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At least 577 records · Page 32
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NASA NTRS1990

Using connected-element interferometer phase-delay data for Magellan navigation in Venus orbit

The pointing accuracy needed to support Magellan's Synthetic Aperture Radar mapping of Venus places stringent requirements on navigation accuracy. This need is met with a combination of two-way Doppler and narrowband delta Very Long Baseline Interferometer (delta VLBI) data, which are capable of determining the spacecraft's orbit to the required level, typically about one-kilometer position uncertainty. Differenced Doppler (two-way Doppler minus three-way Doppler) is also capable of meeting mission navigation requirements, and serves as a backup to narrowband delta VLBI. The Magellan Project specifies that the turn-around time for processing narrowband delta VLBI data must be 12 hours or less, a very difficult requirement to meet operationally. The use of phase-delay data, taken from a Connected-Element Interferometer (CEI) with a 21-km baseline, for Magellan orbit determination was investigated to determine if navigation performance comparable with narrowband delta VLBI and differenced Doppler could be achieved. CEI possesses an operational advantage over delta VLBI data in that the observables are constructed in near-real time, thus greatly reducing the turn-around time needed to process the data, relative to the off-line system used to generate delta VLBI observables. Unfortunately, the results indicate that CEI data are much less powerful than narrowband delta VLBI and differenced Doppler for orbiter navigation, although there was some marginal improvement over the navigation performance obtained when only two-way Doppler data were used.

Thurman, S. W.↗
NASA NTRS1990

Navigational utility of high-precision radio interferometer for Galileo's approach to Jupiter

The effect of very long baseline interferometry (VLBI) measurements of 2-nanoradian (nrad) accuracy has been studied for use in Galileo's approach to Jupiter's moon Io. Of particular interest is reducing the error in the minimum altitude above Io's surface. The nominal tracking strategy includes Doppler, range, and onboard optical data, in addition to VLBI data with 25-nrad accuracy. For nominal data, the altitude error is approximately 250 km with a data cutoff of 19 days before closest approach to Io. A limited number (two to four) of 2-nrad VLBI measurements, simulating a demonstration of improved VLBI data, were found to reduce the altitude error by 10 to 40 percent. Improving the accuracy of the VLBI measurements of the nominal tracking strategy to 2 nrads, to simulate the results from an operational few-nrad VLBI capability, was found to reduce the altitude error by an approximate factor of four. This reduction in altitude error is attributed to the ability that VLBI data give to help determine the along-track component of Jupiter's ephemeris. This capability complements the ability of the onboard optical data to determine the radial and cross-track components of Jupiter's ephemeris.

Folkner, W. M.↗
NASA NTRS2004

The International Global Network of Geodetic Fiducial Stations

Scientific need and technological opportunity require that we move toward implementing a global network of geodetic fiducial stations which feature co-located SLR, VLBI, GNSS, and DORIS instrumentation. Earth science of the next decade will require more accurate global change measurements of sea level topography, sea level change, polar ice mass balance, hydrological and atmospheric mass flux. and topographic deformation, real time mm scale navigation and precision time transfer on a global scale. These scientific requirements have been translated into a goal of mm scale annual stability for the terrestrial reference frame, earth orientation parameters, as well as the orbit and clock determinations tbr the GNSS systems. To meet these challenges, the four geodetic observing systems must be more tightly integrated in technology, location, and analysis. NASA strongly supports the objectives of the IGGOS initiative vis NASA's National Geodetic Observatory and INDIGO programs. The Global networks of GNSS, SLR. and VLBI observatories are for the most part poorly suited for these new demands. These important geodetic networks have evolved with little planning yet these systems are providing essential measurements to a wide swath of society. New signal structures in the GPS and the developing Galileo GNSS will soon require replacement of the GNSS receivers. The SLR network is poorly distributed globally, requires labor intensive observations and analysis, and for the most part relies upon antiquated technology. The VLBI observatories utilize large radio telescopes in remote regions that are poorly distributed globally. Co-location of these networks is sparse and co-location errors contribute significantly to the observing error spectrum. Increasing use of the S and X band by commercial and other government services will also contribute to increased observational errors. The time is upon us for an international effort to develop an optimized global geodetic fiducial network of twelve or more integrated automated geodetic observatories that will provide in near real time high density mea.surements of Earth orientation, geodetic positioning, and GNSS system monitoring. The strategic goal for the optimized global fiducial network should be efficient autonomous operation, real time data streaming to analysis centers with an integrated near real time analysis capability for all four observing systems. These goals are achievable in the next decade given the dramatic technological improvements in all geodetic systems. For VLBI, the BKG TIGO might be viewed as a preliminary prototype of technology for fundamental stations. Mark 5, PC-VSI. e-VLBI broadband development efforts are laying the groundwork to reduce the aperture of the VLBI antennas and to allow for precision mm level co-location of observing systems. Operating costs and enhanced accuracy would accrue from the elimination of hydrogen maser clocks using double differencing, GNSS time transfer technology, GHz broadband recording, real time analysis, and mm wavelength VLBI observations. A global geodetic fiducial network will not be realizable without international efforts and the support of the four services.

