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NASA NTRS1999

A Constellation of Microsatellites Promises to Help in a Range of Geoscience Research

An octet of microsatellites to be launched in 2003 promises to deliver a large amount of useful data for meteorological, climatic, ionospheric, and geodetic research as well as for operational weather forecasting and space weather monitoring. Known as the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), the joint Taiwan-U.S. scientific satellite project makes use of Global Positioning System (GPS) occultation and tracking signals. COSMIC's final operational configuration is depicted in Figure 1. Each of the eight microsatellites in low-Earth-orbit (LEO, shown relative to the high-altitude GPS satellite orbits) will carry in particular an advanced limb-sounding GPS receiver, a Tiny Ionospheric Photometer, and a triband beacon transmitter.

Kuo, Y. H.↗
NASA NTRS1999

Estimation of Non-Seasonal Oceanic Mass Redistribution and its Geodynamic Effects from the Topex/Poseidon Observation

Mass redistribution in terms of angular momentum will change the Earth's rotation under the conservation of angular momentum. It also changes the external gravitational field according to Newton's gravitational law. These small geodynamic variations can be measured very accurately by modem space geodetic techniques. We report on new estimates of the oceanic tidal angular momentum and hence the tidal variations in the Earth's rotation rate and polar motion. These are deduced from the new tidal solutions GOT99 and TPXO.3, plus M2 and Mf estimates (with error bars) from TPXO.4a. Comparisons with observed diurnal/semidiurnal tidal variations are generally similar to previous model estimates. Comparisons of models and observations of Mf polar motion are confused, with neither models nor observations agreeing among themselves or with each other. Part of this is caused by differences in Mf currents; model TPXO.4 shows current-intensive Rossby modes along western boundaries of major basins, which are indeed expected on theoretical grounds. Unfortunately, there are no reliable in situ observations of Mf currents useful for comparing models. On non-tidal oceanic mass redistribution we compute the principal components of the multi-year T/P sea surface height (SSH) field using EOF (empirical orthogonal functions) taking into consideration of the latitude-dependent area weighting. Emphasis is placed upon non-seasonal and interannual oscillations on regional scales, including ENSO and the North Atlantic Oscillation. Steric corrections to the observed SSH due to sea surface temperature changes (which has no geodynamic effects) is now under study using POCM ocean model output.

Chao, Benjamin F.↗
NASA NTRS1999

Monitoring Global Geophysical Fluids by Space Geodesy

Since its establishment on 1/1/1998 by the International Earth Rotation Service, the Coordinating Center for Monitoring Global Geophysical Fluids (MGGF) and its seven Special Bureaus have engaged in an effort to support and facilitate the understanding of the geophysical fluids in global geodynamics research. Mass transports in the atmosphere-hydrosphere-solid Earth-core system (the "global geophysical fluids") will cause the following geodynamic effects on a broad time scale: (1) variations in the solid Earth's rotation (in length-of-day and polar motion/nutation) via the conservation of angular momentum and effected by torques at the fluid-solid Earth interface; (2) changes in the global gravitational field according to Newton's gravitational law; and (3) motion in the center of mass of the solid Earth relative to that of the whole Earth ("geocenter") via the conservation of linear momentum. These minute signals have become observable by space geodetic techniques, primarily VLBI, SLR, GPS, and DORIS, with ever increasing precision/accuracy and temporal/spatial resolution. Each of the seven Special Bureaus within MGGF is responsible for calculations related to a specific Earth component or aspect -- Atmosphere, Ocean, Hydrology, Ocean Tides, Mantle, Core, and Gravity/Geocenter. Angular momenta and torques, gravitational coefficients, and geocenter shift will be computed for geophysical fluids based on global observational data, and from state-of-the-art models, some of which assimilate such data. The computed quantities, algorithm and data formats are standardized. The results are archived and made available to the scientific research community. This paper reports the status of the MGGF activities and current results.

Chao, Benjamin F.↗
NASA NTRS1999

International VLBI Service for Geodesy and Astrometry: 1999 Annual Report

This volume of reports is the 1999 Annual Report of the International VLBI Service for Geodesy and Astrometry -IVS. The individual reports were contributed by VLBI groups in the international geodetic community who constitute the components of IVS. The 1999 Annual Report documents the work of the IVS components for the year ending March 1, 1999, the official inauguration date of IVS. As the newest of the space technique services, IVS decided to publish this Annual Report as a reference to our organization and its components. The entire contents of this Annual Report also appear on the IVS website at: http://ivscc.gsfc.nasa.gov/pub/arl999. The IVS 1999 Annual Report will be a valuable reference for information about IVS and its components. This Annual Report will serve as a baseline from which we can measure the anticipated progress of IVS in coming years.

