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NASA NTRS2001

Short-Arc Analysis of Intersatellite Tracking Data in a Gravity Mapping Mission

A technique for the analysis of low-low intersatellite range-rate data in a gravity mapping mission is explored. The technique is based on standard tracking data analysis for orbit determination but uses a spherical coordinate representation of the 12 epoch state parameters describing the baseline between the two satellites. This representation of the state parameters is exploited to allow the intersatellite range-rate analysis to benefit from information provided by other tracking data types without large simultaneous multiple data type solutions. The technique appears especially valuable for estimating gravity from short arcs (e.g., less than 15 minutes) of data. Gravity recovery simulations which use short arcs are compared with those using arcs a day in length. For a high-inclination orbit, the short-arc analysis recovers low-order gravity coefficients remarkably well, although higher order terms, especially sectorial terms, are less accurate. Simulations suggest that either long or short arcs of GRACE data are likely to improve parts of the geopotential spectrum by orders of magnitude.

Rowlands, David D.↗

NASA NTRS1979

A linear scanning balloon-borne telescope for far-infrared astronomy

A low-background balloon-borne far-infrared (30-300 microns) telescope, utilizing a linear scan mode has dramatically increased the efficiency of mapping large extended areas over more conventional systems. The system consists of a 20 cm linear scanned mirror mounted as an off-axis (Herchelian) telescope designed to achieve an emissivity of 1% or less, a ten position cooled aperture-filter wheel, a cooled Pfund optical system with a composite diamond-germanium bolometer mounted in a spherical cavity, a minimum phase shift ultra-low frequency amplifier, and a 4 micron thick mylar dewar window. This system was used to produce detailed far-infrared maps of the galactic plane. The system specifications and operating parameters will be discussed and preliminary data shown.

Nishimura, T.↗

NASA NTRS2006

Repeat Ground Track Lunar Orbits in the Full-Potential Plus Third-Body Problem

A high degree and order Lunar gravitational field is superimposed on the Earth-Moon Restricted Three Body model to capture the dominating forces on a spacecraft in the vicinity of the Moon. For the synchronously rotating Moon, periodic orbits in this model map repeat ground tracks and represent higher order solutions to the frozen orbit problem. The near-circular, stable or near-stable solutions are found over a wide range of defining characteristics making them suitable for long-lifetime parking applications such as science orbits, crew exploration vehicle parking orbits, and global coverage constellation orbits. A full ephemeris is considered for selected orbits to evaluate the validity of the time-invariant, simplified model. Of the most promising results are the low-altitude families of near-circular, inclined orbits that maintain long-term stability despite the highly non-spherical Lunar gravity. The method is systematic and enables rapid design and analysis of long-life orbits around any tidally-locked celestial body with an arbitrarily high degree and order spherical harmonic gravity field. .

periodic↗

NASA NTRS1980

Gravity and magnetic anomaly modeling and correlation using the SPHERE program and Magsat data

The spherical Earth inversion, modeling, and contouring software were tested and modified for processing data in the Southern Hemisphere. Preliminary geologic/tectonic maps and selected cross sections for South and Central America and the Caribbean region are being compiled and as well as gravity and magnetic models for the major geological features of the area. A preliminary gravity model of the Andeas Beniff Zone was constructed so that the density columns east and west of the subducted plates are in approximate isostatic equilibrium. The magnetic anomaly for the corresponding magnetic model of the zone is being computed with the SPHERE program. A test tape containing global magnetic measurements was converted to a tape compatible with Purdue's CDC system. NOO data were screened for periods of high diurnal activity and reduced to anomaly form using the IGS-75 model. Magnetic intensity anomaly profiles were plotted on the conterminous U.S. map using the track lines as the anomaly base level. The transcontinental magnetic high seen in POGO and MAGSAT data is also represented in the NOO data.

Braile, L. W.↗

NASA NTRS1987

Tail modeling in a stretched magnetosphere. I - Methods and transformations

A new method is developed for representing the magnetospheric field B as a distorted dipole field. Because Delta-B = 0 must be maintained, such a distortion may be viewed as a transformation of the vector potential A. The simplest form is a one-dimensional 'stretch transformation' along the x axis, concisely represented by the 'stretch function' f(x), which is also a convenient tool for representing features of the substorm cycle. One-dimensional stretch transformations are extended to spherical, cylindrical, and parabolic coordinates and then to arbitrary coordinates. It is shown that distortion transformations can be viewed as mappings of field lines from one pattern to another; the final result only requires knowledge of the field and not of the potentials. General transformations in Cartesian and arbitrary coordinates are derived, and applications to field modeling, field line motion, MHD modeling, and incompressible fluid dynamics are considered.

