Recent Results of Radio Interferometric Determinations of a Transcontinental Baseline, Polar Motion, and Earth Rotation

Abstract

Radio interferometric observations of extragalactic radio sources have been made with antennas at the Haystack Observatory in Massachusetts and the Owens Valley Radio Observatory in California during fourteen separate experiments distributed between September 1976 and May 1978. The components of the baseline vector and the coordinates of the sources were estimated from the data from each experiment separately. The root-weighted-mean-square scatter about the weighted mean (“repeatability”) of the estimates of the length of the 3900 km baseline was approximately 7 cm, and of the source coordinates, approximately or less, except for the declinations of low-declination sources. With the source coordinates all held fixed at the best available, a posteriori, values, and the analyses repeated for each experiment, the repeatability obtained for the estimate of baseline length was 4 cm. From analyses of the data from several experiments simultaneously, estimates were obtained of changes in the x component of pole position and in the Earth's rotation (UT1). Comparison with the corresponding results obtained by the Bureau International de l'Heure (BIH) discloses systematic differences. In particular, the trends in the radio interferometric determinations of the changes in pole position agree more closely with those from the International Polar Motion Service (IPMS) and from the Doppler observations of satellites than with those from the BIH.

Earth Rotation Information Derived from Merit and Polaris VLBI Observations

“…It should be noted that the value for the Haystack-OVRO baseline falls i within 2 cm of the value previously published in Robertson et al (1979a), which was based on data taken from 19.76 to 1978. This would seem to indicate that to within the noise of the measurements there has been no motion across this trans-continental baseline in about four years.…”

Miniature interferometer terminals for earth surveying

“…Our limited experience with such short baselines shows, for example, that tropospheric effects can be lowered to the millimeter level (Rogers et al, 1978) and, in appropriate climates, to the tenth millimeter level (Elsmore and Ryle, 1976) for baselines of length up to five kilometers. For long baselines, a series of interferometric measurements by our group (Robertson et al, 1979) involving the Haystack and Owens Valley antennas, separated by nearly 4,000 km, demonstrated that a dozen baseline length determinations, distributed over a one and a half year period, show repeatability at the three centimeter level with the use of only surface measurements of atmospheric parameters. Higher accuracy results might be obtainable through use of water-vapor radiometers (see, for example, Schaper et al, 1970 andMoran andPenfield, 1976) to monitor the watervapor content above each terminal; perhaps the contribution of the troposphere to the uncertainty in baseline-vector determination could thereby be reduced to the centimeter level in all three components even for baselines of transcontinental dimensions.…”

“…Nonetheless this radio interferometric method has seen broad application, especially in astronomy. In geodetic applications, the demonstrated level of repeatability of baseline-length determinations ranges from ~3 mm for ~1 km distances (Rogers et al, 1978) to ~3 cm for transcontinental distances (Robertson et al, 1979). This combination of precision and range should make VLBI a very powerful technique for monitoring the time dependence of regional and continental baselines.…”

Radio interferometry techniques for geodesy

“…The central processing unit (CPU) is a Hewlett Packard 2 1 MX Computer . NOS/NGS personnel have been assisting in developing a new software package and have used the GSFC facilities to analyze Mark I data (Robertson et al, 1978;Robertson et al, 1979). The NOS/NGS expects to have its own processing system operational at NOAA Headquarters in Rockville, Maryland, by the end of 1980.…”

“…Independent clock astronomical radio interferometry, commonly referred to as very long baseline interferometry (VLBI), has produced some extraordinary experimental results during the past few years (e.g., Robertson et al, 1978;Niell et al, 1979). NOS/NGS is working with researchers at academic institutions and other government agencies, particularly the National Aeronautics and Space Administration (NASA), on the development of operational radio interferometric surveying (RIS) systems.…”