ARISE: The Next Generation Space VLBI Mission
We have defined an affordable advanced Space Very Long Baseline Interferometry (VLBI) mission named ARISE (Astronomical Radio Interferometry between Space and Earth).
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On More than Two Decades of Celestial Reference Frame VLBI Observations in the Deep South: IVS-CRDS (1995 - 2021)
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.
VLBI↗
Automatic Processing of Intensives at GSFC VLBI Analysis Center
VLBI Intensive (INT) sessions are conducted to determine the change in Earth rotation, which is measured as a correction to UT1. This correction varies unpredictably over time. Because UT1 is used in precise navigation, particularly GNSS, rapid turnaround is very important. The decrease of time elapsed from observations to obtained results can be shortened with the automatic data processing of new INT sessions at the analysis stage.
VLBI↗
VGOS VLBI Intensives Between MACGO12M and WETTZ13S for the Rapid Determination of UT1-UTC
In this work, we present a status update and preliminary results of the designated research and development VLBI Intensive program VGOS-INT-S, observed between MACGO12M and WETTZ13S for the rapid determination of the Earth’s phase of rotation, expressed via UT1-UTC. Since 2021, 27 Intensive sessions have been observed successfully utilizing a special observation strategy alternating between high- and low-elevation scans for improved determination of delays caused by the neutral atmosphere. Between the end of January and mid of March 2022, VGOS-INT-S was among the most accurate Intensive programs. During this time, eight sessions were observed with an average formal error of 3.1 µs and a bias w.r.t. IERS C04 of 1.1 µs. Later, the session performance decreased due to multiple technical difficulties.
VLBI↗
VLBI clock synchronization tests performed via the ATS-1 and ATS-3 satellites
Clock synchronization experiments were carried out May 10 to June 10, 1971, by the NASA/Goddard Space Flight Center and the Smithsonian Astrophysical Observatory via the ATS-1 and 3 geostationary satellites at the NASA tracking stations Rosman and Mojave, during a VLBI (Very Long Baseline Interferometer) experiment in order to determine the clock-offset between the two stations. Ten microsecond pulses at C-band with very sharp risetime were exchanged by the two stations through the dual transponders of the satellites. At each station, a time-interval counter was started by the transmitted pulse and stopped by the received pulse. The probable error of the difference in the mean values of the clock-offset is 10 nanoseconds.
Ramasastry, J.↗
Upper limit to the 11.4 m flux of Saturn using VLBI.
Summary of a series of interferometric observations of Saturn using large phased dipole arrays at 11.4 m wavelength (26.3 MHz). The observations were made in August 1971 using a VLBI system operated over two baselines. The results obtained are interpreted as negative for both decametric continuum and noise storm emission from source regions much less than the planetary disk size. This leads to an upper limit value of approximately 14 flux units from a source less than 1 arc sec in diameter located in a region plus or minus 40 min in right ascension and 3.5 deg in declination about Saturn's optical position.
Shawhan, S. D.↗
Wind speeds in lower atmosphere of Venus: Status report on possible measurement via differential VLBI tracking of entry probes
The potential of very-long-baseline interferometry (VLBI) is examined for use in the determination of wind speeds in Venus' lower atmosphere via the differential tracking of entry probes. A simplified mathematical model is presented in detail. An incomplete error analysis based on this model permits an educated guess to be made: an uncertainty in wind speed determination of no more than about 100/t m/sec, where t l is the corresponding time resolution in seconds, is an achievable goal, without the use of transponders on the miniprobes. If transponders are available on all probes, there should be little difficulty in estimating wind speeds with useful precision.
