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Full-Text Articles in Engineering
Sea Ice Mapping Method For Seawinds, David G. Long, Hyrum S. Anderson
Sea Ice Mapping Method For Seawinds, David G. Long, Hyrum S. Anderson
Faculty Publications
A sea ice mapping algorithm for SeaWinds is developed that incorporates statistical and spatial a priori information in a modified maximum a posteriori (MAP) framework. Spatial a priori data are incorporated in the loss terms of a Bayes risk formulation. Conditional distributions and priors for sea ice and ocean statistics are represented as empirical histograms that are forced to conform to a set of expected histograms via principal component filtering. Tuning parameters for the algorithm allow adjustments in the algorithm's performance. Results of the algorithm exhibit high correlation with the Remund-Long sea ice mapping algorithm for SeaWinds and the Special …
Large-Scale Inverse Ku-Band Backscatter Modeling Of Sea Ice, David G. Long, Quinn P. Remund
Large-Scale Inverse Ku-Band Backscatter Modeling Of Sea Ice, David G. Long, Quinn P. Remund
Faculty Publications
Polar sea ice characteristics provide important inputs to models of several geophysical processes. Microwave scatterometers are ideal for monitoring these regions due to their sensitivity to ice properties and insensitivity to atmospheric distortions. Many forward electromagnetic scattering models have been proposed to predict the normalized radar cross section (σ˚) from sea ice characteristics. These models are based on very small scale ice features and generally assume that the region of interest is spatially homogeneous. Unfortunately, spaceborne scatterometer footprints are very large (5-50 km) and usually contain very heterogeneous mixtures of sea ice surface parameters. In this paper, we use scatterometer …
An Advanced Ambiguity Selection Algorithm For Seawinds, David G. Long, David W. Draper
An Advanced Ambiguity Selection Algorithm For Seawinds, David G. Long, David W. Draper
Faculty Publications
SeaWinds on QuikSCAT, a spaceborne Ku-band scatterometer, estimates ocean winds via the relationship between the normalized radar backscatter and the vector wind. Scatterometer wind retrieval generates several possible wind vector solutions or ambiguities at each resolution cell, requiring a separate ambiguity selection step to give a unique solution. In processing SeaWinds on QuikSCAT data, the ambiguity selection is "nudged" or initialized using numerical weather prediction winds. We describe a sophisticated new ambiguity selection approach developed at Brigham Young University (BYU) that does not require nudging. The BYU method utilizes a low-order data-driven Karhunen-Loeve wind field model to promote self-consistency. Ambiguity …
High-Resolution Measurements With A Spaceborne Pencil-Beam Scatterometer Using Combined Range/Doppler Discrimination Techniques, David G. Long, Michael W. Spencer, Wu-Yang Tsai
High-Resolution Measurements With A Spaceborne Pencil-Beam Scatterometer Using Combined Range/Doppler Discrimination Techniques, David G. Long, Michael W. Spencer, Wu-Yang Tsai
Faculty Publications
Conically scanning pencil-beam scatterometer systems, such as the SeaWinds radar, constitute an important class of instruments for spaceborne climate observation. In addition to ocean winds, scatterometer data are being applied to a wide range of land and cryospheric applications. A key issue for future scatterometer missions is improved spatial resolution. Pencil-beam scatterometers to date have been real-aperture systems where only range discrimination is used, resulting in a relatively coarse resolution of approximately 25 km. In this paper, the addition of Doppler discrimination techniques is proposed to meet the need for higher resolution. The unique issues associated with the simultaneous application …
Validation Of Sea Ice Motion From Quikscat With Those From Ssm/I And Buoy, David G. Long, Yunhe Zhao, Antony K. Liu
Validation Of Sea Ice Motion From Quikscat With Those From Ssm/I And Buoy, David G. Long, Yunhe Zhao, Antony K. Liu
Faculty Publications
Arctic sea ice motion for the period from October 1999 to March 2000 derived from QuikSCAT and ocean buoy observations. Special Sensor Microwave/Imager (SSM/I) data using the wavelet analysis method agrees well with ocean buoy observations. Results from QuikSCAT and SSM/I are compatible when compared with buoy observations and complement each other. Sea ice drift merged from daily results from QuikSCAT, SSM/I, and buoy data gives more complete coverage of sea ice motion. Based on observations of six months of sea ice motion maps, the sea ice motion maps in the Arctic derived from QuikSCAT data appear to have smoother …
