Aims. We computed a celestial reference frame (CRF) from Very Long Baseline Interferometry (VLBI) Global Observing System (VGOS) data after five years of regular observations carried out under the umbrella of the International VLBI Service for Geodesy and Astrometry. In this paper we evaluate its strengths and weaknesses, document the source selection and scheduling strategies for the individual sessions, and investigate the effect of using this new VGOS CRF in the analysis of individual geodetic VLBI sessions where the radio source positions are fixed to their a priori coordinates. Methods. We estimated the VIE2023-VG CRF in a global adjustment of 155 multi-baseline 24-hour VGOS sessions until 2024.0. We carried out several comparisons with the third version of the International Celestial Reference Frame (ICRF3) adopted by the International Astronomical Union in 2018, and with VIE2023sx CRF which includes VLBI S/X data until 2024.0. Furthermore, we studied the effect of more frequent estimations of tropospheric parameters (30,10, and 5 min for zenith wet delay) on the estimated CRF in the current VGOS network. We evaluated the VIE2023-VG CRF in the geodetic analysis of VGOS sessions by computing the baseline lengths and station positions and statistics on the Earth orientation parameters estimated in the single-session analysis where the source positions were fixed to either the VIE2023-VG CRF or to ICRF3-SX. Results. The current VIE2023-VG CRF is built with 1.39 million VGOS group delays and includes 418 radio sources, where 172 sources (41%) are introduced in only four research and development sessions alone. We show that the VIE2023-VG CRF has excellent source position precision. The median formal error from the least-squares adjustment is 30 mu as for right ascension (scaled by cosine of declination) and 47 mu as for declination. In terms of systematic distortions versus ICRF3-SX, the largest terms in the vector spherical harmonics up to the degree and order two, reach in absolute values around 60 mu as, caused by correlations between the individual terms. Because of the lack of observations in the southern hemisphere, a constraint for a zero slope in declination difference with respect to ICRF3-SX is imposed in the global adjustment. Therefore, VGOS should prioritize the development of southern stations in order to limit the need for such constraints on the frame. Further we show that fixing the a priori CRF to VIE2023-VG CRF instead of the ICRF3-SX in the single-session analysis improves the weighted root mean square of the baseline length by up to 3 mm, especially for the extremely long baselines (>12 000 km), with a weighted mean difference between the baseline length time series up to 2 mm. Therefore, in order to meet the ambitious goal of 1 mm accuracy for a terrestrial reference frame set by the Global Geodetic Observing System, the development of a VGOS-only CRF is required for use in the geodetic analysis of the new generation VGOS data.

Accurate measurement of angular positions on the sky requires a well-defined system of reference, something that in practice is realized by the International Celestial Reference Frame (ICRF) with observations of distant (typical redshift $\sim$1) Active Galactic Nuclei (AGN). At such great distances a subset of these objects exhibit as little as 10$-$50 $\mu$as/year observed parallax or proper motion, thus giving the frame excellent spatial and temporal stability. Until fairly recently the majority of AGN centered imaging was accomplished in the S (2.3 GHz) and X (8.4 GHz) radio frequency bands, however S-band observations for reasons such as sensitivity “plateauing”, increased source structure (jets), and radio frequency interference (RFI) have become less productive. Following spacecraft telemetry moves to higher frequencies and a desire to strengthen JPL's leadership in defining the next-generation of celestial reference frames has motivated the development of a “Quad-band” prototype receiver that operates in X, Ku, K, and Ka band in both right hand (RCP) and left hand (LCP) circular polarization. The goal of this receiver is to achieve less than a 20 % increase in noise over the Jansky Very Large Array (JVLA, NRAO) performance specification, which in such a wide bandwidth represents a revolutionary capability. To evaluate the various technical developments of the 8 GHz$-$40 GHz receiver the feedhorn optical beam was designed to interface to the US based Very Long Baseline Array (VLBA). The receiver's intermediate frequency (IF) spans 4 GHz$-$8 GHz, giving rise to up to eight 4 GHz IF channels for a fully populated instrument. This paper outlines the technical development of a 2$^{1}$/$_{2}$ octave wide (8 GHz$-$40 GHz) X-Ka band prototype receiver, fulfilling a need for super broadband technology within the VLBI network. An important additional benefit of the wideband receiver approach is its simplicity and low cost of operation.