Shapiro, I. I.↗
Lunar baselines and libration from differential VLBI observations of ALSEPs
Differential very-long-baseline interferometric observations of signals from Apollo Lunar Surface Experiment Package telemetry transmitters will yield the relative projected positions of the transmitters with uncertainty of only 1-3 m, set mainly by uncertainty of the lunar ephemeris. Noise and systematic instrumental errors which in the past affected similar observations have been reduced to the equivalent of a few centimeters on the lunar surface by the development of a new type of differential receiver. Continued observations should yield a determination of the motion of the moon about its center of mass with uncertainty less than 1 sec of selenocentric arc. Improvements (by other means) in our knowledge of the moon's orbital motion would allow a further order-of-magnitude refinement in the libration and relative position results obtainable by differential VLBI.
Counselman, C. C., III↗
Precision timing and Very Long Baseline Interferometry (VLBI)
The use of very long baseline interferometry (VLBI) for investigating small angular features in galactic and extragalactic radio sources is discussed. Mathematical models are developed to describe the operation of an interferometer and to show the variations in the resultant fringe frequency. The types of sources to which the interferometer will respond are identified.
Clark, T. A.↗
Demonstration of intercontinental DSN clock synchronization by VLBI
The prototype system for Deep Space Network clock synchronization by VLBI has been demonstrated to operate successfully over intercontinental baselines in a series of experiments between Deep Space Stations at Madrid, Spain, and Goldstone, California. As predicted by analysis and short baseline demonstration, the system achieves reliable synchronization between 26m and 64m antenna stations with 17 and 37K nominal system temperatures using under one million bits of data from each station. Semi-real-time operation is feasible since this small amount of data can be transmitted to JPL and processed within minutes. The system resolution is 50 to 400ns, depending on the amount of data processed and the source intensity. The accuracy is believed to be comparable to the resolution, although it could be independently confirmed to only about 5 microseconds using LORAN C.
Hurd, W. J.↗
ALSEP: Quasar differential VLBI
A program of Apollo Lunar Surface Experiments Package (ALSEP)-Quasar Very Long Baseline Interferometry (VLBI) is being reported. These observations primarily employ a 4 antenna technique, whereby simultaneous observations with two antennas at each end of an intercontinental baseline are used to derive the differential interferometric phase between a compact extragalactic radio source (usually a quasar) and a number of ALSEP transmitters on the lunar surface. A continous ALSEP-quasar differential phase history over a few hour period leads to extremely high angular accuracy in measuring the lunar position against the quasar reference frame. This application of the 4 antenna technique has been underway for more than a year and is now producing high quality data utilizing Deep Space Network (DSN) stations in Australia, Spain, and Goldstone, California, as well as the Spaceflight Tracking and Data Network (STDN) Apollo station at Goldstone.
Slade, M. A.↗
Meter-wavelength VLBI. II - The observations
Observations made during a series of meter-wavelength very-long-baseline (VLBI) experiments conducted during 1971-1973 are reported. A wide variety of objects was observed, including many extragalactic sources known to have compact components, several strong pulsars (especially the Crab nebula pulsar), and a selection of supernova remnants. The experiments are discussed in detail, and tests made to check the consistency of the results are described. The measured correlated flux densities are presented for each source as a function of interferometer baseline and observing frequency. An assessment is made of the measurement errors and of the possibility of confusion within the interferometer beam. The results show that all sources except the pulsars, which are known to have intrinsically small sizes, are resolved on the longest baseline. We also conclude that no supernova remnants except the Crab nebula and Cassiopeia A gave evidence of compact components.
Clark, T. A.↗
An unusually strong radio outburst in Algol - VLBI observations
For 8 hours during a strong radio flare on Jan. 15, 16, 1975, the close binary system Beta Persei (Algol) was observed with a three-station VLBI array operating at 7850 MHz. The size of the radio source was estimated to have been about 1.7 milliarcsec (0.05 AU), based on a model of a uniformly bright disk. The corresponding brightness temperature was nearly 10 billion K, indicating that the emission was probably nonthermal. There was no evidence for expansion of the source; the upper limit on the rate of any expansion was 100 km/s. The position of the Algol radio source with respect to an extragalactic reference frame was also determined from these observations with an uncertainty of about 0.1 arcsec in each coordinate.
