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  • 1
    Publication Date: 2011-08-16
    Description: Nine separate very-long-baseline interferometry experiments, carried out in 1972 and 1973 with radio telescopes 3900 kilometers apart, yielded values for the baseline length with an rms deviation about the mean of less than 20 centimeters. The corresponding fractional spread is about five parts in 100,000,000. Changes in universal time and in polar motion were also determined accurately from these data. The rms scatter of these results with respect to those based on optical methods was 2.9 msec and 1.3 m, respectively. Solid-earth tides were apparently detected, but no useful estimate of their amplitude was extracted.
    Keywords: GEOPHYSICS
    Type: Science; 186; Dec. 6
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  • 2
    Publication Date: 2011-08-16
    Description: Very-long-baseline interferometry experiments, involving observations of extragalactic radio sources, were performed in 1969 to determine the vector separations between antenna sites in Massachusetts and West Virginia. The 845.130-kilometer baseline was estimated from two separate experiments. The results agreed with each other to within 2 meters in all three components and with a special geodetic survey to within 2 meters in length; the differences in baseline direction as determined by the survey and by interferometry corresponded to discrepancies of about 5 meters. The experiments also yielded positions for nine extragalactic radio sources and allowed the hydrogen maser clocks at the two sites to be synchronized.
    Keywords: GEOPHYSICS
    Type: Science; 178; Oct. 27
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  • 3
    Publication Date: 2011-08-18
    Description: Results of measurements of the earth's rotation vector for a 400-day period from late September 1980 to December 1981, for which date from VLBI, satellite laser ranging (SLR), and lunar laser ranging (LLR) were available, are compared. The acquisition of the data and their evaluation are described. VLBI, SLR, and classical astrometric determinations of the X-parameter required to describe the location of the rotation pole on the earth's surface are shown, and VLBI, LLR, and classical astrometric determinations of the angle of rotation about this pole (UT1) are presented. The results indicate that VLBI and SLR, at their present stages of development, yield standard errors under 20 cm in the determinations of X, about twofold smaller than obtained from classical measurements, and that VLBI and LLR yield determination of UT1 with standard errors less than 40 cm, somewhat smaller than that of the corresponding determinations from classical observations. Methods for improving these types of intercomparisons are suggested.
    Keywords: GEOPHYSICS
    Type: Nature (ISSN 0028-0836); 302; April 7
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  • 4
    Publication Date: 2019-07-13
    Description: Advanced methods involving observations of extraterrestrial objects, such as artificial satellites or quasars, may make it possible to monitor geometrical variations in survey networks of regional, continental, and global scale with a spatial resolution of a few centimeters and a temporal resolution of better than one day. However, in connection with a realization of this potential, it is necessary to account for variations in the orientation of the earth in space. The accuracy provided by the conventional approaches for determining the orientation of the earth is not sufficient in this case. However, an appropriate method could be based on the utilization of independent clock radio interferometry (commonly referred to as VLBI) observations of extragalactic radio sources. The project POLARIS (Polar-motion Analysis by Radio Interferometric Sampling) is concerned with an implementation of this method. The MERIT (Monitor Earth Rotation and Intercompare the Techniques of observation and analysis) observations represent an aid to project POLARIS. Attention is given to details regarding these programs and the results obtained thus far.
    Keywords: GEOPHYSICS
    Type: High-precision earth rotation and earth-moon dynamics: Lunar distances and related observations; May 22-27, 1981; Alpes-Maritimes; France
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  • 5
    Publication Date: 2019-07-12
    Description: The 5.5 years of VLBI observations primarily collected under project IRIS are used to search for evidence of the free-core nutation (FCN). The observations are consistent with an irregular excitation process, and a model which assumes a step excitation in the FCN amplitude to about 2.0 milliseconds of arc in late 1985 fits the data well. Theoretical analysis appears to rule out the strong Mexican earthquake of September 19, 1985, as a cause of the excitation.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 13; 949-952
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  • 6
    Publication Date: 2019-08-28
    Description: Results are discussed for radio interferometric observations of extragalactic radio sources with antennas at Haystack Observatory in Massachusetts and the Owens Valley Radio Observatory in California (3900-km baseline) during 14 separate experiments distributed between September 1976 and May 1978. Simultaneous analysis of the data from several experiments yields estimates of changes in the x component of pole position and in earth's rotation (UT1). Comparison with the corresponding results obtained by the Bureau International de l'Heure (BIH) reveals systematic differences. In particular, the trends in the radio interferometric determinations of the changes in pole position are found to agree more closely with those from the International Polar Motion Service and from Doppler observations of satellites than with those from the BIH.
    Keywords: GEOPHYSICS
    Type: Time and the earth''s rotation; Eighty-second Symposium; May 08, 1978 - May 12, 1978; San Fernando; Spain
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