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Ma, C.

Publications and source records attributed to Ma, C..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

Support for the Laboratory for Terrestrial Physics for Dynamics of the Solid Earth (DOSE)

This final report summarizes the accomplishments during the contract period. Under the contract NVI, Inc. provided support to the VLBI group at NASA's Goddard Space Flight Center. The contract covered a period of approximately eight years during which geodetic and astrometric VLBI evolved through several major changes. This report is divided into four sections which correspond to major task areas in the contract: A) Coordination and Scheduling, B) Field System, C) Station Support, and D) Analysis and Research and Development.

Ma, C.↗
NASA NTRS1999

Refinement of Models for VLBI Analysis

Continual refinement of models and estimation methods is an integral part of improving VLBI measurements of Earth orientation and position along with advances in instrumentation and observing strategy. The modeling of the troposphere continues to be challenging with recent improvements in treating the mean gradient and gradient variations affecting the terrestrial reference frame scale and celestial declinations near the current limits of accuracy. Revised models of nutation and solid Earth tides are implemented in CALC 9.0 to conform to the latest IERS Conventions along with other changes to facilitate use by VLBI correlators. Using data since 1995 the celestial reference frame has been extended and improved consistent with the definition and 0.25 mas accuracy limit of the ICRF. The current definition of the VLBI terrestrial reference frame places a floor on the accuracy of station velocities at the 0.4 mm/yr level. The establishment of the IVS (International VLBI Service for Geodesy and Astrometry) in the past year will provide a forum for further improvements of VLBI data analysis.

Ma, C.↗
NASA NTRS1998

El Nino, La Nina and VLBI Measured LOD

VLBI is one of the most important techniques for measuring Earth orientation parameters (EOP), and is unique in its ability to make high accuracy measurements of UT1, and its time derivative, which is related to changes in the length of day, conventionally called LOD. These measurements of EOP give constraints on geophysical models of the solid-Earth, atmosphere and oceans. Changes in EOP are due either to external torques from gravitational forces, or to the exchange of angular momentum between the Earth, atmosphere and oceans. The effect of the external torques is strictly harmonic and nature, and is therefore easy to remove. We analyze an LOD time series derived from VLBI measurements with the goal of comparing this to predictions from AAM, and various ENSO indices. Previous work by ourselves and other investigators demonstrated a high degree of coherence between atmospheric angular momentum (AAM) and EOP. We continue to see this. As the angular momentum of the atmosphere increases, the rate of rotation of the Earth decreases, and vice versa. The signature of the ENSO is particularly strong. At the peak of the 1982-83 El Nino increased LOD by almost 1 ms. This was subsequently followed by a reduction in LOD of 0.75 ms. At its peak, in February of 1998, the 1997-98 El Nino increased LOD by 0.8 msec. As predicted at the 1998 Spring AGU, this has been followed by an abrupt decrease in LOD which is currently -0.4 ms. At this time (August, 1998) the current ENSO continues to develop in new and unexpected ways. We plan to update our analysis with all data available prior to the Fall AGU.

Clark, Thomas A.↗
NASA NTRS1994

Correction to the luni-solar precession from very long baseline interferometry

The redetermination of the luni-solar precession is performed by using Very Long Baseline Interferometry (VLBI) catalogues of extragalactic radio sources containing positions at observation epochs which cover 9 years. The positions of the sources have been determined by the VLBI network of the Crustal Dynamics Project of NASA from measurements covering consecutive half year and one year intervals between 1981 and 1989. In the course of their reduction the International Astronomical Union (IAU) recommended precession terms were applied. On the assumption of an imperfection of the luni-solar precession it is expected that the positions of each source obtained at different epochs but uniformly reduced to the reference system J2000.0 exhibit an apparent proper motion. We attributed this motion tentatively to precession and solved for a correction of the luni-solar precession by making a least squares fit to the position differences between homonymous sources in the set of catalogues. The 18.6 yr terms of nutation entered the process as parameters the values of which were set by choice from the reservoir of recent determinations. Twenty-five sources contributed to the analysis with the effect that the annual and semi-annual catalogues contain 16 sources on the average. The analysis confirmed the slightly overestimated IAU value of the luni-solar precession yielding a mean correction of -3.59 +/- 1.14 mas/yr from the annual and -3.84 +/- 1.16 mas/yr from the semi-annual catalogues. We discuss these secular terms and their dependence on the adopted nutation terms.

