Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Radar observations”

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 1,477 records · Page 82
All sourcesNASA NTRSDOE OSTI
NASA NTRS2016

Optical and Radar Measurements of the Meteor Speed Distribution

The observed meteor speed distribution provides information on the underlying orbital distribution of Earth-intersecting meteoroids. It also affects spacecraft risk assessments; faster meteors do greater damage to spacecraft surfaces. Although radar meteor networks have measured the meteor speed distribution numerous times, the shape of the de-biased speed distribution varies widely from study to study. Optical characterizations of the meteoroid speed distribution are fewer in number, and in some cases the original data is no longer available. Finally, the level of uncertainty in these speed distributions is rarely addressed. In this work, we present the optical meteor speed distribution extracted from the NASA and SOMN allsky networks [1, 2] and from the Canadian Automated Meteor Observatory (CAMO) [3]. We also revisit the radar meteor speed distribution observed by the Canadian Meteor Orbit Radar (CMOR) [4]. Together, these data span the range of meteoroid sizes that can pose a threat to spacecraft. In all cases, we present our bias corrections and incorporate the uncertainty in these corrections into uncertainties in our de-biased speed distribution. Finally, we compare the optical and radar meteor speed distributions and discuss the implications for meteoroid environment models.

Moorhead, A. V.↗
NASA NTRSDate not supplied

Reconstructing Subgrid Cloud Variability Guided by CloudSat/CALIPSO Observations

Predicting realistic cloud subgrid variability remains a challenge for Global Climate Models (GCMs) even though it can play an important role for proper representation of processes pertaining to cloud microphysics, precipitation, and radiation, but also for comparisons with satellite observations which are of much higher resolution than model grids. The diagnostic approach of subgrid cloud variability is commonly handled by subcolumn cloud generators. For a specific GCM, one ideally wants the subgrid variability used for comparisons with satellite observations to be created by the same generator and with the same rules as the one used for model integration. With this in mind, we have embarked in an effort to test and improve cloud subcolumn generators appropriate for GCMs. For this purpose, we use cloud (hydrometeor) products from active observations by the CloudSat radar (CPR) and the CALIPSO lidar (CALIOP). Cloud products from active sensors while suffering significant sampling and coverage drawbacks have the advantage of resolving both horizontal and vertical variability. The main question is: given a profile of cloud condensate mean and variance, can we create a subgrid cloud field that is statistically similar to the observed subgrid cloud field? By “statistically”, we suggest that we do not aspire to reproduce “well” each individual subgrid cloud field of a GCM-scale region, but that our generator performs well for a large ensemble of cases. We simulate radar, passive imager, and radiation flux fields from the observed 2D cloud fields and create one-point statistics; we use the profiles of cloud condensate mean and variance as input to the generator to create subgrid cloud fields; we compare the statistics; we adjust the rules of the generator to create the best possible agreement between the radar, imager and radiation field statistics. In this process, the active observations have actually a dual role: they provide the actual subgrid cloud field which can be reduced to a profile of mean and variance used by the subcolumn generator, but they also provide the rules needed by the generator, such as measures (e.g, decorrelation length) of the vertical overlap of cloud fraction and of the condensate horizontal variability. This presentation will show our progress using Cloudsat and CALIPSO products in this dual fashion.

cloud↗
NASA NTRS1984

Determination of billows and other turbulent structures, part 4.1A

Billows are regular, wave-like arrays of cross-flow vortices that develop in stratified oceanic or atmospheric flows with large shear. Atmospheric billows can become manifest through condensation. Billows are frequently seen in their characteristic cloud forms in the lower atmosphere. Under suitable viewing conditions, billows can also be seen in noctilucent clouds that form near the polar mesosphere during the summer months. Other turbulent structures -- related to billows -- are the Kelvin-Helmholtz instability (KHI) and cat's eye structures that occur in fully developed turbulent shear flows. Shear flows may contain perturbations at many different horizontal wavelengths and vertical scales. Realistic theoretical models have been constructed to study the stability and growth of these perturbations. The extent to which billows and Kelvin-Helmholtz instability have been observed in the atmosphere with the use of radars is outlined. Most of these observations are confined to the troposphere. Suggestions are made for improved radar experiments that are required to detect these structures at higher altitudes.

