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All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

On the application of multifrequency polarimetric radar observations for sea-ice classification

The use of multifrequency polarimetric radar imagery to enhance the ability to separate different sea-ice types using single-frequency, single-polarization synthetic aperture radar (SAR) data is investigated. Backscatter characteristics of six radiometrically and polarimetrically distinct sea-ice types are selected in an unsupervised range-dependent analysis of multifrequency polarimetric SAR data using the maximum a posteriori (MAP) polarimetric classifier. Maximum ice discrimination is achieved with combined C- and L-band full polarimetry, and collocated passive microwave imagery suggests greater than 90 percent classification accuracy. C-band VV-pol alone achieves only 68 percent relative accuracy because it confuses multiyear and rough compressed first year ice. L-band, relative classification accuracy is 75 percent, 83 percent, and 85 percent, using HH-pol, HH- and VV-combined, or the full polarimetry, respectively. P-band is less accurate. Combinations of two frequencies at a single polarization show the greatest improvement over a single channel.

Rignot, Eric↗
NASA NTRS1997

Surface Properties of the Moon, Venus and Small Bodies from Radar Observations

Studies of the moon during the period of the grant revolved around the issues of the possible presence of ice at the lunar poles and the determination of the electrical properties of the maria regoliths. The search for ice at the poles was conducted using measurements of the radar backscatter cross sections and circular polarization ratios measured from 125 m resolution Arecibo radar imagery at 13 cm wavelength obtained by Nicholas Stacy. No clear indication of the presence of ice was found in areas thought to be in permanent shadow from solar radiation. Then Cornell graduate student Greg Black modeled the radar backscattering behavior of the icy Galilean satellites using three wavelength measurements of their radar backscattering properties obtained with the Arecibo and Goldstone radars. The radar scattering properties of Europa, Ganymede, and Callisto are unlike those of any other object observed with planetary radars. They are strongly backscattering with specific radar cross sections that can exceed unity. Polarization ratios are also high, approx. 1.5, indicative of multiple scattering, and the echos follow a diffuse scattering law at all incident angles with no indication of quasi-specular reflections. 3) Most of our effort on small bodies went into developing and investigating methods for long baseline radar synthesis imaging of near-earth asteroids and comets. At X-band, the width of the synthesized beam of the Very Long Baseline Array (VLBA) is approximately 15 m at 0.03AU, a typical close approach distance for near-earth asteroids. A small amount of work was done analyzing Venus data from Arecibo and the Magellan mission.

Campbell, Donald B.↗
NASA NTRS1976

Imaging radar observations of frozen Arctic lakes

A synthetic aperture imaging L-band radar flown aboard the NASA CV-990 remotely sensed a number of ice-covered lakes about 48 km northwest of Bethel, Alaska. The image obtained is a high resolution, two-dimensional representation of the surface backscatter cross section, and large differences in backscatter returns are observed: homogeneous low returns, homogeneous high returns and/or low returns near lake borders, and high returns from central areas. It is suggested that a low return indicates that the lake is frozen completely to the bottom, while a high return indicates the presence of fresh water between the ice cover and the lake bed.

Elachi, C.↗
NASA NTRS1981

Imaging radar observations of volcanic features in Medicine Lake Highland, California

Synthetic aperture L-band radar images of Medicine Lake Highland, California, as obtained from the JPL-NASA aircraft and Seasat orbital systems, are presented. Image interpretation is based on two types of information: slope and topographic effects for geomorphic information, and reflectivity or backscatter in flat terrain which is related to surface roughness and to the surface dielectric constant. Cinder cones and lava tubes are visible (geomorphic features), and three types of lava surfaces are: aa, pahoehoe, and block. In addition, to infer lava flow relative ages, overlapping flow fronts and roughness attenuation observations may be used. It is hoped that this research will improve the understanding of the radar signature of volcanic areas, and will be helpful in interpreting data obtained from spaceborne sensors over the earth and, in particular, Venus.

Farr, T. G.↗
NASA NTRS1999

Global Database of GSSR Mars Delay-Doppler Radar Observations: Analysis for Landing Site Characterization and Rover Trafficability

Earth-based radar data remain an important part of the information set used to select and certify spacecraft landing sites on Mars. Constraints on robotic landings on Mars include: terrain elevation, radar reflectivity, regional and local slopes, rock distribution and coverage, and surface roughness, all of which are addressed by radar data. Indeed, the usefulness of radar data for Mars exploration has been demonstrated in the past. Radar data were critical in assessing the Viking Lander 1 site, and more recently, the Mars Pathfinder landing site.

