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Ma, Chopo

Publications and source records attributed to Ma, Chopo.

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

Integrating Analysis Goals for EOP, CRF and TRF

In a simplified, idealized way the TRF (Terrestrial Reference Frame) can be considered a set of positions at epoch and corresponding linear rates of change while the CRF (Celestial Reference Frame) is a set of fixed directions in space. VLBI analysis can be optimized for CRF and TRF separately while handling some of the complexity of geodetic and astrometric reality. For EOP (Earth Orientation Parameter) time series both CRF and TRF should be accurate at the epoch of interest and well defined over time. The optimal integration of EOP, TRF and CRF in a single VLBI solution configuration requires a detailed consideration of the data set and the possibly conflicting nature of the reference frames. A possible approach for an integrated analysis is described.

Ma, Chopo↗
NASA NTRS2002

Towards a Future ICRF Realization

The data and analysis for the ICRF were completed in 1995 to define a frame to which the Hipparcos optical catalog could be fixed. Additional observations on most of the 608 sources in the overall ICRF catalog have been acquired using a small portion of geodetic observing time as well as astrometric sessions concentrating on the Southern Hemisphere. Positions of new sources have been determined, including approximately 1200 from a VLBA phase calibrator survey. A future ICRF realization will require improved geophysical modeling, sophisticated treatment of position variations and/or source structure, optimized data selection and weighting, and re-identification of defining sources. The motivation for the next realization could be significant improvement in accuracy and density or preparation for optical extragalactic catalogs with microarcsecond precision.

Ma, Chopo↗
NASA NTRS2002

Radio Source Stability and the Observation of Precession-Nutation

Some of the radio sources used by VLBI to materialize the celestial reference frame are known to have apparent motions at the sub-milliarcsecond level, particularly those observed in the early years. On the other hand, state of the art precession-nutation models match the observations at this same level. We investigate to what extent the source instability may contaminate the VLBI determination of precession and nutation corrections in several frequency domains.

Feissel, Martine↗
NASA NTRS2002

The Goddard VLBI SINEX Files

The Goddard VLBI group generates SINEX files for individual 24-hr sessions and multiyear TRF solutions. Each 24-hr file includes the reduced normal matrix for the station positions, EOP, and EOP rates. A global Terrestrial Reference Frame (TRF) SINEX file has the reduced normal matrix for station positions and velocities but no EOP parameters. Both types of normal matrix are free from any datum constraints and are designed for combination solutions that will later apply the necessary datum constraints. The SINEX files also contain blocks that provide the constraints and covariance of well-defined 24-hr or global TRF solutions. These blocks are not yet part of the SINEX 2.0 specification. For the TRF solutions the Goddard VLBI group also generates the corresponding EOP time series. SINEX files and EOP time series are distributed through the IVS. The algorithms for generating the various blocks and the proper use of such SINEX files will be discussed.

Ma, Chopo↗
NASA NTRS2001

GSFC VLBI Analysis Center

This report presents the activities of the Goddard Space Flight Center's Very Long Base Interferometry (VLBI) Analysis Center during the period from March 1, 1999 through December 31, 2000. The center's primary software development, analysis, and research activities axe reported, and the responsible staff members are described. Plans for 2001 are also presented.

Gordon, David↗
NASA NTRS2001

Integration of Reference Frames Using VLBI

Very Long Baseline Interferometry (VLBI) has the unique potential to integrate the terrestrial and celestial reference frames through simultaneous estimation of positions and velocities of approx. 40 active VLBI stations and a similar number of stations/sites with sufficient historical data, the position and position stability of approx. 150 well-observed extragalactic radio sources and another approx. 500 sources distributed fairly uniformly on the sky, and the time series of the five parameters that specify the relative orientation of the two frames. The full realization of this potential is limited by a number of factors including the temporal and spatial distribution of the stations, uneven distribution of observations over the sources and the sky, variations in source structure, modeling of the solid/fluid Earth and troposphere, logistical restrictions on the daily observing network size, and differing strategies for optimizing analysis for TRF, for CRF and for EOP. The current status of separately optimized and integrated VLBI analysis will be discussed.

Ma, Chopo↗
NASA NTRS2001

Integrating Analysis Goals for EOP, CRF and TRF

In a simplified, idealized way the TRF can be considered a set of positions at epoch and corresponding linear rates of change while the CRF is a set of fixed directions in space. VLBI analysis can be optimized for CRF and TRF separately while handling some of the complexity of geodetic and astrometric reality. For EOP time series both CRF and TRF should be accurate at the epoch of interest and well defined over time. The optimal integral EOP, TRF and CRF in a single VLBI solution configuration requires a detailed consideration of the data set and the possibly conflicting nature of reference frames.

