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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Global digital topography mapping using a scanning radar altimeter

The conceptual design of a Scanning Radar Altimeter system capable of collecting less than 300-m spatial and less than 3-m height resolution digital topography data for the entire globe, from an orbital platform, is presented. A 37-GHz frequency SRA system is used to achieve the requisite resolution while reducing antenna length in the along-track dimension. Near-global coverage in a short time period is obtained by scanning the antenna beam cross-track, in a swath of about 100 km. Attention is given to the algorithm that will be used to retrieve pixel height from the return waveform.

Elachi, C.↗
NASA NTRS1987

Spherical harmonic expansion of the Levitus Sea surface topography

Prior information for the stationary sea surface topography (SST) may be needed in altimetric solutions that intend to simultaneously improve the gravity field and determine the SST. For this purpose the oceanographically derived SST estimates are represented by a spherical harmonic expansion. The spherical harmonic coefficients are computed from a least squares adjustment of the data covering the majority of the oceanic regions of the world. Several tests are made to determine the optimum maximum degree of solution and the best configuration of the geometry of the data in order to obtain a solution that fits the data and also provides a good spectral representation of the SST.

Engelis, Theodossios↗
NASA NTRS1988

X-ray diffraction imaging (topography) of electroopticcrystals by synchrotron radiation

Information of special interest to crystal growers and device physicists now available from monochromatic synchrotron diffraction imaging (topography) is reviewed. Illustrations are taken from a variety of electro-optic crystals. Aspects of the detailed understanding of crystal growth processes obtainable from carefully selected samples are described. Finally, new experimental opportunities now available for exploitation are indicated.

Steiner, Bruce↗
NASA NTRS1988

Diffraction imaging (topography) with monochromatic synchrotron radiation

Structural information of special interest to crystal growers and device physicists is now available from high resolution monochromatic synchrotron diffraction imaging (topography). In the review, the importance of superior resolution in momentum transfer and in space is described, and illustrations are taken from a variety of crystals: gallium arsenide, cadmium telluride, mercuric iodide, bismuth silicon oxide, and lithium niobate. The identification and understanding of local variations in crystal growth processes are shown. Finally, new experimental opportunities now available for exploitation are indicated.

Steiner, Bruce↗
NASA NTRS1987

Characteristics of gravity waves generated by surface topography on Venus - Comparison with the VEGA Balloon results

Data from the 1985 VEGA Venus Balloon Mission indicate that the mountainous region known as Aphrodite influences atmospheric motions at balloon float altitudes near 54 km, an altitude located within the middle cloud region. It is shown that stationary gravity waves, generated by surface topography and Doppler-shifted by a wind blowing over the terrain, can propagate upward to the middle cloud layers. Under the right circumstances, waves are amplified considerably in excess of their amplification due to the decrease of density with altitude. The additional amplification is due to resonance that results from variations of static stability and mean zonal wind with altitude. Computed atmospheric propagation characteristics, combined with terrain slopes in Aphrodite estimated from Pioneer Venus radar altimeter data, can be sufficient to produce wind amplitudes at 54 km comparable to those observed by the VEGA-2 balloon as it overflew Aphrodite. The dominant waves have horizontal wavelengths of the order of several hundred kilometers.

Young, Richard E.↗
NASA NTRS1987

The evolution of topography on a comet

The evolution of cometary surface topography is presently modeled in terms of a focusing of thermal energy due to topographical concavity. The model encompasses incident sunlight, shadowing, molecular sublimation, and mutual radiative heating by the walls of a circle-segment cross section trench, which has a 'lensing' effect. After a large number of perihelion passages, the net effect of trench bottom sublimation losses will be the accentuation of topographical concavities. The comet surface sublimation, comet-splitting, and comet surface jet-formation consequences of this model are discussed.

Colwell, Joshua E.↗
NASA NTRS1988

Radii, shapes, and topography of the satellites of Uranus from limb coordinates

The present use of limb coordinates to ascertain radii, shape and local topographic characteristics for the five large Uranian satellites directly measured satellite ellipsoidal shapes, determining radii with subpixel accuracy. While Umbriel, Titania, and Oberon are spherical, Miranda and Ariel are noted to be ellipsoids whose equatorial bulges agree with previously determined mean densities. Miranda's topography exhibits substantial deformation of cratered terrain and the younger coronae, as well as uplift and complex faulting at coronae margins.

