, respectively. This result is in good agreement with the position derived from images taken by the Narrow Angle Camera of the Lunar Reconnaissance Orbiter. Finally, we discuss potential improvements to our approach, which could be used to apply the presented concept to high-precision lunar positioning and studies of the Moon.[Figure not available: see fulltext.]

Simulation studies of new observing concepts for geodetic Very Long Baseline Interferometry

Very Long Baseline Interferometry (VLBI) is a space-geodetic technique in which observations are carried out simultaneously by radio telescopes separated by hundreds or thousands of kilometers. The time difference of signal reception between the telescopes is the basic observable used in geodetic VLBI. This technique is capable of determining all five Earth Orientation Parameters (EOP), which provide the connection between the Earth-fixed and space-fixed reference frames. Currently, there is an ongoing effort concerning the establishment of the VLBI Global Observing System (VGOS), which will significantly improve the present measurement precision and increase the total number of observations per session. This requires the key components of the infrastructure, data handling as well as observation approaches to be upgraded and refined. Thus, the focus of this thesis is set on new observing concepts for VGOS. This includes extensive simulations regarding an improved determination of the rotation of the Earth (UT1-UTC) from one-hour VLBI sessions and investigations on the potential of lunar observations in regular geodetic VLBI sessions. The studies summarized in this work address the main topic from two different aspects, providing valuable insights concerning observations in the VGOS era and stimulating new concepts for space geodesy

Automatic selection of crowdsourced GNSS smartphone data for atmosphere sounding

Smartphones have been transformed into portable GNSS (Global Navigation Satellite System) receivers since Google’s release of the Android 7 operating system. The GNSS data recorded by billions of such devices have great potential for scientific studies, with unprecedented spatiotemporal resolution. However, access to largescale GNSS smartphone data is currently limited and the data processing is challenging. The project CAMALIOT (Application of Machine Learning technology for GNSS IoT data fusion) aims to address these issues to facilitate the usability of crowdsourced GNSS data for weather forecasting and space weather monitoring. The large amount of GNSS data from the CAMALIOT crowdsourcing campaign is of heterogeneous quality. In order to cope with this data processing challenge, we developed an automatic data selection algorithm using machine learning (ML). In this study, the classification performance of different ML models is compared. The importance of different data quality indicators is also examined. Initial results show that the ML-based classifier can achieve an accuracy of 95% on real data from the campaign, without the need to set explicit thresholds for the quality indicators. Based on the selected smartphone GNSS data, tropospheric parameter estimation experiments are conducted and presented as well

Lunar Observations and Geodetic VLBI – A Simulation Study

The recent OCEL (Observing the Chang’E Lander with VLBI) sessions allow the geodetic VLBI community to gain new experience concerning observations of an artificial lunar radio source. Although the analysis of obtained data is still ongoing, the performance of the OCEL sessions, in terms of lunar-based parameters, is still rather unclear. In order to address this and related questions, we carried out Monte Carlo simulations using the c5++ analysis software and OCEL schedules with the purpose to evaluate the accuracy with which the position of an artificial radio source on the surface of the Moon can be determined with geodetic VLBI. We present the results of our study and discuss the limiting factors of this concept. Our simulation results can provide valuable insights concerning global observations of lunar radio transmitters and stimulate new observing ideas for space geodesy