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At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

High resolution lunar radar map at 7.5 meter wavelength

A high-resolution map of lunar radar reflectivity is presented which was obtained at a wavelength of 7.5 m with a radar resolution of 25 to 40 km by a combination of delay-Doppler radar techniques and interferometry. The results are given in the forms of a radar map of normalized echo power and plots of average scattering. A consistent difference in scattering between maria and terrae is observed, along with small differences in scattering between eastern and western maria. Specifically, it is found that the terrae reflect two to four times as much power as the maria, that the weakest echoes come from the youngest and western maria, that a few terra areas have suprisingly weak echoes, and that the average scattering behavior at 7.5 m is remarkably close to the behavior at 68 cm. The results also indicate that the large rayed craters have relatively weak enhancements of 1 to 1.5 over their environs, that the maria and terrae have essentially the same backscattering behavior as a function of incidence angle, and that the average scattering behavior of the moon and the limb darkening at 7.5 m are similar to those at 68 cm.

Thompson, T. W.↗
NASA NTRS1978

Ocean wave detection and direction measurements with microwave radars

The application of synthetic aperture microwave radar imagery to the measurement of ocean wave direction and wavelength is discussed and an airborne experiment conducted in preparation for the SEASAT-A satellite mission is presented. Models for radar backscattering are examined and it is concluded that the Bragg-Rice model, which represents the ocean surface as a combination of periodic surfaces so that the dominant backscatter matches some portion of the wave spectrum, appears most useful. The experiment was conducted with airborne X-band and L-band synthetic aperture radar (SAR) and data was compared with that obtained from a pitch-and-roll buoy and an instrument package mounted on a sea sled. SAR data was converted to wave spectra and it was found to match data from the buoy, while SAR data on wave direction agrees to within two degrees with the buoy and to within one degree with the sea sled sensors. Wave direction data taken by two real aperture radars is also presented and shown to agree with airborne photographs.

Teleki, P. G.↗
NASA NTRS1979

Multibeam synthetic aperture radar for global oceanography

A single-frequency multibeam synthetic aperture radar concept for large swath imaging desired for global oceanography is evaluated. Each beam iilluminates a separate range and azimuth interval, and images for different beams may be separated on the basis of the Doppler spectrum of the beams or their spatial azimuth separation in the image plane of the radar processor. The azimuth resolution of the radar system is selected so that the Doppler spectrum of each beam does not interfere with the Doppler foldover due to the finite pulse repetition frequency of the radar system.

Jain, A.↗
NASA NTRS1979

Geologic interpretation from composited radar and Landsat imagery

Dual polarization L-band (lambda = 25 cm) radar imagery and Landsat multispectral scanner (MSS) data from central Death Valley have been computer processed and combined digitally in order to utilize the complementary information contained in multisensor data sets. Physically, like-polarized radar data gives roughness information on a scale proportional to radar wavelength. Cross-polarized data adds information on the overall roughness, particle shape, and packing density. Visible and near IR reflectivities pertain to surface chemistry. Surficial geologic units in Death Valley are defined on the basis of compositional and textural differences: optical and microwave scattering properties encoded in the multisensor image are sufficient to discriminate most of the units. This synergistic effect is most pronounced for the alluvial fans. Using radar scattering data and measured sun angle, passively-sensed (passive microwave, visible, near IR) images can be corrected for shadowing effects.

Daily, M. I.↗
NASA NTRS1979

Flight investigation of helicopter IFR approaches to oil rigs using airborne weather and mapping radar

Airborne weather and mapping radar is a near-term, economical method of providing 'self-contained' navigation information for approaches to offshore oil rigs and its use has been rapidly expanding in recent years. A joint NASA/FAA flight test investigation of helicopter IFR approaches to offshore oil rigs in the Gulf of Mexico was initiated in June 1978 and conducted under contract to Air Logistics. Approximately 120 approaches were flown in a Bell 212 helicopter by 15 operational pilots during the months of August and September 1978. The purpose of the tests was to collect data to (1) support development of advanced radar flight director concepts by NASA and (2) aid the establishment of Terminal Instrument Procedures (TERPS) criteria by the FAA. The flight test objectives were to develop airborne radar approach procedures, measure tracking errors, determine accpetable weather minimums, and determine pilot acceptability. Data obtained will contribute significantly to improved helicopter airborne radar approach capability and to the support of exploration, development, and utilization of the Nation's offshore oil supplies.

Bull, J. S.↗
NASA NTRS1979

Shuttle orbiter radar cross-sectional analysis

Theoretical and model simulation studies on signal to noise levels and shuttle radar cross section are described. Pre-mission system calibrations, system configuration, and postmission system calibration of the tracking radars are described. Conversion of target range, azimuth, and elevation into radar centered east north vertical position coordinates are evaluated. The location of the impinging rf energy with respect to the target vehicles body axis triad is calculated. Cross section correlation between the two radars is presented.

