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NASA NTRS1992

Global geodesy using GPS without fiducial sites

Baseline lengths and geocentric radii have been determined from GPS data without the use of fiducial sites. Data from the first GPS experiment for the IERS and Geodynamics (GIG '91) have been analyzed with a no-fiducial strategy. A baseline length daily repeatability of 2 mm + 4 parts per billion was obtained for baselines in the Northern Hemisphere. Comparison of baseline lengths from GPS and the global VLBI solution GLB659 (Caprette et al. 1990) show rms agreement of 2.1 parts per billion. The geocentric radius mean daily repeatability for all sites was 15 cm. Comparison of geocentric radii from GPS and SV5 (Murray et al. 1990) show rms agreement of 3.8 cm. Given n globally distributed stations, the n(n - 1)/2 baseline lengths and n geocentric radii uniquely define a rigid closed polyhedron with a well-defined center of mass. Geodetic information can be obtained by examining the structure of the polyhedron and its change with time.

Heflin, Michael↗

NASA NTRS1991

IGARSS '91; Proceedings of the 11th Annual International Geoscience and Remote Sensing Symposium, Helsinki University of Technology, Espoo, Finland, June 3-6, 1991. Vols. 1, 2, 3, & 4

The papers presented at the symposium focus on global monitoring of the earth with emphasis on the solution of environmental problems and utilization of natural resources. Topics discussed include radar forestry modeling, hydrological remote sensing, polarimetric SAR observations of sea ice, optical and IR oceanography, and satellite remote sensing of precipitation and related phenomena. Other subject areas covered are microwave sensing of trace gases; atmospheric pressure, temperature, and humidity; spaceborne sensor technology; geodynamic and geophysical remote sensing; space radar missions; remote sensing of vegetation and crops; and remote sensing of weather systems.

Source record↗

NASA NTRS1992

Atmospheric excitation of polar motion during the GIG '91 measurement campaign

Daily determinations of the earth's pole position from GPS observations obtained during the first GPS IERS and Geodynamics (GIG) experiment are interpreted in terms of atmospheric wind and pressure fluctuations. Evidence is reported for atmospheric excitation of the rapid polar motions observed during the 23-day period from 22 January to 13 February, 1991. It is demonstrated that atmospheric wind and pressure fluctuations are mostly responsible for exciting the rapid polar motions observed during GIG '91.

Gross, Richard S.↗

NASA NTRS1992

Postglacial rebound with a non-Newtonian upper mantle and a Newtonian lower mantle rheology

A composite rheology is employed consisting of both linear and nonlinear creep mechanisms which are connected by a 'transition' stress. Background stress due to geodynamical processes is included. For models with a non-Newtonian upper-mantle overlying a Newtonian lower-mantle, the temporal responses of the displacements can reproduce those of Newtonian models. The average effective viscosity profile under the ice-load at the end of deglaciation turns out to be the crucial factor governing mantle relaxation. This can explain why simple Newtonian rheology has been successful in fitting the uplift data over formerly glaciated regions.

Gasperini, Paolo↗

NASA NTRS1993

Improved Estimation Of Delays In Radio Interferometry

Report describes status of mathematical model of delays in propagation of radio signals that originate at extra-galactic or other distant sources and received at widely separated terrestrial antennas engaged in very-long-baseline interferometry. Implemented in multiparameter estimation computer program MODEST. Needed, in applications as geodynamics and astronomy, to extract significant parameters from observed signal delays. Program and present report are updated versions of MASTERFIT program and accompanying report.

Sovers, Ojars J.↗

NASA NTRS1992

Tectonic connections to interior processes on Venus

The ultimate goal of geophysical/geological exploration of Venus is to relate the present tectonic (and volcanic) state of the lithosphere to interior processes, particularly mantle convection, operating both now and in the past. The Magellan mission has provided a spectacular view of the surface, and upcoming gravity measurements, particularly if the Magellan orbit is circularized, will provide significant constraints on the state of the interior. This extended abstract focuses on several controversial issues regarding venusian tectonics and its relationship to geodynamic mechanisms in the planet's interior. The origin of highlands, coronae diapir structures, and trenches and subduction are discussed.

