Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Echo”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Radar Studies in the Solar System

We aid in study of the solar system by means of ground-based radar. We have concentrated on: (1) developing the ephemerides needed to acquire radar data at Arecibo Observatory and (2) analyzing the resultant data to: test fundamental laws of gravitation; determine the size , shape, topography, and spin vectors of the targets; and study the surface properties of these objects, through their scattering law and polarization characteristics. We are engaged in radar observations of asteroids and comets, both as systematically planned targets and as "targets of opportunity." In the course of the program, we have prepared ephemerides for about 80 asteroids and three comets, and the radar observations have been made or attempted at the Arecibo Observatory, in most cases successfully, and in some cases on more than one apparition. The results of these observations have included echo spectra for the targets and, in some cases, delay - Doppler images and measurements of the total round-trip delay to the targets. Perhaps the most dramatic of these results are the images obtained for asteroids (4179) Toutatis and 1989PB (Castalia), which were revealed to be double-lobed objects by the radar images. Besides these direct results, the radar observations have furnished information on the sizes and shapes of the targets through analysis of the Doppler width of the echoes as a function of time, and on the surface properties (such as composition, bulk density, and roughness) through analysis of the reflectivity and of the polarization state of the echoes. We have also refined the orbits of the observed asteroids as a result of the Doppler (and in some cases delay) measurements from the radar observations. Although the orbits of main-belt asteroids accessible to ground-based radar are quite well known from the available optical data, some near-Earth objects have been seen by radar very soon after their optical discovery (for example, 199OMF, just eight days after discovery). In such cases. the radar results ensure that the object in question can be anticipated and identified at the next apparition. We have also participated in radar studies of the terrestrial planets. The results of these studies have included both planetary topography profiles from the analysis of round-trip delays to points along the target Doppler equator and determinations of the target spin state. The latter is of special interest in the case of Venus, which is very close to, but not on, a multi-body spin-orbit resonance such that Venus rotates 12 times for every 8 Earth orbits and 13 Venus orbits. As a result, Venus presents nearly the same face toward Earth at each inferior conjunction. Our latest results confirm that the spin state of Venus is slightly off the resonance. The delay measurements from planetary 2 ranging have also been used in combination with other types of range data in testing general relativity with increasing accuracy. We have also been engaged in radar studies of planetary satellites. Using our ephemerides, Arecibo made radar observations of the Galilean satellites of Jupiter and of Mars' satellite Phobos during the favorable opposition seasons (1988-1992 for Jupiter and 1990 for Mars). An attempt was also made to observe Deimos, but without detecting an echo. In 1997, an attempt was made to observe Saturn's satellite Titan, using the newly upgraded Arecibo radar system for transmitting and the Goldstone radar for receiving, but no echo was detected. The study of satellites by radar is in many ways similar to that of asteroids. The results from these observations have included characterization of the surface properties from the reflectivity and polarization ratio, as well as (in the case of the large satellites of Jupiter) the variation of reflectivity with incidence angle.

Shaprio, Irwin I.↗
NASA NTRS1998

Re-Observing the First Hours of Supernova 1987A

This project was designed to use archival data from the International Ultraviolet Explorer (IUE) to measure the flux from the shock breakout from SN 1987A, emitted in the first few hours of the event, reflected from neighboring dust clouds in the form of a light echo. Such hot flux from the initial moments of a SN had never been observed before, and this offers a unique opportunity to measure it in reflection. Gilmozzi (1991, in "SN 1987A and Other Supernova") had reported such a detection of using IUE from observations made in 1988 and 1989. Our analysis of these data showed that this detection was very weak, and subject to changes comparable to the size of the signal according to various reasonable means of extracting the spectra. In fact, the features reported by Gilmozzi (1998 in "Ultraviolet Astrophysics Beyond IUE") are not found in a conventional NEWSIPS extraction or extended-source extraction of these data. The 1995 and 1996 IUE data collected for Crotts and Gilmozzi shows a more robust signal (about 20sigma,) from a cloud that echoes brighter in the optical than the 1988 feature. This shows a well-detected signal of about 5 x 10(exp -16) ergs per s/sq cm/Angstroms/arc sq sec at 1300 Angstroms, with a declining slope to redder wavelengths. This surface brightness, while detected with confidence in both large and small SVVT apertures, is still fainter than the reflected flux expected for most models of shock breakout. We have also taken the opportunity to recover the echo signal from exposures taken on Astro-1 and Astro-2 flights of the Ultraviolet Imaging Telescope (UIT) while pointed towards SN 1987A. These data yield a complementary result on the reflected flux, an upper limit of about 5 x 10(exp -17) ergs per s/sq cm/Angstroms/arc sq sec, spread over fainter optically echoing clouds, and over the same band where the signal was detected by IUE. While this appears in contradiction with the IUE result, it is made consistent by the corresponding strengths of the echoes in the optical.

