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

Abstract

New techniques are being developed to improve the spatial and temporal resolutions achievable in measurements of the rotation vector of the Earth. Such improvements are expected to expose fine details of this rotation and to aid in understanding their physical causes. Although not yet fully developed, two of these techniques are now in routine use: radio interferometry and laser ranging. In the former technique, antennas up to thousands of kilometres apart are used to observe compact extragalactic radio sources. The differences in arrival time of the signals propagating from each source to the antennas, measured by the method of very-long-baseline interferometry (VLBI), are sensitive to variations in each of the components of the Earth's rotation vector. In the latter technique, lasers are used to measure the distance (range) to artificial Earth satellites (satellite laser ranging, SLR) and to optical retroreflectors on the Moon (lunar laser ranging, LLR). To assess the accuracy being achieved in the measurement of Earth rotation it is useful to compare results obtained with these different techniques. National programmes and international cooperation, exemplified by the International Astronomical Union/International Union of Geodesy and Geophysics project MERIT^1,2, are encouraging the collection of data suitable for accurate intercomparisons. Here we compare results for a 400-day period from late September 1980 to December 1981, for which data from all three types of measurements are available. We also compare corresponding results from classical astrometric observations which demonstrate the degree of improvement in accuracy afforded by the new techniques.