The International VLBI Service for Geodesy Astrometry (IVS) regularly provides high-quality data to produce Earth Orientation Parameters (EOP), and for the maintenance and realization of the International Terrestrial and Celestial Reference Frames, ITRF and ICRF. The first iteration of the celestial reference frame (CRF) at radio wavelengths, the ICRF1, was adopted by the International Astronomical Union (IAU) in 1997 to replace the FK5 optical frame. Soon after, the IVS began official operations and in 2009 there was a significant increase in data sufficient to warrant a second iteration of the CRF, ICRF2. The most recent ICRF3, was adopted by the IAU in 2018. However, due to the geographic distribution of observing stations being concentrated in the Northern hemisphere, CRFs are generally weaker in the South due to there being fewer Southern Hemisphere observations. To increase the Southern Hemisphere observations, and the density, precision of the sources, a series of deep South observing sessions was initiated in 1995. This initiative in 2004 became the IVS Celestial Reference Frame Deep South (IVS-CRDS) observing program. This paper covers the evolution of the CRDS observing program for the period 1995 to 2021, details the data products and results, and concludes with a summary of upcoming improvements to this ongoing project.

The GGOS Bureau of Networks and Observations works with the IAG Services (IVS, ILRS, IGS, IDS, IGFS, IERS, and PSMSL) to advocate for the expansion and modernization of space geodetic networks for the maintenance and improvement of the reference frame and other applications, as well as for the integration of the techniques. Of particular interest is the integration of gravimetric and tide gauge networks in view of the forthcoming establishment of a new absolute gravity reference frame and of the International Height Reference System/Frame. New sites are being established following the GGOS concept of “core” and co-location sites, and new technologies are being implemented to enhance performance in data yield as well as accuracy. The IAG Committees and Joint Working Groups play an essential role in the Bureau activity. The Standing Committee on Performance Simulations and Architectural Trade-offs (PLATO) uses simulation and analysis techniques to project future network capability and to examine trade-off options. The Committee on Data and Information is working on a strategy for a GGOS metadata system for data products and a more comprehensive long-term plan for an all-inclusive system. The Committee on Satellite Missions is working to enhance communication with the space missions, to advocate for missions that support GGOS goals and to enhance ground systems support. The IERS Working Group on Site Survey and Co-location (also participating in the Bureau) is working to enhance standardization in procedures, outreach and to encourage new survey groups to participate and improve procedures to determine systems’ reference points, a crucial aid in the detection of technique-specific systematic errors. We will give a brief update on the status and projection of the network infrastructure for the next several years, and the progress and plans of the Committees/Working Groups in their critical role in enhancing data product quality and accessibility to the users, scientists and the general community.

The Continuous Very Long Baseline Interferometry (VLBI) Campaign 2017 (CONT17) was observed from November 28 to December 12, 2017 and featured three independent observing networks. Two legacy S/X networks of nominally 14 stations each observed in parallel for the full 15 days of the campaign. One of them was made possible in large part by the participation of the ten-station very long baseline array network of the long baseline observatory. Furthermore, for the 5-day period from December 4 to 8, 2017, a six-station broadband network continuously recorded VLBI global observing system data. The different networks will help probe the accuracy of the VLBI estimates of the Earth orientation parameters and investigate possible network biases. We describe the organizational efforts undertaken to realize CONT17, including the analysis of resources such as recording media, electronic data transfer, and correlation, the assignment of stations to the three networks, and source selection and schedule writing.

Since its creation in 1999, the International VLBI Service for Geodesy and Astrometry (IVS) has documented its progress and current status in the form of annual or biennial reports. The first sixteen years were recorded in the form of annual reports, while starting with the years 2015+2016 the rhythm was changed to two years. Hence, the 2017+2018 Biennial Report constitutes the second installment of the two-yearly reporting scheme, documenting the work of the IVS components for the calendar years 2017 and 2018. As a testament to its usefulness, the general structure of the reporting has remained stable over the years: the individual components of the IVS contributed short reports describing their numerous activities, progress, and future plans. Without the continued input from the VLBI groups of the international geodetic and astrometric community, this publication could not be compiled and the IVS itself would not be able to flourish. So, once again many thanks to all IVS components who contributed to this Biennial Report.