Crotts, Arlin P. S.↗
NASA NTRS1999

The Geoscience Laser Altimeter System (GLAS) for the ICESAT Mission

Accurate measurements of surface heights and atmospheric backscatter have been demonstrated with the SLA, MOLA and LITE space lidar. Recent MOLA measurements of the Mars surface have 40 cm resolution and have reduced the global uncertainty in Mars topography from a few km to approx. 10 m. GLAS is a next generation lidar being developed as part of NASA's Icesat Mission for Earth orbit . The GLAS design combines a 10 cm precision surface lidar with a sensitive dual wavelength cloud and aerosol lidar. GLAS will precisely measure the heights of the Earth's polar ice sheets, determine the height profiles of the Earth's land topography, and profile the vertical backscatter of clouds and aerosols on a global scale. GLAS will fly on a small dedicated spacecraft in a polar orbit at 598 km altitude with an inclination of 94 degrees. GLAS is scheduled to launch in summer 2001 and to operate continuously for a minimum of 3 years with a goal of 5 years. The primary mission for GLAS is to measure the seasonal and annual changes in the heights of the Greenland and Antarctic ice sheets. GLAS will measure the vertical distance to the ice sheet from orbit with 1064 nm pulses from a Nd:Yag laser at 40 Hz. Each 5 nsec wide laser pulse is used for a single range measurement. When over land GLAS will profile the heights of the topography and vegetation. The GLAS receiver uses a I m diameter telescope and a Si APD detector. The detector signal is sampled by an all digital receiver which records each surface echo waveform with I nsec resolution and a stored echo record lengths of either 200, 400, or 600 samples. Analysis of the echo waveforms within the instrument permits discrimination between cloud and surface echoes. Ground based echo analysis permits precise ranging, determining the roughness or slopes of the surface as well as the vertical distributions of vegetation illuminated by the laser, Errors in knowledge of the laser beam pointing angle can bias height measurements of sloped surfaces. For surfaces with 2 deg. slopes, knowledge of pointing angle of the beam centroid to about 8 urad is required to achieve 10 cm height accuracy. GLAS uses a stellar reference system (SRS) to determine the pointing angle of each laser firing relative to inertial space. The SRS uses a high precision star camera oriented toward local zenith whose measurements are combined with a gyroscope to determine the inertial orientation of the SRS optical bench. The far field pattern of each laser pulse is measured with a laser reference system (LRS). Optically measuring each laser far field pattern relative to the star camera and gyroscope permits the angular offsets of each laser pulse to be determined. GLAS will also determine the vertical distributions of clouds and aerosols by measuring atmospheric backscatter profiles at both 1064 and 532 nm. The 1064 nm measurements use an analog detector and profile the height and vertical structure of thicker clouds. Measurements at 532 nm use new highly sensitive photon counting detectors, and measure the height distributions of very thin clouds and aerosol layers. With averaging these can be used to determine the height of the planetary boundary layer. The instrument design and expected performance will be discussed.

