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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

On cumulus mergers

A study of cumulus mergers for three undisturbed untreated days in the summer of 1973 is reported. Mergers produce more than an order of magnitude more rain than unmerged echoes, while mergers of mergers produce still an order of magnitude more rain. Duration, echo area, and rain depths are also compared for merged and unmerged systems, and each day is analyzed individually, indicating a correlation between organization and rain amount. The relevance of mergers to hydrology, weather modification, and the large-scale impacts of convective clouds is discussed.

Simpson, J.↗
NASA NTRS1980

High-altitude resolution stratospheric measurements with the Arecibo 430-MHz radar

Instrumental modification at the Arecibo Observatory are described, which make possible a resolution of 150 m, the maximum allowed by the transmitter bandwidth. Attention is given to digital decoding, the radar controller, and the fact that a complementary coded pulse scheme generating no pulse side lobes has been used for the first time. It is noted that power spectra of the signals corresponding to 256 altitudes are evaluated on line by means of an array processor. A profile of the E-W component of the wind as a function of altitude has been observed, as well as the discrete layered structure of turbulence, especially at higher altitudes; in addition, the maximum range at which an echo has been detected is about 31 km. The results obtained show the importance for stratospheric studies of high resolution when using radar to obtain wind information from the Doppler shift of turbulence echoes.

Woodman, R. F.↗
NASA NTRS1981

Rocket study of auroral processes

Abstracts are presented of previously published reports analyzing data from three Echo 3 rocket flights. Particle experiments designed for the Terrier-Malmute flight, the Echo 5 flight, and the Norwegian Corbier Ferdinand 50 flight are described and their flight performance evaluated. Theoretical studies on auroral particle precipitation are reviewed according to observations made in three regions of space: (1) the region accessible to rockets and low altitude satellites (few hundred to a few thousand kilometers); (2) the region extending from 4000 to 8000 km (S3-3 satellite range); and (3) near the equatorial plane (geosynchronous satellite measurements). Questions raised about auroral arc formation are considered.

Arnoldy, R. L.↗
NASA NTRS1980

Determination of spacecraft attitude using synthetic aperture radar data

A method has been developed for precise sensor platform attitude determination which exploits the availability of synthetic aperture radar (SAR) sensors on future planned NASA earth orbiting spacecraft. Using the ephemeris along with data on planet shape and rotation, the relative motion between sensor and target area and therefore the Doppler frequency shift in the returned echo data is determined. Equations have been developed that relate the Doppler shift to the antenna attitude. The method is tested by comparing attitude predictions from SEASAT SAR digital echo data with available conventionally determined SEASAT attitude information.

Wu, C.↗
NASA NTRS1981

Preliminary results of the study of lightning location relative to storm structure and dynamics

Lightning is being studied relative to storm structure using a VHF space-time discharge mapping system, radar, a cloud-to-ground flash locator, acoustic reconstruction of thunder, and other instrumentation. The horizontal discharge processes within the cloud generally propagate at speeds of 10,000-100,000 m/s. Horizontal extents of lightning were found up to 90 km. In an analysis of a limited number of flashes, lightning occurred in or near regions of high cyclonic shear. Positive cloud-to-ground flashes have been observed emanating from several identifiable regions of severe storms. Lightning echoes observed with 10-cm radar generally are 10-25 dB greater than the largest precipitation echo in the storm.

Rust, W. D.↗
NASA NTRS1982

Radar investigation of asteroids

The dual polarization CW radar system which permits simultaneous reception in the same rotational sense of circular polarization as transmitted (i.e., the "SC" sense) and in the opposite ("OC") sense, was used to observe five previously unobserved asteroids: 2 Pallas, 8 Flora, 22 Kalliope, 132 Aethra, and 471 Papagena. Echoes from Pallas and Flora were easily detected in the OC sense on each of several nights. Weighted mean echo power spectra also show marginally significant responses in the SC sense. An approximately 4.5 standard deviation signal was obtained for Aethra. The Doppler shift of the peak is about 10 Hz higher than that predicted from the a priori trial ephemeris. Calculations are performed to determine whether this frequency offset can be reconciled dynamically with optical positions reported for Aethra.

Ostro, S. J.↗
NASA NTRS1982

Comet Encke - Radar detection of nucleus

The nucleus of the periodic comet Encke was detected in November 1980 with the Arecibo Observatory's radar system (wavelength, 12.6 cm). The echoes in the one sense of circular polarization received imply a radar cross section of 1.1 + or - 0.7 sq km. The estimated bandwidth of these echoes combined with an estimate of the rotation vector of Encke yields a radius for the nucleus of 1.5 +2.3 or -1.0. The uncertainties given are dependent primarily on the range of models considered for the comet and for the manner in which its nucleus backscatters radio waves. Should this range prove inadequate, the true value of the radius of the nucleus might lie outside the limits given.

