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  • 11
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    The short-term prediction of universal time and length of day using atmospheric angular momentum (1994)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: The ability to predict short-term variations in the Earth's rotation has gained importance in recent years owing to more precise spacecraft tracking requirements. Universal time (UT1), that component of the Earth's orientation corresponding to the rotation angle, can be measured by number of high-precision space geodetic techniques. A Kalman filter developed at the Jet Propulsion Laboratory (JPL) optimally combines these different data sets and generates a smoothed times series and a set of predictions for UT1, as well as for additional Earth orientation components. These UT1 predictions utilize an empirically derived random walk stochastic model for the length of the day (LOD) and require frequent and up-to-date measurements of either UT1 or LOD to keep errors from quickly accumulating. Recent studies have shown that LOD variations are correlated with changes in the Earth's axial atmospheric angular momentum (AAM) over timescales of several years down to as little as 8 days. AAM estimates and forecasts out to 10 days are routinely available from meteorological analysis centers; these data can supplement geodetic measurements to improve the short-term prediction of LOD and have therefore been incorporated as independent data types in the JPL Kalman filter. We find that AAM and, to a lesser extent, AAM forecast data are extremely helpful in generating accurate near-real-time estimates of UT1 and LOD and in improving short-term predictions of these quantities out to about 10 days.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 99; B4; p. 6981-6996
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  • 12
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    Extending the ICRF to Higher Radio Frequencies (2002)
    In:  CASI
    Details
    Publication Date: 2004-12-03
    Description: The ICRF forms the basis for all astrometry including use as the inertial coordinate system for navigating deep space missions. This frame was defined using S/X-band observations over the past 20+ years. In January 2002, the VLBA approved our proposal for observing time to extend the ICRF to K-band (24 GHz) and Q-band (43 GHz). The first step will be observations at K- and Q-bands on a subset of ICRF sources. Eventually, K- and Q-band multi-epoch observations will be used to estimate positions, flux density and source structure for a large fraction of the current S/X-band ICRF source list. This work will benefit the radio astronomy community by extending the VLBA calibrator list at these bands. In the longer term, we would also like to extend the ICRF to Ka-band (32 GHz). A celestial reference frame will be needed at this frequency to support deep space navigation. A navigation demonstration is being considered for NASA's Mars 2005 mission. The initial K- and Q-band work will serve to identify candidate sources at Ka-band for use with that mission.
    Keywords: Communications and Radar
    Type: International VLBI Service for Geodesy and Astrometry General Meeting Proceeding; 350-354; NASA/CP-2002-210002
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  • 13
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    The earth's angular momentum budget on subseasonal time scales (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Irregular length of day (LOD) fluctuations on time scales of less than a few years are largely produced by atmospheric torques on the underlying planet. Significant coherence is found between the respective time series of LOD and atmospheric angular momentum (AAM) determinations at periods down to 8 days, with lack of coherence at shorter periods caused by the declining signal-to-measurement noise ratios of both data types. Refinements to the currently accepted model of tidal earth rotation variations are required, incorporating in particular the nonequilibrium effect of the oceans. The remaining discrepancies between LOD and AAM in the 100- to 10-day period range may be due to either a common error in the AAM data sets from different meteorological centers, or another component of the angular momentum budget.
    Keywords: GEOPHYSICS
    Type: Science (ISSN 0036-8075); 255; 321-324
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  • 14
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    The El-Nino, the Southern Oscillation and the earth rotation (1986)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The relationship between length of day (LOD) variations and the Southern Oscillation (SO), and with the associated El Nino phenomenon, is studied. Interannual changes in the LOD are shown to be linearly related to a SO sea level pressure index. A possible physical mechanism for this relationship is the thermal winds caused by changes in the equator to pole heat transports associated with El Nino events and the SO.
    Keywords: GEOPHYSICS
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  • 15
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    High accuracy earth rotation and atmospheric angular momentum (1986)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Recent advances in the measurement and interpretation of earth rotation and polar motion are discussed with attention focusing on short period fluctuations and their relationship to changes in the global atmospheric angular momentum (AAM). Data acquired during the MERIT campaign are used. Fluctuations in the earth's rotation over a time scale of a year or less are dominated by atmospheric effects; agreement is found between changes in length of day and AAM estimates.