Haldemann, A. F. C.↗
NASA NTRS1999

Radar Observations of Convective Systems from a High-Altitude Aircraft

Reflectivity data collected by the precipitation radar on board the tropical Rainfall Measuring Mission (TRMM) satellite, orbiting at 350 km altitude, are compared to reflectivity data collected nearly simultaneously by a doppler radar aboard the NASA ER-2 flying at 19-20 km altitude, i.e. above even the deepest convection. The TRMM precipitation radar is a scanning device with a ground swath width of 215 km, and has a resolution of about a4.4 km in the horizontal and 250 m in the vertical (125 m in the core swath 48 km wide). The TRMM radar has a wavelength of 217 cm (13.8 GHz) and the Nadir mirror echo below the surface is used to correct reflectivity for loss by attenuation. The ER-2 Doppler radar (EDOP) has two antennas, one pointing to the nadir, 34 degrees forward. The forward pointing beam receives both the normal and the cross-polarized echos, so the linear polarization ratio field can be monitored. EDOP has a wavelength of 3.12 cm (9.6 GHz), a vertical resolution of 37.5 m and a horizontal along-track resolution of about 100 m. The 2-D along track airflow field can be synthesized from the radial velocities of both beams, if a reflectivity-based hydrometer fall speed relation can be assumed. It is primarily the superb vertical resolution that distinguishes EDOP from other ground-based or airborne radars. Two experiments were conducted during 1998 into validate TRMM reflectivity data over convection and convectively-generated stratiform precipitation regions. The Teflun-A (TEXAS-Florida Underflight) experiment, was conducted in April and May and focused on mesoscale convective systems mainly in southeast Texas. TEFLUN-B was conducted in August-September in central Florida, in coordination with CAMEX-3 (Convection and Moisture Experiment). The latter was focused on hurricanes, especially during landfall, whereas TEFLUN-B concentrated on central; Florida convection, which is largely driven and organized by surface heating and ensuing sea breeze circulations. Both TEFLUN-A and B were amply supported by surface data, in particular a dense raingauge network, a polarization radar, wind profilers, a mobile radiosonde system, a cloud physics aircraft penetrating the overflown storms, and a network of 10 cm Doppler radars(WSR-88D). This presentation will show some preliminary comparisons between TRMM, EDOP, and WSR-88D reflectivity fields in the case of an MCS, a hurricane, and less organized convection in central Florida. A validation of TRMM reflectivity is important, because TRMM's primary objective is to estimate the rainfall climatology with 35 degrees of the equator. Rainfall is estimated from the radar reflectivity, as well from TRMM's Microwave Imager, which measures at 10.7, 19.4, 21.3, 37, and 85.5 GHz over a broader swath (78 km). While the experiments lasted about three months the cumulative period of near simultaneous observations of storms by ground-based, airborne and space borne radars is only about an hour long. Therefore the comparison is case-study-based, not climatological. We will highlight fundamental differences in the typical reflectivity profiles in stratiform regions of MCS's, Florida convection and hurricanes and will explain why Z-R relationships based on ground-based radar data for convective systems over land should be different from those for hurricanes. These catastrophically intense rainfall from hurricane Georges in Hispaniola and from Mitch in Honduras highlights the importance of accurate Z-R relationships, It will be shown that a Z-R relationship that uses the entire reflectivity profile (rather than just a 1 level) works much better in a variety of cases, making an adjustment of the constants for different precipitation system categories redundant.

Heymsfield, G.↗
NASA NTRS1976

Radar observations of F region equatorial irregularities

The paper presents some results of backscatter observations of the F region irregularities made with the large 50-MHz radar at Jicamarca, Peru, during a few days of observations. The results were obtained by using three observational techniques: the modified range-time-intensity technique, the digital power mapping technique, and the digital raw data recording technique. Backscatter intensity maps as a function of altitude and time are presented, which can be interpreted as radar pictures of F region irregularities. A classification of spread F spectral signatures resulting from approximately 30,000 spectra obtained in sets of 64 simultaneous heights under a variety of conditions is also given.

Woodman, R. F.↗
NASA NTRS1978

Radar observations of a volcanic terrain: Askja Caldera, Iceland

Surface roughness spectra of nine radar backscatter units in the Askja caldera region of Iceland were predicted from computer-enhanced like- and cross-polarized radar images. A field survey of the caldera was then undertaken to check the accuracy of the preliminary analysis. There was good agreement between predicted surface roughness of backscatter units and surface roughness observed in the field. In some cases, variations in surface roughness could be correlated with previously mapped geologic units.

