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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Earth Rotation Parameters from DSN VLBI: 1996

A despcription of the DSN VLBI data set and of most aspects of the data analysis can be found in the IERS Technical Note 17, pp. R-19 to R-32 (see also IERS Technical Note 19, pp. R-21 to R-27). The main changes in this year's analysis form last year's are simply due to including another year's data.

VLBI rotation Earth's rotation troposphere troposp↗
NASA NTRS1995

Astrometry VLBI in Space (AVS

A proposed new space radio astronomy mission for astrometry is described. The Astrometry VLBI (very long baseline) in Space (AVS) nominal mission includes two identical spacecraft, each with a 4-m antenna sending data to a 70-m ground station. The goals of AVS are improving astrometry accuracy to the microarcsecond level and improving the accuracy of the transformation between the inertial radio and optical coordinate reference frames.

astrometry very long baseline interferometry VLBI↗
NASA NTRS1995

VLBI Monitoring of the Nucleus of Centaurus A

A series of 8.4 GHz VLBI images of the nucleus of Centaurus A have been made with a Southern Hemisphere array over more than a 3-year time span. The nuclear radio jet is approximately 50 mas in length, or about 1 pc at the 3.5 Mpc distance of Centaurus A. Sub-luminal motion is seen and structural changes observed on time-scales shorter than four months. Observations at both 4.8 and 8.4 GHz at one epoch allow identification of the core at the southwestern end of the jet.

VLBI Centaurus A↗
NASA NTRS1995

VLBI Phase-Referenced Observations on Southern Hemisphere HIPPARCOS Radio Start

Presented are multiepoch Very Long Baseline Interferometry (VLBI) observations on Southern Hemisphere radio stars phase-referenced to background radio sources. The differential astrometry analysis results in high-precision determinations of proper motions and parallaxes. The astrophysical implications and astrometric consequences of these results are discussed.

Radio Emission Stars VLBI Radio Astronomy Astrophy↗
NASA NTRS1995

Earth Rotation Parameters From DSN VLBI: 1995

A description of the DSN VLBI data set and of last year's analysis can be found in last year's report. Other than including another year's data, the main changes in this year's analysis from last year's are in the use of meteorological data for determining tropospheric parameters and in the weighting of the data to account for the uncertainty in the observables caused by tropospheric effects and source structure. A priori dry zenith tropospheric delays were determined from barometric pressure measurements at the DSN sites, corrected for height differences between the pressure sensor and the antennas. A priori wet zenith tropospheric delays were derived from tables of monthly average wet zenith delays for each station, which are based on historical radiosonde data. The Lanyi function was used for mapping zenith tropospheric delays to observed elevations. the temperature at the top of the boundary layer, a parameter in the Lanyi function, was taken to be the 24-hour average of the surface temperature at the station. Adjustments to the wet troposphere zenith delays were estimated every two to three hours.

Earth rotation, DSN, VLBI↗
NASA NTRS2003

JPL VLBI analysis center IVS annual report for 2002

This report describes the activities of the JPL VLBI analysis center for the year 2002. The highlight for the year was the creation of the first sub-milliarcsecond global celestial reference frames at K-band (24 GHz) and Q-band (43 GHz). Our group also continues to do celestial reference frame, earth orientation and spacecraft navigation work at S (2.3 GHz) and X-bands (8.4 GHz).

VLBI↗
NASA NTRS2023

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↗
NASA NTRSDate not supplied

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↗
NASA NTRS2023

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↗
NASA NTRS1971

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.↗
NASA NTRS1973

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.↗