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At least 1,243 records · Page 69
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

ARC - A source of multisensor satellite data for polar science

The NSF's Antarctic Research Center (ARC) has been established to furnish real-time polar-orbiting satellite data in support of Antarctic field studies, as well as to maintain a multisensor satellite data (MSD) archive for retrospective data analysis. An account is presently given of the ways in which the complementary nature of an MSD set can deepen understanding of Antarctic physical processes. An active microwave SAR with 30-m resolution and a radar altimeter will be added to the ARC resources later in this decade, as will the Earth Observing System.

Van Woert, Michael L.↗
NASA NTRS1985

Potential applications of multipolarization SAR for pine plantation biomass estimation

This study was conducted as a part of the research tasks under the Radar Land Cover Analysis Program. The Radar Land Cover Analysis objective is, through utilization of multisensor data, to gain a basic understanding of the measurements and data characteristics in the visible-IR-microwave regions of the electromagnetic spectrum associated with specific surface features and cover types. Since the results of analysis of data acquired by Shuttle Imaging Radar (SIR-A) and LANDSAT Thematic Mapper (TM) over the study area were reported (NSTL/ERL Report No. 228, December 1984), this study focused on the analysis and evaluation of the L-band multipolarization airborne SAR data acquired over a southeastern pine forest scene. The data acquisition mission was flown on September 8 and 9, 1983. The HH, HV polarizations and the VV, VH polarizations were used on the first and the second day, respectively. Due to instrumentation difficulties, the digital recorder recorded only the second day's data. Because of this, only the VV and VH polarization data were used in this analysis. However, the HH and HV polarization images were available for visual comparison. It appears that SAR digital numbers correlate with the index of green biomass.

Wu, S. T.↗
NASA NTRS1993

SAR terrain classifier and mapper of biophysical attributes

In preparation for the launch of SIR-C/X-SAR and design studies for future orbital SAR, a program has made considerable progress in the development of an SAR terrain classifier and algorithms for quantification of biophysical attributes. The goal of this program is to produce a generalized software package for terrain classification and estimation of biophysical attributes and to make this package available to the larger scientific community. The basic elements of the SAR (Synthetic Aperture Radar) terrain classifier are outlined. An SAR image is calibrated with respect to known system and processor gains and external targets (if available). A Level 1 classifier operates on the data to differentiate: urban features, surfaces and tall and short vegetation. Level 2 classifiers further subdivide these classes on the basis of structure. Finally, biophysical and geophysical inversions are applied to each class to estimate attributes of interest. The process used to develop the classifiers and inversions is shown. Radar scattering models developed from theory and from empirical data obtained by truck-mounted polarimeters and the JPL AirSAR are validated. The validated models are used in sensitivity studies to understand the roles of various scattering sources (i.e., surface trunk, branches, etc.) in determining net backscatter. Model simulations of sigma (sup o) as functions of the wave parameters (lambda, polarization and angle of incidence) and the geophysical and biophysical attributes are used to develop robust classifiers. The classifiers are validated using available AirSAR data sets. Specific estimators are developed for each class on the basis of the scattering models and empirical data sets. The candidate algorithms are tested with the AirSAR data sets. The attributes of interest include: total above ground biomass, woody biomass, soil moisture and soil roughness.

Ulaby, Fawwaz T.↗
NASA NTRS2003

UHF Microstrip Antenna Array for Synthetic- Aperture Radar

An ultra-high-frequency microstrippatch antenna has been built for use in airborne synthetic-aperture radar (SAR). The antenna design satisfies requirements specific to the GeoSAR program, which is dedicated to the development of a terrain-mapping SAR system that can provide information on geology, seismicity, vegetation, and other terrain-related topics. One of the requirements is for ultra-wide-band performance: the antenna must be capable of operating with dual linear polarization in the frequency range of 350 plus or minus 80 MHz, with a peak gain of 10 dB at the middle frequency of 350 MHz and a gain of at least 8 dB at the upper and lower ends (270 and 430 MHz) of the band. Another requirement is compactness: the antenna must fit in the wingtip pod of a Gulfstream II airplane. The antenna includes a linear array of microstrip-patch radiating elements supported over square cavities. Each patch is square (except for small corner cuts) and has a small square hole at its center.