Rastogi, P. K.↗
NASA NTRSDate not supplied

Reconstructing Subgrid Cloud Variability Guided by CloudSat/CALIPSO Observations

Predicting realistic cloud subgrid variability remains a challenge for Global Climate Models (GCMs) even though it can play an important role for proper representation of processes pertaining to cloud microphysics, precipitation, and radiation, but also for comparisons with satellite observations which are of much higher resolution than model grids. The diagnostic approach of subgrid cloud variability is commonly handled by subcolumn cloud generators. For a specific GCM, one ideally wants the subgrid variability used for comparisons with satellite observations to be created by the same generator and with the same rules as the one used for model integration. With this in mind, we have embarked in an effort to test and improve cloud subcolumn generators appropriate for GCMs. For this purpose, we use cloud (hydrometeor) products from active observations by the CloudSat radar (CPR) and the CALIPSO lidar (CALIOP). Cloud products from active sensors while suffering signi􀂦cant sampling and coverage drawbacks have the advantage of resolving both horizontal and vertical variability. The main question is: given a profile of cloud condensate mean and variance, can we create a subgrid cloud field that is statistically similar to the observed subgrid cloud field? By “statistically”, we suggest that we do not aspire to reproduce “well” each individual subgrid cloud field of a GCM-scale region, but that our generator performs well for a large ensemble of cases. We simulate radar, passive imager, and radiation flux fields from the observed 2D cloud fields and create one-point statistics; we use the profiles of cloud condensate mean and variance as input to the generator to create subgrid cloud fields; we compare the statistics; we adjust the rules of the generator to create the best possible agreement between the radar, imager and radiation field statistics. In this process, the active observations have actually a dual role: they provide the actual subgrid cloud field which can be reduced to a profile of mean and variance used by the subcolumn generator, but they also provide the rules needed by the generator, such as measures (e.g, decorrelation length) of the vertical overlap of cloud fraction and of the condensate horizontal variability. This presentation will show our progress using Cloudsat and CALIPSO products in this dual fashion.

clouds↗
NASA NTRS1983

Effects of vegetation canopy on the radar backscattering coefficient

Airborne L- and C-band scatterometer data, taken over both vegetation-covered and bare fields, were systematically analyzed and theoretically reproduced, using a recently developed model for calculating radar backscattering coefficients of rough soil surfaces. The results show that the model can reproduce the observed angular variations of radar backscattering coefficient quite well via a least-squares fit method. Best fits to the data provide estimates of the statistical properties of the surface roughness, which is characterized by two parameters: the standard deviation of surface height, and the surface correlation length. In addition, the processes of vegetation attenuation and volume scattering require two canopy parameters, the canopy optical thickness and a volume scattering factor. Canopy parameter values for individual vegetation types, including alfalfa, milo and corn, were also determined from the best-fit results. The uncertainties in the scatterometer data were also explored.

Mo, T.↗
NASA NTRS1992

First observations of mesospheric dynamics with a partial reflection radar in Hawaii (22 deg N, 160 deg W)

A partial-reflection MF radar was installed at the Pacific Missile Range Facility on Kanai, Hawaii in September 1990. The wind measurements obtained with this radar system in Hawaii during the first year of operation are reviewed. The data reveal a broad spectrum of motions ranging from a mix of the equatorial mesopause semiannual oscillation (MSAO) and the annual mean cycle at middle and high latitudes in the zonal mean structure to low-frequency, tidal, and gravity wave motions at higher frequencies. The zonal mean wind structure is characterized by a downward progression of strong eastward and westward phases of the MSAO from approximately January to July. An eastward maximum of about 60/ms near 80 km during January and February is found, which descends rapidly, and a westward maximum of about 50 ms near 85 km during March and April is found, which descends much more slowly. The second MSAO cycle is greatly suppressed relative to the first due to the reversal of the correlation between this and the annual cycle at higher latitudes from July to December.