Evans, D. L.↗
NASA NTRS1981

Spaceborne and airborne imaging radar observations of sand dunes

An assessment of the ability of orbital synthetic aperture radar (SAR) imaging systems to provide useful information about aeolian features, and to determine how such a system might be constrained by the need to image these features, is presented. Seasat and aircraft radar imagery of five areas of sand dunes are studied, and compared to Landsat imagery and air photos, for two wavelengths (3.0 and 23.5 cm) and incidence angles ranging from 0 to 70 deg. It is shown that the illumination direction of the radar beam is important, since directional dune features must be oriented within 60 deg of perpendicular to the radar illumination direction in order to be imaged. It is concluded that the availability of radar imagery for two directions greatly facilitates interpretation of dune morphology and derivation of conclusions about causative wind regimes.

Blom, R.↗
NASA NTRS1992

Experimental measurement and theoretical modeling of microwave scattering and the structure of the sea surface influencing radar observations from space

The electromagnetic (EM) bias 'epsilon' is an error present in radar altimetry of the ocean due to the nonuniform reflection from wave troughs and crests. The EM bias is defined as the difference between the mean reflecting surface and the mean sea surface. A knowledge of the EM bias is necessary to permit error reduction in mean sea level measurements by satellite radar altimeters. Direct measurements of the EM bias were made from a Shell Offshore oil production platform in the Gulf of Mexico for a six month period during 1989 and 1990. Measurements of the EM bias were made at 5 and 14 Ghz. During the EM bias experiments by Melville et al., a wire wave gauge was used to obtain the modulation of the high frequency waves by the low frequency waves. It became apparent that the EM bias was primarily caused by the modulation of the short waves. This was reported by Arnold et al. The EM bias is explained using physical optics scattering and an empirical model for the short wave modulation. Measurements of the short wave modulation using a wire wave gauge demonstrated a linear dependence of the normalized bias on the short wave modulation strength, M. The theory accurately predicts this dependence by the relation epsilon = -alphaMH sub 1/3. The wind speed dependence of the normalized bias is explained by the dependence of the short wave modulation strength on the wind speed. While other effects such as long wave tilt and curvature will have an effect on the bias, the primary cause of the bias is shown to be due to the short wave modulation. This report will present a theory using physical optics scattering and an empirical model of the short wave modulation to estimate the EM bias. The estimated EM bias will be compared to measurements at C and Ku bands.

Arnold, David↗
NASA NTRS2005

Asteroid Shape Reconstruction From Radar Observations

I estimate near-Earth asteroid 1992 SK's physical properties from radar delay-Doppler images, Doppler-only echo spectra and optical lightcurves. The images are not very strong, but place up to 20 (40 m by 160 m) pixels on the asteroid. The radar tracks are confined to subradar latitudes between 20 and 40 degrees but have complete rotational phase coverage. The echo spectra and optical lightcurves span approx.80 degrees of sky motion, providing geometric leverage to constrain the pole direction. The optical lightcurves are essential to accurate determination of the asteroid's shape and spin state. The asteroid is approx.1.4 km in maximum extent and mildly asymmetric, with an elongation of approx.1.5 and relatively subdued topography. The radar albedo is about 0.13 and the optical albedo about 0.3. The circular polarization ratio for the object is about 0.34, implying typical cm-scale surface roughness. I estimate the asteroid's period to be 7.3182+/-0.0003 hours and its pole direction as (99deg+/-5deg,-3deg+/-5deg) in ecliptic coordinates. The radar-refined orbital solution accurately predicts planetary close approaches between the years 826 and 2690. I have used my model to predict salient characteristics of radar images and optical lightcurves obtainable during the asteroid's March 2006 approach.

Busch, Michael J.↗
NASA NTRS1981

Chatanika radar observations relating to the latitudinal and local time variations of Joule heating

Observations of plasma convection made with the Chatanika incoherent scatter radar have been analyzed to give latitude/local time plots of the electric field contribution (E squared) to thermospheric Joule heating. The data, which plan the invariant latitude range 56 deg to 75 deg, show the presence of strong heating throughout the auroral regions. Of special interest are brief interludes of intense heating (greater than 50 mW/sq m) that are observed at nearly all local times and latitudes in response to magnetospheric disturbances. Further, there seem to be particular regions of the auroral oval where Joule heating seems to be continually enhanced above the broad background. The results of six 24-hour experiments are presented to illustrate summer and winter conditions. A shorter eight hour experiment is also given to show the characteristics of cleft heating, insofar as they are visible to the Chatanika radar.