The International VLBI Service for Geodesy and Astrometry (IVS) is a globally operating service that coordinates and performs Very Long Baseline Interferometry (VLBI) activities through its constituent components. The VLBI activities are associated with the creation, provision, dissemination, and archiving of relevant VLBI data and products. The operational station network of the IVS currently consists of about 40 radio telescopes worldwide, subsets of which participate in regular 24-hour and 1-hour observing sessions. This legacy S/X observing network dates back in large part to the 1970s and 1980s. Because of highly demanding new scientific requirements such as sea-level change but also due to the aging infrastructure, the larger IVS community planned and started to implement a new VLBI system called VGOS (VLBI Global Observing System) at existing and new sites over the past several years. In 2020, a fledgling network of 8 VGOS stations started to observe in operational IVS sessions. We anticipate that the VGOS network will grow over the next couple of years to a global network of 25 stations and will eventually replace the legacy S/X system as the IVS production system. We will provide an overview of the recent developments and anticipated evolution of the geodetic VLBI station infrastructure.

(1) Harvard-Smithsonian Center for Astrophysics, Cambridge, United States (mpearlman@cfa.harvard.edu), (2) NVI, Inc./NASA Goddard Space Flight Center, Greenbelt MD, United States (dirk.behrend-1@nasa.gov), (3) Jet Propulsion Laboratory/California Institute of Technology, Pasadena, CA, United States (Allison.B.Craddock@jpl.nasa.gov), (4) NASA Goddard Space Flight Center, Greenbelt MD, United States (carey.e.noll@nasa.gov), (5) University of Maryland, Baltimore MD, United States (epavlis@umbc.edu), (6) Institut Géographique National, St. Mande, France (jerome.saunier@ign.fr), (7) PSMSL, National Oceanography Centre, Liverpool, United Kingdom (antt@noc.ac.uk), (8) Politecnico di Milano, Milan, Italy (riccardo.barzaghi@polimi.it), (9) Bundesamt für Kartographie und Geodäsie, Frankfurt, Germany (Daniela.Thaller@bkg.bund.de), (10) GeoForschungsZentrum, Potsdam, Germany (benjamin.maennel@gfz-potsdam.de), (11) Rise Research Institutes of Sweden, Borås, Sweden (sten.bergstrand@ri.se), (12) Institut für Erdmessung, Hannover, Germany (mueller@mbox.ife.uni-hannover.de)

The GGOS Bureau of Networks and Observations works with the IAG Services (IVS, ILRS, IGS, IDS, IGFS, and PSMSL) to advocate for the expansion and upgrade of space geodesy networks for the maintenance and improvement of the reference frame and other applications, as well as for the integration with other techniques, including absolute gravity and sea level measurements from tide gauges. New sites are being established following the GGOS concept of “core” and co-location sites, and new technologies are being implemented to enhance performance in data yield as well as accuracy. The Bureau continues to meet with organizations to discuss possibilities, including partnerships, for new and expanded participation. The GGOS Network continues to grow as new stations join every year. The Bureau holds meetings frequently, providing the opportunity for representatives from the services to meet and share progress and plans, and to discuss issues of common interest. It also monitors the status and projects the evolution of the network based on information from the current and expected future participants. Of particular interest at the moment is the integration of gravity and tide gauge networks and the forthcoming establishment of the new absolute gravity reference frame. The IAG Committees and Joint Working Groups play an essential role in the Bureau activity. The Standing Committee on Performance Simulations and Architectural Trade-offs (PLATO) uses simulation and analysis techniques to project future network capability and to examine trade-off options. The Committee on Data and Information is working on a strategy for a GGOS metadata system for data products and a more comprehensive long-term plan for an all-inclusive system. The Committee on Satellite Missions is working to enhance communication with the space missions, to advocate for missions that support GGOS goals and to enhance ground systems support. The IERS Working Group on Site Survey and Co-location (also participating in the Bureau) is working to enhance standardization in procedures, outreach and to encourage new survey groups to participate and improve procedures to determine systems’ reference points, a crucial aid in the detection of technique-specific systematic errors. We will give a brief update on the status and projection of the network infrastructure of the next several years, and the progress and plans of the Committees/Working Group in their critical role in enhancing data product quality and accessibility to the users.