Abshire, James B.↗
NASA NTRS2000

Radar Observations of Asteroid 10115 (1992 SK)

The approach of 1992 SK to within 0.056 AU of Earth on 1999 March 26 provided an excellent opportunity for radar observations. We observed it daily between March 22-26 at Goldstone at a transmitter frequency of 8560 MHz (3.50 cm). Weighted, optimally smoothed sums of cw (Doppler-only) echoes achieve a signal-to-noise ratio of about 300. Our highest resolution cw setup places up to approx. 30 0.5-Hz Doppler cells on the target at echo powers exceeding two standard deviations of the noise. Our highest resolution imaging setup places about 20 0.25-us (37.5 m) x 2.0 Hz pixels on 1992 SK at echo powers above the same threshold. We completed nearly 300 runs with each setup and obtained thorough rotational phase coverage (period = 7.320 h, P. Pravec, pers. comm.). The dispersion of the echoes in time delay indicates a lower bound on 1992 SK's maximum pole-on breadth of approx. 0.4 km that is consistent with the echo bandwidth and rotation period. Variations in the cw spectral bandwidths and shapes and in the delay-Doppler images as a function of rotation phase are evident on each day, are consistent in bandwidth with each other and with the rotation period, and indicate that the asteroid is asymmetric and modesty elongated.

Benner, L. A. M.↗
NASA NTRS2000

The Geoscience Laser Altimeter System (GLAS) for the ICESAT Mission

The Laser In space Technology Experiment, Shuttle Laser Altimeter and the Mars Observer Laser Altimeter have demonstrated accurate measurements of atmospheric backscatter and Surface heights from space. The recent MOLA measurements of the Mars surface have 40 cm vertical resolution and have reduced the global uncertainty in Mars topography from a few km to about 5 m. The Geoscience Laser Altimeter System (GLAS) is a next generation lidar for Earth orbit being developed as part of NASA's Icesat Mission. The GLAS design combines a 10 cm precision surface lidar with a sensitive dual wavelength cloud and aerosol lidar. GLAS will precisely measure the heights of the Earth's polar ice sheets, establish a grid of accurate height profiles of the Earth's land topography, and profile the vertical backscatter of clouds and aerosols on a global scale. GLAS is being developed to fly on a small dedicated spacecraft in a polar orbit with a 590 630 km altitude at inclination of 94 degrees. GLAS is scheduled to launch in the summer 2001 and to operate continuously for a minimum of 3 years with a goal of 5 years. The primary mission for GLAS is to measure the seasonal and annual changes in the heights of the Greenland and Antarctic ice sheets. GLAS will continuously measure the vertical distance from orbit to the Earth's surface with 1064 nm pulses from a ND:YAG laser at a 40 Hz rate. Each 5 nsec wide laser pulse is used to produce a single range measurement, and the laser spots have 66 m diameter and about 170 m center-center spacings. When over land GLAS will profile the heights of the topography and vegetation. The GLAS receiver uses a 1 m diameter telescope and a Si APD detector. The detector signal is sampled by an all digital receiver which records each surface echo waveform with I nsec resolution and a stored echo record lengths of either 200, 400, or 600 samples. Analysis of the echo waveforms within the instrument permits discrimination between cloud and surface echoes. Ground based echo analysis permits precise ranging, determining the roughness or slopes of the surface as well as the vertical distributions of vegetation illuminated by the laser. Accurate knowledge of the laser beam's pointing angle is needed to prevent height biases when over sloped surfaces. For surfaces with 2 deg. slopes, knowledge of pointing angle of the beam's centroid to about 8 urad is needed to achieve 10 cm height accuracy. GLAS uses a stellar reference system (SRS) to determine the pointing angle of each laser firing relative to inertial space. The SRS uses a high precision star camera oriented toward local zenith and a gyroscope to determine the inertial orientation of the SRS optical bench. The far field pattern of each laser is measured pulse relative to the star camera with a laser reference system (LRS). Optically measuring each laser far field pattern relative to the orientation of the star camera and gyroscope permits the precise pointing angle of each laser pulse to be determined. GLAS will also determine the vertical distributions of clouds and aerosols by measuring the vertical profile of laser energy backscattered by the atmosphere at both 1064 and 532 nm. The 1064 nm measurements use the Si APD detector and profile the height and vertical structure of thicker clouds. The measurements at 532 nm use new highly sensitive photon counting, detectors, and measure the height distributions of very thin Clouds and aerosol layers. With averaging these can be used to determine the height of the planetary boundary layer. The instrument design and expected performance will be discussed.