Stern, David P.↗

NASA NTRS2022

Kelvin-Helmholtz Vortices as an Interplay of Magnetosphere-Ionosphere Coupling

The solar wind-magnetosphere interaction drives diverse physical processes on the flanks of Earth’s magnetopause, and in turn these processes couple to the ionosphere. We investigate simultaneous multipoint in-situ spacecraft and ground-based measurements to determine the role of Kelvin-Helmholtz waves at the Earth’s magnetopause and the low-latitude boundary layer in the magnetosphere-ionosphere coupling process. Nonlinear Kelvin-Helmholtz waves develop into flow vortices that twist and/or shear flux tube magnetic fields, thereby generating localized field-aligned currents. Kelvin-Helmholtz vortices on the dusk (dawn) flanks of the magnetosphere generate clockwise (counter-clockwise) rotations and upward (downward) field-aligned currents inside the flux tubes, consistent with the region-1 field-aligned current. We present in-situ MMS and Cluster spacecraft observations of Kelvin-Helmholtz vortices at the magnetopause that map to the poleward edge of the auroral regions. The FAST spacecraft and the ground-based magnetometers from which spherical elementary currents (acting as a proxy for vertical currents) can be calculated observe corresponding field-aligned current signatures. This study demonstrates the role played by the Kelvin-Helmholtz waves in linking magnetopause boundary fluctuations to ionospheric phenomena.

K-J Hwang↗

NASA NTRS1985

Topographic mapping of the moon

Contour maps of the moon have been compiled by photogrammetric methods that use stereoscopic combinations of all available metric photographs from the Apollo 15, 16, and 17 missions. The maps utilize the same format as the existing NASA shaded-relief Lunar Planning Charts (LOC-1, -2, -3, and -4), which have a scale of 1:2,750,000. The map contour interval is 500 m. A control net derived from Apollo photographs by Doyle and others was used for the compilation. Contour lines and elevations are referred to the new topographic datum of the moon, which is defined in terms of spherical harmonics from the lunar gravity field. Compilation of all four LOC charts was completed on analytical plotters from 566 stereo models of Apollo metric photographs that cover approximately 20 percent of the moon. This is the first step toward compiling a global topographic map of the moon at a scale of 1:5,000,000.

Wu, S. S. C.↗

NASA NTRS1993

Linear structures on the small inner satellites of Saturn

Shape modelling methods developed for non-spherical worlds were applied to four satellites of Saturn: Prometheus, Pandora, Janus, and Epimetheus. This results in the first detailed shaded relief maps of their surfaces. Ridges and valleys are described with their implications for satellite history and asteroid 951 Gaspra. They probably result from fracturing during break-up of parent bodies and/or later large impacts. Prometheus and perhaps Gaspra may be coated with debris from parent body fragmentation as well as more recent regolith. These four satellites are covered by relatively few useful images, so the shapes are imperfectly known and positions of features on maps may be wrong by up to a few tens of degrees in some areas (worst where a feature is seen only near a limb). Nevertheless, these shape models are more useful than the previous triaxial ellipsoid models.

Stooke, Philip J.↗

NASA NTRS2014

Geant4 Predictions of Energy Spectra in Typical Space Radiation Environment

Accurate knowledge of energy spectra inside spacecraft is important for protecting astronauts as well as sensitive electronics from the harmful effects of space radiation. Such knowledge allows one to confidently map the radiation environment inside the vehicle. The purpose of this talk is to present preliminary calculations for energy spectra inside a spherical shell shielding and behind a slab in typical space radiation environment using the 3D Monte-Carlo transport code Geant4. We have simulated proton and iron isotropic sources and beams impinging on Aluminum and Gallium arsenide (GaAs) targets at energies of 0.2, 0.6, 1, and 10 GeV/u. If time permits, other radiation sources and beams (_, C, O) and targets (C, Si, Ge, water) will be presented. The results are compared to ground-based measurements where available.

Sabra, M. S.↗

NASA NTRS1977

A harmonic analysis of lunar topography

A global lunar topographic map has been derived from existing earth-based and orbital observations supplemented in areas without data by a linear autocovariance predictor. Of 2592 bins, each 5 deg square, 1380 (64.7% by area) contain at least one measurement. A spherical harmonic analysis to degree 12 yields a mean radius of 1737.53 plus or minus 0.03 km (formal standard error) and an offset of the center of figure of 1.98 plus or minus 0.06 km toward (19 plus or minus 2) deg S, (194 plus or minus 1) deg E. A Bouguer gravity map, derived from a 12-degree free-air gravity model and the present topography data, is presented for an elevation of 100 km above the mean surface. It is confirmed that the low-degree gravity harmonics are determined primarily by surface height variations and only secondarily by lateral density variations.