Kamoun, P. G.↗
NASA NTRS1982

ISEE 1 observations of VLF line radiation in the earth's magnetosphere

VLF line radiation in the magnetosphere is analyzed in an attempt to clarify the role of radiation from electric power lines in magnetospheric wave-particle interactions. Observations were made by the ISEE 1 satellite from October 1977 through August 1979 between L = 2 and L = 8 and 50 to 110 deg W longitude, a region encompassing the magnetic field lines linking the Eights, Siple and Roberval stations and in which VLF chorus activity had been linked to power line radiation. Line radiation was detected on 5 of 90 orbits, in all cases at frequencies below 4 kHz. The one event with a high S/N ratio exhibited lines changing frequency at rates from 22 to 6 Hz/min over a 9-min period. The radiation was detected at a time when whistler mode echoing was quite pronounced on lower L shells, and thus may have been a scattered component of line radiation echoing between hemispheres. It is concluded that very little of the background VLF wave energy in the outer magnetosphere is contained in line radiation, although the catalytic role of line radiation in controlling wave-particle interactions remains to be assessed.

Bell, T. F.↗
NASA NTRS1982

Delay-Doppler radar observations of Saturn's rings

Delay-Doppler observations at 12.6 cm which have been used to estimate the relative radar reflectivities of Saturn's classical ring sections show that the A and B rings are responsible for most of the radar echo, and that the average radar reflectivity per unit of projected area of the A ring is 90% as large as the B ring. No firm evidence is found for radar backscattering from particles interior to the B ring, exterior to the A ring, of from the planet itself. Unexpectedly large amounts of power at Doppler shifts near the center of the echo spectrum, which had been reported at 3.5 and 12.6 cm for ring plane tilt angles greater than 24.4 deg are not apparent in spectra for those wavelengths obtained at ring plane tilt angles smaller than 21.4 deg.

Ostro, S. J.↗
NASA NTRS1982

Demonstration of radar reflector detection and ground clutter suppression using airborne weather and mapping radar

A navigation system which utilizes minimum ground-based equipment is especially advantageous to helicopters, which can make off-airport landings. Research has been conducted in the use of weather and mapping radar to detect large radar reflectors overland for navigation purposes. As initial studies have not been successful, investigations were conducted regarding a new concept for the detection of ground-based radar reflectors and eliminating ground clutter, using a device called an echo processor (EP). A description is presented of the problems associated with detecting radar reflectors overland, taking into account the EP concept and the results of ground- and flight-test investigations. The echo processor concept was successfully demonstrated in detecting radar reflectors overland in a high-clutter environment. A radar reflector target size of 55 dBsm was found to be adequate for detection in an urban environment.

Anderson, D. J.↗
NASA NTRS1982

Large sounding rockets as a base for probing the distant magnetosphere

The Echo program's use of large sounding rockets to inject electron beams into the outer trapping region where the earth's plasma sheet interacts with the inner magnetosphere is discussed. The beams pass through the plasma sheet and the trapping region, reflect at the southern hemisphere conjugate point and return to the vicinity of the rocket where they are detected and analyzed for acceleration and diffusion processes which may also be responsible for the radiation belts. The motion of electron beams in the magnetosphere is discussed from the standpoint of single particle motion, and the trajectories of electrons injected with known energy, known angle between the electron velocity vector and the local magnetic field vector, and known position are examined. The interaction between the rocket motion and the electron drift motion is described, and the advantages and drawbacks of beam detection by optical methods and by throwaway detectors are compared. Planned Echo experiments using the Space Shuttle are discussed

Winckler, J. R.↗
NASA NTRS1983

Correlated measurements of UHF radar signatures, RF radiation, and electric field changes from lightning

During Storm Hazards - 82, simultaneous measurements were made of radar echoes, fast and slow field changes and RF radiation from lightning near the Wallops Flight Facility. RF radiation and radar echoes were also obtained during periods when the NASA F106 research aircraft was struck by lightning. These data are presently being used to better understand the electrical processes which occur during strikes to the aircraft. Preliminary conclusions based on data obtained in 1982 verify that the events recorded aboard the aircraft occurred during lightning but also indicate that they occur with surprising frequency very early in the flash.

