a b s t r a c tSince the 1970s Very Long Baseline Interferometry (VLBI) has proven to be a primary space-geodetic technique by determining precise coordinates on the Earth, by monitoring the variable Earth rotation and orientation with highest precision, and by deriving many other parameters of the Earth system. VLBI provides an important linkage to astronomy through, for instance, the determination of very precise coordinates of extragalactic radio sources. Additionally, it contributes to determining parameters of relativistic and cosmological models. After a short review of the history of geodetic VLBI and a summary of recent results, this paper describes future perspectives of this fascinating technique. The International VLBI Service for Geodesy and Astrometry (IVS), as a service of the International Association of Geodesy (IAG) and the International Astronomical Union (IAU), is well on its way to fully defining a next generation VLBI system, called VLBI2010. The goals of the new system are to achieve on scales up to the size of the Earth an accuracy of 1 mm in position and of 0.1 mm/year in velocity. Continuous observations shall be carried out 24 h per day 7 days per week in the future with initial results to be delivered within 24 h after taking the data. Special sessions, e.g. for monitoring the Earth rotation parameters, will provide the results in near real-time. These goals require a completely new technical and conceptual design of VLBI measurements. Based on extensive simulation studies, strategies have been developed by the IVS to significantly improve its product accuracy through the use of a network of small (∼12 m) fast-slewing antennas. A new method for generating high precision delay measurements as well as improved methods for handling biases related to radio source structure, system electronics, and deformations of the antenna structures has been developed. Furthermore, as of January 2012, the construction of ten new VLBI2010 sites has been funded, with good prospects for one dozen more antennas, which will improve the geographical distribution of geodetic VLBI sites on Earth and provide an important step toward a global VLBI2010 network. Within this paper, the Global Geodetic Observing System (GGOS) of the IAG will also be introduced and the contribution of VLBI to GGOS will be described.

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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. Derome, M., et al. (2018). Demonstration of a broadband very long baseline interferometer system: A new instrument for high-precision space geodesy.

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