Ma, Chopo↗
NASA NTRS2001

Towards a Future ICRF Realization

The data and analysis for the ICRF were completed in 1995 to define a frame to which the Hipparcos optical catalog could be fixed. Additional observations on most of the 608 sources in the overall ICRF catalog have been acquired using a small portion of geodetic observing time as well as astrometric sessions concentrating on the southern hemisphere. Positions of new sources have been determined, including approx.1200 from a VLBA phase calibrator survey. A future ICRF realization will require improved geophysical modeling, sophisticated treatment of position variations and/or source structure, optimized data selection and weighting, and reidentification of defining sources. The motivation for the next realization could be significant improvement in accuracy and density or preparation for optical extragalactic catalogs with microarcsecond precision.

Ma, Chopo↗
NASA NTRS2000

A Proposed Astrometric Observing Program for Densifying the ICRF in the Northern Hemisphere

The International Celestial Reference Frame (ICRF) could be of significant importance to the astronomy community for observing weak objects angularly close to ICRF sources with the phase-referencing technique. However, the current distribution of the ICRF sources is found to be largely non-uniform, which precludes the wide use of the ICRF as a catalog of calibrators for phase-referencing observations. We show that adding 150 new sources at appropriate sky locations would reduce the distance to the nearest ICRF source for any randomly-chosen location in the northern sky from up to 13 deg to up to 6 deg, close to the requirement of the phase-referencing technique. Accordingly, a set of 150 such sources, selected from the Jodrell Bank-VLA Astrometric Survey and filtered out using the Very Long Baseline Array Calibrator Survey, has been proposed for observation to the European VLBI Network (EVN) extended with additional geodetic stations. The use of the EVN is essential to this project since most of the new sources will be weaker and thus difficult to observe with standard geodetic networks.

Charlot, Patrick↗
NASA NTRS2000

Improvement of VLBI EOP Accuracy and Precision

In the CORE program, EOP measurements will be made with several different networks, each operating on a different day. It is essential that systematic differences between EOP derived by the different networks be minimized. Observed biases between the simultaneous CORE-A and NEOS-A sessions are about 60-130 micro(as) for PM, UT1 and nutation parameters. After removing biases, the observed rms differences are consistent with an increase in the formal precision of the measurements by factors ranging from 1.05 to 1.4. We discuss the possible sources of unmodeled error that account for these factors and the biases and the sensitivities of the network differences to modeling errors. We also discuss differences between VLBI and GPS PM measurements.

MacMillan, Daniel↗
NASA NTRS2000

VLBI Contribution to ITRF2000

The VLBI data set spanning Aug. 1919 to the present is a major constituent of ITRF2000 because of the time span, the geometric strength and the accuracy of the modeling. Formal velocity errors for the best stations are less than 0.3 mm/yr while the probable frame definition error for station velocities is approximately 0.4 mm/yr. The primary systematic errors are the troposphere and tidal loading (solid Earth and ocean). However, the scale of the VLBI TRF is probably only weakly affected because of continued improvements in the tropospheric mapping function and gradient estimation. There are some weaknesses in the spatial distribution of data, particularly in the southern hemisphere, and the quality of the mobile VLBI results is now suboptimal. The deployment of the TIGO system to Chile will improve the geometric coverage. The current state of the art of VLBI analysis will be discussed.

Ma, Chopo↗
NASA NTRS1999

The Celestial Reference Frame

The conceptual basis of reference frames defined by extragalactic objects is straightforwaxd: that the universe as a whole does not rotate so very distant objects cannot have an overall rotational motion. Experimentally, the global rotation of the universe is less than 10(exp -12) arcsecond/yr as inferred from the 3K microwave background radiation. At the distance of 10(exp 8) parsecs, even if an object were moving transversely at the speed of light, its angular velocity would be less than 0.6 x 10(exp -3) arcsecond/yr, while an object moving at a physically more reasonable speed comparable to the Sun would show a motion of 10(exp -6) arcsecond/yr, entirely undetectable by current technology. Since neither systematic universal motion nor random motion at such great distance is measurable, it is reasonable to construct a static celestial reference frame on the basis that such objects axe fixed in the sky.

Ma, Chopo↗
NASA NTRS1999

GSFC VLBI Analysis Center Annual Report

The GSFC VLBI group, located at NASA's Goddard Space Flight Center in Greenbelt, MD, is a part of the NASA Space Geodesy Program. Since its inception in the mid 1970's, this group has been involved with and been a leader in most aspects of geodetic and astrometric VLBI. Current major activities include coordination of the international geodetic observing program; coordination and analysis of the CORE program; VLBI technique development; and all types of data processing, analysis, and research activities.