We present K-band (24 GHz) images of 731 compact extragalactic radio sources with submilliarcsecond resolution, based on radio interferometric observations made with the Very Long Baseline Array of 10 telescopes during 29 day long sessions spanning from 2015 to 2018 and recorded at 2048 Mbps. Many of these sources are imaged with submilliarcsecond resolution for the first time at frequencies above X band (8 GHz). From each of the K-band images, we derive the following source properties: peak brightness, core and total flux density, the ratio of peak and core to total flux (compactness measure), radial source extent, structure index, source size, and jet direction. The vast majority of sources are imaged at multiple epochs, providing insights into their temporal behavior. The use of K band was motivated by the fact that the sources are generally intrinsically more compact at higher frequencies, as well as by the factor of 3 improvement in interferometer resolution relative to the historically standard S/X band (2.3/8.4 GHz) used for a large amount of reference frame and calibrator work. Lastly, as most of the sources imaged here are in the K-band component of the third International Celestial Reference Frame, these images serve to characterize the objects used in that International Astronomical Union standard.

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.

Sagittarius A* (Sgr A*) is a strong, compact radio source believed to be powered by a super-massive black hole at the galactic center. Extinction by dust and gas in the galactic plane prevents observing it optically, but its position and proper motion have previously been estimated using radio interferometry. We present new VLBI absolute astrometry measurements of its precise position and proper motion in the frame of the third realization of the International Celestial Reference Frame, ICRF3. The observations used were made at 52 epochs on the VLBA at K-band (24 GHz) between June 2006 and August 2022. We find the proper motion of Sgr A* to be -3.128 $\pm$ 0.042 mas/yr in right ascension and -5.584 $\pm$ 0.075 mas/yr in declination, or 6.400 $\pm$ 0.073 mas/yr at a position angle of 209.26 $\pm$ 0.51 degrees. We also find its J2000 ICRF3 coordinates at the 2015.0 proper motion epoch to be 17$^h$45$^m$40.034047$^s$ $\pm$ 0.000018$^s$, -29$^o$00'28.21601'' $\pm$ 0.00044''. In galactic coordinates, Sgr A* shows proper motion of -6.396 $\pm$ 0.071 mas/yr in galactic longitude and -0.239 $\pm$ 0.045 mas/yr in galactic latitude, indicating solar motion of 248.0 $\pm$ 2.8 km/sec in the galactic plane and 9.3 $\pm$ 1.9 km/sec towards the north galactic pole.

The third realization of the International Celestial Reference Frame (ICRF3) was adopted in August 2018 and includes positions of extragalactic objects at three frequencies: 8.4 GHz, 24 GHz, and 32 GHz. In this paper, we present celestial reference frames estimated from Very Long Baseline Interferometry measurements at K-band (24 GHz) including data until June 2022. The data set starts in May 2002 and currently consists of more than 120 24h observing sessions performed over the past 20 years. Since the publication of ICRF3, the additional observations of the sources during the last four years allow maintenance of the celestial reference frame and more than 200 additional radio sources ensure an expansion of the frame. A study of the presented solutions is carried out helping us to understand systematic differences between the astrometric catalogs and moving us towards a better next ICRF solution. We compare K-band solutions (VIE-K-2022b and USNO-K-2022July05) computed by two analysts with two independent software packages (VieVS and Calc/Solve) and describe the differences in the solution strategy. We assess the systematic differences using vector spherical harmonics and describe the reasons for the most prominent ones.

Accurate measurement of angular positions on the sky requires a well-defined system of reference that is realized with accessible objects. The purpose of this study is to review the international standard realization of such a system, the International Celestial Reference Frame (ICRF). The ICRF uses the Very Long Baseline Interferometry (VLBI) technique as it has the highest resolution of any current astrometric technique for reference frames in order to observe Active Galactic Nuclei (AGN) which are at such great distances (typical redshift ∼1) that there is currently no observed parallax or proper motion of these objects thus giving the frame excellent stability. We briefly review the history of the transition from the Fundamental Katalog 5 (FK5) optical frame to VLBI-based frames with attention to each of the three generations: ICRF-1, ICRF-2, and ICRF-3. We present some of the more prominent applications of the ICRF and outline the methods used to construct the ICRF. Next we discuss in more detail the current standard ICRF-3—which is the first frame to be realized at multiple wavelengths (S/X, K, X/Ka-bands)—including an estimate of its accuracy and limiting errors. We conclude with an overview of future plans for improving the ICRF.

The 50-meter Large Millimeter Telescope (LMT) operating on the Sierra Negra in Mexico is the largest single- dish millimeter-wave telescope in the world. Although designed to work in the 3 mm and 1 mm bands, there is significant potential for LMT observations at centimeter wavelengths. Here, we summarize the scientific case and operational arguments for a K-band receiver system on the LMT, describe several of the unique technical challenges that the proposed installation would entail, and mention some possible solutions to these challenges.