Cooper, D. W.↗
NASA NTRS1979

SEASAT-A synthetic aperture radar performance

The SEASAT-A satellite synthetic aperture radar (SAR) is presented, and its performance since it began operation in July 1978, is examined. The imaging radar system, the first to be designed as an orbital scientific instrument, was intended to generate continuous imagery over a 100-km swath at 25-m resolution from an altitude of 800 km. SAR image quality is measured in terms of resolution, the range of detectable radar backscatter, and the contrast ratio, each of which is affected by antenna, sensor, data link, and recording/processor subsystem parameters. Microdensitometer scans of output image film of actual SEASAT-A SAR data reveal a range resolution of less than 25 m, an azimuth resolution of approximately 40 m, a noise equivalent dynamic range of from 9.6 to -21 + or - 5 dB, and an image contrast ratio of 7.5 dB. SEASAT-A SAR performance is concluded to have confirmed the utility of spaceborne radar sensing.

Jordan, R. L.↗
NASA NTRS1979

The use of radar at non-attenuating wavelengths as a tool for the estimation of rain attenuation at frequencies above 10 GHz

The paper describes radar techniques and modeling procedures for estimating slant path attenuation and site diversity performance. The estimation results of an ongoing experiment at Wallops Island, Virginia are examined. At this location a high resolution S-band radar (0.4 deg beamwidth) monitors the reflectivity at contiguous pulse volumes approximately colocated with an earth space-path along which the Comstar beacon signal at 28.56 GHz is received. Radar estimated levels of rain attenuation at 28.56 GHz are compared with directly measured values both on a case by case basis as well as statistically for summer and winter periods. Use is made of ground disdrometer measurements in the radar estimation scheme.

Goldhirsh, J.↗
NASA NTRS1980

Radar altimeter mean return waveforms from near-normal-incidence ocean surface scattering

Under assumptions common in radar altimetry, the mean backscattered return power for a short-pulse radar and near-normal-incidence scattering from a rough ocean surface is given by the convolution of several terms. For a nearly Gaussian transmitted pulse shape scattered from a nearly Gaussian distributed sea surface, a small-argument series expansion of one of the terms within the convolution leads to a several-term power series expansion for the mean return waveform. Specific expressions are given for the first four terms. These results, which require much less computer time than would the otherwise necessary numerical convolution, are useful for data analysis from current or past radar altimeters and for design studies of future systems. Several representative results are presented for an idealized Seasat radar altimeter.

Hayne, G. S.↗
NASA NTRS1980

KU-Band rendezvous radar performance computer simulation model

The preparation of a real time computer simulation model of the KU band rendezvous radar to be integrated into the shuttle mission simulator (SMS), the shuttle engineering simulator (SES), and the shuttle avionics integration laboratory (SAIL) simulator is described. To meet crew training requirements a radar tracking performance model, and a target modeling method were developed. The parent simulation/radar simulation interface requirements, and the method selected to model target scattering properties, including an application of this method to the SPAS spacecraft are described. The radar search and acquisition mode performance model and the radar track mode signal processor model are examined and analyzed. The angle, angle rate, range, and range rate tracking loops are also discussed.

Griffin, J. W.↗
NASA NTRS1980

Seasat views North America, the Caribbean, and Western Europe with imaging radar

Forty-one digitally correlated Seasat synthetic-aperture radar images of land areas in North America, the Caribbean, and Western Europe are presented to demonstrate this microwave orbital imagery. The characteristics of the radar images, the types of information that can be extracted from them, and certain of their inherent distortions are briefly described. Each atlas scene covers an area of 90 X 90 kilometers, with the exception of the one that is the Nation's Capital. The scenes are grouped according to salient features of geology, hydrology and water resources, urban landcover, or agriculture. Each radar image is accompanied by a corresponding image in the optical or near-infrared range, or by a simple sketch map to illustrate features of interest. Characteristics of the Seasat radar imaging system are outlined.

Ford, J. P.↗
NASA NTRS1981

Determination of rain rate from a spaceborne radar using measurements of total attenuation

Studies shows that path-integrated rain rates can be determined by means of a direct measurement of attenuation. For ground based radars this is done by measuring the backscattering cross section of a fixed target in the presence and absence of rain along the radar beam. A ratio of the two measurements yields a factor proportional to the attenuation from which the average rain rate is deduced. The technique is extended to spaceborne radars by choosing the ground as reference target. The technique is also generalized so that both the average and range-profiled rain rates are determined. The accuracies of the resulting estimates are evaluated for a narrow beam radar located on a low earth orbiting satellite.