Gordon, David↗
NASA NTRS1999

Global Velocities from VLBI

Precise geodetic Very Long Baseline Interferometry (VLBI) measurements have been made since 1979 at about 130 points on all major tectonic plates, including stable interiors and deformation zones. From the data set of about 2900 observing sessions and about 2.3 million observations, useful three-dimensional velocities can be derived for about 80 sites using an incremental least-squares adjustment of terrestrial, celestial, Earth rotation and site/session-specific parameters. The long history and high precision of the data yield formal errors for horizontal velocity as low as 0.1 mm/yr, but the limitation on the interpretation of individual site velocities is the tie to the terrestrial reference frame. Our studies indicate that the effect of converting precise relative VLBI velocities to individual site velocities is an error floor of about 0.4 mm/yr. Most VLBI horizontal velocities in stable plate interiors agree with the NUVEL-1A model, but there are significant departures in Africa and the Pacific. Vertical precision is worse by a factor of 2-3, and there are significant non-zero values that can be interpreted as post-glacial rebound, regional effects, and local disturbances.

Ma, Chopo↗
NASA NTRS1998

Prospect of Continuous VLBI Measurement of Earth Rotation in Monitoring Geophysical Fluids

Large-scale mass transports in the geophysical fluids of the Earth system excite Earth's rotational variations in both length-of-day and polar motion. The excitation process is via the conservation of angular momentum. Therefore Earth rotation observations contain information about the integrated angular momentum (consisting of both the mass term and the motion term) of the geophysical fluids, which include atmosphere, hydrosphere, mantle, and the outer and inner cores. Such global information is often important and otherwise unattainable depending on the nature of the mass transport, its magnitude and time scale. The last few years have seen great advances in VLBI measurement of Earth rotation in precision and temporal resolution. These advances have opened new. areas in geophysical fluid studies, such as oceanic tidal angular momentum, atmospheric tides, Earth librations, and rapid atmospheric angular momentum fluctuations. Precision of 10 microseconds in UTI and 200 microarcseconds in polar motion can now be achieved on hourly basis. Building upon this heritage, the multi-network geodetic VLBI project, Continuous Observation of the Rotation of the Earth (CORE), promises to further these studies and to make possible studies on elusive but tell-tale geophysical processes such as oscillatory modes in the core and in the atmosphere. Currently the early phase of CORE is underway. Within a few years into the new mellinnium, the upcoming space gravity missions (such as GRACE) will measure the temporal variations in Earth's gravitational field, thus providing complementary information to that from Earth rotation study for a better understanding of global geophysical fluid processes.

Chao, Benjamin F.↗
NASA NTRS1998

Current Limitations on VLBI Accuracy

The contribution of VLBI to geophysics and geodesy arises from its ability to measure distances between stations in a network and to determine the orientation of stations in the network as well as the orientation of the network with respect to the external reference frame of extragalactic radio objects. Integrating nearly two decades of observations provides useful information about station positions and velocities and the orientation of the Earth, but the complications of the real world and the limitations of observing, modeling and analysis prevent recovery of all effects. Of the factors that limit the accuracy of seemingly straightforward geodetic parameters, the neutral propagation medium has been subject to the greatest scrutiny, but the treatment of the mapping function, the wet component and spatial/temporal inhomogeneities is still improving. These affect both the terrestrial scale and consistency over time. The modeling of non-secular site motions (tides and loading) has increased in sophistication, but there are some differences between the models and the observations. VLBI antennas are massive objects, so their behavior is quite unlike GPS monuments, but antenna deformations add some (generally) unmodeled signal. Radio sources used in geodetic VLBI observations are selected for strength and (relative) absence of structure, but apparent changes in position can leak into geodetic parameters. A linear rate of change of baseline or site parameters is the simplest model and its error improves with time span. However, in most cases the VLBI data distribution is insufficient to look for real non-linear behavior that might affect the average rate. A few sites have multiple VLBI antennas, and some show small differences in rate. VLBI intrinsically measures relative positions and velocities, but individual site positions and velocities are generally more useful. The creation of the VLBI terrestrial reference frame, which transforms relative information into individual results, is an empirical process that has intrinsic errors. While UT1 is uniquely measured by VLBI, the geographical distribution and availability of VLBI stations, especially in the southern hemisphere, and the consistency of the VLBI terrestrial reference frame may limit the accuracy of Earth orientation measurements. The effects of particular error sources on geodetic and geophysical parameters derived from VLBI data will be illustrated.

Ma, Chopo↗