Thomas, P. C.↗
NASA NTRS1988

Large-scale topography of Io - Implications for internal structure and heat transfer

Recent stereographic methods have been applied to Voyager 1 images in order to determine the shape and large-scale topography of Io. The best triaxial figure of semiaxes of 1830.0, 1818.7, and 1815.3 kilometers is consistent with a differentiated satellite in hydrostatic equilibrium. It is suggested that many of the broad topographic swells and basins noted may be due to isostatic responses to thermal changes in the lithosphere-asthenosphere system. Results support the picture of increasing heat flow converting the basal lithosphere into a lower-density asthenosphere, resulting in isostatic uplift. It is shown that lithospheric thicknesses ranging from 5-100 km may results in elevation variations of up to 1 km.

Gaskell, Robert W.↗
NASA NTRS1988

Observations and wind-forced model simulations of the mean seasonal cycle in tropical Pacific sea surface topography

This paper quantitatively evaluates simulations of the mean seasonal cycle in the tropical Pacific Ocean, using a multiple vertical-mode version of the Busalacchi and O'Brien (1980) linear model. The period studied (1979-1981) includes several coincident oceanic and surface wind data sets, as well as a fairly regular seasonal cycle leading into the 1982-1983 El Nino. Simulated mean seasonal cycles in dynamic height and sea level are compared with observed variations in expendable bathythermograph and island tide gauge data averaged over the same period. It was found that, although all simulations show characteristic features of the mean meridional ridge-trough structure in surface topography, simulated north and south equatorial ridges at 20 deg N and 20 deg S are much higher than those observed. Only weak equatorial ridges were generated near 4 N, and none of the simulations exhibited a significant equatorial trough. These discrepancies are attributed to limitations in model physics and to the wind forcing.

Mcphaden, Michael J.↗
NASA NTRS1988

Ocean topography experiment (TOPEX) radar altimeter

A spaceflight qualified Radar Altimeter capable of achieving the TOPEX Mission measurement precision requirement of 2-centimeters, is provided and its performance (Engineering Assessment) will be evaluated after launch and continuously during its 3-year mission operational period. Information will be provided to JPL about the calibration of the TOPEX Radar Altimeter. The specifications for the required data processing algorithms which will be necessary to convert the Radar Altimeter mission telemetry data into the geophysical data will also be provided. The stringent 2 cm precision requirement for ocean topography determination from space necessitated examining existing Radar Altimeter designs for their applicability towards TOPEX. As a result, a system configuration evolved using some flight proven designs in conjunction with needed improvements which include: (1) a second frequency or channel to remove the range delay or apparent height bias caused by the electron content of the ionosphere; (2) higher transmit pulse repetition frequencies for correlation benefits at higher sea states to maintain precision; and (3) a faster microprocessor to accommodate two channels of altimetry data. Additionally, examination of past altimeter programs associated data processing algorithms was accomplished to establish the TOPEX-class Radar Altimeter data processing algorithms, and the necessary direction was outlined to begin to generate these for the TOPEX Mission.

Rossi, L. C.↗
NASA NTRS1988

Extraction of topography from side-looking satellite systems - A case study with SPOT simulation data

A simulated data set was used to evaluate techniques for extracting topography from side-looking satellite systems for an area of northwest Washington state. A negative transparency orthophotoquad was digitized at a spacing of 85 microns, resulting in an equivalent ground distance of 9.86 m between pixels and a radiometric resolution of 256 levels. A bilinear interpolation was performed on digital elevation model data to generate elevation data at a 9.86-m resolution. The nominal orbital characteristics and geometry of the SPOT satellite were convoluted with the data to produce simulated panchromatic HRV digital stereo imagery for three different orbital paths and techniques for reconstructing topographic data were developed. Analyses with the simulated HRV data and other data sets show that the method is effective.

