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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 145 records · Page 8
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NASA NTRS1974

A simulation of synthetic aperture radar imaging of ocean waves

A simulation of radar imaging of ocean waves with synthetic aperture techniques is presented. The modelling is simplistic from the oceanographic and electromagnetic viewpoint in order to minimize the computational problems, yet reveal some of the physical problems associated with the imaging of moving ocean waves. The model assumes: (1) The radar illuminates a one-dimensional, one harmonic ocean wave. (2) The scattering is assumed to be governed by geometrical optics. (3) The radar is assumed to be down-looking, with Doppler processing (range processing is suppressed due to the one-dimensional nature of the problem). (4) The beamwidth of the antenna (or integration time) is assumed to be sufficiently narrow to restrict the specular points of the peaks and troughs of the wave. The results show that conventional processing of the image gives familiar results if the ocean waves are stationary. When the ocean wave dispersion relationship is satisfied, the image is smeared due to the motion of the specular points over the integration time. In effect, the image of the ocean is transferred to the near field of the synthetic aperture.

Swift, C. T.↗
NASA NTRS1972

A radar image of Venus.

Radar scans of Venus have yielded a brightness map of a large portion of the surface. The bright area in the south (alpha) and the twin such areas in the north (beta and delta) were first discovered by spectral analysis of radar echos. When range-gating is also applied, their shapes are revealed, and they are seen to be roundish and about 1000 km across. Although radar brightness can be the result of either intrinsic reflectivity or surface roughness, polarization studies show these features to be rough (to the scale of the wavelength, 12.5 cm). Dark, circular areas can also be seen, many with bright central spots. The dark areas are probably smooth. The blurring of the equatorial strip is an artifact of the range-Doppler geometry; all resolution disappears at the equator. Another artifact of the method is the 'ghost', in the south, of the images of beta and delta. Such ghosts appear only at the eastern and western extremes of the map.

Goldstein, R. M.↗
NASA NTRSDate not supplied

Radar Measurements of Orbital Debris from the Haystack Ultra-wideband Satellite Imaging Radar (HUSIR): 2020 to 2022

For over three decades, the NASA Orbital Debris Program Office (ODPO) has partnered with the U.S. Department of Defense and the Massachusetts Institute of Technology’s Lincoln Laboratory (MIT/LL) to collect radar data on the orbital debris (OD) environment. These radar measurements are used to statistically measure cm and sub-cm debris in low Earth orbit (LEO) that is used to define the current debris environment, monitor how the environment evolves, and support predictions of the future debris environment. Currently, the principal ground-based radar sensor used by the ODPO is the MIT/LL Haystack Ultrawideband Satellite Imaging Radar (HUSIR). HUSIR provides data on OD down to approximately 5 mm in size at altitudes below 1000 km. This paper will deliver an overview of recent HUSIR measurements of the LEO debris environment for calendar years 2020 to 2022. Process improvements will be highlighted along with results of interest, including the effects of the 15 November 2021 Russian direct ascent anti-satellite (ASAT) test on the defunct COSMOS 1408 spacecraft.

Jessica A. Arnold↗
NASA NTRSDate not supplied

Radar Measurements of Orbital Debris from the Haystack Ultra-wideband Satellite Imaging Radar (HUSIR): 2020-2021

For over three decades, the NASA Orbital Debris Program Office (ODPO) has partnered with the U.S. Department of Defense and the Massachusetts Institute of Technology’s Lincoln Laboratory (MIT/LL) to collect radar data on the orbital debris (OD) environment. These radar measurements are used to statistically measure centimeter (cm) and sub-cm debris in low Earth orbit (LEO) that is used to define the current debris environment, monitor how the environment evolves, and support predictions of the future debris environment. Currently, the principal ground-based radar sensor used by the ODPO is the MIT/LL Haystack Ultrawideband Satellite Imaging Radar (HUSIR). HUSIR provides data on OD down to approximately 5 mm in size at altitudes below 1000 km. This paper will deliver an overview of recent HUSIR measurements of the LEO debris environment for calendar years 2020 to 2022. Process improvements will be highlighted along with results of interest, including the effects of the 15 November 2021 Russian direct ascent anti-satellite (ASAT) test on the defunct Cosmos 1408 spacecraft.