Banks, P. M.↗
NASA NTRS1986

Simultaneous rocket and MST radar observation of an internal gravity wave breaking in the mesosphere

In June, 1983, the Structure and Atmospheric Turbulence Environment (STATE) rocket and Poker Flat Mesophere-Stratosphere-Troposphere radar campaign was conducted to measure the interaction between turbulence, electron density and electron density gradient that has produced unusually strong MST radar echoes from the summer mesosphere over Poker Flat, Alaska. Analysis or radar wind measurements and a concurrent wind and temperature profile obtained from a rocket probe carrying a three-axis accelerometer are given. The two data sets provide a fairly complete (and in some cases, redundant) picture of the breaking (or more correctly, the saturation) of a large-amplitude, low-frequency, long-wavelength internal gravity wave. The data show that small-scale turbulence and small-scale wave intensity is greatest at those altitudes where the large-scale wave-induced temperature lapse rate is most negative or most nearly unstable, but the wind shear due to the large-scale wave is a minimum. A brief review of linear gravity-wave theory is presented as an aid to the identification of the gravity-wave signature in the radar and rocket data. Analysis of the time and height cross sections of wind speed and turbulence intensity observed by the Poker Flat MST radar follows. Then, the vertical profile of temperature and winds measured by a rocket probe examined. Finally, the use of the independent data sets provided by the rocket and the radar are discussed and implications for theories of wave saturation are presented.

Smith, S. A.↗
NASA NTRS1978

Incoherent scatter radar observations of irregular structure in mid-latitude sporadic E layers

The basic experiments used phase-coded pulses to record electron density profiles with a resolution of 600 m in range and 300 m in horizontal extent, while scanning in azimuth. Data from incoherent scatter radar were compared with simultaneous ionosonde observations. Observations of sporadic E layers by incoherent scatter radar were discussed in terms of the effects of the neutral wind system acting on metallic ions. Several features were noted in the data, which support the wind shear mechanism of layer formation. The sporadic E layers often contained a pronounced small-scale structure, especially at times when partially transparent echoes were observed by the ionosonde. Under specific conditions, the ions in a meteor trail can be converged by a shear in the neutral wind into a relatively small irregularity at the center of a sporadic E layer.

Miller, K. L.↗
NASA NTRS1979

Viking bistatic radar observations of the Hellas basin on Mars - Preliminary results

Preliminary reduction of Viking bistatic radar data gives root-mean-square surface slopes in the Hellas basin on Mars of about 4 deg on horizontal scales averaged over 10 centimeters to 100 meters. This roughness decreases slightly with position along the ground track, south to north. The dielectric constant in this area appears to be approximately 3.1, greater than the Martian average. These values are characteristic of lunar maria and are similar to those found near the Viking Lander site in Chryse with the use of earth-based radar.

Simpson, R. A.↗
NASA NTRS1984

VHF Radar Observations in the Stratosphere and Mesosphere During a Stratospheric Warming

The SOUSY-VHF-radar was used to carry out measurements during minor and a major stratospheric warming in February and March 1980, respectively. Echoes have been received from the stratosphere up to an altitude of about 30 km continuously during day and night, whereas echoes from the mesosphere were restricted to the daytime and occurred sporadically at different heights within the altitude range from 60 to 90 km. The three dimensional velocity vector was derived from Doppler measurements made in three different antenna beam directions with a height resolution of 1.5 km. In particular, the results obtained during disturbed conditions show the change of the zonal winds at mesospheric heights from westerly to easterly. A spectral analysis reveals a diurnal and a weaker semidiurnal tide of the zonal wind component.

Ruster, R.↗
NASA NTRS1985

VHF radar observation of the middle atmosphere at Syowa Station, Antarctica

The newly developed pulsed Doppler radars with 50 and 112 MHz were installed at Syowa Station (69 deg 00'S, 39 deg 35'E geographic; 70.0 deg S, 80.2 deg E geomagnetic) in 1982 and 1983, respectively. They have the nominal peak power of 15 kW; the narrow antenna beams (4 deg in the horizontal plane) in two different directions (approximately geomagnetic south and geographic south with a crossing angle of about 33 deg), the three operation modes (spectrum, double-pulse and meteor mode). The radars were designed to measure the intensity and Doppler velocity of auroral radar echoes due to the 3- and 1.34-m irregularities appearing often in the disturbed E region and also to detect the meteor echoes in the 80 to 100 km altitudes, thereby to clarify how the middle atmosphere in the polar region behaves in response to the energy input from the magnetosphere, especially during a substorm. Some initial results obtained through the radar operation during 1982 to 1983 are presented to show that this system is useful for continuous monitoring of the lower auroral ionosphere.

Igarashi, K.↗
NASA NTRS1997

Mars Delay-Doppler Radar Observations With GSSR: Global Analysis for Landing Site Selection and Characterization

Earth-based radar data remain an important part of the information set used to select and certify spacecraft landing sites on Mars. Constraints on robotic landings on Mars include: terrain elevation, radar reflectivity. regional and local slopes, rock distribution and coverage, and surface roughness, all of which are addressed by radar data. Indeed, the usefulness of radar data for Mars exploration has been demonstrated in the past. Radar data were critical in assessing the Viking Lander I site, and more recently, the Mars Pathfinder landing site.

Haldemann, A. F. C.↗