The Continuous VLBI Campaign 2017 (CONT17) was observed from November 28 to December 12, 2017. Unlike previous CONT campaigns organized under the auspices of the IVS, CONT17 was not a single-network effort but featured three independent observing networks. Two legacy S/X networks of nominally 14 stations each observed in parallel for the full 15 days of the campaign. This was made possible in large part by the participation of the ten-station VLBA network of the LBO. Furthermore, for the five-day period from December 4-8, 2017 a six-station broadband network continuously recorded VGOS data. The different networks will help probe the accuracy of the VLBI estimates of the EOP and investigate possible network biases. In this paper, we describe the coordination effort undertaken to make CONT17 a successful endeavor. This includes the assignment of stations to the three networks based on EOP simulations, analysis of media, e-transfer, and correlation resources, as well as schedule writing, among other things.

A prototype broadband geodetic very long baseline interferometry system has been implemented, and measurements of the baseline length over approximately two years, between December 2014 and January 2017, have been made in the process of exercising the system, developing operational procedures, and assessing geodetic precision for the new broadband observing concept. In addition to developing a broadband signal chain and installing the instrumentation on both a new 12‐m antenna at the Goddard Geophysical and Astrophysical Observatory and the 18‐m Westford antenna at the Massachusetts Institute of Technology Haystack Observatory, it was necessary to develop new correlation and analysis procedures to process the four‐band, dual‐linear‐polarization data. A geodetic analysis of the data from 19 sessions that were observed during this period yielded a weighted root‐mean‐square deviation of the baseline length residuals about the weighted mean of 1.6 mm. These results validate several of the expectations set forth for the vision of the next‐generation geodetic very long baseline interferometry system.

The second realization of the International Celestial Reference Frame (ICRF2) is based on Very Long Baseline Interferometry (VLBI) data at radio frequencies in X band and S band. The European Space Agency's Gaia mission, launched on 2013 December 19, started routine scientific operations in 2014 July. By scanning the whole sky, it is expected to observe ~500000 Quasi Stellar Objects in the optical domain. This means that, in the future, two extragalactic celestial reference frames, at two different frequency domains, will coexist. It will thus be important to align them very accurately. In 2012, the Laboratoire d'Astrophysique de Bordeaux (LAB) selected 195 sources from ICRF2 that will be observed by Gaia and should be suitable for aligning the radio and optical frames: they are called ICRF2-Gaia transfer sources. The LAB submitted a proposal to the International VLBI Service (IVS) to regularly observe these ICRF2-Gaia transfer sources at the same rate as Gaia observes them in the optical realm, e.g., roughly once a month. Of the 195 sources, all but one have been successfully observed in the 12 months prior to 2015 September 01. Table1 lists the 195 ICRF2-Gaia transfer sources. Beginning in 2003 June, the Goddard VLBI group developed a program to purposefully monitor when sources were observed and to increase the observations of "under-observed" sources. In 2013 March, we added all 195 ICRF2-Gaia transfer sources to the IVS source monitoring program with an observation target of 12 successful sessions per year. (1 data file).