Walter, H. G.↗
NASA NTRS1994

Comparison of terrestrial reference frame velocities determined from SLR and VLBI

The most recent site velocity solutions from the Very Long Baseline Interferometry (VLBI) analysis group at the Goddard Space Flight Center and the Satellite Laser Ranging (SLR) analysis group at the University of Texas Center for Space Research are compared. The results indicate that the horizontal velocities agree, after removing a velocity transformation of a few millimeters per year, to the 2-3 millimeter per year level in an rms sense. The velocities of the best observed sites agree to nearly 1 mm/yr in each horizontal component. In addition, when only sites with adjusted velocities are compared, the agreement of epoch site positions is improved over previous studies to the subcentimeter level in the horizontal components. The sites in the western United States agree at the few millimeter level.

Watkins, M. M.↗
NASA NTRS1994

Evaluation of very long baseline interferometry atmospheric modeling improvements

We determine the improvement in baseline length precision and accuracy using new atmospheric delay mapping functions and MTT by analyzing the NASA Crustal Dynamics Project research and development (R&D) experiments and the International Radio Interferometric Surveying (IRIS) A experiments. These mapping functions reduce baseline length scatter by about 20% below that using the CfA2.2 dry and Chao wet mapping functions. With the newer mapping functions, average station vertical scatter inferred from observed length precision (given by length repeatabilites) is 11.4 mm for the 1987-1990 monthly R&D series of experiments and 5.6 mm for the 3-week-long extended research and development experiment (ERDE) series. The inferred monthly R&D station vertical scatter is reduced by 2 mm or by 7 mm is a root-sum-square (rss) sense. Length repeatabilities are optimum when observations below a 7-8 deg elevation cutoff are removed from the geodetic solution. Analyses of IRIS-A data from 1984 through 1991 and the monthly R&D experiments both yielded a nonatmospheric unmodeled station vertical error or about 8 mm. In addition, analysis of the IRIS-A exeriments revealed systematic effects in the evolution of some baseline length measurements. The length rate of change has an apparent acceleration, and the length evolution has a quasi-annual signature. We show that the origin of these effects is unlikely to be related to atmospheric modeling errors. Rates of change of the transatlantic Westford-Wettzell and Richmond-Wettzell baseline lengths calculated from 1988 through 1991 agree with the NUVEL-1 plate motion model (Argus and Gordon, 1991) to within 1 mm/yr. Short-term (less than 90 days) variations of IRIS-A baseline length measurements contribute more than 90% of the observed scatter about a best fit line, and this short-term scatter has large variations on an annual time scale.

Macmillan, D. S.↗
NASA NTRS1994

A radio/optical reference frame. 5: Additional source positions in the mid-latitude southern hemisphere

We report new accurate radio position measurements for 30 sources, preliminary positions for two sources, improved radio postions for nine additional sources which had limited previous observations, and optical positions and optical-radio differences for six of the radio sources. The Very Long Baseline Interferometry (VLBI) observations are part of the continuing effort to establish a global radio reference frame of about 400 compact, flat spectrum sources, which are evenly distributed across the sky. The observations were made using Mark III data format in four separate sessions in 1988-89 with radio telescopes at Tidbinbilla, Australia, Kauai, USA, and Kashima, Japan. We observed a total of 54 sources, including ten calibrators and three which were undetected. The 32 new source positions bring the total number in the radio reference frame catalog to 319 (172 northern and 147 southern) and fill in the zone -25 deg greater than delta greater than -45 deg which, prior to this list, had the lowest source density. The VLBI positions have an average formal precision of less than 1 mas, although unknown radio structure effects of about 1-2 mas may be present. The six new optical postion measurements are part of the program to obtain positions of the optical counterparts of the radio reference frame source and to map accurately the optical on to the radio reference frames. The optical measurements were obtained from United States Naval Observatory (USNO) Black Birch astrograph plates and source plates from the AAT, and Kitt Peak National Observatory (KPNO) 4 m, and the European Southern Observatory (ESO) Schmidt. The optical positions have an average precision of 0.07 sec, mostly due to the zero point error when adjusted to the FK5 optical frame using the IRS catalog. To date we have measured optical positions for 46 sources.