We report results from multi-epoch radio astrometry of the Cassini spacecraft with the Very Long Baseline Array (VLBA). These observations are part of a program to determine a series of accurate positions for the Saturn system barycenter in the inertial International Celestial Reference Frame (ICRF) and to use these position measurements to improve our knowledge of Saturn's orbit in the planetary ephemeris. Our VLBA observations cover the full duration of the orbital phase of the Cassini mission, from Saturn orbit insertion in 2004 to the end of mission in 2017. This period covers more than one-third of Saturn's orbital period, allowing us to obtain good orbit constraints for Saturn, particularly on the inclination and ascending node longitude. During the early years of Cassini's orbital mission our VLBA data dominated the determination of orbit orientation, while later in the mission range measurements become more significant. The orientation of Saturn's orbit is now known to approximately 0.25 milli-arcseconds (1.25 nrad), an order of magnitude improvement since the start of Cassini observations. Continuing improvements in the ICRF position accuracy for our phase reference sources, and possible improvements in the final orbit solutions for Cassini, may lead to a still better Saturn orbit over the coming years.

We report a new result of a packaged low noise amplifier (LNA) module with wide bandwidth of 5 to 35 GHz and low noise temperature performance of 10 -18 K, while operated at 10 K ambient. The LNA used 3-stages of sub-50 nm gate length, 100% indium channel content indium phosphide (InP) high electron mobility transistors (HEMTs). Wideband cryogenic LNAs are important for future radio astronomy observatories. To our knowledge these results represent the lowest noise achieved in a wideband amplifier from 5–35 GHz.

The Very Long Baseline Array (VLBA) is a ten antenna radio interferometer with baseline lengths up to 8000 km. It can provide astrometric measurements of spacecraft orbiting planets and other objects in our solar system with subnrad precision (5 nrad = 1 milli-arcsec). These measurements can be used to create a time series of positions for solar system objects in the inertial International Celestial Reference Frame, which in turn can be combined with other data to refine the planetary ephemeris. An accurate solar system ephemeris is critical for interplanetary spacecraft navigation, dynamical studies and tests of gravitational theories, the analysis of pulsar timing observations, predictions of transits, eclipses, and occultations, and other applications. We are using VLBA observations of the Juno spacecraft in orbit about Jupiter to provide accurate positions for the Jupiter system barycenter for the ephemeris development group at the Jet Propulsion Laboratory, using observing and data reduction techniques developed for similar observations of the Cassini spacecraft while it orbited Saturn from 2004 until 2017. The VLBA observations of Cassini helped to improve the accuracy of Saturn's orbit by nearly an order of magnitude, and we expect that our observations of Juno will produce a similar improvement in our knowledge of Jupiter's orbit. Astrometric positions are particularly useful in constraining the orientation (inclination and longitude of ascending node) of an orbit, while range measurements are most useful in constraining the semi-major axis and ellipticity. Juno's orbit around Jupiter has a longer period than initially planned due to a concern about the spacecraft main engine. The resulting extended mission duration will improve our constraints on Jupiter's orbit inclination beyond that originally expected. Our VLBA observations of Juno are scheduled during approximately every third or fourth perijove pass. During these times the Juno spacecraft is continuously tracked by the Deep Space Network and the most precise solutions for the orbit of Juno about Jupiter are available. A good spacecraft orbit solution is needed to transfer our spacecraft sky positions to planet system barycenter positions. VLBA astrometry of planetary spacecraft has previously been applied to Mars orbiting space-craft, and will also be used during the OSIRISREx mission to improve the accuracy of the orbit of the potentially hazardous asteroid Bennu.

We have been using the Very Long Baseline Array (VLBA) to measure accurate sky positions of the Juno spacecraft since its insertion into orbit around Jupiter in July 2016. These positions are referenced to the inertial International Celestial Reference Frame. During Juno's perijove passes the relative positions of the spacecraft and the Jupiter system barycenter are extremely well determined from Doppler tracking by the Deep Space Network (DSN). Combining our VLBA measurements with DSN tracking gives us accurate Jupiter barycenter positions, which in turn can be used to improve our knowledge of Jupiter's orbit (particularly its orientation) as part of the JPL planetary ephemeris. A problem with the main engine has kept Juno in its current 53.5-day orbit instead of the originally planned 14-day orbit. This will extend the mission duration in Jupiter orbit to nearly 5 years, well over 1/4 of Jupiter's orbital period, which in turn will allow us to obtain stronger constraints on orbital inclination and ascending node longitude. DSN range measurements complement the astrometric measurements by providing the best constraints on orbit semi-major axis and eccentricity. We expect to reduce the uncertainty in the orientation of Jupiter's orbit with respect to the International Celestial Reference Frame from approximately 20 milliarcseconds (mas) to approximately 0.2 mas (1 nrad), based on the improvement in Saturn's orbit obtained from similar VLBA measurements of the Cassini spacecraft. Our initial VLBA observations of Juno, combined with the Ulysses flyby in 1992, have already improved the Jupiter ephemeris accuracy by about a factor of 4.