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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

Geodetic Reference Instrument Transponder for Small Satellites (Gritss)

The accuracy of the Terrestrial Reference Frame is limited by systematic errors in tying the contributions from the different geodetic techniques. Local survey uncertainties are limited due to the inaccessibility of the instrument reference points. NASA is implementing a technology demonstration mission called Geodetic Reference Instrument Transponder for Small Satellites(GRITSS) that uses a small satellite as a space-based ref-refence point for tying together collocated geodetic stations. The technology being demonstrated uses a novel idea of upconverting the Global Positioning System (GPS) signals received at the satellite and transponding them to a VLBI Global Observing System (VGOS) antenna ground station. This approach does not require the satellite to be in view of more than one VLBI station at a time, allowing the use of Low Earth Orbits and an inexpensive CubeSat. The demonstration will initially involve only the NASA VGOS stations in Maryland, Hawaii, and Texas, but may be expanded later in the mission to include other international VGOS stations. This presentation will provide an overview of the GRITSS mission and the measurement concept.

S M Merkowitz↗
NASA NTRSDate not supplied

Geodetic Reference Instrument Transponder for Small Satellites (GRITTS)

The accuracy of the Terrestrial Reference Frame is limited by systematic errors in tying the contributions from the different geodetic techniques. Local survey uncertainties are limited due to the inaccessibility of the instrument reference points. NASA is implementing a technology demonstration mission called Geodetic Reference Instrument Transponder for Small Satellites(GRITSS) that uses a small satellite as a space-based ref-refence point for tying together collocated geodetic stations. The technology being demonstrated uses a novel idea of upconverting the Global Positioning System (GPS) signals received at the satellite and transponding them to a VLBI Global Observing System (VGOS) antenna ground station. This approach does not require the satellite to be in view of more than one VLBI station at a time, allowing the use of Low Earth Orbits and an inexpensive CubeSat. The demonstration will initially involve only the NASA VGOS stations in Maryland, Hawaii, and Texas, but may be expanded later in the mission to include other international VGOS stations. This presentation will provide an overview of the GRITSS mission and the measurement concept.

S. M. Merkowitz↗
NASA NTRSDate not supplied

Geodetic Reference Instrument Transponder for Small Satellites (GRITTS)

The accuracy of the Terrestrial Reference Frame is limited by systematic errors in tying the contributions from the different geodetic techniques. Local survey uncertainties are limited due to the inaccessibility of the instrument reference points. NASA is implementing a technology demonstration mission called Geodetic Reference Instrument Transponder for Small Satellites(GRITSS) that uses a small satellite as a space-based ref-refence point for tying together collocated geodetic stations. The technology being demonstrated uses a novel idea of upconverting the Global Positioning System (GPS) signals received at the satellite and transponding them to a VLBI Global Observing System (VGOS) antenna ground station. This approach does not require the satellite to be in view of more than one VLBI station at a time, allowing the use of Low Earth Orbits and an inexpensive CubeSat. The demonstration will initially involve only the NASA VGOS stations in Maryland, Hawaii, and Texas, but may be expanded later in the mission to include other international VGOS stations. This presentation will provide an overview of the GRITSS mission and the measurement concept.

S M Merkowitz↗
NASA NTRS2000

Dynamical Reference Frame: Current Relevance and Future Prospects

Planetary and lunar ephemerides are no longer used for the determination of inertial space. Instead, the new fundamental reference frame, the International Celestial Reference Frame (ICRF), is inherently less susceptible to extraneous, non-inertial rotations than a dynamical reference frame determined by the ephemerides would be. Consequently, the ephemerides are now adjusted onto the ICRF, and they are fit to two modern, accurate observational data types: ranging (radar, lunar laser, spacecraft) and Very Long Baseline Interferometry (VLBI) (of spacecraft near planets). The uncertainties remaining in the inner planet ephemerides are on the order of 1 kilometer, both in relative positions between the bodies and in the orientation of the inner system as a whole. The predictive capabilities of the inner planet ephemerides are limited by the uncertainties in the masses of many asteroids. For this reason, future improvements to the ephemerides must await determinations of many asteroid masses. Until then, it will be necessary to constantly update the ephemerides with a continuous supply of observational data.

VLBI (VERY LONG BASELINE INTERFEROMETRY)↗
NASA NTRS1996

Combinations of EOP Measurements: COMB95 and POLE95

A Kalman filter has been used to combine independent measurements of the Earth's orientation taken by optical astrometry and the space-geodetic techniques of LLR, SLR, VLBI, and GPS.