Abshire, James B.↗
NASA NTRS2002

Ship Radar Observations of a Developing Hurricane in the East Pacific

During the recently completed East Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System (EPIC) field program, the NOAA research vessel Ronald H. Brown (RHB) was deployed in the east Pacific Inter Tropical Convergence Zone (ITCZ) for approximately 3 weeks near 10 deg. N, 95 deg. W. One of the principal objectives of the EPIC-ITCZ program was to observe the modulation of convection by synoptic-scale easterly waves and the air-sea coupling process in this poorly sampled region of the east Pacific. Data from the experiment will be used as validation to improve forecast models. The RHB carried a variety of platforms during EPIC to sample atmospheric and oceanic phenomena, including a scanning C-band Doppler radar, radiation flux instrumentation, air-sea flux system, Doppler lidar, 35 GHz cloud radar, UHF wind profiler, sea surface temperature (SST) sensors, as well as standard surface meteorological instrumentation and a suite of rain gauges. This presentation will focus on the analysis of C-band radar data that was collected on 10 September, 2001 as the ship passed through an easterly wave which later developed into hurricane Ivo. The ship captured approximately 12 hours of convection associated with the tropical disturbance. During this period, the domain sampled by the radar (approximately 71,000 sq km) contained a significant number of echo features. Specifically, the fraction of the domain containing radar echo above 10 dBZ reached 80% for over 2 hours and remained near 60% for a continuous six hour period. Animation of radar images showed distinct rotation in echo features associated with the easterly wave passage. Despite an approximate 4 C drop in surface air temperature and sustained winds approaching 20 m/ s, the SST remained nearly constant throughout the observation period (approx. 29.5 C). Peak values of latent and sensible heat flux exceeded 400 and 100 W /sq m, respectively. The radar documented the change in precipitation vertical structure as the ship passed through regions of significant convection with echo tops approaching 14 km and 30 dBZ echo tops extending to near 9 km embedded in regions that were predominantly stratiform in nature. Single Doppler retrievals (Extended Velocity Azimuth Display) were conducted continuously at 10-minute resolution for approximately 4 hours in the latter part of the observation period, and documented the transition from convection (low-level convergence, upper level divergence) to stratiform (mid-level convergence sandwiched between upper and lower level divergence) kinematic structure.

Cifelli, Robert↗
NASA NTRS2008

Lessons Learned from Previous Space-Borne Sounders as a Guide to Future Sounder Development

Space-borne radio sounding is considered to be the gold standard for electron-density (N(sub e)) measurements compared to other techniques even under low-density conditions, such as N(sub e) < 1/cu cm, when other techniques are known to experience difficulties. These reliable measurements are not restricted to in-situ N(sub e) determinations since a spaceborne sounder can provide vertical N(sub e) profiles (N(sub e)(h)) from the spacecraft altitude to the altitude of maximum N(sub e). Near-conjunction studies involving the International Satellites for Ionospheric Studies (ISIS) satellites in the topside ionosphere and Dynamics Explorer 2 (DE 2) near the altitude of the F-region peak density have verified that, even at the greatest distance from the sounder, the ISIS-derived N(sub e)(h) profiles agree with the DE-2 Langmuir-probe measurements to within about 30% over a density range of more than two decades. Space-borne sounders can also provide N(sub e) profiles along the magnetic-field B, by inverting echoes that are ducted along field-aligned irregularities (FAI), and can provide information about the terrain beneath the satellite by examining surface reflections in the frequency range above the ionospheric penetration frequency. Many nations have launched rocket and satellite radio sounders in geospace over more than 4 decades and there have been sounders on space-probes and in orbit around other planets. Here we will summarize some of the lessons learned from these accomplishments by analyzing data from radio sounders on the Alouette and ISIS satellites and the OEDIPUS and other rockets in the terrestrial ionosphere, the IMAGE satellite in the terrestrial magnetosphere, the Ulysses space probe in Jupiter's 10 plasma torus and the MARSIS satellite in orbit around Mars. The emphasis will be on information deduced concerning (1) fundamental plasma processes and gradients in N, and B in the vicinity of the sounders from sounder-stimulated plasma resonances and short-range echoes involving ion as well as electron motions, (2) the importance of the antenna orientation relative to B for the detection of different plasma resonances, (3) sounder-stimulated plasma phenomena, including FAI, when special plasma conditions are satisfied, (4) the minimum power required for long-range echoes, as indicated by echoes from frequency components of the transmitted pulse and by multiple ducted echoes, and (5) the terrain beneath the satellite from surface reflections. Knowledge of these results should enable the optimum design of a future sounder to satisfy specific science requirements with minimal spacecraft resources.