Russell, J. L.↗
NASA NTRS1994

Radio optical reference frame. 6: Additional source positions in the northern hemisphere

Radio and optical positions for northern hemisphere extragalactic sources are reported. Milliarcsecond (mas) accurate radio positions of 106 sources north of -2 deg declination are derived from Mark III Very Long Baseline Interferometry (VLBI) observations taken during ten experiments from 1990 January through 1990 October. The results presented supplement an ongoing project to define and maintain an all-sky radio/optical reference frame of 400 or more extragalactic sources with mas accurate radio and optical positions. Radio positions for 34 new sources are presented along with improved radio positions for 72 sources already in the reference frame catalog. An additional nine sources have been determined to be unsuitable reference frame objects. Radio observations of nine calibration sources tie the new positions to the existing catalogue. The radio positions of the new sources have formal mean errors of approximately 0.7 mas in right ascension and approximately 1.0 mas in declination. Sources for which we report improved radio positions now have formal mean errors of approximately 0.5 mas in both coordinates, an improvement in some cases by as much as 75%. Positions in the FK5 system have also been obtained for the optical counterparts of an additional five northern hemisphere radio sources using prime focus plates from the Kitt Peak National Observatory's 4 m telescope and a Ritchey-Chretien focus plate from the Calar Alto 2.2 m telescope. The optical positions have internal accuracies of about 0.03 sec and differ from the radio positions by about 0.07 sec on the average.

Fey, A. L.↗
NASA NTRS1993

Global Positions and Velocities from One Year of GPS Data

A generalized no-fiducial approach has been developed to obtain global positions and velocities without fixing any individual position or velocity components. The method used can be applied to any global geodetic technique and proceeds in three general steps. First, daily solutions derived with weak constraints are combined to yield one global set of positions and velocities. Second, 14 minimal constraints are applied to remove uncertainties due to the loosely defined reference frame. Third, transformation from one reference frame to another is accomplished with a 14 parameter transformation. One year of daily FLINN solutions, each made with data from about 40 sites, have been combined to yield our best fit global model...

Heflin, M. B.↗
NASA NTRS1993

NASA Space Geodesy Program: GSFC data analysis, 1992. Crustal Dynamics Project VLBI geodetic results, 1979 - 1991

The Goddard VLBI group reports the results of analyzing 1648 Mark 3 data sets acquired from fixed and mobile observing sites through the end of 1991, and available to the Crustal Dynamics Project. Two large solutions were used to obtain Earth rotation parameters, nutation offsets, radio source positions, site positions, site velocities, and baseline evolution. Site positions are tabulated on a yearly basis for 1979 to 1995, inclusive. Site velocities are presented in both geocentric Cartesian and topocentric coordinates. Baseline evolution is plotted for 200 baselines, and individual length determinations are presented for an additional 356 baselines. This report includes 155 quasar radio sources, 96 fixed stations and mobile sites, and 556 baselines.

Ryan, J. W.↗
NASA NTRS1992

A radio optical reference frame. IV - Additional radio source positions in the Northern Hemisphere

Seventy-seven extragalactic radio sources north of -2 deg declination have been observed using Mark III VLBI during 13 experiments from 1988 October to 1989 November. Radio positions derived from these observations supplement an ongoing project to define and maintain an all-sky radio/optical reference frame of 400 or more extragalactic sources with milliarcsecond (mas) accurate radio and optical positions. Positions for 11 new sources are presented along with improved positions for 54 sources already in the reference-frame catalog. An additional five sources have been determined to be unsuitable reference-frame objects. Observations of seven calibration sources tie the new positions to the existing catalog. The radio positions of the new sources have formal mean errors of about 0.7 mas in each coordinate. Sources for which improved positions are reported now have formal errors not greater than 1 mas, an improvement in some cases by as much as 85 percent. The complementary optical program is addressed briefly.

Fey, A. L.↗
NASA NTRS1992

A radio optical reference frame. III - Additional radio and optical positions in the Southern Hemisphere

Radio and optical positions are presented for southern hemisphere extragalactic sources from the Parkes 2.7 GHz survey. Sixty-one sources were observed with Mark III VLBI at 8.4 GHz between Tidbinbilla, Australia, and Hartebeesthoek, South Africa. The results presented are part of the effort to establish a global reference frame of 400 extragalactic radio sources. Radio positions with about 10 milliarcsec errors have been estimated for 39 sources not previously in the present radio reference frame catalog, and provisional positions were obtained for two additional sources, bringing the total number of catalog sources to 276. The principal source of error is the uncalibrated ionosphere. Of the remaining sources five were completely undetected, six were either too faint or too resolved, and nine had previous catalog positions. Optical positions on the FK5 system have also been measured for four southern sources using prime focus plates from the Anglo-Australian 4 m telescope with an accuracy of 0.06 arcsec. This raises to 40 the number of radio sources with accurately measured positions for their optical counterparts.

