J. W. Ryan

Precision Geodesy Using the Mark-III Very-Long-Baseline Interferometer System

Very-long-baseline interferometry (VLBI) has been used to make precise measurements of the vector separation between widely separated antennas. The system for acquiring and processing VLBI data known as Mark-III is described. Tests of the system show it to have millimeter-level accuracy on short baselines; measurements of baselines longer than a few hundred kilometers suggest that accuracy is limited by the uncertainty in the calibration of tropospheric path delay to the level of a few centimeters. VLBI experiments conducted between 1976 and 1983 have demonstrated the stability of the North American plate by showing that there is no change in the distance between easternl-California and Massachusetts at the level of a few millimeters per year or greater. Experiments made from 1980 to 1984 indicate that the distance from Massachusetts to Sweden is increasing by 1.7 ± 1 cm/year where the quoted standard deviation includes the estimated effects of systematic atic errors

Very-Long-Baseline Radio Interferometry: The Mark III System for Geodesy, Astrometry, and Aperture Synthesis

The Mark III very-long-baseline interferometry (VLBI) system allows recording and later processing of up to 112 megabits per second from each radio telescope of an interferometer array. For astrometric and geodetic measurements, signals from two radio-frequency bands (2.2 to 2.3 and 8.2 to 8.6 gigahertz) are sampled and recorded simultaneously at all antenna sites. From these dual-band recordings the relative group delays of signals arriving at each pair of sites can be corrected for the contributions due to the ionosphere. For many radio sources for which the signals are sufficiently intense, these group delays can be determined with uncertainties under 50 picoseconds. Relative positions of widely separated antennas and celestial coordinates of radio sources have been determined from such measurements with 1 standard deviation uncertainties of about 5 centimeters and 3 milliseconds of arc, respectively. Sample results are given for the lengths of baselines between three antennas in the United States and three in Europe as well as for the arc lengths between the positions of six extragalactic radio sources. There is no significant evidence of change in any of these quantities. For mapping the brightness distribution of such compact radio sources, signals of a given polarization, or of pairs of orthogonal polarizations, can be recorded in up to 28 contiguous bands each nearly 2 megahertz wide. The ability to record large bandwidths and to link together many large radio telescopes allows detection and study of compact sources with flux densities under 1 millijansky.

Comparison of VLBI and SLR geocentric site coordinates

The geocentric coordinates for 18 pairs of SLR and VLBI sites are compared. After a seven-parameter frame adjustment, the two coordinate sets have weighted rms differences of 15, 22, and 22 mm for X, Y, and Z, respectively, consistent with the formal errors being too small by a factor of about two.

Radio-source positions from VLBI

Positions of 85 compact extragalactic radio sources and the Galactic object Beta Persei (Algol) have been determined in the J2000.0 coordinate system for analysis of VLBI observations made with the bandwidth-synthesis technique. Twenty-four of these sources were observed with the Mark I VLBI system in 37 sessions distributed between April 1972 and May 1978, and 82 of the sources were observed with the Mark III system in 85 sessions distributed between August 1979 and December 1982. Each session spanned at least 24 hr. Standard errors for the estimated positions on the sky of the about 10 sources frequently observed with the Mark I system are about 1 mas, except for the declinations of nearly equatorial sources, where these errors approach 5 mas. Corresponding uncertainties for the about 20 sources frequently observed with the Mark III system are 0.3 and 2 mas, respectively. 27 references.

Earth rotation measurement yields valuable information about the dynamics of the Earth system

Intensive work in recent years has demonstrated the value of high precision and high temporal resolution in collecting Earth rotation data, paving the way for continuous measurement that will open exciting new research areas in geophysics as the new millennium unfolds. Very long baseline interferometry (VLBI) will continue to be a principal technique. However, it will have greatly improved precision and temporal resolution, making possible more in-depth studies of geophysical phenomena affecting Earth rotation. Originally developed as a radio astronomical technique for high-resolution mapping of distant radio sources, VLBI, when “turned around,” has become a valuable tool in geodesy. Because of its past success, a new VLBI project, Continuous Observation of the Rotation of the Earth (CORE), was launched last year. It is being implemented in phases by the NASA Goddard Space Flight Center in collaboration with the U.S. Naval Observatory (USNO) [see Ma et al., 1997].

Measurements of the VLBI experiments during the first campaign of the Asian-Pacific space geodynamics (APSG) program

During the first campaign of the Asian-Pacific space geodynamics (APSG) program in October 1997, two VLBl experiments were successfully organized and coordinated by the Astrometry and Geodesy VLBl Group of Shanghai Astronomical Observatory, the Chinese Academy of Sciences, cooperated with geodetic VLBl group, GSFC, NASA, USA. Six VLBl stations participated in the experiments, including Seshan and Urumqi station of China, Gilcreek station in Alaska and Kokee station in Hawaii of USA, Kashima station of Japan and Hobart station at Tasmania of Australia. Baseline lengths are from 1 900 to 11 000 km and the mean relative uncertainty of the baseline length measurements is 1.0 x 10-9. In addition, the rates of the baseline lengths among the six stations and their three-dimensional velocities are solved out via global analysis of the two APSG sessions and the historical observations of the six stations as well as other VLBl observations from the global observation network. These results are appreciable to the studies of the modern crustal movement in the Asian-Pacific region. Especially,an 8 mm/a eastward motion and a 14 mm/a north by northeast motion are detected respectively for Seshan and Urumqi stations relative to the stable part of the Eurasian plate. The motions directly illustrate the effect of the northward movement of Indian plate on the modern crustal motions of the northwestern and the eastern part of China, which is of important significance to the study of the modern crustal motion of China.