The International VLBI Service for Geodesy and Astrometry (IVS) has observed the weekly IVS-R1 (R1) and IVS-R4 (R4) series of sessions since 2002. These regular series are generally stable in the sense that the networks were designed to be similar from week to week. The uniformity of these series has allowed researchers to conduct many scientific investigations. The IVS also observes with other networks that allow continued sampling of data from all VLBI stations, but the R1 and R4 observing sessions are a dominant part of overall VLBI observing accounting for 1841 sessions out of a total of 3129 24-hour sessions from 2002.0 to 2020.0 (where the last R1 and R4 in 2019 was on December 30 and December 26, respectively). In this paper, we investigate the evolution of these series in terms of their observing networks. We also discuss the construction of the R1 and R4 networks and the scheduling of these sessions. The performance of these networks in terms of the formal precision of polar motion have improved by factors of 2–3 over the period from 2002.0 to 2018.0. UT1 precision improved by a factor of about 1.2–1.5. The main reason for this improvement is the increased size of the networks. We also discuss the effect on this improvement arising from changes in the data rate and the number of observed sources. There is some degradation in performance after 2018 that is most likely due to a decline in the number of available network stations.

Frequent, low-latency measurements of the Earth’s rotation phase, expressed as UT1 $$-$$ UTC  critically support the current estimate and short-term prediction of this highly variable Earth orientation parameter (EOP). Very long baseline interferometry (VLBI) Intensive sessions provide the required data. However, the Intensive UT1 $$-$$ UTC measurement accuracy depends on the accuracy of numerous models, including the VLBI station position. Intensives observed with the Maunakea (Mk) and Pie Town (Pt) stations of the Very Long Baseline Array (VLBA) illustrate how a geologic event (i.e., the $$M_w$$ 6.9 Hawai‘i Earthquake of May 4th, 2018) can cause a station displacement and an associated offset in the values of UT1 $$-$$ UTC measured by that baseline, rendering the data from the series useless until it is corrected. Using the nonparametric Nadaraya–Watson estimator to smooth the measured UT1 $$-$$ UTC values before and after the earthquake, we calculate the offset in the measurement to be 75.7 ± 4.6 $$\upmu $$ s. Analysis of the sensitivity of the Mk-Pt baseline’s UT1 $$-$$ UTC measurement to station position changes shows that the measured offset is consistent with the 67.2 ± 5.9 $$\upmu $$ s expected offset based on the 12.4 ± 0.6 mm total coseismic displacement of the Maunakea VLBA station determined from the displacement of the co-located global navigation satellite system (GNSS) station. GNSS station position information is known with a latency on the order of tens of hours and thus can be used to correct the a priori position model of a co-located VLBI station such that it can continue to provide accurate measurements of the critical EOP UT1 $$-$$ UTC as part of Intensive sessions. In the absence of a co-located GNSS receiver, the VLBI station position model would likely not be updated for several months, and a near real-time correction would not be possible. This contrast highlights the benefit of co-located GNSS and VLBI stations in support of the monitoring of UT1 $$-$$ UTC with single-baseline Intensives.

Every three years, the International VLBI Service for Geodesy and Astrometry (IVS) has carried out a continuous observing campaign (CONT) with the goal of demonstrating state-of-the art Very Long Baseline Interferometry (VLBI) observing since 2002. In 2017, the campaign was unique in two respects: three independent networks ran simultaneously and one of the networks was a demonstration of new VLBI Global Observing System (VGOS) technology. Two 14-station legacy station networks, CONT17-L1 and CONT17-L2 observed in S/X mode and the third network, CONT17-VGOS, consisted of five stations observing in VGOS broadband mode. We investigated the differences between Earth Orientation Parameters (EOPs) and scale parameters estimated from the simultaneous observing sessions of the three networks. Prior studies could not determine VLBI EOP precision or VLBI network biases within the VLBI technique. For CONT17, EOP (X-pole, Y-pole, and UT1) biases between the CONT17-L2 and CONT17-L1 networks were $$-11\pm 12$$ $$\mu $$ as, $$-19\pm 13$$ $$\mu $$ as, $$-1.1\pm 1.0$$ $$\mu $$ s, which are at the 1-sigma level. X-pole, Y-pole, and UT1 biases between the CONT17-VGOS and CONT17-L1 networks were $$-108\pm 24$$ $$\mu $$ as, $$-17\pm 23$$ $$\mu $$ as, $$1.4\pm 0.6$$ $$\mu $$ s. Three corner hat analysis of the CONT17-L1, CONT17-L2, and Global Navigation Satellite System (GNSS) series yielded X and Y polar motion precisions of CONT17-L1: 19.8 $$\mu $$ as, 22.4 $$\mu $$ as; CONT17-L2: 41.0 $$\mu $$ as, 35.2 $$\mu $$ as; GNSS: 19.8 $$\mu $$ as, 15.0 $$\mu $$ as. Session scale parameter precisions of the CONT17-L1 and CONT17-L2 networks based on the standard deviation of their respective scale series are 0.45 ppb and 0.30 ppb. Baseline length repeatabilities for each of the networks indicate that the length precision versus length of the VGOS network is about 0.4 ppb compared to 0.5 ppb for the CONT17-L2 network and 0.6 ppb for the CONT17-L1 network for baselines of length less than 10,000 km.