Russell, J. L.↗
NASA NTRS1992

Crustal dynamics project data analysis, 1991: VLBI geodetic results, 1979 - 1990

The Goddard VLBI group reports the results of analyzing 1412 Mark II data sets acquired from fixed and mobile observing sites through the end of 1990 and available to the Crustal Dynamics Project. Three large solutions were used to obtain Earth rotation parameters, nutation offsets, global source positions, site velocities, and baseline evolution. Site positions are tabulated on a yearly basis from 1979 through 1992. Site velocities are presented in both geocentric Cartesian coordinates and topocentric coordinates. Baseline evolution is plotted for 175 baselines. Rates are computed for earth rotation and nutation parameters. Included are 104 sources, 88 fixed stations and mobile sites, and 688 baselines.

Ma, C.↗
NASA NTRS1991

An evaluation of water vapor radiometer data for calibration of the wet path delay in very long baseline interferometry experiments

The internal consistency of the baseline-length measurements derived from analysis of several independent VLBI experiments is an estimate of the measurement precision. The paper investigates whether the inclusion of water vapor radiometer (WVR) data as an absolute calibration of the propagation delay due to water vapor improves the precision of VLBI baseline-length measurements. The paper analyzes 28 International Radio Interferometric Surveying runs between June 1988 and January 1989; WVR measurements were made during each session. The addition of WVR data decreased the scatter of the length measurements of the baselines by 5-10 percent. The observed reduction in the scatter of the baseline lengths is less than what is expected from the behavior of the formal errors, which suggest that the baseline-length measurement precision should improve 10-20 percent if WVR data are included in the analysis. The discrepancy between the formal errors and the baseline-length results can be explained as the consequence of systematic errors in the dry-mapping function parameters, instrumental biases in the WVR and the barometer, or both.

Kuehn, C. E.↗
NASA NTRS1991

A radio optical reference frame. II - Additional radio and optical source positions in the Northern Hemisphere

VLBI observations from 12 sessions have been made of a total of 103 sources north of -2 deg declination. These data give the first extension to the radio/optical reference frame catalog already defined by the data in the NASA Crustal Dynamics Project database prior to July 1988. The new observations add 53 new sources to the radio reference frame catalog, improve the positions of 29 from the previous version, and show another seven sources to be unsuitable reference frame objects. Observations of 14 calibration sources tie the new positions to the existing catalog. The formal mean errors for the radio positions of the additional sources are about 0.8 mas in each coordinate. Optical positions for eight sources have been measured in the FK5 system, bringing the total number of sources in the catalog with accurate positions for their optical counterparts to 36. The new optical positions have an accuracy of about 50 mas.

Russell, J. L.↗
NASA NTRS1991

Comparison of VLBI and SLR geocentric site coordinates

Results are reported from a systematic comparison of the geocentric coordinates determined for 18 pairs of VLBI and satellite laser ranging (SLR) sites. The data and results are presented in tables and briefly characterized. The rms differences in the X, Y, and Z coordinates are found, after a 7-parameter frame adjustment, to be 15, 22, and 22 mm, respectively. The potential usefulness of a combined VLBI-SLR reference frame for spacecraft tracking and similar applications is indicated.

Ray, J. R.↗
NASA NTRS1990

Measurement of horizontal motions in Alaska using very long baseline interferometry

Results are presented on an analysis of VLBI measurements performed between 1984 and 1990 by means of a network of 53 sites in Alaska, the Yukon Territory, and the conterminous United States to determine the extent of horizontal motions in Alaska. Results are presented in two ways, one showing the evolution of individual baselines and the other yielding site velocities; both approaches use VLBI data from other permanent stations in order to define a global reference frame. It was found that VLBI sites within the Alaska-Aleutian subduction boundary zone (Yakataga, Kodiak, and Sand Point) had higher instantaneous velocities relative to eastern North America than the interior sites of Alaska. The results of Yakataga data modeling suggests that the observed motion is the result of elastic straining of the overriding plate due to a locked main thrust zone with a component of oblique slip.

Ma, C.↗