Clark, T. A.↗
Meter-wavelength VLBI. III - Pulsars
Observations of pulsars, especially the Crab Nebula pulsar, made in very long baseline interferometry (VLBI) experiments are discussed. Based on a crude 144 MHz visibility curve which is consistent with a Gaussian brightness distribution, the measured visibilities at 196, 111, and 74 MHz were interpreted to yield apparent angular diameters (at half-power) of about 0.03 sec, 0.07 sec, and 0.18 sec, respectively. These sizes scale approximately as wavelength-squared, and the 74 MHz size agrees with recent observations using interplanetary scintillation techniques. The total flux densities lie on the extrapolation from higher frequencies of the pulsing flux densities. Variations in the total flux density up to 25 per cent were observed. A lack of fine structure other than the pulsar in the nebula is indicated by the simple visibility curves. The pulse shapes are similar to single-dish measurements at 196 MHz but reveal a steady, nonpulsing component at 111 MHz. The ratio of pulsing to total power was approximately equal to one-half but varied with time. It was found that four strong, low-dispersion pulsars were only slightly resolved.
Vandenberg, N. R.↗
VLBI Observations of ALSEP Transmitters
The technique of differential very-long-baseline inteferometry (VLBI) Apollo Lunar Surface Experiments Package was used to measure the relative positions of the (ALSEP) transmitters at the Apollo 12, 14, 15, 16, and 17 lunar landing sites with uncertainties less than 0.005 sec of the geocentric arc. These measurements yielded improved determinations of the selenodetic coordinates of the Apollo landing sites, and of the physical libration of the moon.
Counselman, C. C.↗
Meter-wavelength VLBI. IV - Temporal and spatial scattering of the Crab Nebula pulsar's radiation
The spatial and temporal phenomena observed in the Crab Nebula pulsar's radiation are reviewed, with emphasis on the recent VLBI observations of the apparent angular size and the ratio of pulsing to total power. Three models for scattering (single thin screen, two screen, and extended medium) are discussed and shown to be deficient, under the usual assumption of a uniform scattering medium, in explaining the observed combination of temporal and angular broadening. A new model is proposed, in which the temporal broadening is dominated by scattering in the small filament which lies in the nebula in front of the pulsar, while the extended interstellar medium causes the angular broadening. This model explains the otherwise discordant angular and temporal effects and also the lack of an apparent source-size expansion with pulse phase.
Vandenberg, N. R.↗
Lunar dynamics and selenodesy - Results from analysis of VLBI and laser data
Very long base line interferometry (VLBI) observations of lunar radio transmitters have been combined with data from laser ranging to lunar retroreflectors to estimate simultaneously (1) parameters in models of the lunar orbit and libration and (2) the selenodetic coordinates of the radio transmitters and retroreflectors. For the ratio of the mass of the sun to that of the earth plus moon, a value of approximately 328,900.5 is obtained. The lunar moment-of-inertia ratios beta and gamma are also determined. The values and most of the uncertainties obtained for the third-degree harmonics of the moon's gravity field are comparable to those which have been obtained by others from observations of lunar-orbiting spacecraft. However, the results for the two third-degree gravity coefficients have much less uncertainty than determinations based on laser data alone. For the relative position vectors of the lunar radio transmitters and the retroreflectors, the estimates have uncertainties of about 30 m along the earth-moon direction and about 10 m in each of the two transverse coordinates.
King, R. W.↗
VLBI Validation Project
Two training and three operational Deep Space Network experiments conducted in support of the very long baseline interferometry (VLBI) validation project are reported. Vector measurements using common benchmark locations at continental distances (Goldstone, Haystack, and the McDonald Observatory) with an accuracy goal on the order of 5 cm. were obtained and compared with laser ranging systems data.
Ross, W. J.↗