NASA maintains and operates a global network of Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), and Global Navigation Satellite System ground stations as part of the NASA Space Geodesy Program. The NASA Space Geodesy Network (NSGN) provides the geodetic products that support Earth observations and the related science requirements as outlined by the US National Research Council (NRC in Precise geodetic infrastructure: national requirements for a shared resource, National Academies Press, Washington, 2010 . http://nap.edu/12954 , Thriving on our changing planet: a decadal strategy for Earth observation from space, National Academies Press, Washington, 2018 . http://nap.edu/24938 ). The Global Geodetic Observing System (GGOS) and the NRC have set an ambitious goal of improving the Terrestrial Reference Frame to have an accuracy of 1 mm and stability of 0.1 mm per year, an order of magnitude beyond current capabilities. NASA and its partners within GGOS are addressing this challenge by planning and implementing modern geodetic stations colocated at existing and new sites around the world. In 2013, NASA demonstrated the performance of its next-generation systems at the prototype next-generation core site at NASA’s Goddard Geophysical and Astronomical Observatory in Greenbelt, Maryland. Implementation of a new broadband VLBI station in Hawaii was completed in 2016. NASA is currently implementing new VLBI and SLR stations in Texas and is planning the replacement of its other aging domestic and international legacy stations. In this article, we describe critical gaps in the current global network and discuss how the new NSGN will expand the global geodetic coverage and ultimately improve the geodetic products. We also describe the characteristics of a modern NSGN site and the capabilities of the next-generation NASA SLR and VLBI systems. Finally, we outline the plans for efficiently operating the NSGN by centralizing and automating the operations of the new geodetic stations.

Aims. The IVS Working Group on Galactic Aberration (WG8) was established to investigate issues related to incorporating the effect of Galactic aberration in IVS analysis. The circular motion of the solar system barycenter around the Galactic center causes a change in aberration, which in the case of geodetic VLBI observing is over time scales of several decades. One of the specific goals was to recommend a Galactic aberration model to be applied by the IAU ICRF3 working group in the generation of ICRF3 as well as in other IVS analysis. Studies made by working group members have shown that the three-dimensional acceleration vector of the solar system barycenter can be estimated from VLBI delay observations. Methods. Among the working group members, three methods were used to estimate the acceleration vector. One is to directly estimate the acceleration vector as a global parameter. The second is to estimate the acceleration vector from source proper motions determined from estimated source position time series. A third method estimated a global reference frame scale parameter for each source and derived the acceleration vector from these estimates. The acceleration vector estimate consists of a galactocentric component along with the non-galactocentric components. Results. The geodetic reference frame VLBI estimates of the galactocentric aberration constant from the different working group members are in the range 5.1–6.4 μas yr−1. These estimates are relatively close to independent estimates of 4.8–5.4 μas yr−1 that can be derived from astrometric measurements of proper motions and parallaxes of masers in the Milky Way galaxy. Based on the most recent geodetic VLBI solutions, we find an upper bound of 0.8 μas yr−1 for the non-galactocentric component of the secular aberration. Conclusions. The working group made a recommendation only for the galactocentric component of the observed acceleration vector. For the recommended galactocentric aberration constant, the working group chose a geodetic value to be consistent with geodetic VLBI applications. The recommended value 5.8 μas yr−1 was estimated directly in a global solution that used the ICRF3 solution data set: 1979–May 2018.