Vandenberg, Nancy R.↗
NASA NTRS1999

Source of Global Scale Variations in the Midday Vertical Content of Ionospheric Metal Ions

An analysis of long baseline NIMBUS 7 SBUV (Solar Backscatter UV Spectrometer) observations of the latitudinal variation of the noontime vertical Mg' content above approx. 70 km have revealed seasonal, solar activity and magnetic activity dependencies in the Mg+ content. The distributions were categorized in terms of magnetic coordinates partially because transport processes lifting metallic ions from the main meteor ionization layer below 100 km up into the F- region and down again are controlled by electrodynamical processes. Alternatively, the Nimbus Mg+ distributions may simply be a result of ion/neutral chemistry changes resulting from atmospheric changes and not dynamics. In such a case magnetic control would not dominate the distributions. Using in situ satellite measurements of metal ions from the Atmosphere Explorer satellites in the region above the main meteor layer and published sounding rocket measurements of the main metallic ion layers, the effects of the dynamics on the vertical content are delineated. The consequences of atmospheric changes on the vertical content are explored by separating the Nimbus measurements in a geodetic frame of reference.

Joiner, J.↗
NASA NTRS1999

Co-Seismic Mass Dislocation and Its Effect on Earth's Rotation and Gravity

Mantle processes often involve large-scale mass transport, ranging from mantle convection, tectonic motions, glacial isostatic adjustment, to tides, atmospheric and oceanic loadings, volcanism and seismicity. On very short time scale of less than an hour, co-seismic event, apart from the "shaking" that is the earthquake, leaves behind permanent (step-function-like) dislocations in the crust and mantle. This redistribution of mass changes the Earth's inertia tensor (and hence Earth's rotation in both length-of-day and polar motion), and the gravity field (in terms of spherical harmonic Stokes coefficients). The question is whether these effects are large enough to be of any significance. In this paper we report updated calculation results. The calculation uses the normal mode summation scheme, applied to 15,814 major earthquakes that occurred during 1976-1998, according to source mechanism solutions given by the Harvard Central Moment Tensor catalog. Compared to the truly large ones earlier in the century, the earthquakes we study are individually all too small to have left any discernible signature in geodetic records of Earth rotation or global gravity field. However, their collective effects continue to exhibit an extremely strong statistical tendencies. For example, earthquakes conspire to decrease J(sub 2) and J(sub 22) while shortening LOD, resulting in a rounder and more compact Earth. Strong tendency is also seen in the earthquakes trying to "nudge" the Earth rotation pole towards about 140 degree E, roughly opposite to the observed polar drift direction. The geophysical significance and implications will be further studied.

Chao, Benjamin F.↗
NASA NTRS1999

Calculation and Error Analysis of a Digital Elevation Model of Hofsjokull, Iceland from SAR Interferometry

Two ascending European Space Agency (ESA) Earth Resources Satellites (ERS)-1/-2 tandem-mode, synthetic aperture radar (SAR) pairs are used to calculate the surface elevation of Hofsjokull, an ice cap in central Iceland. The motion component of the interferometric phase is calculated using the 30 arc-second resolution USGS GTOPO30 global digital elevation product and one of the ERS tandem pairs. The topography is then derived by subtracting the motion component from the other tandem pair. In order to assess the accuracy of the resultant digital elevation model (DEM), a geodetic airborne laser-altimetry swath is compared with the elevations derived from the interferometry. The DEM is also compared with elevations derived from a digitized topographic map of the ice cap from the University of Iceland Science Institute. Results show that low temporal correlation is a significant problem for the application of interferometry to small, low-elevation ice caps, even over a one-day repeat interval, and especially at the higher elevations. Results also show that an uncompensated error in the phase, ramping from northwest to southeast, present after tying the DEM to ground-control points, has resulted in a systematic error across the DEM.

Barton, Jonathan S.↗
NASA NTRS1999

The GPS Transmit Antenna Phase Center and Its Impact on Precise Geophysical Applications

The ionospheric-free (Lc) phase center of the GPS transmit antenna is modeled as a fixed constant in most geodetic software packages. We estimated a phase center offset for all GPS satellites and found large deviations from the nominal values. In particular, meter level deviation was observed for PRN 13 and somewhat smaller anomalies were observed for PRNs 6, 15, 19 and 31. We will present the estimation results for all GPS satellites and describe the estimation process as well as an error budget. We will present evidence that estimating the phase center offset improves the GPS orbits and discuss the impact of phase center mismodeling on various high-precision applications.