Thomas, Robert F.↗
NASA NTRS1973

Backscattering from a two-scale rough surface with application to radar sea return

A two-scale composite surface scattering theory was developed without using the noncoherent assumption. The surface is assumed electrically homogeneous and finitely conducting; the surface roughness may be nonuniform geometrically. The special forms of the terms for excluding the non-coherent assumption and the meanings of these terms are discussed. To gain insight into the mechanisms of backscattering, the results are compared with those obtained by previous theories. The comparison with NRL data shows satisfactory agreement for both horizontal and vertical polarization, especially for incident angles larger than 30 deg. For smaller incident angles, NASA/JSC data have been chosen for comparison and close agreement is again observed.

Chan, H. L.↗
DOE OSTI2021

Mixed‐Phase Clouds Over the Southern Ocean as Observed From Satellite and Surface Based Lidar and Radar

Abstract This study investigates the occurrence of mixed‐phase clouds (MPC, i.e., cloud layers containing both liquid and ice water at sub‐freezing temperatures) over the Southern Ocean (SO) using space‐ and surface‐based lidar and radar observations. The occurrence of supercooled clouds is dominated by geometrically thin (<1 km) layers that rarely contain ice. We diagnose layers that are geometrically thicker than 1 km to contain ice ~65% and ~4% of the time from below by surface remote sensors and from above by orbiting remote sensors, respectively. We examine the discrepancy in MPC occurrence statistics as diagnosed from below and above the cloud layer. From above, we find that MPC occurrence has a gradient associated with the Antarctic Polar Front near 55°S with a rare occurrence of satellite‐derived MPC south of that latitude. In contrast, surface sensors find ice in 33% of supercooled liquid water layers. We infer using observing system simulation experiments and data analysis that space‐based lidar cannot identify the occurrence of MPC except when secondary ice‐forming processes operate in convection that is, sufficiently strong to loft ice crystals to cloud tops. We conclude that the CALIPSO phase statistics of MPC have a severe low bias in MPC occurrence. Based on surface‐based statistics in the SO, we present a parameterization of the frequency of MPC as a function of cloud top temperature that differs substantially from that used in recent climate model simulations.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1990

Synthetic Aperture Radar Imagery of Airports and Surrounding Areas: Denver Stapleton International Airport

This is the third in a series of three reports which address the statistical description of ground clutter at an airport and in the surrounding area. These data are being utilized in a program to detect microbursts. Synthetic aperture radar (SAR) data were collected at the Denver Stapleton Airport using a set of parameters which closely match those which are anticipated to be utilized by an aircraft on approach to an airport. These data and the results of the clutter study are described. Scenes of 13 x 10 km were imaged at 9.38 GHz and HH-, VV-, and HV-polarizations, and contain airport grounds and facilities (up to 14 percent), cultural areas (more than 50 percent), and rural areas (up to 6 percent). Incidence angles range from 40 to 84 deg. At the largest depression angles the distributed targets, such as forest, fields, water, and residential, rarely had mean scattering coefficients greater than -10 dB. From 30 to 80 percent of an image had scattering coefficients less than -20 dB. About 1 to 10 percent of the scattering coefficients exceeded 0 dB, and from 0 to 1 percent above 10 dB. In examining the average backscatter coefficients at large angles, the clutter types cluster according to the following groups: (1) terminals (-3 dB), (2) city and industrial (-7 dB), (3) warehouse (-10 dB), (4) urban and residential (-14 dB), and (5) grass (-24 dB).

