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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
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NASA NTRS1975

Effects of long-term rotation and hypergravity on developing rat femurs

Male and female Sprague-Dawley rats derived from a single mating were raised for three generations under constant centrifugation at 1.03 G (Rotation Controls) and at 2 G. When the third generation rats were 3 months old, they were sacrificed, and their femurs removed. After fixation and cleaning, the femurs were then measured for length and diameter. Then right femurs were sectioned longitudinally, left femurs transversely. After staining with Hematoxylin and Eosin, right femurs were examined for ossification patterns and left femurs were measured for cortical thickness. All rotation control rats showed marked stimulation of ossification in the femoral head, and males showed significant cortical thinning when compared to non-rotated earth gravity controls. All 2 G femurs showed decreased length and aspect (L/D) ratios, and increased cortical thickness/diameter ratios when compared to earth controls or rotation controls. Ossification of the femoral head was slightly advanced, while the distal epiphyseal plate was thinned.

Smith, S. D.↗
NASA NTRS1993

Geocenter location and variations in earth orientation using global positioning system measurements

We have studied the use of GPS ground and flight tracking data to measure short-period earth orientation variations and changes in geocenter location. Comparisons between GPS-estimated earth rotation variations and those calculated from ocean tide models suggest that observed subdaily variations in earth rotation are dominated by oceanic tidal effects. Our preliminary GPS estimates for geocenter location agree with an independent satellite laser ranging estimates to 10-15 cm. Covariance analysis predicts that temporal resolution of GPS estimates for earth orientation and geocenter improves significantly when data collected from low earth-orbiting satellites as well as from ground sites are combined. The low-earth GPS tracking data enhance the accuracy and resolution for measuring high-frequency global geodynamical signals over time scales less than 1 day.

Malla, R. P.↗
NASA NTRS2012

VLBI: A Fascinating Technique for Geodesy and Astrometry

Since the 1970s Very Long Baseline Interferometry (VLBI) has proven to be a primary space-geodetic technique by determining precise coordinates on the Earth, by monitoring the variable Earth rotation and orientation with highest precision, and by deriving many other parameters of the Earth system. VLBI provides an important linkage to astronomy through, for instance, the determination of very precise coordinates of extragalactic radio sources. Additionally, it contributes to determining parameters of relativistic and cosmological models. After a short review of the history of geodetic VLBI and a summary of recent results, this paper describes future perspectives of this fascinating technique. The International VLBI Service for Geodesy and Astrometry (IVS), as a service of the International Association of Geodesy (IAG) and the International Astronomical Union (IAU), is well on its way to fully defining a next generation VLBI system, called VLBI2010. The goals of the new system are to achieve on scales up to the size of the Earth an accuracy of 1 mm in position and of 0.1 mm/year in velocity. Continuous observations shall be carried out 24 h per day 7 days per week in the future with initial results to be delivered within 24 h after taking the data. Special sessions, e.g. for monitoring the Earth rotation parameters, will provide the results in near real-time. These goals require a completely new technical and conceptual design of VLBI measurements. Based on extensive simulation studies, strategies have been developed by the IVS to significantly improve its product accuracy through the use of a network of small (approx 12 m) fast-slewing antennas. A new method for generating high precision delay measurements as well as improved methods for handling biases related to radio source structure, system electronics, and deformations of the antenna structures has been developed. Furthermore, as of January 2012, the construction of ten new VLBI2010 sites has been funded, with good prospects for one dozen more antennas, which will improve the geographical distribution of geodetic VLBI sites on Earth and provide an important step toward a global VLBI2010 network. Within this paper, the Global Geodetic Observing System (GGOS) of the IAG will also be introduced and the contribution of VLBI to GGOS will be described.

Geodesy↗
NASA NTRS1960

Determination of Azimuth Angle at Burnout for Placing a Satellite Over a Selected Earth Position

Expressions are presented for relating the satellite position in the orbital plane with the projected latitude and longitude on a rotating earth surface. An expression is also presented for determining the azimuth angle at a given burnout position on the basis of a selected passage position on the earth's surface. Examples are presented of a satellite launched eastward and one launched westward, each passing over a selected position sometime after having completed three orbits. Incremental changes from the desired latitude and longitude due to the earth's oblateness are included in the iteration for obtaining the azimuth angles of the two examples. The results for both cases are then compared with those obtained from a computing program using an oblate rotating earth. Changes from the selected latitude and longitude resulting from incremental changes from the burn-out azimuth angle and latitude are also analyzed.

