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Proceedings of the IEEE

Issue 12 • Date Dec 1994

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Displaying Results 1 - 10 of 10
  • Advances in polarization diversity lidar for cloud remote sensing

    Page(s): 1907 - 1914
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    As soon as high-energy pulsed lasers became available in the mid-1960s, atmospheric scientists began assessing the information contents of various light detection and ranging (lidar) techniques in the field and laboratory. A particularly promising approach, which has recently gained increased stature as a result of growing interests in previously overlooked cloud types important to climate research (e.g. high-altitude cirrus), involves the polarization analysis of the backscattered laser return. Polarization diversity lidars collect data simultaneously in two or more channels to remotely determine the thermodynamic cloud phase, structure, and boundaries, and infer a variety of other climatically important cloud microphysical properties. As illustrated by the description of the mobile University of Utah Polarization Diversity Lidar system, commercially available dual-wavelength transmitters and improved electronic technologies to process high-resolution multichannel lidar signals represent significant advantages over earlier devices used in cloud studies. Polarization diversity can be added rather economically to more specialized lidars employing spectroscopic techniques, for other forms of atmospheric probing, such as Raman, differential absorption, and high spectral resolution lidars, in order to enhance instrument accuracy and versatility for cloud an aerosol research. Future engineering challenges remain in the “hardening” of compact lidar systems for extraterrestrial deployment, the achievement of unattended eye-safe operations, and in the improved integration of multiple remote sensor data streams and supporting in situ measurements to more fully characterize cloud systems and their effects on the Earth's radiation balance View full abstract»

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  • Millimeter-wave remote sensing of ozone and trace constituents in the stratosphere

    Page(s): 1915 - 1929
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    The application of millimeter-wave heterodyne spectroscopy to measurements of ozone and trace constituents in the upper atmosphere (between about 20- and 70-km altitude) is discussed. Brief, introductory reviews, with references, of the chemistry of ozone creation and destruction are given. The technical considerations involved in making the measurements are discussed. A millimeter-wave instrument for measuring stratospheric ozone is described as an example of the application of the technical considerations to instrument design. Results achieved with this instrument are reviewed View full abstract»

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  • Mapping the world's topography using radar interferometry: the TOPSAT mission

    Page(s): 1774 - 1786
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    Global-scale topographic data are of fundamental importance to many Earth science studies, and obtaining these data is a priority for the Earth science community. Several groups have considered the requirements for such a data set, and a consensus assessment is that many critical studies would be enabled by the availability of a digital global topographic model with accuracies of 2 and 30 m in the vertical and horizontal directions, respectively. Radar interferometric techniques have been used to produce digital elevation models at these accuracies and are technologically feasible as the centerpiece of a spaceborne satellite mission designed to map the world's land masses, which we denote TOPSAT. A radar interferometer is formed by combining the radar echoes received at a pair of antennas displaced across-track, and specialized data processing results in the elevation data. Two alternative implementations, one using a 2 cm-λ radar, and one using a 24 cm-λ radar, are technologically feasible. The former requires an interferometer baseline length of about 15 m to achieve the required accuracy, and this could be built on a single spacecraft with a long extendible boom. The latter necessitates a kilometers long baseline, and would thus be best implemented using two spacecraft flying in formation. Measurement errors are dominated by phase noise, due largely to signal-to-noise ratio considerations, and attitude errors in determining the baseline orientation. For the 2-m accuracy required by TOPSAT, the orientation must be known to 1 arc-second. For the single-spacecraft approach, where attitude would be determined by star tracking systems, this performance is just beyond the several arc-second range of existing instruments. For the dual-spacecraft systems, though, differential global positioning satellite measurements possess sufficient accuracy. Studies indicate that similar performance can be realized with either system View full abstract»

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  • Design of a bistatic dual-Doppler radar for retrieving vector winds using one transmitter and a remote low-gain passive receiver

    Page(s): 1861 - 1872
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    A bistatic dual-Doppler network consisting of an existing single transmitting pencil-beam weather radar and a remotely located, nontransmitting, passive bistatic receiver with a low-gain antenna was constructed and tested during 1993. High-quality dual Doppler vector winds were retrieved from this system. The windfields were compared with those collected with a traditional two-transmitter dual-Doppler system. Several interesting engineering challenges relating to frequency and timing synchronization were resolved in order to retrieve successfully Doppler velocities at a remote bistatic site. Frequency synchronization was achieved by using extremely stable local oscillators, linked by both Global Positioning Satellite (GPS) signals and transmitter sidelobe coupling. Both methods provided the necessary one part in 109 coherence necessary for calculating accurate Doppler velocities. Timing/gating synchronization with submicrosecond accuracy was achieved by using local oscillators at each site linked with GPS and sidelobe coupling. The successful testing of this system demonstrates that inexpensive and practical bistatic multiple-Doppler networks can be deployed. These systems can provide three-dimensional vector winds for a number of purposes in research, aviation, media, weather prediction, education, meteorological modeling and severe weather detection View full abstract»

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  • Ground penetrating radar as a subsurface environmental sensing tool

    Page(s): 1802 - 1822
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    Ground penetrating radar (GPR) is considered as an environmental tool. The basic concepts involved in GPR are introduced briefly including the antennas, propagation, target scattering, and mapping. Target identification is important when using GPR since the scatterer can only be observed by evacuation. This is discussed in terms of mapping and complex natural resonances. GPR has been used and is being considered as a tool for the detection of a wide variety of subterranean features. A very brief description of the various applications of GPR is presented. In terms of environmental sensing, it has been applied to detect buried tanks, landfill debris, water levels, and contaminated fluids. The detection of various military devices also represent a serious environmental concern including landmines and unexploded ordnance. There are also possible applications involving the detection of buried utilities highway voids, grave sites. It has been used for examining archeological sites. The above list is far from complete because of the ever-expanding use of GPR View full abstract»