Onstott, Robert G.↗
NASA NTRS2020

The Evolution of Lidar Networks: A US Perspective

Atmospheric aerosols and clouds play an important role in climate by directly scattering and absorbing sunlight. Aerosol-cloud interactions modify the particle properties causing indirect effects, alter aerosol deposition and rainfall, and contribute substantially to the uncertainties in predicting climate effects. Aerosols also affect air quality and both constituents modulate boundary layer dynamics to a certain extent, in turn impacting aerosol transport and aerosol-cloud interactions. The spatiotemporal distribution of aerosol and cloud layers is thus important, and lidar remains the primary instrument for determining the vertical distribution of aerosols and thin clouds. Lidar still provides important information for opaque cloud layers by determining base heights. Combined lidar and radar data provide a comprehensive coverage of aerosol and cloud vertical distributions, and thus locations where and when they interact. Lidar data provide a means of constraining column aerosol loading observations (e.g. AOD) to vertical extents. In addition, lidar has proven effective at determining a proxy for boundary layer height by examining aerosol gradients and/or cloud base heights in lidar profiles. Polarized lidars provide additional information on particle shape, allowing estimates of cloud phase and separation of dust or smoke from sulfate and sea salt aerosols. Given the importance of lidar observations and a critical maturation in technology and retrieval techniques, several organizations began to build operational lidar networks around 2000. The European Aerosol Research Lidar Network (EARLINET) was started as a research focused network of advanced lidars across Europe. The Asian Dust Lidar Network (ADNET) was also developed as a regional network providing lidar profiles of dust and pollution across Eastern Asia. In the US, the NASA Micro Pulse Lidar Network (MPLNET) was created to provide global lidar profiling at key sites in the NASA Aerosol Robotic Network (AERONET). The Network for the Detection of Atmospheric Composition Change (NDACC) pre-dates these networks and many sites have lidar, but it is not strictly a lidar network and at the time focused less on the lower troposphere. Finally, there were already existing ceilometer networks operated by various meteorological agencies, but in particular here in the US the profile data has not been available. The ceilometer networks were used to provide only clouds base heights and estimates of PBL height. Thus, the distinguishing feature between lidar and ceilometer networks was historically the ability to actually provide profile information (in addition to differences in wavelength, and advanced retrievals such as the raman technique). As time progressed, each lidar network matured and developed more operational capabilities and data sets, coupled with viable data centers providing DAAC services and access to near-real-time (NRT) data. In 2008 under WMO guidance, the Global Atmospheric Watch (GAW) Aerosol Lidar Observation Network (GALION) was formed as a global network made up of the existing lidar networks. The goal was to share information, best practices, and develop frameworks and techniques for quality data. GALION grew to include several other regional lidar networks and this has led to a vast increase in quality lidar sites worldwide.

Welton, Ellsworth J.↗
DOE OSTI2021

Arctic observations and numerical simulations of surface wind effects on Multi-Angle Snowflake Camera measurements

Abstract. Ground-based measurements of frozen precipitation are heavily influenced by interactions of surface winds with gauge-shield geometry. The Multi-Angle Snowflake Camera (MASC), which photographs hydrometeors in free-fall from three different angles while simultaneously measuring their fall speed, has been used in the field at multiple midlatitude and polar locations both with and without wind shielding. Here, we present an analysis of Arctic field observations – with and without a Belfort double Alter shield – and compare the results to computational fluid dynamics (CFD) simulations of the airflow and corresponding particle trajectories around the unshielded MASC. MASC-measured fall speeds compare well with Ka-band Atmospheric Radiation Measurement (ARM) Zenith Radar (KAZR) mean Doppler velocities only when winds are light (≤5ms-1) and the MASC is shielded. MASC-measured fall speeds that do not match KAZR-measured velocities tend to fall below a threshold value that increases approximately linearly with wind speed but is generally <0.5ms-1. For those events with wind speeds ≤1.5ms-1, hydrometeors fall with an orientation angle mode of 12∘ from the horizontal plane, and large, low-density aggregates are as much as 5 times more likely to be observed. Simulations in the absence of a wind shield show a separation of flow at the upstream side of the instrument, with an upward velocity component just above the aperture, which decreases the mean particle fall speed by 55 % (74 %) for a wind speed of 5 m s−1 (10 m s−1). We conclude that accurate MASC observations of the microphysical, orientation, and fall speed characteristics of snow particles require shielding by a double wind fence and restriction of analysis to events where winds are light (≤5ms-1). Hydrometeors do not generally fall in still air, so adjustments to these properties' distributions within natural turbulence remain to be determined.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1977

The theory and measurement of noncoherent microwave scattering parameters

The radar equation for incoherent scenes is derived and scattering coefficients are introduced in a systematic way to account for the complete interaction between the incident wave and the random scene. Intensity (power) and correlation techniques similar to that for coherent targets are proposed to measure all the scattering parameters. The sensitivity of the intensity technique to various practical realizations of the antenna polarization requirements is evaluated by means of computer simulated measurements, conducted with a scattering characteristic similar to that of the sea. It was shown that for scenes satisfying reciprocity one must admit three new cross-correlation scattering coefficients in addition to the commonly measured autocorrelation coefficients.