Phillips, R. J.↗

NASA NTRS1992

Orbital, rotational, and climatic interactions: Lessons from Earth and Mars

Though variations in orbital and rotational parameters of the Earth and Mars are widely recognized as plausible sources of significant climatic variation on 10(exp 3) to 10(exp 8) yr timescales, many aspects of the connection between orbital, rotational, and climatic variations remain poorly understood. In general, the orbital histories are very well known, the rotational histories (especially for Mars) are very poorly known. A brief review is given of recent progress in computing orbital and rotational secular variations, and in connecting them to climatic change. The emphasis is on highlighting those areas that limit the present understanding. It is obvious that mass redistributions associated with climatic change (glaciation) are a source of crustal deformation and geodynamic change on the Earth, and may have played similar roles on Mars. It is much less appreciated, that rates, phases, and amplitudes of deformation of the deep interior of the planet can influence climate. The mantle and core, if decoupled, would precess at different rates, and even with plausible coupling strengths, some degree of differential precession is possible.

Bills, Bruce G.↗

NASA NTRS1992

STS-52 Space Shuttle mission report

The STS-52 Space Shuttle Program Mission Report provides a summary of the Orbiter, External Tank (ET), Solid Rocket Booster/Redesigned Solid Rocket Motor (SRB/RSRM), and the Space Shuttle main engine (SSME) subsystem performance during the fifty-first flight of the Space Shuttle Program, and the thirteenth flight of the Orbiter vehicle Columbia (OV-102). In addition to the Orbiter, the flight vehicle consisted of the following: an ET (designated as ET-55/LWT-48); three SSME's, which were serial numbers 2030, 2015, and 2034 in positions 1, 2, and 3, respectively; and two SRB's, which were designated BI-054. The lightweight RSRM's that were installed in each SRB were designated 360L027A for the left SRB and 360Q027B for the right SRB. The primary objectives of this flight were to successfully deploy the Laser Geodynamic Satellite (LAGEOS-2) and to perform operations of the United States Microgravity Payload-1 (USMP-1). The secondary objectives of this flight were to perform the operations of the Attitude Sensor Package (ASP), the Canadian Experiments-2 (CANEX-2), the Crystals by Vapor Transport Experiment (CVTE), the Heat Pipe Performance Experiment (HPP), the Commercial Materials Dispersion Apparatus Instrumentation Technology Associates Experiments (CMIX), the Physiological System Experiment (PSE), the Commercial Protein Crystal Growth (CPCG-Block 2), the Shuttle Plume Impingement Experiment (SPIE), and the Tank Pressure Control Experiment (TPCE) payloads.

Fricke, Robert W., Jr.↗

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

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 NTRS1992

Precision GPS ephemerides and baselines

The emphasis of this grant was focused on precision ephemerides for the Global Positioning System (GPS) satellites for geodynamics applications. During the period of this grant, major activities were in the areas of thermal force modeling, numerical integration accuracy improvement for eclipsing satellites, analysis of GIG '91 campaign data, and the Southwest Pacific campaign data analysis.

Source record↗

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 NTRS1993

Eighth International Workshop on Laser Ranging Instrumentation

The Eighth International Workshop for Laser Ranging Instrumentation was held in Annapolis, Maryland in May 1992, and was sponsored by the NASA Goddard Space Flight Center in Greenbelt, Maryland. The workshop is held once every 2 to 3 years under differing institutional sponsorship and provides a forum for participants to exchange information on the latest developments in satellite and lunar laser ranging hardware, software, science applications, and data analysis techniques. The satellite laser ranging (SLR) technique provides sub-centimeter precision range measurements to artificial satellites and the Moon. The data has application to a wide range of Earth and lunar science issues including precise orbit determination, terrestrial reference frames, geodesy, geodynamics, oceanography, time transfer, lunar dynamics, gravity and relativity.

Degnan, John J.↗

NASA NTRS1993

Laser tracking for vertical control

The Global Laser Tracking Network has provided LAGEOS ranging data of high accuracy since the first MERIT campaign in late 1983 and we can now resolve centimeter-level three dimensional positions of participating observatories at monthly intervals. In this analysis, the station height estimates have been considered separately from the horizontal components, and can be determined by the strongest stations with a formal standard error of 2 mm using eight years of continuous observations. The rate of change in the vertical can be resolved to a few mm/year, which is at the expected level of several geophysical effects. In comparing the behavior of the stations to that predicted by recent models of post-glacial rebound, we find no correlation in this very small effect. Particular attention must be applied to data and survey quality control when measuring the vertical component, and the survey observations are critical components of the geodynamic results. Seasonal patterns are observed in the heights of most stations, and the possibility of secular motion at the level of several millimeters per year cannot be excluded. Any such motion must be considered in the interpretation of horizontal inter-site measurements, and can help to identify mechanisms which can cause variations which occur linearly with time, seasonally, or abruptly.