Bar-Sever, Y. E.↗
NASA NTRS2000

Distribution of Slip at the Northern Sumatran Fault System

We model spatial variations in horizontal displacements of 117 geodetic sites measured during annual surveys in 1989-1996 with the Global Positioning System (GPS) as elastic strain across a locked strike-slip fault to infer the contemporary slip rate, locking depth, and location of the Sumatran fault (SF) in northern Sumatra (1 S-3 N). GPS-derived slip rate estimates increase slightly northward from 23 plus or minus 3 mm/yr at 0.8 deg S to 26 plus or minus 2mm/yr at 2.7 N. They agree with geologic estimates north of the Equator, but at 0.5 S they are about 10 mm/yr higher. Strain appears to be distributed asymmetrically about the fault. South of 2 N, about 5 mm/yr of shear is required within the offshore forearc, west of the fault, to achieve a closer agreement of fault locations inferred from GPS velocities with geologically identified traces of the SF. Locking depth estimates are on the order of 10-20 km. The western branch of the major fault bifurcation near 1 N slips at a rate five times higher than the eastern branch. The two main strands of the fault at the northwestern tip of Sumatra (5.5 N) appear to be nearly free of horizontal strain; significant slip must occur away from the two strands, probably further east at two other geologically active branches. The Banda Aceh embayment is extruded to the northwest at a rate of 5 plus or minus 2 mm/yr. Within the estimated velocity uncertainties of several mm/yr, fault-normal deformation along the SF is insignificant. Almost strain free, the northern part of the back-arc basin is part of a rigid Sunda shelf, while the northern forearc is subjected to 8 plus or minus 5 x 10 (exp -8)/yr of extension nearly parallel to the arc.

Genrich, J. F.↗
NASA NTRS1999

Breadth of Scientific Activities and Network Station Specifications in the IGS

This presentation provides a brief overview of the scientific activities of the International GPS Service (IGS). This was an approved activity of the International Association of Geodesy (IAG) with official start of service on 1 Jan 1994. The mission of the IGS is "To provide a service to support geodetic and geophysical research activities, through GPS data and data products." The presentation explains the concept of the IGS working group, and pilot projects, and reviews the current working groups and pilot projects.

Moore, A. W.↗
NASA NTRS1997

Using Laser Altimetry to Detect Topographic Change in Long Valley Caldera, California

Long Valley caldera California, is a site of extensive volcanism, persistent seismicity, and uplift of a resurgent dome, currently at a rate of about 3 cm/year. Airborne laser altimetry was used to determine the surface topography of the region in 1993. A repeat mission occurred in 1995. Three different laser altimeters were flown, dubbed ATLAS, SLICER and RASCAL. Data processing consists of the combination of the aircraft trajectory and attitude data with the laser range, the determination of an atmospheric delay, laser pulse timing errors, laser system biases, and data geolocation to obtain the position of the laser spot on the ground. Results showed that using the ATLAS and SLICER instruments, the elevation of an overflown lake is determined to precisions of 3.3 cm and 2.9 cm from altitudes of 500 m and 3 km above the ground, and about 10 cm using the RASCAL instrument from 500 m above ground. Comparison with tide gauge data showed the laser measurements are able to resolve centimeter-level changes in the lake elevation over time. Repeat pass analysis of tracks over flat surfaces indicate no systematic biases affect the measurement procedure of the ATLAS and SLICER instruments. Comparison of GPS and laser-derived elevations of easily-identifiable features in the caldera confirm the horizontal accuracy of the measurement is within the diameter of the laser footprint, and vertical accuracy is within the error inherent in the measurement. Crossover analysis shows that the standard error of the means at track intersection points within the caldera, and dome (i.e., where zero and close to the maximum amount of uplift is expected) are about I cm, indicating elevation change at the 3 cm/year level should be detectable. We demonstrate one of the powerful advantages of scanning laser altimetry over other remote sensing techniques; the straightforward creation of precise digital elevation maps of overflown terrain. Initial comparison of the 1993-1995 data indicates uplift occurred, but filtering is required to remove vegetation effects. Although research continues to utilize the full potential of laser altimetry data, the results constitute a successful demonstration that the technique may be used to perform geodetic monitoring of surface topographic change.