Jessica A Arnold↗
NASA NTRS1991

Status Of Imaging Radar Polarimetry

Report pulls together information on imaging radar polarimetry from a variety of sources. Topics include theory, equipment, and experimental data. Reviews state of the art, examines current applicable developments in radar equipment, describes recording and processing of radar polarimetric measurements, and discusses interpretation and application of resulting polarimetric images.

Van Zyl, Jakob J.↗
NASA NTRS1986

Use of speckle for determining the response characteristics of Doppler imaging radars

An optical model is developed for imaging optical radars such as the SAR on Seasat and the Shuttle Imaging Radar (SIR-B) by analyzing the Doppler shift of individual speckles in the image. The signal received at the spacecraft is treated in terms of a Fresnel-Kirchhoff integration over all backscattered radiation within a Huygen aperture at the earth. Account is taken of the movement of the spacecraft along the orbital path between emission and reception. The individual points are described by integration of the point source amplitude with a Green's function scattering kernel. Doppler data at each point furnishes the coordinates for visual representations. A Rayleigh-Poisson model of the surface scattering characteristics is used with Monte Carlo methods to generate simulations of Doppler radar speckle that compare well with Seasat SAR data SIR-B data.

Tilley, D. G.↗
NASA NTRS1986

Airborne and spaceborne radar images for geologic and environmental mapping in the Amazon rain forest, Brazil

Spaceborne and airborne radar image of portions of the Middle and Upper Amazon basin in the state of Amazonas and the Territory of Roraima are compared for purposes of geological and environmental mapping. The contrasted illumination geometries and imaging parameters are related to terrain slope and surface roughness characteristics for corresponding areas that were covered by each of the radar imaging systems. Landforms range from deeply dissected mountain and plateau with relief up to 500 m in Roraima, revealing ancient layered rocks through folded residual mountains to deeply beveled pediplain in Amazonas. Geomorphic features provide distinct textural signatures that are characteristic of different rock associations. The principle drainages in the areas covered are the Rio Negro, Rio Branco, and the Rio Japura. Shadowing effects and low radar sensitivity to subtle linear fractures that are aligned parallel or nearly parallel to the direction of radar illumination illustrate the need to obtain multiple coverage with viewing directions about 90 degrees. Perception of standing water and alluvial forest in floodplains varies with incident angle and with season. Multitemporal data sets acquired over periods of years provide an ideal method of monitoring environmental changes.

Ford, John P.↗
NASA NTRS2013

Automatic Focusing for a 675 GHz Imaging Radar with Target Standoff Distances from 14 to 34 Meters

This paper dicusses the issue of limited focal depth for high-resolution imaging radar operating over a wide range of standoff distances. We describe a technique for automatically focusing a THz imaging radar system using translational optics combined with range estimation based on a reduced chirp bandwidth setting. The demonstarted focusing algorithm estimates the correct focal depth for desired targets in the field of view at unknown standoffs and in the presence of clutter to provide good imagery at 14 to 30 meters of standoff.

automatic focus↗
NASA NTRS1988

The optimal polarizations for achieving maximum contrast in radar images

There is considerable interest in determining the optimal polarizations that maximize contrast between two scattering classes in polarimetric radar images. A systematic approach is presented for obtaining the optimal polarimetric matched filter, i.e., that filter which produces maximum contrast between two scattering classes. The maximization procedure involves solving an eigenvalue problem where the eigenvector corresponding to the maximum contrast ratio is an optimal polarimetric matched filter. To exhibit the physical significance of this filter, it is transformed into its associated transmitting and receiving polarization states, written in terms of horizontal and vertical vector components. For the special case where the transmitting polarization is fixed, the receiving polarization which maximizes the contrast ratio is also obtained. Polarimetric filtering is then applies to synthetic aperture radar images obtained from the Jet Propulsion Laboratory. It is shown, both numerically and through the use of radar imagery, that maximum image contrast can be realized when data is processed with the optimal polarimeter matched filter.