Bills, B. G.↗

NASA NTRS1974

A stabilized large-aperture far-infrared telescope gondola

Description of a recently developed balloon-borne 1-m far-infrared telescope designed to carry out astronomical observations and to map radiation from selected areas of the sky. The telescope optical system consists of a conventional Cassegrain arrangement with an f/2 aluminum primary mirror, spherically figured, which feeds a pyrex secondary mirror to produce an f/13.8 beam at the focal plane with a scale of 15.2 arc sec/mm. Three focal planes are provided. The infrared detectors consist of an array of four gallium-doped germanium bolometers fitted within a two-liter helium dewar. An altitude-azimuth mount design is used for attitude control, with the pointing objectives being achieved in two steps. The azimuth control system is capable of operation in a position mode, in which the sensing element is a servo-driven null magnetometer always pointed north, and in an inertial mode, in which the gyro is uncaged and becomes the azimuth reference. The elevation system also operates in both position and inertial modes. An auxiliary position control, powered by an independent battery and controlled by a separate tone command system, is provided in case of a primary system difficulty.

Hazen, N. L.↗

NASA NTRS1989

The figure of Phobos

The figure of Phobos is presently modeled by a sixth-degree-and-order spherical harmonic expansion that fits the observed surface to an accuracy of a few hundred m, as validated through the use of control points, limbs, terminators, map projections, and predicted rotational properties. Only the region around the large crater, Stickney, exhibits lower accuracy. The forced libration amplitude obtained closely matches the observed value; this supports the view of Phobos as possessing a uniform density distribution.

Duxbury, Thomas C.↗

NASA NTRS1978

Mars synthetic topographic mapping

Topographic contour maps of Mars are compiled by synthesizing data acquired from various scientific experiments of the Mariner 9 mission, including S-band radio occultation, the UV spectrometer, the IR radiometer, the IR interferometer spectrometer, and television imagery, as well as earth-based radar information. The entire planet is mapped at scales of 1:25,000,000 and 1:5,000,000 using Mercator, Lambert, and polar stereographic map projections. For the computation of map projections, a biaxial spheroid figure is adopted. The semimajor and semiminor axes are 3393.4 and 3375.7 km, respectively, with a polar flattening of 0.0052. For the computation of elevations, a topographic datum is defined by a gravity field described in terms of spherical harmonics of fourth order and fourth degree combined with a 6.1-mbar occultation pressure surface. This areoid can be approximated by a triaxial ellipsoid with semimajor axes of A = 3394.6 km and B = 3393.3 km and a semiminor axis of C = 3376.3 km. The semimajor axis A intersects the Martian surface at longitude 105 deg W. The dynamic flattening of Mars is 0.00525. The contour interval of the maps is 1 km. For some prominent features where overlapping pictures from Mariner 9 are available, local contour maps at relatively larger scales were also compiled by photogrammetric methods on stereo plotters.

Wu, S. S. C.↗

NASA NTRS2008

Mapping Snow Grain Size over Greenland from MODIS

This paper presents a new automatic algorithm to derive optical snow grain size (SGS) at 1 km resolution using Moderate Resolution Imaging Spectroradiometer (MODIS) measurements. Differently from previous approaches, snow grains are not assumed to be spherical but a fractal approach is used to account for their irregular shape. The retrieval is conceptually based on an analytical asymptotic radiative transfer model which predicts spectral bidirectional snow reflectance as a function of the grain size and ice absorption. The analytical form of solution leads to an explicit and fast retrieval algorithm. The time series analysis of derived SGS shows a good sensitivity to snow metamorphism, including melting and snow precipitation events. Preprocessing is performed by a Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm, which includes gridding MODIS data to 1 km resolution, water vapor retrieval, cloud masking and an atmospheric correction. MAIAC cloud mask (CM) is a new algorithm based on a time series of gridded MODIS measurements and an image-based rather than pixel-based processing. Extensive processing of MODIS TERRA data over Greenland shows a robust performance of CM algorithm in discrimination of clouds over bright snow and ice. As part of the validation analysis, SGS derived from MODIS over selected sites in 2004 was compared to the microwave brightness temperature measurements of SSM\I radiometer, which is sensitive to the amount of liquid water in the snowpack. The comparison showed a good qualitative agreement, with both datasets detecting two main periods of snowmelt. Additionally, MODIS SGS was compared with predictions of the snow model CROCUS driven by measurements of the automatic whether stations of the Greenland Climate Network. We found that CROCUS grain size is on average a factor of two larger than MODIS-derived SGS. Overall, the agreement between CROCUS and MODIS results was satisfactory, in particular before and during the first melting period in mid-June. Following detailed time series analysis of SGS for four permanent sites, the paper presents SGS maps over the Greenland ice sheet for the March-September period of 2004.

