We measured the components of the 31-m-long vector between the two very-long-baseline interferometry (VLBI) antennas at the Kokee Park Geophysical Observatory (KPGO), Hawaii, with approximately 1 mm precision using phase delay observables from dedicated VLBI observations in 2016 and 2018. The two KPGO antennas are the 20 m legacy VLBI antenna and the 12 m VLBI Global Observing System (VGOS) antenna. Independent estimates of the vector between the two antennas were obtained by the National Geodetic Survey (NGS) using standard optical surveys in 2015 and 2018. The uncertainties of the latter survey were 0.3 and 0.7 mm in the horizontal and vertical components of the baseline, respectively. We applied corrections to the measured positions for the varying thermal deformation of the antennas on the different days of the VLBI and survey measurements, which can amount to 1 mm, bringing all results to a common reference temperature. The difference between the VLBI and survey results are 0.2 ± 0.4 mm, −1.3 ± 0.4 mm, and 0.8 ± 0.8 mm in the East, North, and Up topocentric components, respectively. We also estimate that the Up component of the baseline may suffer from systematic errors due to gravitational deformation and uncalibrated instrumental delay variations at the 20 m antenna that may reach ± 10 and −2 mm, respectively, resulting in an accuracy uncertainty on the order of 10 mm for the relative heights of the antennas. Furthermore, possible tilting of the 12 m antenna increases the uncertainties in the differences in the horizontal components to 1.0 mm. These results bring into focus the importance of (1) correcting to a common reference temperature the measurements of the reference points of all geodetic instruments within a site, (2) obtaining measurements of the gravitational deformation of all antennas, and (3) monitoring local motions of the geodetic instruments. These results have significant implications for the accuracy of global reference frames that require accurate local ties between geodetic instruments, such as the International Terrestrial Reference Frame (ITRF).

The next-generation VLBI system called VGOS (VLBI Global Observing System) has been designed and built as a significant improvement over the legacy geodetic VLBI system to meet the accuracy and stability goals set by the Global Geodetic Observing System (GGOS). Improved geodetic products are expected as the VGOS technique transitions from demonstration to operational status, which is underway. Since 2019, a network of nine VGOS stations has been observing bi-weekly under the auspices of the International VLBI Service for Geodesy and Astrometry (IVS) to generate standard geodetic products. These products, together with the mixed-mode VLBI observations that tie the VGOS and legacy networks together will be contributions to the next realization of the International Terrestrial Reference Frame (ITRF2020). Moreover, since 2020 a subset of 2 to 4 VGOS stations has also been observing in a VLBI Intensive-like mode to assess the feasibility of Earth rotation (UT1) estimation using VGOS. Intensives are daily legacy VLBI observations that are run on a daily basis using a single baseline between Kokee Park Geophysical Observatory, Hawaii, and Wettzell Observatory, Germany, made with the goal of near-real-time monitoring of UT1. In this presentation, we will describe the VGOS observations, correlation, post-processing, and preliminary geodetic results, including UT1. We will also compare the VGOS estimates to estimates from legacy VLBI, including estimates from mixed-mode observations, to explore the precision and accuracy of the VGOS products.

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 GGAO 12-m and Westford 18-m antennas are instrumented with the four-band broadband signal chain to provide VGOS capability. These antennas have been making VGOS geodetic observations for more than a year. Preliminary analysis of the thirteen sessions, which range in duration from one to 15 hours, gives a weighted RMS deviation of 2 mm from the mean baseline length (601 km). The 12-m VGOS antenna at Kokee Park Geophysical Observatory on Kauai, HI, was completed in early 2016, and the broadband signal chain, built by MIT Haystack Observatory, was installed. Observations with the GGAO12M and Westford antennas began in February. These will increase in duration from one to 24 hours with completion of the Commissioning Phase for KOKEE12M expected in May 2016.

New mapping functions based on in situ meteorological parameters have been developed for calculating the radio path length through the atmosphere at elevations down to 3°. The hydrostatic component of the mapping function is related to the geopotential height of the 200 mb isobaric pressure level above the site and provides a factor of two improvement in accuracy and precision over previous hydrostatic mapping functions at mid-latitudes. The wet component of the mapping function is calculated from the vertical profile of wet refractivity at the site but will provide an improvement of only about twenty-five percent. However, since the effect of known errors in the hydrostatic mapping function dominates that from the wet component, except near the equator, implementation of these mapping functions should reduce the contribution of the atmosphere to errors in estimates by VLBI and GPS of both the vertical component of site position and the radio propagation delay due to water vapor in the atmosphere.

Analysis activities at Haystack Observatory are directed towards improving the accuracy of geodetic measurements, whether these are from VLBI, GNSS, SLR, or any other technique. Those analysis activities that are related to technology development are reported elsewhere in this volume. In this report, a preliminary analysis of the first geodetic sessions with the new broadband geodetic VLBI system is reported.

A relatively inexpensive 16 Gbps data-recording system based on commercial off-the-shelf technology and open-source software has recently been developed. Combining this recorder with the parallel development of broadband Very Long Baseline Interferometer (VLBI) instrumentation is enabling dramatically improved sensitivity for both astronomical and geodetic VLBI. In this article, we describe the VLBI system and the results of a demonstration experiment that illustrates a number of cutting-edge technologies that can be deployed in the near future to significantly enhance the power of the VLBI technique.