Swartz, A. A.↗
NASA NTRS1976

A Seasat-A Synthetic Aperture Imaging Radar System

The Seasat-A Synthetic Aperture Imaging Radar System is the first radar system of its kind designed for the study of ocean wave patterns from orbit. The basic requirement of this system is to generate continuous radar imagery with a 100-km swath with 25 m resolution from an orbital altitude of 800 km. These requirements impose unique system design problems and their solutions will be stated. The end to end data system will be described including interactions of the spacecraft, antenna, sensor, telemetry link, and data processor. The synthetic aperture radar system generates a large quantity of data (110 megabits per second) requiring the use of a dedicated data link. The data link selected for use with the synthetic aperture radar is an analog link with stable local oscillator encoding. The problems associated in telemetering the radar information with sufficient fidelity to synthesize an image on the ground will be described as well as the selected solutions to the problems.

Jordan, R. L.↗
NASA NTRS1976

The Seasat-A Synthetic Aperture Imaging Radar System

The Seasat-A Synthetic Aperture Imaging Radar System is the first radar system of its kind designed for orbital use. The requirement of this system is to generate continuous radar imagery with a 100-km swath with 25 m resolution from an orbital altitude of 800 km. These requirements impose unique system design problems and a description of the implementation will be given. The end to end data system will be described including interactions of the spacecraft, antenna, sensor, telemetry link, and data processor. The synthetic aperture radar system generates a large quantity of data (110 megabits per second) requiring the use of a dedicated data link. The data link selected for use with the synthetic aperture radar is an analog link with stable local oscillator encoding. The problems associated in telemetering the radar information with sufficient fidelity to synthesize an image on the ground will be described as well as the selected solutions to the problems. The interactions between the antenna attitude control system, rotation of the earth and the data processor will be described as well as proposed solutions, both optical and digital, to generate final imagery with the required 25 m resolution.

Jordan, R. L.↗
NASA NTRS1987

Spaceborne imaging radar on EOS

A multiparameter synthetic aperture imaging radar is planned as a facility instrument for the Earth Orbiting System (EOS). This sensor will operate at L, C, and X and at all possible polarizations (HH, VV, HV, VH), thus allowing the acquisition of detailed information about the surface physical and electrical properties. When combined with the visible and IR imaging spectrometry data and the surface topography, a full description of the surface structure, composition, thermal properties and physical properties could then be extracted.

Elachi, Charles↗
NASA NTRS1999

Original Size of the Sudbury Structure: Evidence from Field Investigations and Imaging Radar

This paper summarizes results of continuing studies of the original size of the Sudbury impact structure, including imaging radar and field investigations of supposed "Sudbury breccia" north of the Sudbury Igneous Comples (SIC). Imaging radar acquired from Canada Centre for Remote Sensing (CCRS) aircraft, European Space Agency Remote Sensing Satellite (ERS-1), and RADARSAT shows no evidence of outer rings concentric with the North Range. Illumination directions are such that these rings, presumably extension fractures, would be conspicuous by look azimuth highlighting if they existed. Field mapping supports this interpretation, showing that supposed ring fractures occupied by Huronian sediments are essentially synclines older than the 1850 Ma impact and are not related to the impact. Field investigations of "Sudbury breccia" north of the SIC shows that most if not all of it is inside or along contacts with diabase dykes of the Sudbury Swarm (ca. 1238 Ma), and hence is far too young to be related to the impact. A recently-discovered occurrence of "Sudbury breccia" south of the SIC, near Creighton, is similarly associated with a NW-trending diabase dyke cutting the SIC, supporting the post-impact age of the breccia. It is concluded that the original north rim of the Sudbury crater was not more than 5 to 10 km north of the present North Range SIC contact, and that published estimates of the crater size (ca 200 km diameter) are incorrect.