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  • ESTAR: a synthetic aperture microwave radiometer for remote sensing applications

    Page(s): 1787 - 1801
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    ESTAR represents a new technology being developed for passive microwave remote sensing of the environment from space. The instrument employs an interferometric technique called aperture synthesis in which the coherent product from pairs of antennas is measured as a function of pair spacing. Substantial reductions in the antenna aperture needed for a given spatial resolution can be achieved with this technique. As a result, aperture synthesis could lead to practical passive microwave remote sensing instruments in space to measure parameters such as soil moisture and ocean salinity which require observations at long wavelengths and, therefore, large antennas. ESTAR is an L-band, aircraft built as part of research to develop this technique ESTAR is a hybrid real-and-synthetic aperture radiometer which employs stick antennas to achieve resolution along track and uses aperture synthesis to achieve resolution across track. Experiments to validate the instrument's ability to measure soil moisture have recently been conducted at the USDA watersheds at Walnut Gulch in Arizona and the Little Washita River in Oklahoma. The results of both experiments indicate that a valid image reconstruction and calibration have been obtained for this remote sensing technique View full abstract»

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  • Environmental characterization with magnetics and STOLS

    Page(s): 1823 - 1834
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    The application of techniques utilizing total field magnetic anomaly measurements can contribute significantly to the environmental characterization of lands contaminated with hazardous, toxic, or radioactive wastes. By addressing the correlated problem of detection and characterization of HTRW waste containers, predominantly made of ferromagnetic materials, high-density magnetics systems can be used to characterize a wide range of buried hazardous materials. Ferromagnetic materials such as iron and steel, induce an observable secondary magnetic field which can be measured and modeled to yield estimates of location, depth, and mass of the ferromagnetic objects. As a passive geophysical technique magnetics is logistically tractable, allowing for development of systems which acquire high density data over large areas. The STOL magnetics detection system was developed for this purpose, and is outfitted with seven total field magnetometers which are integrated with a differential GPS to provide 20 Hz magnetic-field measurements positioned with nominal absolute position accuracy of 30 cm. Processing of magnetic data requires the removal of the Earth's primary field and the interpolation onto a mesh. Image processing techniques are applied to enhance and segment the data for anomaly delineation. Isolated anomalies are modeled under the point dipole assumption to produce estimates of object position, depth, and mass View full abstract»

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  • Millimeter-wave radars for remotely sensing clouds and precipitation

    Page(s): 1891 - 1906
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    Millimeter-wave radars have been used since the early 1950s to study clouds and precipitation, but until recently these early instruments were limited to simple backscatter power measurements and were plagued by hardware problems. However, development of solid-state millimeter-wave componentry and high-power klystron amplifiers has spurred the evolution of reliable, coherent radars operating up to 95 GHz. In addition, advances in digital signal processing technology have resulted in single-card processors that can simultaneously execute algorithms to compute reflectivity, Doppler, and polarimetric quantities in real time. A review of the current state of the art in millimeter-wave cloud radars is presented, including a discussion of transmitters, antennas, and receiver components. Two radar systems built by the University of Massachusetts are described, including a mobile, dual-frequency (33- and 95-GHz) polarimetric radar, and an airborne 95-GHz polarimetric radar that was recently flown in a cooperative experiment with the University of Wyoming. Spaceborne applications are also discussed, especially the use of satellite-based 95-GHz radars for measuring the vertical distribution of clouds View full abstract»

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  • Airborne scatterometers: investigating ocean backscatter under low and high-wind conditions

    Page(s): 1835 - 1860
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    Attempting to understand and predict weather on a local and global basis has challenged both the scientific and engineering communities. One key parameter in understanding the weather is the ocean surface wind vector because of its role in the energy exchange at the air-sea surface. scatterometers, radars that measure the reflectivity of a target offer a tool with which to remotely monitor these winds from tower-, aircraft-, and satellite-based platforms. This paper introduces three current airborne scatterometer systems, and presents data collected by these instruments under low-, moderate-, and high-wind conditions. The paper focuses on airborne scatterometers because of their ability to resolve submesoscale variations in wind fields. Discrepancies between existing theory and the observations are noted and the concerns in measuring low-wind speeds discussed. Finally, the application of using this technology for estimating the surface-wind vector during a hurricane is demonstrated View full abstract»

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  • The ELDORA/ASTRAIA airborne Doppler weather radar: goals, design, and first field tests

    Page(s): 1873 - 1890
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    Describes the development and first operation of a innovative research tool for observing atmospheric storms: an airborne X-band Doppler radar. The radar has been built jointly by the National Center for Atmospheric Research (NCAR) in Boulder, CO and the Centre de Recherches en Physique de l'Environnement Terrestre et Planetaire (CRPE) in Paris, France. The radar is called ELDORA/ASTRAIA for Electra Doppler Radar/Analyse Stereoscopique par Radar Aeroporte sur Electra. The radar is designed to provide high-resolution measurements of the air motion and rainfall characteristics of very large storms: storms which are frequently too large and/or too remote to be adequately observed by ground-based radars. This paper includes discussions of the scientific impetus and design criteria as well as the engineering solutions to these design needs. The design options and tradeoffs of the resulting capabilities are discussed. The paper concludes with an evaluation of the performance of the system at its first field test, conducted as a part of the international global warming experiment in the equatorial Pacific Ocean known as the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). This evaluation illustrates that the major design goals for the radar - collection of relatively noise-free velocity and reflectivity data using a rapid scanning radar - have been adequately met. Future options for further improvements to the radar are discussed View full abstract»

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North Carolina State University