Lyapustin, Alexei↗

NASA NTRS1980

A three-dimensional body-fitted coordinate system for flow field calculations on asymmetric nosetips

A three dimensional body-fitted coordinate system developed for use in the calculation of inviscid flows over ablated, asymmetric reentry vehicle nosetips is described. Because of the potential geometric asymmetries, no standard coordinate system (e.g., spherical, axisymmetric reference surface-normal) is capable of being closely aligned with the nosetip surface. To generate a 3-D, body-fitted coordinate system an analytic mapping procedure is applied that is conformal within each meridional plane of the nosetip; these transformations are then coupled circumferentially to yield a three dimensional coordinate system. The mappings used are defined in terms of hinge points, which are points selected to approximate the body contours in each meridional plane. The selection of appropriate hinge points was automated to facilitate the use of the resulting nosetip flow field code.

Hall, D. W.↗

NASA NTRS2011

Cancer Risk Map for the Surface of Mars

We discuss calculations of the median and 95th percentile cancer risks on the surface of Mars for different solar conditions. The NASA Space Radiation Cancer Risk 2010 model is used to estimate gender and age specific cancer incidence and mortality risks for astronauts exploring Mars. Organ specific fluence spectra and doses for large solar particle events (SPE) and galactic cosmic rays (GCR) at various levels of solar activity are simulated using the HZETRN/QMSFRG computer code, and the 2010 version of the Badhwar and O Neill GCR model. The NASA JSC propensity model of SPE fluence and occurrence is used to consider upper bounds on SPE fluence for increasing mission lengths. In the transport of particles through the Mars atmosphere, a vertical distribution of Mars atmospheric thickness is calculated from the temperature and pressure data of Mars Global Surveyor, and the directional cosine distribution is implemented to describe the spherically distributed atmospheric distance along the slant path at each elevation on Mars. The resultant directional shielding by Mars atmosphere at each elevation is coupled with vehicle and body shielding for organ dose estimates. Astronaut cancer risks are mapped on the global topography of Mars, which was measured by the Mars Orbiter Laser Altimeter. Variation of cancer risk on the surface of Mars is due to a 16-km elevation range, and the large difference is obtained between the Tharsis Montes (Ascraeus, Pavonis, and Arsia) and the Hellas impact basin. Cancer incidence risks are found to be about 2-fold higher than mortality risks with a disproportionate increase in skin and thyroid cancers for all astronauts and breast cancer risk for female astronauts. The number of safe days on Mars to be below radiation limits at the 95th percent confidence level is reported for several Mission design scenarios.

Kim, Myung-Hee Y.↗

NASA NTRS1992

Analysis of crustal structure of Venus utilizing residual Line-of-Sight (LOS) gravity acceleration and surface topography data. A trial of global modeling of Venus gravity field using harmonic spline method

To construct Venus' gravity disturbance field (or gravity anomaly) with the spacecraft-observer line of site (LOS) acceleration perturbation data, both a global and a local approach can be used. The global approach, e.g., spherical harmonic coefficients, and the local approach, e.g., the integral operator method, based on geodetic techniques are generally not the same, so that they must be used separately for mapping long wavelength features and short wavelength features. Harmonic spline, as an interpolation and extrapolation technique, is intrinsically flexible to both global and local mapping of a potential field. Theoretically, it preserves the information of the potential field up to the bound by sampling theorem regardless of whether it is global or local mapping, and is never bothered with truncation errors. The improvement of harmonic spline methodology for global mapping is reported. New basis functions, a singular value decomposition (SVD) based modification to Parker & Shure's numerical procedure, and preliminary results are presented.

Fang, Ming↗

NASA NTRS1999

Improved Gravitational Models of Mars from Radio Tracking of Mars Global Surveyor

After a long period of aerobraking, Mars Global Surveyor entered a low altitude, near-circular and polar orbit about Mars on February 4, 1999. Since that time, the spacecraft has been tracked routinely by the antennae of the Deep Space Network (DSN). The X band tracking data for the first time provide us: with uniform low-altitude coverage over the entire planet. In terms of both quality and geographic distribution, these data supersede the S Band tracking obtained by the Viking Orbiters and Mariner 9. We have used all available tracking obtained by Mars Global Surveyor to determine improved models; of the Man geopotential to 70 x 70 in spherical harmonies. The new models provide an order of magnitude improvement in both accuracy and detail. For instance, the Valles Marimeris canyon system clearly appears in the gravity anomaly maps: with the new model. In this paper, we discuss the derivation of these new models, and we evaluate their performance in terms of observed features and orbit quality. We also discuss the use of altimeter data from the Mars Orbiter Laser Altimeter (MOLA) in the form of altimeter crossovers, and how these data can contribute to the determination of geopotential models for Mars.

Lemoine, Frank G.↗