Meneghini, R.↗
NASA NTRS1980

Radar image enhancement and simulation as an aid to interpretation and training

Greatly increased activity in the field of radar image applications in the coming years demands that techniques of radar image analysis, enhancement, and simulation be developed now. Since the statistical nature of radar imagery differs from that of photographic imagery, one finds that the required digital image processing algorithms (e.g., for improved viewing and feature extraction) differ from those currently existing. This paper addresses these problems and discusses work at the Remote Sensing Laboratory in image simulation and processing, especially for systems comparable to the formerly operational SEASAT synthetic aperture radar.

Frost, V. S.↗
NASA NTRS1980

Ground radar detection of meteoroids in space

A special test to lower the detection threshold for satellite fragments potentially dangerous to spacecraft was carried out by NORAD for NASA, using modified radar software. The Perimeter Acquisition Radar Attack Characterization System, a large, planar face, phased radar, operates at a nominal 430 MHz and produces 120 pulses per second, 45 of which were dedicated to search. In a time period of 8.4 hours of observations over three days, over 6000 objects were detected and tracked of which 37 were determined to have velocities greater than escape velocity. Six of these were larger objects with radar cross sections greater than 0.1 sq m and were probably orbiting satellites. A table gives the flux of both observed groups.

Kessler, D. J.↗
NASA NTRS1981

A comparison of the thermal and radar characteristics of Mars

Results of thermal infrared sensing and radar observations of the Martian surface are compared for the region centered on +22 deg latitude. Values of the apparent thermal inertia of the surface were derived from surface brightnesses observed by the Viking Orbiter Infrared Thermal Mapper, while radar cross sections were obtained in earth-based experiments at wavelengths of 3.8, 12.5 and 70 cm. A correlation between the thermal inertia and radar cross section values is observed which is strongest with the 70-cm radar data except between longitudes of 10 and 90 deg, where a slight anticorrelation is found. The mixing of small (much less than 70 cm) rocks into a surface of fine material can account for the data between 90 and 370 deg longitude, with as much as 50% of the surface covered by rock in Syrtis Major, Isidis Planitia and parts of Elysium Planitia, and little or no rock cover near Olympus Mons, Elysium Mons, and Amazonis Planitia. The remaining longitudes may be explained in terms of the effects of atmospheric dust on the surface temperature or the effects of local variations in large-scale roughness or scattering from rocks. Data from the 90 to 370 deg longitude region are consistent with the division of the Martian surface into two types of terrain, possibly related to the erosional or depositional nature of the regions.

Jakosky, B. M.↗
NASA NTRS1980

Navigation errors encountered using weather-mapping radar for helicopter IFR guidance to oil rigs

In 1978 a joint NASA-FAA helicopter flight test was conducted to examine the use of weather-mapping radar for IFR guidance during landing approaches to oil rig helipads. The following navigation errors were measured: total system error, radar-range error, radar-bearing error, and flight technical error. Three problem areas were identified: (1) operational problems leading to pilot blunders, (2) poor navigation to the downwind final approach point, and (3) pure homing on final approach. Analysis of these problem areas suggests improvement in the radar equipment, approach procedure, and pilot training, and gives valuable insight into the development of future navigation aids to serve the off-shore oil industry.

Phillips, J. D.↗
NASA NTRS1981

Viking bistatic radar experiment - Summary of first-order results emphasizing north polar data

Initial results of bistatic radar observations of Mars made by the Viking Orbiter spacecraft are presented with particular emphasis on the previously unstudied polar regions. Bistatic radar scattering experiments were performed in near-equatorial regions by the Viking Orbiter 1, while other regions of the planet were observed by the polar-orbiting Orbiter 2, with scattered signals received by stations of the NASA Deep Space Network. In the equatorial region, the bistatic radar estimates of rms surface slope are found to be in qualitative agreement with results obtained using earth-based transmitter-receivers, showing a nearly 2:1 decrease in rms surface roughness between 20 and 22 deg N with no appreciable change in dielectric constant. Data in the north polar region reveal variations in surface roughness from 1 to 6 deg, with areas of smooth material generally located within rougher environs, surface roughness decreasing along Vastitas Borealis in the direction of the north pole, and north polar cap rms roughnesses on the order of 2.5-3.0 deg. Trends in radar reflectivity suggest a decreasing surface density with increasing latitude, consistent with a layer of seasonal CO2 or H2O snow increasing in depth as the pole is approached.

Simpson, R. A.↗
NASA NTRS1981

Planetary radar studies

Progress made in studying the evolution of Venusian craters and the evolution of infrared and radar signatures of lunar crater interiors is reported. Comparison of radar images of craters on Venus and the Moon present evidence for a steady state Venus crater population. Successful observations at the Arecibo Observatory yielded good data on five nights when data for a mix of inner and limb areas were acquired. Lunar craters with radar bright ejects are discussed. An overview of infrared radar crater catalogs in the data base is included.

Thompson, T. W.↗