Lowmman, Paul D., Jr.↗
NASA NTRS1985

Remote sensing of the earth with spaceborne imaging radars

Recent scientific and technological developments are reviewed in the field of earth observation with spaceborne imaging radars. Such developments, beginning with Seasat in 1978 and continuing with the Space Shuttle in 1981 and 1984, were made possible by the use of new large spaceborne lightweight planar array antennas (2 x 10 m) with printed radiating elements. Transmitters were solid-state 1-kW peak power units operating at L-band (1.2 GHz). Images were obtained to monitor sea ice, soil moisture, and geologic, biologic and oceanographic features. Optical and digital processing was done to achieve high resolution (25 to 40 m). More advanced systems are under development, including multispectral, multipolarization imaging radar systems for flight in the late 1980s. An overview of planned activities in the 1980s is given.

Elachi, C.↗
NASA NTRS1973

Imaging radars provide terrain texture and roughness parameters in semi-arid environments

Geoscience potentialities and limitations of available imaging radars are examined for alluvial valley-playa terrain environments. In such arid environments, a dual-sensor combination of an imaging radar with aerial photography is shown to provide a practical method for monitoring gross changes in surface textures of alluvial fans and playa surface conditions. It also appears feasible to infer surface materials and relative surface roughness with an improved degree of interpretive reliability.

Macdonald, H. C.↗
NASA NTRS1991

Alphonsus crater - Floor fracture and dark-mantle deposit distribution from new 3.0-cm radar images

The lunar crater Alphonsus is characterized by numerous fractures or graben, and by endogenic dark-halo craters. Existing maps of fractures from analysis of lunar photography may be biased by the east-west solar illumination. This paper presents new high-resolution, dual-polarization 3.0-cm wavelength radar images of Alphonsus with radar illumination from northerly directions, and uses these data to better map the locations of both the graben and a variety of dark-mantle deposits. The distribution of fractures, and several graben which cut the crater floor and central ridge, are cited as possible evidence for simultaneous, post-Imbrium uplift of both structures. Some of the endogenic dark halo deposits are more extensive in depolarized radar images than in photographs; these extensions are attributed in some cases to more distant emplacement of pyroclastic material, and in others to fortuitous connections with smoother, less cratered portions of the Alphonsus floor.

Zisk, Stanley H.↗
NASA NTRS1986

SIR-B - The second Shuttle Imaging Radar experiment

On October 5, 1984, the second Shuttle Imaging Radar (SIR-B) was launched into orbit aboard the Space Shuttle Challenger. SIR-B is part of an evolutionary radar program designed to progressively develop a multifrequency, multipolarization synthetic aperture radar with a variable earth-imaging geometry. The SIR-B instrument is an upgraded version of SIR-A, with the additional capability of tilting the antenna mechanically to acquire imagery at variable incidence angles ranging from 15 to 60 deg. The variable look angle capability provided a means of acquiring multiple incidence angle imagery over specific targets on successive days of the mission. These data are being used to classify surface features by their backscatter signatures as a function of incidence angle and for topographic mapping. In addition to the antenna tilt capability, a digital data-handling system was added to increase the dynamic range, the resolution was improved by a factor of two over SIR-A, and a calibration subsystem was added to improve the radiometric accuracy of the data. The mission had a number of problems, including loss of the primary digital data path between the Shuttle and the ground. In spite of these problems, approximately 20 percent of the planned digital data were collected over the 8-day Shuttle mission corresponding to an areal coverage of about 6.4 million sq km.

Cimino, J.↗