Kalman filter Kalman Earth's orientation LLR SLR V↗
NASA NTRS1994

(abstract) Asteroids and Comets: Future Imaging Opportunities with Earth-Based Radar Systems

Major improvements currently underway to the Arecibo 305 m antenna and 13 cm wavelength radar system will currently increase the radar system's sensitivity by a factor of about 20. Recent upgrades to the Goldstone 3.5 cm wavelength radar system have also improved its sensitivity. While the Arecibo system will have significantly greater sensitivity than Goldstone, the Arecibo antenna's limited declination coverage of -2(deg) to +38(deg) means that the Goldstone system will be important for the investigation of near-Earth objects with declinations outside this range. Over one hundred mainbelt and near-Earth asteroids per year will be observable with the new Arecibo system, and there will be a number of good imaging opportunities each year. While only a few imaging opportunities are predicted for comets over the next 10 years, the system will be able to respond to unexpected cometary opportunities. We are currently investigating the applicability of VLBI techniques to the direct synthesis of images of asteroids and comets illuminated by a radar signal.

asteroids comets radar imaging Arecibo Goldstone V↗
NASA NTRS1994

Sub-Daily Polar Motion During Epoch '92 with GPS

Data from a worldwide Global Positioning System (GPS) tracking network spanning six days during the EPOCH '92 campaign are used to estimate variations of the Earth's pole position every 30 minutes. The resulting polar motion time series is compared with estimates derived from very long baseline interferometry (VLBI) observations.

Global Positioning System (GPS) VLBI polar motion↗
NASA NTRS2006

Use of Very Long Baseline Array Interferometric Data for Spacecraft Navigation

The main VLBI technique that is used at JPL is known as the Delta Differential One-way Ranging ((Delta)DOR). Two DSN antennas simultaneously track a source, and alternate between sources. The signals recorded at the antennas from each source are correlated to obtain the delay in arrival to the two antennas, and the delays are differenced to remove common-source errors. An alternative technique is to use carrier phase differences between antennas. This is routinely done by the Very Large Baseline Array (VLBA) as part of source imaging. The VLBA capabilities are used for scientific research, but also have the potential to be used for navigation. Two main experiments were performed with the VLBA and JPL spacecraft. This paper describes and analyzes these experiments and discusses the possible uses of VLBA tracking for spacecraft navigation.

Very Large Baseline Array (VLBA)↗
NASA NTRS2012

A Celestial Reference Frame at X/ka-Band (8.4/32 Ghz) for Deep Space Navigation

Deep space tracking and navigation are done in a quasi-inertial reference frame based upon the angular positions of distant active galactic nuclei (AGN). These objects, which are found at extreme distances characterized by median redshifts of z = 1, are ideal for reference frame definition because they exhibit no measurable parallax or proper motion. They are thought to be powered by super massive black holes whose gravitational energy drives galactic sized relativistic jets. These jets produce synchrotron emissions which are detectable by modern radio techniques such as Very Long baseline Interferometry (VLBI).

very long baseline interferometry (VLBI)↗
NASA NTRS2010

Use of Reference Frames for Interplanetary Navigation at JPL

Navigation of interplanetary spacecraft is typically based on range, Doppler, and differential interferometric measurements made by ground-based telescopes. Acquisition and interpretation of these observations requires accurate knowledge of the terrestrial reference frame and its orientation with respect to the celestial frame. Work is underway at JPL to reprocess historical VLBI and GPS data to improve realizations of the terrestrial and celestial frames. Improvements include minimal constraint alignment, improved tropospheric modeling, better orbit determination, and corrections for antenna phase center patterns.

International Celestial Reference Frame (ICRF)↗
NASA NTRS2000

The Pearson-Readhead Survey from space

The VSOP Space VLBI mission uses the HALCA spacecraft, launched from Japan in February 1997, in conjunction with gound radio observatories around the world to create a high resolution radio-wavelength imaging facility

Radio↗
NASA NTRS1996

Errors in the J 3 Part of Nutation Series

This paper points out that the nutation terms of Zhu and Gorten due to the tidal potential of degree 3 are erroneous. Due to the high accuracy of Very Long Baseline Interferometry (VLBI), present theoretical nutation models should consider amplitudes of theorder of a few micro arcseconds.

nutation↗
NASA NTRSDate not supplied

On The Processing of Log Files for Monitoring Antenna Health

In order to improve the quality of geodetic results, we have developed an infrastructure for timely processing of telemetry from IVS observing stations. We check every hour for new log files with telemetry from both VLBI observing sessions, single dish experiments, and stow-in data collection and automatically process them. The telemetry data we use is the system temperature, phase calibration phases and amplitudes, system equivalent flux density, and the differences between formatter clock and GPS clock. For the system temperature and phase calibration, processing includes filtering out outliers and computing averages and rms of the scatter in each scan. Furthermore, for the phase calibration we also compute the group delay and detect spurious signals. Cleaned and post-processed telemetry is archived. Our process detects abnormalities, such as, anomalously high system temperature, unstable phase calibration phases, jumps in the GPS and formatter clock differences, and others. With our procedure, the latency of detection of station abnormalities is reduced to less than two hours. Early detection of abnormalities reduces the amount of affected data since station personnel get early alerts. We discuss our experience of running this system since 2022.

Phase Calibration↗