Fritts, David C.↗
NASA NTRS1989

EISCAT observations during MAC/SINE and MAC/Epsilon

The EISCAT incoherent scatter radar facility in Tromsoe, Norway was operated during the MAC/SINE campaign for 78 hours in the period 10 June to 17 July 1987, and during the MAC/Epsilon campaign for 90 hours in the period 15 October to 5 November 1987. The VHF (224 MHz) radar operations during MAC/SINE yielded most interesting observations of strong coherent echoes from the mesopause region. Characteristic data of these polar mesospheric summer echoes are presented. The UHF (933 MHz) radar operations during MAC/Epsilon were done with 18 deg off zenith beam and allows the deduction of meridonal and horizontal wind components as well as radial velocity spectra in addition to the usual electron density profiles in the D and lower E regions. Some results from the VHF and UHF radars indicating the presence of gravity waves are examined.

Roettger, J.↗
DOE OSTI2023

Time Resolved Reflectivity Measurements of Convective Clouds

National Aeronautics and Space Administration's Investigations of Convective Updrafts (INCUS) mission aims to document convective mass flux through changes in the radar reflectivity (ΔZ) in convective cores captured by a constellation of three Ka-band radars sampling the same convective cells over intervals of 30, 90, and 120 s. Here, high spatiotemporal resolution observations of convective cores from surface-based radars that use agile sampling techniques are used to evaluate aspects of the INCUS measurement approach using real observations. Analysis of several convective cells confirms that large coherent ΔZ structure with measurable signal (>5 dB) can occur in less than 30 s and are correlated with underlying convective motions. The analysis indicates that the INCUS mission radar footprint and along track sampling are adequate to capture most of the desirable ΔZ signals. This unique demonstration of reflectivity time-lapse provides the framework for estimating convective mass flux independent from Doppler techniques with future radar observations.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS2012

Recent observations of clouds and precipitation by the Airborne Precipitation Radar 2nd Generation in support of the GPM and ACE missions

In this paper we illustrate the unique dataset collected during the Global Precipitation Measurement Cold-season Precipitation Experiment (GCPEx, US/Canada Jan/Feb 2012). We will focus on the significance of these observations for the development of algorithms for GPM and ACE, with particular attention to classification and retrievals of frozen and mixed phase hydrometeors.

Im, Eastwood↗
DOE OSTI2021

Remote Sensing of Electric Fields Observed Within Winter Precipitation During the 2020 Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) Field Campaign

Aircraft electric fields from NASA's Lightning Instrument Package (LIP) were coupled with other airborne and ground-based, and in situ measurements to understand electrification in winter clouds that did not produce lightning. The measurements were made during seven research flights by a NASA ER-2 during the 2020 Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) campaign. Observed total electric field magnitudes were as high as 80 V m -1 and variability in the electric field was observed along the flight path of the ER-2, indicating horizontal and/or vertical inhomogeneity in the cloud's electrical structure. X-band airborne radar data indicated 20-dBZ echo tops above 5 km in regions where electrification exceeded 10 V m -1 . In these regions, 85-GHz brightness temperatures (TB) from an airborne radiometer were lower than 265 K, with the lowest T B (~210 K) associated with ice scattering collocated with the strongest electric fields. In situ microphysical measurements from the NASA P-3 aircraft on February 7 indicated that regions near strong electric field contained supercooled water, rimed ice hydrometeors, ice water p- content as high as 1 g m -3 , liquid water content as high as 0.15 g m -3 , and supersaturation as high as 3.5%. Furthermore, these observations support the role of mixed phase microphysics in the generation of electric fields in clouds. In three case studies, ground based S-band polarimetric radar observed depolarization streaks in differential reflectivity near areas where the strongest electrification was observed. This observation reinforces the utility of depolarization streaks to identify areas of electrification prior to lightning occurrence.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1984

Observations of frontal zone structures with a VHF Doppler radar and radiosondes, part 1.2A

The SOUSY-VHF-Radar is a pulsed coherent radar operating at 53.5 MHz and located near Bad Lauterbert, West Germany. Since 1977, the facility, operated by the Max-Planck-Institut fur Aeronomie, has been used to make a series of frontal passage observations in the spring and fall. Experiments in winter have been difficult because part of the transmitting and receiving array is usually covered by snow during that part of the year. Wavelengths around 6 m are known to be sensitive to the vertical temperature structure of the atmosphere (GREEN and GAGE, 1980; RASTOGI and ROTTGER, 1982). Thus, it has been possible to use radars operating at frequencies near 500 MHz to locate the tropopause. Comparisons between radar data and radiosonde data have shown that there is a large gradient in the radar reflectivity at the height where the radiosonde tropopause occurs. An experiment carried out by ROTTGER (1979) on March 15 to 16, 1977, showed that the radar's sensitivity to the vertical temperature structure could also be used to locate the position of fronts. The SOUSY-VHF-Radar consists of a transmitting array, also used for receiving in some configurations, that can be scanned in the off-vertical direction but not at sufficiently low elevation angles to study the horizontal extent of structures.