Issues related to digital-backend (DBE) systems can be difficult to evaluate in either local tests or actual VLBI experiments. The 2nd DBE intercomparison workshop at Haystack Observatory on 25-26 October 2012 provided a forum to explicitly address validation and interoperability issues among independent global developers of DBE equipment. This special report discusses the workshop. It identifies DBE systems that were tested at the workshop, describes the test objectives and procedures, and reports and discusses the results of the testing.

The recent development of a relatively inexpensive 16-Gbps data-recording system based on commercial off-the-shelf technology and open-source software, along with parallel development in broadband Very Long Baseline Interferometry (VLBI) techniques, is enabling dramatically improved sensitivity for both astronomical and geodetic VLBI. The system is described, including the results of a demonstration VLBI experiment that illustrates a number of cutting-edge technologies that can be deployed in the near future to significantly enhance the power of the VLBI technique.

For the past three years the role of the Westford antenna in geodetic VLBI has been two-fold. Over this time its primary purpose has been to participate in standard S/X-band geodetic VLBI observations. In its secondary role the Westford antenna has been converted into a research instrument, facilitating the development of the broadband geodetic VLBI observing technique. As a research instrument, the Westford antenna incorporates a commercially-available ETS-Lindgren 3164 quadridge antenna as a radio telescope feed. The system also uses the VLBI2010 data acquisition system that incorporates digital backends (DBEs) implementing a polyphase lter bank processor. The process of converting the station from its mode of operations to a research instrument often introduces subtle anomalies that must be diagnosed prior to broadband observing. Furthermore, this bifurcation of the station’s role is not in line with the goals of the VLBI2010 specications. Until recently it has not been possible for the Westford station to serve as both an operational and research instrument without conversion for two reasons: poor sensitivity and incompatibility of backend baseband lter bandwidths. The poor sensitivity of the Westford antenna as a broadband radio telescope is in large part due to the commercial broadband feed which was readily available when the proof-of-concept VLBI2010 observations were initiated. However, with the materialization of the quadridge ared horn (QRFH) by the California Institute of Technology and with the improvements in the DiFX software correlator, the necessary components are now available to upgrade the Westford station to full-broadband capability while adhering to the mandate to maintain backwards compatibility with the legacy S/X systems. In this paper we will present the path forward for upgrading the Westford site to full-broadband capability while maintaining S/X compatibility.

The International VLBI Service for Geodesy and Astrometry (IVS) is well on the way to fully defining a next generation VLBI system, called VLBI2010. The goals of the new system are to achieve 1-mm position accuracy over a 24-h observing session and to carry out continuous observations, with initial results to be delivered within 24 h after taking the data. 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, and improved methods for handling biases related to system electronics, deformations of the antenna structures, and radio source structure. To test many of the proposed strategies, NASA is sponsoring a proof-of-concept development effort using IVS antennas near Washington, DC, and Boston, MA. Furthermore, as of Feb. 2009, the construction of ten new VLBI2010 sites has already been funded, which will improve the geographical distribution of geodetic VLBI sites and provide an important step towards a global VLBI2010 network.

For the past three years the role of the Westford antenna in geodetic VLBI has been two-fold. Over this time its primary purpose has been to participate in standard S/X-band geodetic VLBI observations. In its secondary role the Westford antenna has been converted into a research instrument, facilitating the development of the broadband geodetic VLBI observing technique. As a research instrument, the Westford antenna incorporates a commercially-available ETS-Lindgren 3164 quadridge antenna as a radio telescope feed. The system also uses the VLBI2010 data acquisition system that incorporates digital backends (DBEs) implementing a polyphase lter bank processor. The process of converting the station from its mode of operations to a research instrument often introduces subtle anomalies that must be diagnosed prior to broadband observing. Furthermore, this bifurcation of the station’s role is not in line with the goals of the VLBI2010 specications. Until recently it has not been possible for the Westford station to serve as both an operational and research instrument without conversion for two reasons: poor sensitivity and incompatibility of backend baseband lter bandwidths. The poor sensitivity of the Westford antenna as a broadband radio telescope is in large part due to the commercial broadband feed which was readily available when the proof-of-concept VLBI2010 observations were initiated. However, with the materialization of the quadridge ared horn (QRFH) by the California Institute of Technology and with the improvements in the DiFX software correlator, the necessary components are now available to upgrade the Westford station to full-broadband capability while adhering to the mandate to maintain backwards compatibility with the legacy S/X systems. In this paper we will present the path forward for upgrading the Westford site to full-broadband capability while maintaining S/X compatibility.