Skopinski, T. H.↗
NASA NTRS2001

Meteorological Data for Geodynamics Use: IERS Special Bureau for the Atmosphere

During the life of this contract, the PI set up and operated a data center known as the "Special Bureau for the Atmosphere" of the International Earth Rotation Service (IERS) at Atmospheric and Environmental Research, Inc., and in conjunction with the U.S. National Oceanic and Atmospheric Administration. The role of the center is to calculate, archive, analyze, and distribute atmospheric data related to Earth rotation, polar motion and other motions of the Earth. We have produced data from 4 different operational meteorological centers. We have also produced data from atmospheric reanalyzes, spanning back about a half a century. The center is one of several centers under the Global Geophysical Fluids Center (GGFC) of the IERS; the GGFC is centered at NASA's Goddard Space Flight Center. Our data center has made data available to a large variety of scientists worldwide.

Salstein, David A.↗
NASA NTRS1992

Measuring the earth to within an inch using GPS satellites

In 1991, NASA-JPL coordinated the GPS for International Earth Rotation Service and Geodynamics '91, or GIG'91, three-week experiment; this had as its fundamental purpose the demonstration of GPS monitoring of earth rotation. Distance precisions of 3-20 ppb, or several cm across the diameter of the earth, were demonstrated by several analysis groups by means of their software systems; these results are approximately 100 times more precise than commercial GPS software. While satellite laser ranging and VLBI have been capable of such precision for several years, GPS is more cost-effective in many cases.

Blewitt, Geoffrey↗
NASA NTRS1993

Research on Earth's rotation and the effect of atmospheric pressure on vertical deformation and sea level variability

The work done under NASA grant NAG5-485 included modelling the deformation of the earth caused by variations in atmospheric pressure. The amount of deformation near coasts is sensitive to the nature of the oceanic response to the pressure. The PSMSL (Permanent Service for Mean Sea Level) data suggest the response is inverted barometer at periods greater than a couple months. Green's functions were constructed to describe the perturbation of the geoid caused by atmospheric and oceanic loading and by the accompanying load-induced deformation. It was found that perturbation of up to 2 cm are possible. Ice mass balance data was used for continental glaciers to look at the glacial contributions to time-dependent changes in polar motion, the lod, the earth's gravitational field, the position of the earth's center-of-mass, and global sea level. It was found that there can be lateral, non-hydrostatic structure inside the fluid core caused by gravitational forcing from the mantle, from the inner core, or from topography at the core/mantle or inner core/outer core boundaries. The nutational and tidal response of a non-hydrostatic earth with a solid inner core was modeled. Monthly, global tide gauge data from PSMSL was used to look at the 18.6-year ocean tide, the 14-month pole tide, the oceanic response to pressure, the linear trend and inter-annual variability in the earth's gravity field, the global sea level rise, and the effects of post glacial rebound. The effects of mantle anelasticity on nutations, earth tides, and tidal variation in the lod was modeled. Results of this model can be used with Crustal Dynamics observations to look at the anelastic dissipation and dispersion at tidal periods. The effects of surface topography on various components of crustal deformation was also modeled, and numerical models were developed of post glacial rebound.

Wahr, John↗
NASA NTRS2019

A Relationship Between Blue and Near‐IR Global Spectral Reflectance and the Response of Global Average Reflectance to Change in Cloud Cover Observed From EPIC

We performed a detailed analysis of Earth Polychromatic Imaging Camera (EPIC) spectral data. We found that the vector composed of blue and near-infrared (NIR) reflectance follows a counterclockwise closed-loop trajectory from 0 to 24 UTC as Earth rotates. This non-linear relationship was not observed by any other satellites due to limited spatial or temporal coverage of either low earth orbit (LEO) or geostationary (GEO) satellites. We found that clouds play an important role in determining the non-linear relationship in addition to the well-known cloud free land-ocean reflectance contrast in the two bands. The non-linear relationship is the result of three factors: (1) a much larger cloud free land-ocean contrast in the NIR band compared to the blue band; (2) significantly larger difference between cloudy-land and cloudy-ocean reflectance in theNIR band compared to the blue band; (3) the periodic variation of fractions of clear land, clear ocean, cloudy land, and cloudy ocean in the sunlit hemisphere as Earth rotates. We found that the green vegetation contributes significantly to the NIR global average reflectance when the South and North Americas appear and disappear in the EPIC's field-of-view. The blue and NIR relationship can be useful for exoplanet research. Clouds impose a strong impact on global spectral reflectance and the reflectance response to a change in cloud cover depends on whether the change is over land or over the ocean. On average, an increase of 0.1 in cloud coverage will lead to a 7%increase in spectrally integrated global average reflectance.