Benson, Robert F.↗
NASA NTRS2007

Ground Clutter as a Monitor of Radar Stability at Kwajalein,RMI

There are many applications in which the absolute and day-to-day calibration of radar sensitivity is necessary. This is particularly so in the case of quantitative radar measurements of precipitation. While absolute calibrations can be done periodically using solar radiation, variations that occur between such absolute checks are required to maintain the accuracy of the data. The authors have developed a method for h s purpose using the radar on Kwajalein Atoll, which has been used to provide a baseline calibration for control of measurements of rainfall made by the Tropical Rainfall Measuring Mission 0T.he method u ses echoes from a multiplicity of ground targets. The average clutter echoes at the lowest elevation scan have been found to be remarkably stable from hour to hour, day to day, and month to month within better than +1 dB. They vary significantly only after either deliberate system modifications, equipment failure or unknown causes. A cumulative probability distribution of echo reflectivities (Ze in dBZ) is obtained on a daily basis. This CDF includes both the precipitation and clutter echoes. Because the precipitation echoes at Kwajalein rarely exceed 45 dBZ, selecting an upper percentile of the CDF associated with intense clutter reflectivities permits monitoring of radar stability. The reflectivity level at which the CDF attains 95% is our reference. Daily measurements of the CDFs have been made since August 1999 and have been used to correct the 7 M years of measurements and thus enhance the integrity of the global record of precipitation observed by TRMM. The method also has potential applicability to other pound radar sites.

Silberstein, David S.↗
NASA NTRS2012

Modeling Radar Scatter from Icy and Young Rough Lunar Craters

For lunar orbital synthetic aperture radars, such as the Chandrayaan Mini-RF operating at S- band (13-cm) wavelength and the Lunar Reconnaissance Orbiter Mini-RF operating at S- band and X-band (3-cm) wavelengths, it is important to understand the radar backscattering characteristics of the icy and young, rough craters. Assuming a mixing model consisting of diffuse and quasi-specular scattering components, we have modeled the opposite-sense circular (OC) and same-sense circular (SC) backscattering characteristics. The specular component, consisting of only OC echoes, represents the echoes from the surface and subsurface layers that are oriented perpendicular to the radar's line-of-sight. The diffuse component, consisting of both SC and OC echoes, represents the echoes associated with either rocks or ice. Also, diffuse echoes have backscatter that is proportional to the cosine of the incidence angle. We modeled how these two (specular and diffuse) radar scattering components could be modulated by factors such as surface roughness associated with young craters. We also modeled how ice radar scattering components could be modulated by a thin regolith covering, and/or by the situation where ice occupies small patches within a larger radar pixel. We tested this modeling by examining 4 nonpolar craters and 12 polar craters using LRO Mini-RF data. Results indicate that icy and young rough craters can be distinguished based upon their SC enhancements (Alpha) and OC enhancements (Gamma). In addition, we also examined the craters that have unusual circular polarization ratios (CPRs) that likely result from a double bounce mode of scattering. Blocky fresh craters, icy craters, and craters exhibiting double bounce scattering can be separated based on the values of Alpha, Gamma, the ratio of Alpha/Gamma and the weighted sum of Alpha and Gamma.

lunar craters↗
DOE OSTI2020

The effect of magnetic susceptibility and magnetic field strength on porosity estimates determined from low-field nuclear magnetic resonance

