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Eubanks, T. M.

Publications and source records attributed to Eubanks, T. M..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Small Penetrator Instrument Concept for the Advancement of Lunar Surface Science

Fundamental scientific objectives concerning the surface and subsurface material and dynamics of the Moon are the drivers for the use and advancement of penetrators, which emplace a suite of scientific instruments by impact into a planetary surface, typically at velocities of dozens to hundreds of meters per second. Small lunar penetrators are poised to become a valuable new tool for lunar science and exploration during the next decade. These low-cost ballistic probes can be deployed in large numbers from orbit, or from descending robotic or crewed vehicles, in order to explore and characterize the diversity of extreme lunar shallow subsurface environments. In this paper, we describe the general overview of penetrator objectives, potential instrumentation, and how these would benefit the advancement of lunar science at various extreme environments.

47 OTHER INSTRUMENTATION↗
NASA NTRS1998

A Kalman-Filter-Based Approach to Combining Independent Earth-Orientation Series

An approach. based upon the use of a Kalman filter. that is currently employed at the Jet Propulsion Laboratory (JPL) for combining independent measurements of the Earth's orientation, is presented. Since changes in the Earth's orientation can be described is a randomly excited stochastic process, the uncertainty in our knowledge of the Earth's orientation grows rapidly in the absence of measurements. The Kalman-filter methodology allows for an objective accounting of this uncertainty growth, thereby facilitating the intercomparison of measurements taken at different epochs (not necessarily uniformly spaced in time) and with different precision. As an example of this approach to combining Earth-orientation series, a description is given of a combination, SPACE95, that has been generated recently at JPL.

Gross, Richard S.↗
NASA NTRS1997

Mars Dynamics from Earth-Based Tracking of the Mars Pathfinder Lander

Measurements of Mars' rotational variations can be conducted via Earth-based radio tracking observations of the Mars Pathfinder lander during an extended mission. Two-way range measurements between an Earth tracking station and the lander will enable precise monitoring of the planet's orientation, allowing details of Mars' internal structure and global surface/atmosphere interactions to be determined. An analysis has been performed to investigate the accuracy with which key physical parameters of Mars can be determined using the Earth-based radio tracking measurements. Acquisition of such measurements over one Martian year should enable determination of Mars' polar moment of inertia to 1% or better, providing a strong constraint on radial density profiles (and hence on the iron content of the core and mantle) and on long-term variations of the obliquity, which influences the climate. Variations in Mars length of day and polar motion should also be detectable, and will yield information on the seasonal cycling of carbon dioxide between the atmosphere and the surface.

Folkner, W. M.↗
NASA NTRS1996

Mars Dynamics from Earth-Based Tracking of the Mars Pathfinder Lander

Measurements of Mars' rotational variations can be conducted via Earth-based radio tracking observations of the Mars Pathfinder lander during an extended mission. Two-way range measurements between an Earth antenna and the lander will enable precise monitoring of the planet's orientation and length-of-day variation, allowing details of Mars' internal structure and global surface/atmosphere interactions to be determined with precision for the first time.

two-way↗
NASA NTRS1994

Angular momentum exchange among the solid Earth, atmosphere, and oceans: A case study of the 1982-1983 El Nino event