EPIC↗
NASA NTRS1993

Use of global positioning system measurements to determine geocentric coordinates and variations in Earth orientation

Geocentric tracking station coordinates and short-period Earth-orientation variations can be measured with Global Positioning System (GPS) measurements. Unless calibrated, geocentric coordinate errors and changes in Earth orientation can lead to significant deep-space tracking errors. Ground-based GPS estimates of daily and subdaily changes in Earth orientation presently show centimeter-level precision. Comparison between GPS-estimated Earth-rotation variations, which are the differences between Universal Time 1 and Universal Coordinated Time (UT1-UTC), and those calculated from ocean tide models suggests that observed subdaily variations in Earth rotation are dominated by oceanic tidal effects. Preliminary GPS estimates for the geocenter location (from a 3-week experiment) agree with independent satellite laser-ranging estimates to better than 10 cm. Covariance analysis predicts that temporal resolution of GPS estimates for Earth orientation and geocenter improves significantly when data collected from low Earth-orbiting satellites as well as from ground sites are combined. The low Earth GPS tracking data enhance the accuracy and resolution for measuring high-frequency global geodynamical signals over time scales of less than 1 day.

R P Malla↗
NASA NTRS1995

VLBI data, acquisition, environmental effects

During this quadrennium, very long baseline interferometry (VLBI) data acquisition and system development has focused on improving the accuracy of the system sufficiently to allow the determination of reliable estimates of height variations. Associated with this aim has been improvements in the determinations of horizontal velocity fields, monitoring water vapor delay using interferometric methods, and improvements to Earth rotation measurements. The primary aims of the improvements to height measurement accuracy have been to directly measure the contemporary magnitudes of post glacial rebound, and to determine a height reference system for measuring global sea level rise. High frequency Earth rotation studies have been carried out to better define the transformation parameters from an inertial coordinate system to an Earth fixed one, and to better understand the coupling between the components of the atmosphere-ocean-solid Earth system. Two major VLBI campaigns were carried out in support of these studies: (1) Epoch-92 in July 1992 and (2) Cont-94 in January 1994. Each of these campaigns lasted approximately two weeks and involved multiple VLBI networks operating simultaneously in addition to other space geodetic systems operating during these periods. Two major compilations of the VLBI results (and results from other space geodetic systems) have been published during this quadrennium.

Herring, Thomas A.↗
NASA NTRS1979

Geodetic interferometry submission for the IUGG Quadrennial Report

The present paper reviews the activities of two major VLBI (Very Long Baseline Interferometry) groups, within the United States, which have been concentrating on geodetic measurements for a number of years. The activities have resulted in the development of transportable VLBI terminals for regional surveying applications; development of VLBI techniques for obtaining station positions; establishment of relative epochs and rates of the hydrogen maser clocks at the stations; the acquisition of polar motion and earth rotation data in support of deep space tracking functions; and the development of VLBI techniques necessary for tectonic measurements on trans- and intercontinental baselines and for astrometry, polar-motion and earth-rotation observations.

Clark, T. A.↗
NASA NTRS2006

Revised Atmospheric Angular Momentum Series Related to Earth's Variable Rotation under Consideration of Surface Topography