LaBrecque, John↗
NASA NTRS1973

The application of VLB interferometry to earth measurements

Results of a four-phase development of capability in very long-baseline interferometry (VLBI) at the Smithsonian Astrophysical Observatory (SAO) are summarized. The objectives of the VLBI program at SAO are to: (1) develop the capability to collect, process, and analyze VLBI data; (2) determine experimentally the accuracy now attainable with VLBI and, within the constraints imposed by this accuracy, initiate a program of VLBI observations for geophysical applications; (3) identify all significant error sources and devise means for eliminating or correcting these errors; and (4) develop a long range plan for applying VLBI techniques to a program of earth physics measurements.

Michelini, R. D.↗
NASA NTRS1989

On principles, methods and recent advances in studies towards a GPS-based control system for geodesy and geodynamics

Although Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) are becoming increasingly important tools for geodynamic studies, their future role may well be fulfilled by using alternative techniques such as those utilizing the signals from the Global Positioning System (GPS). GPS, without the full implementation of the system, already offers a favorable combination of cost and accuracy and has consistently demonstrated the capability to provide high precision densification control in the regional and local areas of the VLBI and SLR networks. This report reviews VLBI and SLR vis-a-vis GPS and outlines the capabilities and limitations of each technique and how their complementary application can be of benefit to geodetic and geodynamic operations. It demonstrates, albeit with a limited data set, that dual-frequency GPS observations and interferometric type analysis techniques make possible the modelling of the GPS orbits for several days with an accuracy of a few meters. The use of VLBI or SLR sites as fiducial stations together with refinements in the orbit determination procedures can greatly reduce the systematic errors in the GPS satellite orbits used to compute the positions of non-fiducial locations. In general, repeatability and comparison with VLBI of the GPS determined locations are of the order of between 2 parts in 10 to the 7th power and 5 parts in 10 to the 8th power for baseline lengths less than 2000 km. This report is mainly a synthesis of problems, assumptions, methods and recent advances in the studies towards the establishment of a GPS-based system for geodesy and geodynamics and is one phase in the continuing effort for the development of such a system. To some, including the author, it seems reasonable to expect within the next few years that more evidence will show GPS to be as a powerful and reliable a tool as mobile VLBI and SLR are today, but largely more economical.

Delikaraoglou, Demitris↗
NASA NTRS1990

Geodesy by radio interferometry - The application of Kalman filtering to the analysis of very long baseline interferometry data

The application of Kalman filtering techniques to the analysis of VLBI data is discussed. The VLBI observables are geometrically related to the geodetic and astrometric parameters which can be determined from them. However, contributions to the observables from the clocks at, and the atmospheres above, the VLBI sites must be accounted for if reliable estimates of geodetic and astrometric parameters are to be obtained. Here, an implementation of a Kalman filter to account for stochastic behavior on those parameters which vary during the course of a VLBI experiment is discussed. Both the nature of the stochastic processes which should be used in the model for the VLBI data and the implementation of the Kalman filter estimator are considered. It is concluded that the Kalman filter is appropriate for analyzing VLBI data.

Herring, Thomas A.↗
NASA NTRS1991

Basic research for the geodynamics program

The mathematical models of space very long base interferometry (VLBI) observables suitable for least squares covariance analysis were derived and estimatability problems inherent in the space VLBI system were explored, including a detailed rank defect analysis and sensitivity analysis. An important aim is to carry out a comparative analysis of the mathematical models of the ground-based VLBI and space VLBI observables in order to describe the background in detail. Computer programs were developed in order to check the relations, assess errors, and analyze sensitivity. In order to investigate the estimatability of different geodetic and geodynamic parameters from the space VLBI observables, the mathematical models for time delay and time delay rate observables of space VLBI were analytically derived along with the partial derivatives with respect to the parameters. Rank defect analysis was carried out both by analytical and numerical testing of linear dependencies between the columns of the normal matrix thus formed. Definite conclusions were formed about the rank defects in the system.