Dunn, Peter↗

NASA NTRS1993

Test results from LAGEOS-2 optical characterization using pulsed lasers

The Laser Geodynamic Satellite-2 (LAGEOS-2) has undergone extensive optical testing at NASA Goddard Space Flight Center during 1989. The techniques included measuring the far field diffraction pattern using cw and pulsed lasers. In the pulsed measurement technique, response of the satellite was studied by measuring the far-field diffraction pattern (FFDP) as a function of pulsewidth, wavelength, polarization, position in the FFDP, detector/processing techniques, and satellite orientation. The purpose of the pulsed laser testing was two-fold: (1) to characterize the satellite optical response with the detector and signal processing electronics currently used in most SLR stations using the portable laser ranging standard, and (2) to characterize the satellite response for various conditions using the highest bandwidth optical detector (streak camera) available for the next generation of satellite laser ranging (SLR) technology. The portable ranging standard employed multiple measurement devices and an optical calibration scheme to eliminate range-dependent and amplitude-dependent systematics. These precautions were taken to eliminate/minimize instrumental errors and provide maximum accuracy. For LAGEOS orbit (6000 Km), ground stations are located 34 to 38 Mu radians off the axis of the return signal from the satellite; therefore, an optical mask was used to restrict the field of view (FOV) of detection to this annular region of the FFDP. The two measurement techniques were implemented using an aperture sharing scheme and complemented each other by providing mutual verification.

Varghese, Thomas K.↗

NASA NTRS1993

Historical MOBLAS system characterization

This paper is written as a direct response to the published NASA Laser Geodynamic Satellite (LAGEOS) orbital solution SL7.1, in order to close the data information loop with an emphasis on the NASA Mobile Laser Ranging System's (MOBLAS) LAGEOS full rate data since November 1, 1983. A preliminary analysis of the supporting information (i.e. satellite laser ranging system eccentricities and system dependent range and time bias corrections) contained in SL7.1 indicated centimeter (cm) level discrepancies. In addition, a preliminary analysis of the computed monthly MOBLAS range biases from SL7.1 appear to show cm level systematic trends, some of which appear to be 'real', particularly in the 1984 to 1987 time period. This paper is intended to be a reference document for known MOBLAS systematic errors (magnitude and direction) and for supporting MOBLAS information (eccentricities, hardware configurations, and potential data problem periods). Therefore, this report is different than your typical system characterization report, but will be more valuable to the user. The MOBLAS error models and supporting information contained in this paper will be easily accessible from the Crustal Dynamics Data Information System (CDDIS).

Husson, Van S.↗

NASA NTRS1993

Ishtar deformed belts: Evidence for deformation from below?

The mountain belts of Ishtar Terra are unique on Venus. Models for their formation include mantle upwelling, mantle downwelling, and horizontal convergence. The present forms of these models are too simple to predict surface strain, topography, or gravity. More detailed models will require specific constraints as imposed by geologic relations. In order to develop specific constraints for geodynamic models, we examine the geology of Ishtar Terra as viewed in Magellan SAR imagery in an attempt to interpret regional surface strain patterns. In this paper, we present geologic and structural relations that leads us to postulate that Ishtar deformed belts result from shear forces within the mantle acting on the lithosphere, and not by horizontal forces from colliding plates. We propose that the surface strains result from differential strain and displacement of domains within the upper mantle, and that further analysis of Ishtar deformation may allow us to identify individual domains within the mantle, and to constrain displacement trajectories between domains.

Hansen, V. L.↗

NASA NTRS1993

Constraints on the differentiation of the Earth from the coupled Sm-146,147-Nd-142,143 systematics

The coupled Sm-Nd systematics are a powerful (albeit analytically challenging) tool for investigating the geodynamic history of the Earth. We have previously reported evidence for a 33 ppm difference of an Isua sample relative to our terrestrial standard. Interpretation yields a formation age range for the depleted mantle (DM) source reservoir of 4.45-4.55 Ga. This is consistent with an epoch of LREE-enriched melt extraction from the mantle (proto-crust formation), soon after magma ocean freeze-up following the putative Moon forming giant impact.

Jacobsen, S. B.↗