Earth orientation from lunar laser ranging and an error analysis of polar motion services

Abstract

The rotational orientation of the earth has been determined from lunar laser ranging (LLR) measurements acquired at McDonald Observatory in Texas. The values and uncertainties of UT0 are calculated from 1971 through May 1982 in four forms: raw results on 799 separate days, Kalman‐filtered values, Gaussian‐smoothed values, and Fourier‐smoothed values. The effects of the different smoothings are assessed. LLR, being dominated by a single‐station operation, is sensitive to one projection of polar motion: the component along the McDonald meridian. Variation‐of‐latitude results are presented. For the polar motion component along the McDonald meridian, analyses of the LLR data assess the errors of polar motion sources: Bureau Internationale de l'Heure (BIH), Defense Mapping Agency Hydrographic/Topographic Center (DMAHTC Doppler), the International Polar Motion Service (IPMS), LAGEOS/Goddard Space Flight Center (GSFC), LAGEOS/University of Texas (Texas), and the Lerch analysis of artificial satellite data. The rms errors, of both the LLR data and the determination of polar motion by the various services, decrease in recent times. For data taken from August 1976 through May 1982, Doppler and BIH values are of comparable quality (estimated error: Doppler, 20±5 cm; BIH, 22±4 cm), with IPMS results being somewhat larger (38±5 cm). LAGEOS polar motion error drops in early 1979 in both analyses (GSFC, 14±7 cm; Texas, 18±6 cm, for data from February 1979 through May 1981). This drop parallels instrumental improvements and the increased number of stations in the laser system. The Doppler polar motion error (17±5 cm) is especially low for the period May 1977 through February 1981; the analysis of Lerch (February 1979 through May 1981) gives an error estimate of 14±7 cm. All analyses are consistent with 11 cm as an estimate for the combined LLR modeling, fitting, and instrumental error (noise) for August 1976 to May 1982.