Source record↗
NASA NTRS1983

Crustal dynamics project observations: 1982 results and plans for 1983

The 1982 Crustal Dynamics Project observations by fixed and mobile SLR and VLBI systems are reviewed. Plate motion measurements between North America and Europe were conducted by both techniques and SLR measurements were also made between North America, the Pacific, Australia and South American plates. Regional deformation measurements by VLBI and SLR systems were restricted to the western United States in 1982, including a number of important intercomparison baseline measured by both techniques. In 1983 the observing program grows significantly, with new SLR systems in Mexico, Easter Island, the Pacific and Italy. New VLBI systems will include a dedicated VLBI site at Weltzell, in Germany. Two highly mobile SLR and two highly mobile VLBI systems will greatly increase the regional deformation measurements in California and through the Basin and Range, where more than 25 sites will be occupied in 1983.

Frey, H.↗
NASA NTRS1983

Comparison of earth rotation as inferred from radio interferometric, laser ranging and astrometric observations

Results of measurements of the earth's rotation vector for a 400-day period from late September 1980 to December 1981, for which date from VLBI, satellite laser ranging (SLR), and lunar laser ranging (LLR) were available, are compared. The acquisition of the data and their evaluation are described. VLBI, SLR, and classical astrometric determinations of the X-parameter required to describe the location of the rotation pole on the earth's surface are shown, and VLBI, LLR, and classical astrometric determinations of the angle of rotation about this pole (UT1) are presented. The results indicate that VLBI and SLR, at their present stages of development, yield standard errors under 20 cm in the determinations of X, about twofold smaller than obtained from classical measurements, and that VLBI and LLR yield determination of UT1 with standard errors less than 40 cm, somewhat smaller than that of the corresponding determinations from classical observations. Methods for improving these types of intercomparisons are suggested.

Robertson, D. S.↗
NASA NTRS1987

Radio structure in the inner 1 arcsecond of NGC 1068

High-resolution VLA and VLBI observations of NGC 1068 at several different wavelengths are reported. These observations provide new information about the triple radio source in the central arcsec (75 pc) of the galaxy. The 21-cm European VLBI Network observations at 0.05-arcsec resolution detect a core of flux density 89 + or - 10 mJy. This core is dominated by a component less than 20 marcsec (1.5 pc) in diameter that has a brightness temperature of about 10 to the 8th K. Two more components may have been detected by the VLBI observations, one on either side of the compact core. At 0.07-arcsec resolution, the 1.3-cm VLA map of the central arcsec of NGC 1068 reveals a C-shaped radio morphology on scales from 10 to 50 pc. A comparison of the relative locations of the three components in the VLA and VLBI maps shows that the compact core in the VLBI map probably should be identified with the central component in the 1.3- and 2-cm VLA maps. If this identification is correct, the central component in the VLA maps is the actual nucleus of the galaxy. The core of the inner triple radio source may contain a significant flat-spectrum component. The presence of a compact core and the evidence for bending strengthen the argument that the inner triple radio source is produced by a mechanism similar to that operating in powerful radio galaxies. For a minimum-energy configuration, magnetic-field strengths of about 1 mG and energy densities of about 100 nerg/cu cm are found for the components of this source.

Ulvestad, James S.↗
NASA NTRS1987

The Spring 1985 high precision baseline test of the JPL GPS-based geodetic system

The Spring 1985 High Precision Baseline Test (HPBT) was conducted. The HPBT was designed to meet a number of objectives. Foremost among these was the demonstration of a level of accuracy of 1 to 2:10 to the 7th power, or better, for baselines ranging in length up to several hundred kilometers. These objectives were all met with a high degree of success, with respect to the demonstration of system accuracy in particular. The results from six baselines ranging in length from 70 to 729 km were examined for repeatability and, in the case of three baselines, were compared to results from colocated VLBI systems. Repeatability was found to be 5:10 to the 8th power (RMS) for the north baseline coordinate, independent of baseline length, while for the east coordinate RMS repeatability was found to be larger than this by factors of 2 to 4. The GPS-based results were found to be in agreement with those from colocated VLBI measurements, when corrected for the physical separations of the VLBI and CPG antennas, at the level of 1 to 2:10 to the 7th power in all coordinates, independent of baseline length. The results for baseline repeatability are consistent with the current GPA error budget, but the GPS-VLBI intercomparisons disagree at a somewhat larger level than expected. It is hypothesized that these differences may result from errors in the local survey measurements used to correct for the separations of the GPS and VLBI antenna reference centers.