Levine, D. M.↗
NASA NTRS1982

Radar images of the earth from space

The operational principles and imagery available from microwave SAR radars for earth observations from aircraft, the Shuttle, and Seasat are explored. Using microwave frequencies is noted to offer imagery access in day or night, all-weather conditions. SAR radar functions by obtaining a series of reflected signals over a single path, with data processing combining the echoes into an image corresponding to what may be obtained with a large antenna. A stable, reference signal is added to the incoming signals in order to establish the phase and amplitude of echoes. Because the wavelengths of the images can be precisely controlled, Doppler shifts can be detected, thus allowing point by point scattering analysis in two-dimensions. The Shuttle SIR-A and the Seasat radar feature a ground resolution of 25 m. Applications of the satellite systems to terrain, ice mapping, and for a Venus radar mapper mission are described.

Elachi, C.↗
NASA NTRS1983

A concentrated outbreak of tornadoes, downbursts and microbursts, and implications regarding vortex classification

A remarkable case of severe weather occurred near Springfield, Illinois on 6 August 1977. Aerial and ground surveys revealed that 17 cyclonic vortices, an anticyclonic vortex, 10 downbursts and 19 microbursts occurred in a limited (20 km x 40 km) area, associated with a bow-shaped radar echo. About half of the vortices appeared to have occurred along a gust front. Some of the others appear to have occurred within the circulation of a mesocyclone accompanying the bow echo, but these vortices seem to have developed specifically in response to localized boundary-layer vorticity generation associated with horizontal and vertical wind shears on the periphery of microbursts. Some of these vortices, and other destructive vortices in the literature, do not qualify as tornadoes as defined in the Glossary of Meteorology. A more pragmatic definition of a tornado is suggested.

Forbes, G. S.↗
NASA NTRS1983

First VHF auroral radar interferometer observations

Auroral plasma instabilities were investigated using the radar interferometer technique based on observations obtained with a 49.92 MHz, 20-25 KW peak power pulsed radar located in Ithaca, NY (42.5 degrees N, 76.4 degrees W). Strong auroral echoes obtained during several highly active periods were analyzed. Phase differences between the signals received on the two antennas were utilized to accurately determine the E-W position, within the scattering volume, of localized scattering centers, and changes in this phase were used to determine the corresponding velocity. The radial (essentially N-S) motion was described by the signal Doppler shift. It is found that these data provide detailed information on the turbulent structure of the echoing region and show clearly that different features in the Doppler power spectrum often represent signals coming from different locations. It is concluded that these data can be utilized to determine full horizontal velocity vectors and hence the horizontal electric field, usually with a time resolution of the order of 15-30 s.

Providakes, J. F.↗
NASA NTRS1982

Automated preprocessing of spaceborne SAR data

An efficient algorithm has been developed for estimation of the echo phase delay in spaceborne synthetic aperture radar (SAR) data. This algorithm utilizes the spacecraft ephemeris data and the radar echo data to produce estimates of two parameters: (1) the centroid of the Doppler frequency spectrum f(d) and (2) the Doppler frequency rate. Results are presented from tests conducted with Seasat SAR data. The test data indicates that estimation accuracies of 3 Hz for f(d) and 0.3 Hz/sec for the Doppler frequency rate are attainable. The clutterlock and autofocus techniques used for estimation of f(d) and the Doppler frequency rate, respectively are discussed and the algorithm developed for optimal implementation of these techniques is presented.

Curlander, J. C.↗
NASA NTRS1983

Correlated measurements of UHF radar signatures, RF radiation and electric field changes from lightning

During Storm Hazards - 82, simultaneous measurements are made of radar echoes, fast and slow field changes and RF radiation from lightning. Radio frequency radiation and radar echoes are also obtained during periods when the research aircraft is struck by lightning. These data are presently used to better understand the electrical processes which occur during strikes to the aircraft. Preliminary conclusions verify that the events recorded aboard the aircraft occurred during lightning but also indicate that they occur with surprising frequency very early in the flash.

Levine, D. M.↗
NASA NTRS1984

Radar investigation of asteroids

The initial radar observations of the mainbelt asteroids 9 Metis, 27 Euterpe, and 60 Echo are examined. For each target, data are taken simultaneously in the same sense of circular polarization as transmitted as well as in the opposite (OC) sense. Estimates of the radar cross sections provide estimates of the circular polarization ratio, and the normalized OC radar cross section. The circular polarization ratio, is comparable to values measured for other large S type asteroids and for a few much smaller, Earth approaching objects, most of the echo is due to single reflection backscattering from smooth surface elements.

Ostro, S. J.↗