The presence of materials with high magnetic susceptibility are known to have an impact on 1H nuclear magnetic resonance (NMR) measurements and in laboratory data this often results in poor estimates of porosity from NMR data. To quantify and understand the cause of the poor NMR porosity estimations, in this laboratory study, we examine the effect of magnetic susceptibility, NMR measurement parameters, and NMR instrument design on NMR measurements. Data were collected with two instruments, each with a different Larmor frequency (2 MHz and 485 kHz), on water-saturated unconsolidated sediments with magnetic susceptibility values ranging from 3.6 × 10 -6 to 7020 × 10 -6 SI. The results show that for materials with low magnetic susceptibility (<378 × 10 -6 SI), the gravimetric porosity is accurately predicted from the NMR measurements. For the samples with high magnetic susceptibility (>987 × 10 -6 SI) the gravimetric porosity is poorly predicted from the 2 MHz NMR measurements made at all echo times (from 0.2 to 3.0 ms). In contrast, the gravimetric porosity is more accurately predicted at an echo time of 0.2 ms for measurements made using the 485 kHz instrument, although at larger echo times (>1.0 ms), the porosity estimate becomes poor. The 485 kHz NMR instrument has non-zero internal magnetic field gradients, similar to those found in borehole instruments, in contrast to the 2 MHz NMR instrument, which has a homogeneous applied magnetic field. Finally, we conclude that differences is the magnetic field strength and higher magnetic field inhomogeneities in the 485 kHz NMR instrument contribute to a reduction of the impact of inhomogeneities in the magnetic field caused by materials with high magnetic susceptibility, allowing for improved porosity estimation. These results indicate that NMR measurements collected at short echo times in low, inhomogeneous static fields, e.g., borehole instruments, may provide accurate estimates of porosity in water saturated sediments, even in the presence of magnetic minerals.

58 GEOSCIENCES↗
NASA NTRS1974

Atlas of lunar radar maps at 70-cm wavelength

The intensity distribution of lunar radar echoes has been mapped for two-thirds of the earth-visible lunar surface at a wavelength of 70 cm. The depolarizing effects of the lunar surface were observed by simultaneously receiving the radar echoes in opposite polarizations. These echoes were mapped with areal resolutions of 25-100 sq km. Mapping with this resolution confirmed that the young craters have enhanced returns. A few craters were found to have enhanced echoes only from their rims. Backscattering differences were also observed between various areas within a mare, between different highland areas, and between maria and adjacent highlands. These scattering differences were interpreted with a simple model, which assumed that the surface backscattered with varying amounts of quasi-specular and diffuse power. Only an increase in the diffuse power was needed to give the numerical values of the enhancements.

Thompson, T. W.↗
NASA NTRS1974

An investigation of wave-particle interactions and particle dynamics using electron beams injected from sounding rockets

Electrons with energy up to 40 kV have been injected into semi-trapped orbits from sounding rockets at Wallops Island, Virginia, and at Fort Churchill, Manitoba, Canada. By directing the rocket trajectory to have a horizontal component which in direction and speed matched the bounce displacement of the injected electrons, it was possible to detect conjugate echoes at Wallops and possibly at Churchill, and to study the distribution of the echoes in space, time and energy. By combining observations of many echoes, a composite picture can be obtained of the beam patterns. Atmospheric scattering at the conjugate point of Wallops Island has been extensively studied, and the process produces a scale width of 5-10 m, but with an echo intensity which is only 10% of theoretical estimates.

Winckler, J. R.↗
NASA NTRS1977

Method for measuring biaxial stress in a body subjected to stress inducing loads

A method is described for measuring stress in test articles including the steps of obtaining for a calibrating specimen a series of transit time differentials between the second wave echo for a longitudinal wave and the first wave echo for each of a pair of shear waves propagated through the specimen as it is subjected to known stress load of a series of stress loads for thus establishing a series of indications of the magnitudes for stress loads induced in the specimen, and thereafter obtaining a transit time differential between the second wave echo for a longitudinal wave and the first wave echo for each of a pair of shear waves propagated in the planes of the stress axes of a test article and comparing the transit time differential thus obtained to the series of transit time differentials obtained for the specimen to determine the magnitude of biaxial stress in the test article.