Davidson, John M.↗
NASA NTRS1988

North America-Pacific plate boundary, an elastic-plastic megashear - Evidence from very long baseline interferometry

Data obtained by Mark III VLBI measurements of radio signals from permanent and mobile VLBI sites for 5.5 years of observations, starting in October 1982, were used to derive a picture of the earth crust deformation near the North America-Pacific plate boundary. The data, which included the vector positions of the VLBI sites and their rate of change, were used for comparison with a number of lithospheric deformation models based upon the concept that the motions of points near the North America-Pacific plate boundary are a linear combination of North America and Pacific velocities. The best of these models were found to fit 95 percent of the variance in 139 VLBI length and transverse velocity observations. Instantaneous shear deformation associated with plate tectonics is apparently developing in a zone 450 km wide paralleling the San Andreas Fault; some of this deformation will be recovered through elastic rebound, while the rest will be permanently set through plastic processes. Because the VLBI data have not been collected for a significant fraction of the earthquake cycle, they cannot discriminate between elastic and plastic behaviors.

Ward, Steven N.↗
NASA NTRS1988

Mark III very long baseline interferometry positions of eight stellar systems and astrometric comparisons

Accurate positions of eight stellar systems have been obtained from Mark III VLBI observations performed with multistation arrays, and the quality of the celestial frames used by different techniques is compared. The satisfactory agreement found between VLBI and optical positions indicates that the preliminary FK5 optical frame defined by Fricke (1982) is aligned with the VLBI extragalactic reference frame. The VLBI and VLA right ascensions agree, while a large discrepancy is noted between the VLBI and VLA declinations.

Lestrade, Jean-Francois↗
NASA NTRS1991

JPL 1990-3: A 5-nrad extragalactic source catalog based on combined radio interferometric observations

A combined analysis merges 17,000 Deep Space Network (DSN) Very Long Baseline Interferometric (VLBI) observations with 303,000 observations from the Crustal Dynamics Project (CDP) and the International Radio Interferometric Surveying (IRIS) project. Observations from the Radio Reference Frame Development (RRFD) and Time and Earth Motion Precision Observations (TEMPO) programs through late 1990 form the DSN VLBI data set. The combined analysis yields angular coordinates of extragalactic radio sources with a precision of a few nanoradians, as compared with 5 to 10 nrad precision for coordinates derived in the past solely from DSN data. The improvement in the combined analysis is due to the new Mark III DSN data, as well as to increased statistical strength from the large volume of observations from non-DSN experiments. Such a unified analysis is made possible by recent improvements in parameter estimation software efficiency. The terrestrial reference frame is based on joint VLBI experiments using both DSN and CDP antennas, and on specifying the coordinates of VLBI antennas in a proper geocentric coordinate system by means of Global Positioning System (GPS) collocation of VLBI, LLR, and SLR (Laser Ranging) sites.

Sovers, O. J.↗
NASA NTRS1991

Geodesy by radio interferometry - Water vapor radiometry for estimation of the wet delay

An important source of error in VLBI estimates of baseline length is unmodeled variations of the refractivity of the neutral atmosphere along the propagation path of the radio signals. This paper presents and discusses the method of using data from a water vapor radiomete (WVR) to correct for the propagation delay caused by atmospheric water vapor, the major cause of these variations. Data from different WVRs are compared with estimated propagation delays obtained by Kalman filtering of the VLBI data themselves. The consequences of using either WVR data or Kalman filtering to correct for atmospheric propagation delay at the Onsala VLBI site are investigated by studying the repeatability of estimated baseline lengths from Onsala to several other sites. The repeatability obtained for baseline length estimates shows that the methods of water vapor radiometry and Kalman filtering offer comparable accuracies when applied to VLBI observations obtained in the climate of the Swedish west coast. For the most frequently measured baseline in this study, the use of WVR data yielded a 13 percent smaller weighted-root-mean-square (WRMS) scatter of the baseline length estimates compared to the use of a Kalman filter. It is also clear that the 'best' minimum elevationi angle for VLBI observations depends on the accuracy of the determinations of the total propagation delay to be used, since the error in this delay increases with increasing air mass.