Larsen, M. F.↗
NASA NTRS1990

Radar characteristics of the equatorial plains of Venus from Goldstone observations - Implications for interpretation of Magellan data

Based on analyses of recent high resolution observations of the approximately 1000 km diameter Heng-O ring structure by the Goldstone Solar System Radar, it is predicted that the structure is a large corona. Calibrated values of radar backscatter coefficient indicate lateral variations of a factor of 4 in Fresnel reflectivity in the vicinity of Heng-O, as well as in other areas of the plains, at scales of tens of km. It is predicted that areas with very low reflectivity values, which occur in a variety of geological settings, are soil-dominated, with bulk densities as low as 1.5 g/cu cm.

Plaut, Jeffrey J.↗
NASA NTRS2017

Evolution of Circular Polarization Ratio (CPR) Profiles of Kilometer-scale Craters on the Lunar Maria

When sufficiently large impact craters form on the Moon, rocks and unweathered materials are excavated from beneath the regolith and deposited into their blocky ejecta. This enhances the rockiness and roughness of the proximal ejecta surrounding fresh impact craters. The interior of fresh craters are typically also rough, due to blocks, breccia, and impact melt. Thus, both the interior and proximal ejecta of fresh craters are usually radar bright and have high circular polarization ratios (CPR). Beyond the proximal ejecta, radar-dark halos are observed around some fresh craters, suggesting that distal ejecta is finer-grained than background regolith. The radar signatures of craters fade with time as the regolith grows.

moon↗
NASA NTRS2004

Multi-Wavelength Observations of Asteroid 2100 Ra-Shalom: Visible, Infrared, and Thermal Spectroscopy Results

The August 2003 apparition of asteroid 2100 Ra-Shalom brought together a collaboration of observers with the goal of obtaining rotationally resolved multiwavelength spectra at each of 5 facilities: infrared spectra at the NASA Infrared Telescope Facility (Clark and Shepard), radar images at Arecibo (Shepard and Clark), thermal infrared spectra at Palomar (Lim, McConnochie and Bell), visible spectra at McDonald Observatory (Vilas, Lederer and Jarvis), and visible lightcurves at Ondrojev Observatory (Pravec). The radar data was to be used to develop a high spatial resolution physical model to be used in conjunction with spectral data to investigate compositional and textural properties on the near surface of Ra Shalom as a function of rotation phase. This was the first coordinated multi-wavelength investigation of any Aten asteroid. There are many reasons to study near-Earth asteroid (NEA) 2100 Ra-Shalom: 1) It has a controversial classification (is it a C- or K-type object)? 2) There would be interesting dynamical ramifications if Ra-Shalom is a K-type because most K-types come from the Eos family and there are no known dynamical pathways from Eos to the Aten population. 3) The best available spectra obtained previously may indicate a heterogeneous surface (most asteroids appear to be fairly homogeneous). 4) Ra-Shalom thermal observations obtained previously indicated a lack of regolith, minimizing the worry of space weathering effects in the spectra. 5) Radar observations obtained previously hinted at interesting surface structures. 6) Ra-Shalom is one of the largest Aten objects. And 7) Ra-Shalom is on a short list of proposed NEAs for spacecraft encounters and possible sample returns. Preliminary results from the visible, infrared, and thermal spectroscopy measurements will be presented here.

Clark, Beth Ellen↗
NASA NTRS1987

High resolution radar map of the Moon

Previous radar mappings of the Moon at 70 cm wavelength in the late 1960's by Thompson have been replaced with a new set of observations using the 430 MHz radar at the Arecibo Observatory, Puerto Rico. Radar resolution was reduced to 2 to 5 km radar cell size and a beam-sweep, limb-to-limb calibration was conducted. Advances in computer technology provided the principle means of improving lunar radar mapping at this wavelength. Observation techniques and data processing are described and scattering differences found in the orthographic projection of the radar data are discussed.

Thompson, T. W.↗