Clotfelter, W. N.↗
NASA NTRS1985

Rain volume estimation over areas using satellite and radar data

An investigation of the feasibility of rain volume estimation using satellite data following a technique recently developed with radar data called the Arera Time Integral was undertaken. Case studies were selected on the basis of existing radar and satellite data sets which match in space and time. Four multicell clusters were analyzed. Routines for navigation remapping amd smoothing of satellite images were performed. Visible counts were normalized for solar zenith angle. A radar sector of interest was defined to delineate specific radar echo clusters for each radar time throughout the radar echo cluster lifetime. A satellite sector of interest was defined by applying small adjustments to the radar sector using a manual processing technique. The radar echo area, the IR maximum counts and the IR counts matching radar echo areas were found to evolve similarly, except for the decaying phase of the cluster where the cirrus debris keeps the IR counts high.

Doneaud, A. A.↗
NASA NTRS1985

Mean Winds and Tides over Urbana, Illinois During November 16-22, 1981

The Urbana Meteor Radar uses a high power pulse transmitter to achieve high echo rates, and an interferometer to spatially locate the individual echoes. The irregularly spaced observations of range, radial velocity, and direction cosines are reduced to a rectangular grid, evenly spaced in time and height, of estimates of the zonal and meridional wind. This is accomplished using the method of GROVES whereby a constant horizontal wind is fitted, using least squares, to all echoes occurring within a fixed time/height interval. This interpolation region is then stepped in time and height. the size of the interpolation region is 2 hours by 5 km, the time step is approximately 11 minutes, and the height step is 2 km. The filtering of high temporal and spatial frequencies is somewhat reduced by weighting the echoes according to their distance from the center of the interpolation box.

Tetenbaum, D.↗
NASA NTRS1986

Radar glory from buried craters on icy moons

Three ice-covered moons of Jupiter, in comparison with rocky planets and earth's moon, produce radar echoes of astounding strengths and bizarre polarizations. Scattering from buried craters can explain these and other anomalous properties of the echoes. The role of such craters is analogous to that of the water droplets that create the apparition known as 'the glory', the optically bright region surrounding an observer's shadow on a cloud. Both situations involve the electromagnetic phenomenon of total internal reflection at a dielectric interface, operating in a geometry that strongly favors exact backscattering. Dim surface craters are transformed into bright glory holes by being buried under somewhat denser material, thereby increasing the intensity of their echoes by factors of hundreds. The dielectric interface thus formed at the crater walls nicely accounts for the unusual polarizations of the echoes.

Eshleman, Von R.↗
NASA NTRS1987

Meteor observation by the Kyoto meteor radar

The Kyoto Meteor Radar is a monostatic coherent pulsed Doppler radar operating on the frequency of 31.57 MH. The system is computer controlled and uses radio interferometry for echo height determination. The antenna, an improvement, can be directed either to the north or the east. The system has been continuously collecting data on winds at meteor heights by radar observation. The meteor echo rate was also measured, the echo rate distribution with height and the daily variation in height integrated echo rate are discussed. Investigations of atmospheric tides are being pursued by cooperative observations. A novel approach to the study of gravity waves was attempted using the meteor radar which is able to detect the horizontal propagation of the waves by observing the changing phase through the region illuminated by the radar.

Kato, Susumu↗
NASA NTRS1989

Radar observations of Comet Halley

Five nights of Arecibo radar observations of Comet Halley are reported which reveal a feature in the overall average spectrum which, though weak, seems consistent with being an echo from the comet. The large radar cross section and large bandwidth of the feature suggest that the echo is predominantly from large grains which have been ejected from the nucleus. Extrapolation of the dust particle size distribution to large grain sizes gives a sufficient number of grains to account for the echo. The lack of a detectable echo from the nucleus, combined with estimates of its size and rotation rate from spacecraft encounters and other data, indicate that the nucleus has a surface of relatively high porosity.

Campbell, D. B.↗