The IVS Working Group on Galactic Aberration (WG8) was established to investigate issues related to incorporating the effect of galactic aberration in IVS analysis. Secular aberration drift is caused by the acceleration of the Solar System barycenter. It is mainly due to the rotation of the barycenter about the center of the Milky Way galaxy. Studies made by working group members have shown that aberration can be estimated from VLBI geodetic data. The VLBI estimates of the aberration amplitude are in the range 5.1 to 6.4 μas/yr. These estimates are close to independent estimates of 4.8 to 5.4 μas/yr that were derived from astrometric measurements of proper motions and parallaxes of masers in the Milky Way galaxy. For the recommended aberration constant, a geodetic value 5.8 μas/yr based on data until May 2018 was chosen by the Working Group in order to be consistent with geodetic VLBI applications, specifically for the generation of the ICRF3 solution. In this paper, we discuss the investigation of the Working Group and its findings.

Analysis of the time series at the 3-4 multi-technique GGOS sites to analyze and compare the spectral content of the space geodetic and gravity time series. Evaluate the level of agreement between the space geodesy measurements and the physical tie vectors.

Conclusions and Future Work  We have compared polar motion series from VLBI, GNSS, and SLR where the reference frames were aligned to ITRF2008. Three objectives of the comparisons are 1) to determine biases between the techniques, 2) to determine the precisions of each technique via a 3-cornered hat analysis after removing the relative biases, and 3) to evaluate the long-term stability of Earth orientation parameter (EOP) series. Between VLBI and GPS and SLR, there are systematic variations ranging from 20 to 60 μas in peak-topeak amplitude. These may be caused by VLBI or SLR network dependent effects, including network station changes in these networks over the period from 2002-2016. We also determined the polar motion bias and precision of the most recent IVS VLBI CONT campaign in 2014. These 2-week observing campaigns are designed to provide the highest quality results that can be produced at the time.

Continuous (CONT) VLBI campaigns have been carried out about every 3 years since 2002. The basic idea of these campaigns is to acquire state-of-the-art VLBI data over a continuous time period of about 2 weeks to demonstrate the highest accuracy of which the current VLBI system is capable. In addition, these campaigns support scientific studies such as investigations of high-resolution Earth rotation, reference frame stability, and daily to sub-daily site motions. The size of the CONT networks and the observing data rate have increased steadily since 1994. Performance of these networks based on reference frame scale precision and polar motion/LOD comparison with global navigation satellite system (GNSS) earth orientation parameters (EOP) has been substantially better than the weekly operational R1 and R4 series. The precisions of CONT EOP and scale have improved by more than a factor of two since 2002. Polar motion precision based on the WRMS difference between VLBI and GNSS for the most recent CONT campaigns is at the 30 as level, which is comparable to that of GNSS. The CONT campaigns are a natural precursor to the planned future VLBI observing networks, which are expected to observe continuously. We compare the performance of the most recent CONT campaigns in 2011 and 2014 with the expected performance of the future VLBI global observing system network using simulations. These simulations indicate that the expected future precision of scale and EOP will be at least 3 times better than the current CONT precision.

We simulated future networks of VLBI+SLR sites to assess their performance. The objective is to build a global network of geographically well distributed, co-located next-generation sites from each of the space geodetic techniques. The network is being designed to meet the GGOS terrestrial reference frame goals of 1 mm in accuracy and 0.1 mm/yr in stability. We simulated the next generation networks that should be available in five years and in ten years to assess the likelihood that these networks will meet the reference frame goals. Simulations were based on the expectation that 17 broadband VLBI stations will be available in five years and 27 stations in ten years. We also consider the improvement resulting from expanding the network by six additional VLBI sites to improve the global distribution of the network. In the simulations, the networks will operate continuously, but we account for station downtime for maintenance or because of bad weather. We ran SLR+VLBI combination TRF solutions, where site ties were used to connect the two networks in the same way as in combination solutions with observed data. The strengths of VLBI and SLR allows them to provide the necessary reference frame accuracy in scale, geocenter, and orientation. With the +10-year extended network operating for ten years, simulations indicate that scale, origin, and orientation accuracies will be at the level of 0.02 ppb, 0.2 mm, and 6 μas. Combining the +5-year and +10-year network realizations will provide better estimates of accuracy and estimates of stability.

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).