The atmospheric angular momentum is closely related to variations in the Earth rotation. The atmospheric excitation function (AEF), or namely atmospheric effective angular momentum function, is introduced in studying the atmospheric excitation of the Earth's variable rotation. It may be separated into two portions, i.e, the "wind" terms due to the atmospheric motion relative to the mantle and the "pressure" terms due to the variations of atmospheric mass distribution evident through surface pressure changes. The AEF wind terms during the period of 1948-2004 are re-processed from the NCEP/NCAR (National Centers for Environmental Prediction-National Center for Atmospheric Research) reanalysis 6-hourly wind and pressure fields. Some previous calculations were approximate, in that the wind terms were integrated from an isobaric lower boundary of 1000 hPa. To consider the surface topography effect, however, the AEF is computed by integration using the winds from the Earth's surface to 10 hPa, the top atmospheric model level, instead of from 1000 hPa. For these two cases, only a minor difference, equivalent to approx. 0.004 milliseconds in length-of-day variation, exists with respect to the axial wind term. However, considerable differences, equivalent to 5-6 milliarcseconds in polar motion, are found regarding equatorial wind terms. We further compare the total equatorial AEF (with and without the topographic effect) with the polar motion excitation function (PMEF) during the period of 1980-2003. The equatorial AEF gets generally closer to the PMEF, and improved coherences are found between them when the topography effect is included. Keywords: Atmospheric angular momentum, Atmospheric excitation function, Earth rotation, Topography, Wind, Pressure.

Zhou, Y. H.↗
NASA NTRS1983

Normal mode study of the earth's rigid body motions

In this paper it is shown that the earth's rigid body (rb) motions can be represented by an analytical set of eigensolutions to the equation of motion for elastic-gravitational free oscillations. Thus each degree of freedom in the rb motion is associated with a rb normal mode. Cases of both nonrotating and rotating earth models are studied, and it is shown that the rb modes do incorporate neatly into the earth's system of normal modes of free oscillation. The excitation formula for the rb modes are also obtained, based on normal mode theory. Physical implications of the results are summarized and the fundamental differences between rb modes and seismic modes are emphasized. In particular, it is ascertained that the Chandler wobble, being one of the rb modes belonging to the rotating earth, can be studied using the established theory of normal modes.

Chao, B. F.↗
NASA NTRS1992

Crustal dynamics project data analysis, 1991: VLBI geodetic results, 1979 - 1990

The Goddard VLBI group reports the results of analyzing 1412 Mark II data sets acquired from fixed and mobile observing sites through the end of 1990 and available to the Crustal Dynamics Project. Three large solutions were used to obtain Earth rotation parameters, nutation offsets, global source positions, site velocities, and baseline evolution. Site positions are tabulated on a yearly basis from 1979 through 1992. Site velocities are presented in both geocentric Cartesian coordinates and topocentric coordinates. Baseline evolution is plotted for 175 baselines. Rates are computed for earth rotation and nutation parameters. Included are 104 sources, 88 fixed stations and mobile sites, and 688 baselines.

Ma, C.↗
NASA NTRS2002

The Global Geophysical Fluids Center of IERS (and its Special Bureau for Mantle)

The Global Geophysical Fluids Center (GGFC) was established by the International Earth Rotation Service (IERS) on IERS's 10th anniversary day January 1, 1998, in an effort to expand IERS's services to the scientific community. Under the GGFC, eight Special Bureaus (SB) have been selected, each to be responsible for research and data service activities pertaining to mass transports and related geophysical processes in specific components of the Earth system, or "global geophysical fluids," including the atmosphere, oceans, solid Earth, core, and geophysical processes of gravity, loading, tides and hydrological cycles. GGFC and the SBs have the responsibility of supporting, facilitating, and providing services to the worldwide research community, in areas related to the variations in Earth rotation, gravity field and geocenter that are caused by mass transport in the global geophysical fluids. These minute variations have been observed by various space geodetic techniques, as effective remote sensing tools, with ever increasing precision/accuracy and temporal/spatial resolution. The GGFC and SBs have organized dedicated workshops and special sessions at international conferences, published articles, and held regular business meetings. The SBs also maintain individual website for data services and information exchanges. See URL . In particular, the SB for Mantle focuses on large-scale mass redistributions that occur in the mantle in association with various dynamic processes, including seismic activities, the post-glacial rebound, and mantle convections.

Chao, Benjamin F.↗
NASA NTRS1984

Analysis of the Earth's variable rotation

The objective was to analyze the Earth's rotation, and to compare the observations with the atmospheric and seismic excitation functions. The variation in the Earth's rotation can be separated into the length-of-day (LOD) variation and the polar motion. Possible driving mechanisms include: atmospheric/oceanic circulations, seismic activities, solar-lunar tides, mantle convection, core-mantle coupling, and solar activities. The major problem of concern was the identification of these dynamical processes as primary driving mechanisms for the various features in the variation of the Earth's rotation.

Chao, B. F.↗