Claassen, J. P.↗
NASA NTRS1985

Sources of scattering from vegetation canopies at 10 GHz

High-resolution backscatter measurements were made at X-band, with vertical polarization, and incidence angles of 30 and 50 deg, for winter wheat, soybeans, and corn during the summer of 1984. Plants were observed both under natural conditions and with partial-to-complete defoliation. The resolution volume of the measurement system is better than 20 x 20 x 11.25 cu cm within the entire range of measurements. Results are presented as a radar image, which also provides information on the spatial distribution of the (attenuated) scattering inside the canopy observed under natural conditions. Measured results indicate that: (1) for the full-grown wheat plants, heads are the dominant scatterers at the early heading stage and the ground is the dominant scatterer at the later growth stages; (2) for the soybeans, returns are mostly from the upper 30-cm portion of the plants, and leaves and stems dominate the radar signal at 30 and 50 deg incidence angles, respectively; (3) for the corn plants, leaves are the single dominant scatterers at both angles of incidence; and (4) ground returns are relatively unimportant as they are highly attenuated by the vegetation cover.

Wu, L.-K.↗
NASA NTRS1986

Radiometric calibration of the Shuttle Imaging Radar (SIR-C) system

The radiometric calibration accuracy of the Shuttle Imaging Radar (SIR-C) sensor is discussed. The analysis includes the antenna, RF electronics, the digital data handling system, the platform attitude control, attitude determination accuracy, and orbit effects. The radiometric distortion of the image products by the ground processing system used for the image formation is also considered. Since the SIR-C system is a dual-frequency quad-polarized system (i.e., 8 channels), the amplitude and phase error is considered over all possible operating modes and environments for absolute and relative (long-term and short-term) calibration within a channel and across channels.

Curlander, John C.↗
NASA NTRS2002

Doppler Radar and Cloud-to-Ground Lightning Observations of a Severe Outbreak of Tropical Cyclone Tornadoes

Data from a single WSR-88D Doppler radar and the National Lightning Detection Network are used to examine the characteristics of the convective storms that produced a severe tornado outbreak within Tropical Storm Beryl's remnants on 16 August 1994. Comparison of the radar data with reports of tornadoes suggests that only 12 cells produced the 29 tornadoes that were documented in Georgia and the Carolinas on that date. Six of these cells spawned multiple tornadoes, and the radar data confirm the presence of miniature supercells. One of the cells was identifiable on radar for 11 hours, spawning tornadoes over a time period spanning approximately 6.5 hours. Time-height analyses of the three strongest supercells are presented in order to document storm kinematic structure and evolution. These Beryl mini-supercells were comparable in radar-observed intensity but much more persistent than other tropical cyclone-spawned tornadic cells documented thus far with Doppler radars. Cloud-to-ground lightning data are also examined for all the tornadic cells in this severe swarm-type tornado outbreak. These data show many of the characteristics of previously reported heavy-precipitation supercells. Lightning rates were weak to moderate, even in the more intense supercells, and in all the storms the lightning flashes were almost entirely negative in polarity. No lightning at all was detected in some of the single-tornado storms. In the stronger cells, there is some evidence that lightning rates can decrease during tornadogenesis, as has been documented before in some midlatitude tornadic storms. A number of the storms spawned tornadoes just after producing their final cloud-to-ground lightning flashes. These findings suggest possible benefits from implementation of observing systems capable of monitoring intracloud as well as cloud-to-ground lightning activity.

McCaul, Eugene W., Jr.↗
NASA NTRS1990

Synthetic aperture radar observations of the Greenland ice sheet

Results of preliminary analyses of aircraft polarimetric SAR data acquired over the Greenland Ice Sheet are presented. Data were collected in August 1989 by the Jet Propulsion Laboratory (JPL) multifrequency, polarimetric SAR using the NASA DC-8 aircraft over southern Greenland. Data of this kind are the first to be acquired over an ice sheet. They are complementary to the limited coverage provided by the SEASAT satellite SAR in 1978, and more recent aircraft X-band SAR image coverage. Frequency and polarization dependencies observed in the P-, L-, and C-band image products are attributed to large-scale variations in the snow and ice surface characteristics. At this time of year, during the ablation season, ice topography exerts a strong influence upon drainage and other hydrological features on the ice sheet surface. Systematic trends in backscatter strength observed across regions of changing snow facies are suggestive of a capability to map areas of snow wetness. Trends observed at C-band indicate that algorithms could possibly be developed which have the ability to delineate areas of significant melt.