The 1982-1983 El Nino/Southern Oscillation (ENSO) event was accompanied by the largest interannual variation in the Earth's rotation rate on record. In this study we demonstrate that atmospheric forcing was the dominant cause for this rotational anomaly, with atmospheric angular momentum (AAM) integrated from 1000 to 1 mbar (troposphere plus stratosphere) accounting for up to 92% of the interannual variance in the length of day (LOD). Winds between 100 and 1 mbar contributed nearly 20% of the variance explained, indicating that the stratosphere can play a significant role in the Earth's angular momentum budget on interannual time scales. Examination of LOD, AAM, and Southern Oscillation Index (SOI) data for a 15-year span surrounding the 1982-1983 event suggests that the strong rotational response resulted from constructive interference between the low-frequency (approximately 4-6 year) and quasi-biennial (approximately 2-3 year) components of the ENSO phenomenon, as well as the stratospheric Quasi-Biennial Oscillation (QBO). Sources of the remaining LOD discrepancy (approximately 55 and 64 microseconds rms residual for the European Centre for Medium-Range Forecasting (EC) and U.S. National Meteorological Center (NMC) analyses) are explored; noise and systematic errors in the AAM data are estimated to contribute 18 and 33 microseconds, respectively, leaving a residual (rms) of 40 (52) microseconds unaccounted for by the EC (NMC) analysis. Oceanic angular momentum contributions (both moment of inertia changes associated with baroclinic waves and motion terms) are shown to be candidates in closing the interannual axial angular momentum budget.

Dickey, J. O.↗
NASA NTRS1994

The short-term prediction of universal time and length of day using atmospheric angular momentum

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.

Freedman, A. P.↗
NASA NTRS1993

The Short-Term Prediction of Universal Time and Length-Of-Day Using Atmospheric Angular Momemtum

Methods to combine the diverse set of geodetic measurements of Earth orientation that are currently available, and to interpolate and extrapolate these data to generate an optimal estimate of Earth orientation, have been under development at JPL for a number of years. The strategy currently in use is a Kalman filtering scheme based on Earth orentation parameters and their excitation functions that incorporates stochastic models of the important physical processes and takes into account the variable form, quality, and temporal density of the data provided by diffent measurement services.

Universal Time↗
NASA NTRS1990

Interannual and decade fluctuations in the earth's rotation

The main findings of a study of earth rotation changes on the longer interannual and decade range of time scales are summarized. The earth rotation data provide important constraints on models of the internal structure and dynamics of the core and lower mantle. Torques at the core-mantle interface are due to time varying fluid motions in the core. A torque of about 0.2 Hadley units is found in the axial component during the 1980 period.

Dickey, Jean O.↗
NASA NTRS1990

Forecasting atmospheric angular momentum and length-of-day using operational meteorological models

Forecasts of zonal wind fields produced by the medium-range forecast model of the U.S. National Meteorological Center are used to create predictions of the atmosphere's angular momentum at lead times of 1-10 days. The skill of these forecasts, which are of interest to those concerned with monitoring changes in the length-of-day for navigational purposes, is assessed, and the regions in the atmosphere that contribute most importantly to forecast errors are identified.

Rosen, R. D.↗
NASA NTRS1988

Causes of rapid motions of the earth's pole

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.

Eubanks, T. M.↗
NASA NTRS1988

The long term stability of VLBI earth orientation measurements

Tectonic motions will, in general, change the orientation as well as the length of baselines used in Very Long Baseline Interferometry (VLBI), and will thus cause slow divergences between earth orientation results obtained with different VLBI networks, as well as between VLBI results and those obtained by satellite laser ranging and lunar laser ranging. Such drifts (on the order of a milliarcsecond/year) are inherently interesting as well as being significant in combinations of orientation results from different sources. The geodetic study of tectonic motions is also closely connected to research into the nature and causes of systematic errors in data from the modern techniques of space geodesy. Both a special coordinate system found to be of use in the analysis of VLBI data and tectonic motion estimates for a VLBI baseline stretching from California to Australia are described.

Eubanks, T. M.↗
NASA NTRS1986

An analysis and intercomparison of VLBI nutation estimates

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.

Eubanks, T. M.↗
NASA NTRS1986

The atmospheric excitation of earth orientation changes during MERIT

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.

Eubanks, T. M.↗
NASA NTRS1986

Intercomparison of UT1 measurements during the MERIT campaign period

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.

Spieth, M. A.↗
NASA NTRS1986

Intercomparison of polar motion measurements during the MERIT period

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.

Steppe, J. A.↗
NASA NTRS1986

High accuracy earth rotation and atmospheric angular momentum

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.

Dickey, J. O.↗