The next generation geodetic VLBI instrument is being developed with a goal of 1 mm position uncertainty in twenty-four hours. The broadband signal chain, which is essential for obtaining the required delay accuracy from a network of relatively small antennas, has been implemented on the 12-meter antenna at the Goddard Space Flight Center, Maryland, USA, and on the 18-meter Westford antenna at Haystack Observatory, Massachusetts, USA. Data have been obtained in four 512 MHz bands spanning the range 3.2 to 9.9 GHz using commercially available broadband feeds, LNAs, digital back ends, and recorders. The first geodetic-style observing session has been completed. While demonstrating that the broadband hardware functions as expected, the six-hour session has illuminated areas of the scheduling, correlation, and post-correlation process that require improvement. 1. The VLBI2010 Broadband Observing System The potential of the broadband delay and some of the expected challenges in processing the data were presented in the report on the Proof-of-Concept (PofC) system in the Proceedings for the previous IVS General Meeting [1]. Since that time a fundamental element of the VLBI2010 concept, a fast-slewing 12-meter antenna [Figure 1], has been installed adjacent to the 5-meter antenna at the Goddard Geophysical and Astronomical Observatory (GGAO) on the grounds of the Goddard Space Flight Center, and the PofC instrumentation has been replaced with production versions of the broadband signal chain. The principal replaced components are the feed, digital back end, and recorder, each of which are described in the following paragraphs. A significant improvement to the VLBI2010 system is the incorporation of the quadruple-ridged flared horn (QRFH) feed in place of the Lindgren feed that was used for the PofC demonstration. The QRFH feed was developed at Caltech [2] and provides the two desirable features for geodetic VLBI, beamwidth and phase center that are largely independent of frequency over the 2-14 GHz range. Equally important, this is achieved using only one low noise amplifier (LNA) per polarization. As with all proposed feeds, the QRFH output is dual linear polarizations. Different versions of the QRFH feed are used for the two antennas due to the different f/D ratios. The digital back end, designated RDBE-H (hereafter referred to as RDBE), is a completely new design developed by NRAO-Socorro and MIT Haystack Observatory [3]. Features new to the IVS 2012 General Meeting Proceedings 13 Arthur Niell et al.: First Broadband Results with a VLBI2010 System Figure 1. The MIT 12-m antenna installed at Goddard Space Flight Center. This is the first antenna fully configured for VLBI2010 operation. RDBE compared to the DBE1 used previously are selectable channel output, improved threshold setting for quantization, adjustable attenuators for setting power levels on input, and control and synchronous detection of an external noise diode for measurement of system temperature. The output of the RDBE is via 10 Gigabit Ethernet in Mark 5B format for recording on the Mark 5C. Conversion of the RF signal to IF is accomplished by the UpDown Converter, a component used in the PofC that did not require any improvement. Another step in the move to VLBI2010 is the use of the DiFX software correlator [4]. This has required the addition of the capability of converting the native output into Mark IV format, which has been accomplished by the creation of a program difx2mark4, which parallels difx2fits for conversion to the astronomical data format. A basic assumption in the broadband concept is that the data from the four bands and both linear polarizations will be fit coherently for the delay observable and for the differential ionosphere dispersion. This, and the use of all available phasecal tones in a channel, have required significant modifications and additions to the estimation program fourfit. The broadband concept is that data from four bands spanning approximately 2.2 GHz to up to 14 GHz will provide sufficient phase accuracy to estimate the VLBI delay and differential ionosphere with no ambiguity. Thus a total of four parallel hardware paths are required. The signal chain that has been designed to implement this concept is shown in Figure 2.

Analysis activities at Haystack Observatory are directed towards improving the accuracy of geodetic measurements, whether these are from VLBI, GNSS, SLR, or any other technique. Those analysis activities that are related to technology development are reported elsewhere in this volume. In this report we present some preliminary results of an analysis of a 24-hour broadband VGOS session from May 22, 2013. The data were calibrated to obtain correlated flux densities at X-band for a subset of the sources, and a geodetic solution was obtained. Both analyses led to improvements in the DiFX correlator and the post-correlation software.

A digital backend based on the ROACH board has been developed jointly by the National Radio Astronomy Observatory and MIT Haystack Observatory. The RDBE will have both Polyphase Filterbank and Digital Downconverter personalities. The initial configuration outputs sixteen 32-MHz channels, comprised of half the channels from the PFB processing of the two IF inputs, for use in the VLBI2010 geodetic system and in the VLBA sensitivity upgrade project. The output rate is 2x109 bits/second (1x10(exp 9) bits/sec = 1 Gbps) over a 10 GigE connection to the Mark 5C with the data written in Mark 5B format on disk.

The next generation geodetic VLBI instrument is being developed with a goal of 1 mm position uncertainty in twenty-four hours. We have implemented a proof-of-concept system for a possible VLBI2010 signal chain, from feed through recorder, on the Westford (Massachusetts, USA) 18-m and MV-3 (Maryland, USA) 5-m antennas. Data have been obtained in four 512 MHz bands spanning the range 3.5 to 11 GHz to investigate the sensitivity and phase delay capability of the system. Using a new phase cal design, the phases have been aligned across four bands spanning 2 GHz with an RMS deviation of approximately eight degrees. Several components of the system will be improved for the prototype version of VLBI2010, including the feed, digital backend, and recorder, and these will be installed on a 12-m antenna that has been purchased and is ready for installation at the Goddard Space Flight Center outside of Washington, D.C., USA, site of the MV-3 antenna.