Hofton, M. A.↗
NASA NTRS1998

The Surface Displacement Field of the November 8, 1997, Mw7.6 Manyi (Tibet) Earthquake Observed with ERS InSAR Data

ERS2 radar data acquired before and after the Mw7.6, Manyi (Tibet) earthquake of November 8, 1997, provide geodetic information about the surface displacement produced by the earthquake in two ways. (1) The sub-pixel geometric adjustment of the before and after images provides a two dimensional offset field with a resolution of approx, 1m in both the range (radar line of sight) and azimuth (satellite track) directions. Comparison of offsets in azimuth and range indicates that the displacement along the fault is essentially strike-slip and in a left-lateral sense. The offset map reveals a relatively smooth and straight, N78E surface rupture that exceeds 150 km in length, consistent with the EW plane of the Harvard CMT solution. The rupture follows the trace of a quaternary fault visible on satellite imagery (Tapponnier and Molnar, 1978; Wan Der Woerd, pers. comm.). (2) Interferometric processing of the SAR data provides a range displacement map with a precision of a few millimeters. The slip distribution along the rupture reconstructed from the range change map is a bell-shaped curve in the 100-km long central section of the fault with smaller, local maxima near both ends. The curve shows that the fault slip exceeds 2.2 m in range, or 6.2 in strike-slip, along a 30-km long section of the fault and remains above 1 m in range, approx. 3 m strike-slip, along most of its length. Preliminary forward modeling of the central section of the rupture, assuming a uniform slip distribution with depth, indicates that the slip occur-red essentially between 0 and the depth of 10 km, consistent with a relatively shallow event (Velasco et al., 1998).

Peltzer, G.↗
NASA NTRS1999

Highlights of Recent Developments in the International GPS Service and Perspectives for Future Directions

In December 1998, the IGS Governing Board officially changed the name of this IAG service from 'International GPS Service for Geodynamics to simply the 'International GPS Service'. This change of name reflects the fact that today the IGS supports numerous scientific projects outside the traditional geodetic and geodynamic disciplines. A number of IGS projects and working groups have been established, each concentrating on a particular science application, such as the ionosphere, atmosphere, reference frame, precise time transfer, etc. These activities are enabled and simulated by the IGS and directly contribute to the continuing development of the service. The IGS is currently poised to respond to evolving user requirements that focus on timeliness and reliability of data and products, particularly in support of a slate of Low Earth Orbiter missions over the next decade. Perspectives on the future of the IGS will be developed based on current directions as well as anticipated external influences, such as GPS satellite modernization, GLONASS, availability of global communications, and plans for the European GALILEO (Global Navigation Satellite System - GNSS). We will address development of user friendly interfaces and IGS product tutorials.

Neilan, R.↗
NASA NTRS1997

Using Laser Altimetry to Detect Topographic Change at Long Valley Caldera, California

Long Valley caldera, California, is a site of extensive volcanism, persistent seismicity, and uplift of a resurgent dome, currently at a rate of approximately 3 cm/year. Airborne laser altimetry was used to determine the surface topography of the region in 1993. A repeat mission occurred in 1995. Three different laser altimeters were flown, dubbed ATLAS, SLICER and RASCAL. Data processing consists of the combination of the aircraft trajectory and attitude data with the laser range, the determination of an atmospheric delay, laser pulse timing errors, laser system biases, and data geolocation to obtain the position of the laser spot on the ground. Results showed that using the ATLAS and SLICER instruments, the elevation of an overflown lake is determined to precisions of 3.3 cm and 2.9 cm from altitudes of 500 m and 3 km above the ground, and approximately 10 cm using the RASCAL instrument from 500 m above ground. Comparison with tide gauge data showed the laser measurements are able to resolve centimeter-level changes in the lake elevation over time. Repeat pass analysis of tracks over flat surfaces indicate no systematic biases affect the measurement procedure of the ATLAS and SLICER instruments. Comparison of GPS and laser-derived elevations of easily-identifiable features in the caldera confirm the horizontal accuracy of the measurement is within the diameter of the laser footprint, and vertical accuracy is within the error inherent in the measurement. Crossover analysis shows that the standard error of the means at track intersection points within the caldera and dome (i.e., where zero and close to the maximum amount of uplift is expected) are about 1 cm, indicating elevation change at the 3 cm/year level should be detectable. We demonstrate one of the powerful advantages of scanning laser altimetry over other remote sensing techniques; the straightforward creation of precise digital elevation maps of overflown terrain. Initial comparison of the 1993-1995 data indicates uplift occurred, but filtering is required to remove vegetation effects. Although research continues to utilize the full potential of laser altimetry data, the results constitute a successful demonstration that the technique may be used to perform geodetic monitoring of surface topographic change.