Elgered, G.↗
NASA NTRS1992

Demonstration of a joint US-Russian very long baseline interferometry tracking capability

This article discusses results of the first very long baseline interferometric (VLBI) measurements between antennas of the NASA DSN and the Russian three-station spacecraft tracking network. The VLBI systems of the U.S. and Russian tracking networks are described, and their compatibility for joint U.S.-Russian measurements is discussed. The results of a series of VLBI measurements involving Deep Space Stations and Russian tracking antennas are presented. The purpose of these first observations is to establish the compatibility of the two VLBI recording systems and verify that data recorded on these systems can be successfully correlated. The delay and delay rate observables produced by correlation of the recorded data are then used to estimate the locations of the Russian tracking stations relative to the Deep Space Stations. These first experiments, carried out at 1.7 GHz, are precursors to a future series of observations at 2 and 8 GHz, which will provide far more accurate station location estimates. The capability of the VLBI systems for joint U.S.-Russian spacecraft navigation measurements is also discussed.

Kroger, P. M.↗
NASA NTRS1994

International mission planning for space Very Long Baseline Interferometry

Two spacecraft dedicated to Very Long Baseline Interferometry (VLBI) will be launched in 1996 and 1997 to make observations using baselines between the space telescopes and many of the world's ground radio telescopes. The Japanese Institute of Space and Astronautical Science (ISAS) will launch VSOP (VLBI Space Observatory Program) in September 1996, while the Russian Astro Space Center (ASC) is scheduled to launch RadioAstron in 1997. Both spacecraft will observe radio sources at frequencies near 1.7, 4.8, and 22 GHz; RadioAstron will also observe at 0.33 GHz. The baselines between space and ground telescopes will provide 3-10 times the resolution available for ground VLBI at the same observing frequencies. Ground tracking stations on four continents will supply the required precise frequency reference to each spacecraft measure the two-way residual phase and Doppler on the ground-space link, and record 128 Megabit/s of VLBI data downlinked from the spacecraft. The spacecraft data are meaningless without cross-correlation against the data from Earth-bound telescopes, which must take place at special-purpose VLBI correlation facilities. Therefore, participation by most of the world's radio observatories is needed to achieve substantial science return from VSOP and RadioAstron. The collaboration of several major space agencies and the ground observatories, which generally follow very different models for allocation of observing time and for routine operations, leads to great complexity in mission planning and in day-to-day operations. This paper describes some of those complications and the strategies being developed to assure productive scientific missions.

Ulvestad, James S.↗
NASA NTRS1994

Morphological changes in the NGC 6251 jet

The giant radio galaxy NGC 6251 is a particularly good object for observational tests of relativistic jet models. Due to its high declination and approximately 0.5 Jy radio nucleus, high-quality Very Long Baseline Interferometry (VLBI) images of the central regions of the source can be made with northern hemisphere arrays. In addition, the large-scale radio morphology strongly suggests that the radio axis lies close to the plane of the sky, so Doppler boosting should be less extreme than in the core-dominated superluminal sources. Earlier 18 cm VLBI observations of NGC 6251 revealed an unexpectedly large jet/counterjet brightness ratio and small transverse motion of a feature in the parsec-scale jet. These early results are difficult to reconcile with the simplest symmetric relativistic jet models. In this paper we present a third-epoch 18 cm VLBI image of the parsec-scale radio jet in NGC 6251, and compare jet morphology over a 5 year time span. The jet shows a minor brightness peak at nearly the same distance from the core as the '25 mas knot' seen in the first- and second-epoch VLBI images. This feature is much less pronounced in the third epoch, and a relatively bright, new knot has appeared approximately 12 mas from the core. If this new component had a constant brightness during the 5 years separating the first and third observing epochs, then it must have moved away from the core with an apparent speed of at least 1.2c (compared with an upper limit of 0.23c for motion of the 25 mas knot). However, we cannot yet rule out a local brightening of the inner jet in favor of a new moving component. We determine a lower limit for the jet/couterjet brightness ratio of 100:1 within 6 mas of the core. We also present a new Very Large Array (VLA) image of the kpc-scale jet with 3 sec resolution, made from data obtained during the VLBI observations. The rate of decrease in jet surface brightness from parsec to kiloparsec scales is similar to jets in known superluminal radio sources.

Jones, Dayton L.↗