Jezek, K.↗
NASA NTRS2007

Modeling Lunar Radar Scattering from Icy Regoliths

The Apollo 15, 16, and 17 core tubes show that the uppermost few meters of the lunar regolith are interlaced layers of a fine grained powders and blocky crater ejecta. The layers of crater ejecta have dielectric constants in the range of 7-9 while the fine-grained powders has dielectric constant on the order of 2.7. These differences in dielectric constant, in turn, create radar reflections that are both refracted and reflected back through the space-regolith interface. Note that for a dielectric constant of 2.7 for the lunar regolith, radio waves incident on the lunar surface at the angle of 30-degrees from the normal will propagate in the regolith at an angle of 18-degrees. At the limb, radio waves incident on the lunar surface at an angle near 90-degrees from the normal will propagate in the regolith at an angle of about 37-degrees. These angles are within the range where radar backscatter is in the quasi-specular regime. When these buried crater ejecta layers are modeled using Hagfors' formulation (Hagfors,1963), echo powers match the behavior observed for average lunar backscatter at centimeter wavelengths for higher (30 to 90) angles of incidence. In addition, Hagfors et al. (1965) conducted an experiment where the Moon was illuminated at 23-cm wavelength with circular polarization and the differences were observed in orthogonal linear polarizations. Modeling of these observations and assuming again that the buried crater ejecta scatter in a quasi-specular manner, echo differences in horizontal and vertical linear polarizations are in good agreement with the observations.

Moon↗
NASA NTRS1994

Electron density measurements during the NLC-91 campaign

A Super Arcas rocket, MISTI B, containing DC and RF probes, was launched as a part of the PMSE (Polar Mesosphere Summer Echoes) Salvo during the NLC-91 (Noctilucent Cloud) campaign to measure electron density irregularities with high spatial resolution. Measurements of large and small scale structures in the electron density were made on rocket ascent and descent at the altitudes of 86.5 and 88.5 +/- 0.5 km corresponding to the two altitudes of strongest backscatter recorded by the nearby CUPRI (Cornell University Portable Radar Interferometer) radar. Power spectra of the fluctuations shows two different structuring and scattering mechanisms exist at altitudes only 1 km apart. Since the rocket apogee was 89 km, the rocket was in the height range 88.5 +/- 0.5 km for 30 seconds giving an unusual measurement of horizontal structure over a distance of 5.5 km. Using the simultaneous DC and RF probe measurements of electron depletions and sharp gradient in the lower layer, the role of aerosols in creating these depletions and gradients is speculated upon.

Ulwick, J. C.↗
NASA NTRS2004

Windsat Validation Using Seawinds, Windrad and POLSCAT Measurements

Global mapping of near surface ocean wind vectors is crucial for many oceanographic and atmospheric studies. The US Navy together with the National Polar Orbiting Environmental Satellite System (NPOESS) launched the Windsat with multi-frequency polarimetric radiometers in January 2003 to demonstrate the passive polarimetry for large spatial coverage of ocean surface wind vector measurements from space. We derived the geophysical model function (GMF) for Windsat polarimetric brightness temperature measurements using six months of matchup dataset. The Windsat GMF was compared with the aircraft radiometer and radar measurements and the SeaWinds scatterometer winds with good agreement up to about 20 m/s wind speed.

scatterometer↗
NASA NTRS2017

Active and Passive 3D Vector Radiative Transfer with Preferentially-Aligned Ice Particles

For the purposes of interpreting active (radar) and passive (radiometer) microwave and millimeter wave remote sensing data, we have constructed a consistent radiative transfer modeling framework to simulate the responses for arbitrary sensors with differing sensing geometries and hardware configurations. As part of this work, we have implemented a recent method for calculating the electromagnetic properties of individual ice crystals and snow flakes. These calculations will allow us to exploit polarized remote sensing observations to discriminate different particles types and elucidate dynamics of cloud and precipitating systems.

Adams, Ian S.↗