Ungar, Stephen G.↗
NASA NTRS1990

Evolution of topography on comets. II - Icy craters and trenches

The determination of the effects of topography on the sublimation rates of comets and other icy bodies is presently approached via a model of ice heating and sublimation from topographical features. The energy balance equation is solved for cylindrical trenches and spherical craters; the model encompasses shadowing, solar heating, the trapping of thermal radiation and sublimed gas molecules, and reflection of sunlight within the cavity. Generally, an enhancement is found in the net sublimation rate for trenches and craters farther from the sun than some critical distance which depends on the albedo.

Colwell, Joshua E.↗
NASA NTRS1990

Fractal image analysis - Application to the topography of Oregon and synthetic images.

Digitized topography for the state of Oregon has been used to obtain maps of fractal dimension and roughness amplitude. The roughness amplitude correlates well with variations in relief and is a promising parameter for the quantitative classification of landforms. The spatial variations in fractal dimension are low and show no clear correlation with different tectonic settings. For Oregon the mean fractal dimension from a two-dimensional spectral analysis is D = 2.586, and for a one-dimensional spectral analysis the mean fractal dimension is D = 1.487, which is close to the Brown noise value D = 1.5. Synthetic two-dimensional images have also been generated for a range of D values. For D = 2.6, the synthetic image has a mean one-dimensional spectral fractal dimension D = 1.58, which is consistent with the results for Oregon. This approach can be easily applied to any digitzed image that obeys fractal statistics.

Huang, Jie↗
NASA NTRS1990

Global digital topography mapping with a synthetic aperture scanning radar altimeter

Global digital topography data of the land surface is of importance in a variety of geoscientific and application disciplines. Such a database, with a spatial resolution of 150 to 500 m and height accuracy of 5 m or better can be acquired from an orbiting platform using a synthetic aperture scanning radar altimeter. Near-global coverage can be achieved within 14 days from an orbiting platform in a polar or near-polar orbit.

Elachi, C.↗
NASA NTRS1990

Australian topography from Seasat overland altimetry

Retracking of overland returns from the Seasat altimeter using algorithms originally developed for recovering elevations over ice has led to the successful recovery of high quality continental topography over Australia and other continents. Cross-over analysis both before and after orbit adjustment shows the altimetric data over land to have a 2-3 m quality. Direct comparison of gridded Seasat data with surface data re-averaged in the same way shows excellent agreement except where Seasat data are sparse, due either to poor track spacing or to dropouts caused by loss of tracker lock over steeply sloping ground. These results suggest that useful topographic data can be derived from Seasat and the more recent Geosat altimeters for parts of the world where surface data are few or of poor quality.

Frey, Herbert↗
NASA NTRS1990

The evolution of Tharsis: Implications of gravity, topography, and tectonics

Dominating the Western Hemisphere of Mars, the Tharsis rise is an elongate area centered on Syria Planum that ascends as much as 8 to 10 km above the datum. It is intensely fractured by long, narrow grabens that extend radially hundreds of kilometers beyond the rise and is ringed by mostly concentric wrinkle ridges that formed over 2,000 km from the center of the rise. Its size, involving a full hemisphere of Mars, gives it a central role in the thermo-tectonic evolution of the planet and has stimulated a number of studies attempting to determine the sequence of events responsible for this feature. The constraints that gravity and topography data place on the current structure of Tharsis, along with insights into its development derived from comparisons of detailed regional mapping of faulting with theoretical deformation models are reviewed. Finally, a self-consistent model for the structure of Tharsis is proposed.

Banerdt, W. B.↗
NASA NTRS1990

Dynamical effects from equation of state on topographies and geoid anomalies due to internal loading

Using a Cartesian compressible model, the effects of compressibility on geoid anomalies and of deformations on the topographic signatures caused by internal loading in the mantle are quantitatively investigated by examining different types of the equation of state. Also considered are different types of density laws with variations of thermodynamical parameters taken into play. It is shown that surface topographies are not changed much by the effects of mantle compressibility and that the deformation of bottom boundary is not significantly influenced except for long wavelengths and large viscosity contrasts between the upper and lower mantles. On the other hand, mantle compressibility can greatly affect geoid signals with horizontal wavelengths exceeding 10,000 km.

Hong, Hanjing↗