Hofton, M. A.↗
NASA NTRS2001

A New Era in Geodesy and Cartography: Implications for Landing Site Operations

The Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) global dataset has ushered in a new era for Mars local and global geodesy and cartography. These data include the global digital terrain model (Digital Terrain Model (DTM) radii), the global digital elevation model (Digital Elevation Model (DEM) elevation with respect to the geoid), and the higher spatial resolution individual MOLA ground tracks. Currently there are about 500,000,000 MOLA points and this number continues to grow as MOLA continues successful operations in orbit about Mars, the combined processing of radiometric X-band Doppler and ranging tracking of MGS together with millions of MOLA orbital crossover points has produced global geodetic and cartographic control having a spatial (latitude/longitude) accuracy of a few meters and a topographic accuracy of less than 1 meter. This means that the position of an individual MOLA point with respect to the center-of-mass of Mars is know to an absolute accuracy of a few meters. The positional accuracy of this point in inertial space over time is controlled by the spin rate uncertainty of Mars which is less than 1 km over 10 years that will be improved significantly with the next landed mission.

Duxbury, T. C.↗
NASA NTRS2000

Deep Internal Structure of Mars and the Geophysical Package of Netlander

Our present understanding of the interior structure of Mars is mostly based on the interpretation of gravity and rotation data, the chemistry of the SNC (shergottites, nakhlites, chassignites) meteoroids, and a comparison with the much better-known interior structure of the Earth. However geophysical information from previous missions have been insufficient to determine the deep internal structure of the planet. Therefore the state and size of the core and the depth and type of mantle discontinuities are unknown. Most previous seismic experiments have indeed failed, either due to a launch failure (as for the Optimism seismometer onboard the small surface stations of Mars 96) or after failure on Mars (as for the Viking 1 seismometer). The remaining Viking 2 seismometer did not produce a convincing marsquake detection, basically due to too strong wind sensitivity and too low resolution in the teleseismic frequency band. After almost a decade of continuous activity and proposals, the first network mission to Mars, NetLander (NL), is expected to be launched between 2005 and 2007. One of the main scientific objectives of this four-lander network mission will be the determination of the internal structure of the planet using a geophysical package. This package will have a seismometer, a magnetometer, and a geodetic experiment, allowing a complementary approach that will yield many new constraints on the mineralogy and temperature of the mantle and core of the planet.

Lognonne, P.↗
NASA NTRS2000

A Lifetime of Geodesy and Geophysics: In Rememberence of Bill Kaula

In the early 1960's the secrets that knowledge of the Earth's gravity field would eventually reveal about the processes that govern our planet were yet to be appreciated. It was the beginning of a new science known as space geodesy, which arose at a time when most efforts were devoted to understanding how to extract precise measurements of Earth structure and motions from an orbiting spacecraft. Bill Kaula was central to that beginning and showed the way for many who were to follow, both in time and in the development of approaches most likely to yield results. Bill laid out the theory, analyzed the data, and argued strenuously for a spacecraft mission devoted to measuring gravity to make it all come true in the way he knew it really could. That mission, GRACE, was a long time coming and Bill would not see its final staging, but his influence in making it happen was everywhere. With time, the concepts for measuring the static gravity field of the Earth and terrestrial planets became well advanced, although not universally agreed upon, and certainly not by Bill, who was always eager to argue and challenge traditional methods and thinking. The extension of space geodetic techniques to the planets and the development of new techniques to measure time variations in gravity have recently brought geodesy even closer to the geophysical processes that Bill sought to understand. This presentation will contain a little geodesy, a little history, and a little reminiscing about the leader in our field.

Smith, David E.↗
NASA NTRS2001

MOLA Science Team A Mars' Year of Topographic Mapping with the Mars Orbiter Laser Altimeter

Mars Orbiter Laser Altimeter (MOLA) has operated at Mars for a full Mars year and provided a new geodetic and geophysical view of the planet. As the spacecraft enters into the Extended Mission, MOLA will concentrate its observations on the seasonal variability of the icecaps and martian clouds. Additional information is contained in the original extended abstract.

Smith, David E.↗