The second realization of the International Celestial Reference Frame (ICRF2) is based on Very Long Baseline Interferometry (VLBI) data at radio frequencies in X band and S band. The European Space Agency's Gaia mission, launched on 2013 December 19, started routine scientific operations in 2014 July. By scanning the whole sky, it is expected to observe ~500000 Quasi Stellar Objects in the optical domain. This means that, in the future, two extragalactic celestial reference frames, at two different frequency domains, will coexist. It will thus be important to align them very accurately. In 2012, the Laboratoire d'Astrophysique de Bordeaux (LAB) selected 195 sources from ICRF2 that will be observed by Gaia and should be suitable for aligning the radio and optical frames: they are called ICRF2-Gaia transfer sources. The LAB submitted a proposal to the International VLBI Service (IVS) to regularly observe these ICRF2-Gaia transfer sources at the same rate as Gaia observes them in the optical realm, e.g., roughly once a month. Of the 195 sources, all but one have been successfully observed in the 12 months prior to 2015 September 01. Table1 lists the 195 ICRF2-Gaia transfer sources. Beginning in 2003 June, the Goddard VLBI group developed a program to purposefully monitor when sources were observed and to increase the observations of under-observed sources. In 2013 March, we added all 195 ICRF2-Gaia transfer sources to the IVS source monitoring program with an observation target of 12 successful sessions per year. (1 data file).

Continuous VLBI campaigns (CONT) started in 1994 with the goal of demonstrating stateof-the-art VLBI over a continuous period of time. The first CONT was followed by campaigns in 1995 and 1996. After a six year hiatus, CONT campaigns were organized approximately every three years from 2002 through 2014. In this paper we primarily focus on the cornerstones of each CONT campaign. Specifically, we review the developments in networks, scheduling techniques, recording media, correlation, and other resources used. A timeline of the history of the CONTs and the goals for future campaigns will be presented. The CONTs used a significant amount of IVS resources to produce a large volume of high quality data and demonstrated the advantages of continuous observing which will soon be realized with VGOS.

The second realization of the International Celestial Reference Frame (ICRF2), which is the current fundamental celestial reference frame adopted by the International Astronomical Union, is based on Very Long Baseline Interferometry (VLBI) data at radio frequencies in X band and S band. The European Space Agency’s Gaia mission, launched on 2013 December 19, started routine scientific operations in 2014 July. By scanning the whole sky, it is expected to observe ∼500,000 Quasi Stellar Objects in the optical domain an average of 70 times each during the five years of the mission. This means that, in the future, two extragalactic celestial reference frames, at two different frequency domains, will coexist. It will thus be important to align them very accurately. In 2012, the Laboratoire d’Astrophysique de Bordeaux (LAB) selected 195 sources from ICRF2 that will be observed by Gaia and should be suitable for aligning the radio and optical frames: they are called ICRF2-Gaia transfer sources. The LAB submitted a proposal to the International VLBI Service (IVS) to regularly observe these ICRF2-Gaia transfer sources at the same rate as Gaia observes them in the optical realm, e.g., roughly once a month. We describe our successful effort to implement such a program and report on the results. Most observations of the ICRF2-Gaia transfer sources now occur automatically as part of the IVS source monitoring program, while a subset of 37 sources requires special attention. Beginning in 2013, we scheduled 25 VLBI sessions devoted in whole or in part to measuring these 37 sources. Of the 195 sources, all but one have been successfully observed in the 12 months prior to 2015 September 01. Of the sources, 87 met their observing target of 12 successful sessions per year. The position uncertainties of all of the ICRF2-Gaia transfer sources have improved since the start of this observing program. For a subset of 24 sources whose positions were very poorly known, the uncertainty has decreased, on average, by a factor of four. This observing program is successful because the two main goals were reached for most of the 195 ICRF2-Gaia transfer sources: observing at the requested target of 12 successful sessions per year and improving the position uncertainties to better than 200 μas for both R.A. and decl. However, scheduling some of the transfer sources remains a challenge because of network geometry and the weakness of the sources, and this will be one focus of future sessions used in this ongoing program.