    Keywords: GEOPHYSICS
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  • 16
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    Intercomparison of UT1 measurements during the MERIT campaign period (1986)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Universal time (UT1) measurements obtained using VLBI, lunar laser ranging (LLR), and BIH optical astronomy are compared. The JPL Kalman filter for the earth rotation and the polar motion is utilized to smooth one data set or combine data sets for intercomparison. The differences between raw UT1 data and independently smoothed data are employed to assess the accuracy of the measured series and the sufficiency of the error budget. Systematic errors in periodic signatures and length of day estimates are analyzed. The data reveal that the techniques of LLR and VLBI agree to within their formal errors. It is determined that the residual error for the LLR is too large and the residual error estimated for the BIH data is too small.
    Keywords: GEOPHYSICS
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  • 17
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    The atmospheric excitation of earth orientation changes during MERIT (1986)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Geodetic estimates of earth orientation variations detected during the MERIT campaign (September 1983-November 1984) are compared with corresponding meteorological data. The geodetic data were obtained using VLBI, lunar laser ranging, and satellite laser ranging and the meteorological data were from the NMC in the U.S. and the European Centre for Medium Range Weather Forecasting (ECMRWF) in the U.K. The effects of changes in pressure, wind, and the inverted barometer ocean response on the excitation of the polar motion and the length of day are examined. The comparison between the meteorological and geodetic data reveals that the equatorial vector component of the atmospheric angular momentum (AAM) has significant annual and semiannual variations. Good correlation is detected between the geodetic polar motion data and the NMC pressure inverted barometer data combined with the ECMRWF wind estimates and semiannual agreement with the NMC pressure data is observed. There is also good correlation between the ECMRWF and NMC polar vector component of the AAM data and the geodetic length of day estimates, and good semiannual agreement with NMC pressure data is noted.
    Keywords: GEOPHYSICS
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  • 18
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    Intercomparison of polar motion measurements during the MERIT period (1986)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Polar motion estimates obtained by satellite laser ranging (SLR) and VLBI are analyzed and compared. The JPL Kalman filter was utilized to smooth the polar motion estimates. Variations in the smoothing residuals for the X and Y components of polar motion for SLR and VLBI are examined. It is observed that the rms differences between the SLR and VLBI estimates for X and Y are about 2-3 marcsec. Power spectra of the differences are studied in order to measure possible periodic systematic errors. The data reveal that the SLR and VLBI polar motion data have a rms difference of about 15-20 marcsec and no significant systematic errors.
    Keywords: GEOPHYSICS
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  • 19
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    An analysis and intercomparison of VLBI nutation estimates (1986)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Nutation estimates from long duration VLBI experiments conducted by the Deep Space Network and reduced at JPL were compared with similar estimates from the IRIS/Polaris data reduced at Harvard. The two series were found to have an rms difference of 1.6 milliarcsec or less, and both exhibited the existence of seasonal errors in the IAU 1980 nutation theory. Most of the observed seasonal discrepancies could be removed by changing the period of the free core resonance to 431.5 solar days. The VLBI data constrain the resonance damping time to be at least one decade, and possibly much longer. Any free core nutation has an amplitude of less than 1 milliarcsec. Crude estimates of the atmospheric forcing of this resonance indicate that meteorological forcing could easily explain the observed free core nutation.
    Keywords: GEOPHYSICS
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  • 20
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    Causes of rapid motions of the earth's pole (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Analysis of new, highly accurate geodetic data reveals rapid motions of the earth's pole, with peak-to-peak variations of about 0.002 to 0.020 seconds of arc, fluctuating on timescales between two weeks and several months. Comparison with meteorological excitation estimates shows that these motions are at least partially driven by surface air pressure changes as modified by the response of sea level to atmospheric loading. Such geodetic measurements thus potentially provide a novel means of observing the dynamics of the atmosphere and oceans at very low spatial wavenumbers.
    Keywords: GEOPHYSICS
    Type: Nature (ISSN 0028-0836); 334; 115-119
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