Image Reconstruction And Enhanced Resolution Imaging From Irregular Samples, David G. Long, David S. Early
Image Reconstruction And Enhanced Resolution Imaging From Irregular Samples, David G. Long, David S. Early
Faculty Publications
While high resolution, regularly gridded observations are generally preferred in remote sensing, actual observations are often not evenly sampled and have lower-than-desired resolution. Hence, there is an interest in resolution enhancement and image reconstruction. This paper discusses a general theory and techniques for image reconstruction and creating enhanced resolution images from irregularly sampled data. Using irregular sampling theory, we consider how the frequency content in aperture function-attenuated sidelobes can be recovered from oversampled data using reconstruction techniques, thus taking advantage of the high frequency content of measurements made with nonideal aperture filters. We show that with minor modification, the algebraic …
An Iterative Approach To Multisensor Sea Ice Classification, David G. Long, Mark R. Drinkwater, Quinn P. Remund
An Iterative Approach To Multisensor Sea Ice Classification, David G. Long, Mark R. Drinkwater, Quinn P. Remund
Faculty Publications
Characterizing the variability in sea ice in the polar regions is fundamental to an understanding of global climate and the geophysical processes governing climate changes. Sea ice can be grouped into a number of general classes with different characteristics. Multisensor data from NSCAT, ERS-2, and SSM/I are reconstructed into enhanced resolution imagery for use in ice-type classification. The resulting twelve-dimensional data set is linearly transformed through principal component analysis to reduce data dimensionality and noise levels. An iterative statistical data segmentation algorithm is developed using maximum likelihood (ML) and maximum a posteriori (MAP) techniques. For a given ice type, the …
Azimuth Variation In Microwave Scatterometer And Radiometer Data Over Antarctica, David G. Long, Mark R. Drinkwater
Azimuth Variation In Microwave Scatterometer And Radiometer Data Over Antarctica, David G. Long, Mark R. Drinkwater
Faculty Publications
While designed for ocean observation, scatterometer and radiometer data have proven very useful in a variety of cryosphere studies. Over large regions of Antarctica, ice sheet and bedrock topography and the snow deposition, drift, and erosional environment combine to produce roughness on various scales. Roughness ranges from broad, basin-scale ice-sheet topography at 100 km wavelengths to large, spatially coherent dune fields at 10 km wavelength to erosional features on the meter scale known as sastrugi. These roughness scales influence the microwave backscattering and emission properties of the surface, combining to introduce azimuth-angle dependencies in the satellite observation data. In this …
Improved Resolution Backscatter Measurements With The Seawinds Pencil-Beam Scatterometer, David G. Long, Michael W. Spencer, Chialin T. Wu
Improved Resolution Backscatter Measurements With The Seawinds Pencil-Beam Scatterometer, David G. Long, Michael W. Spencer, Chialin T. Wu
Faculty Publications
The SeaWinds scatterometer was launched on the NASA QuikSCAT spacecraft in June 1999 and is planned for the Japanese ADEOS-II mission in 2000. In addition to generating a global Ku-band backscatter data set useful for a variety of climate studies, these flights will provide ocean-surface wind estimates for use in operational weather forecasting. SeaWinds employs a compact "pencil-beam" design rather than the "fan-beam" approach previously used with SASS on Seasat, NSCAT on ADEOS-I, and the AMI scatterometer on ERS-1, 2. As originally envisioned and reported, the resolution of the SeaWinds backscatter measurements were to be antenna-beamwidth limited. In order to …
Accuracy Of Scatterometer-Derived Winds Using The Cramer-Rao Bound, David G. Long, Travis E. Oliphant
Accuracy Of Scatterometer-Derived Winds Using The Cramer-Rao Bound, David G. Long, Travis E. Oliphant
Faculty Publications
A wind scatterometer makes measurements of the normalized radar-backscatter coefficient O of the ocean surface. To retrieve the wind, a geophysical model function (GMF), which relates O to the near-surface wind, is used. The wind vector can be estimated using maximum-likelihood techniques from several O measurements made at different azimuth angles. The probability density of the measured O is assumed to be Gaussian with a variance that depends on the true O and therefore, depends on the wind through the GMF. With this model for wind estimation, the Cramer-Rao (C-R) bound is derived for wind estimation, and its implications for …
Postlaunch Sensor Verification And Calibration Of The Nasa Scatterometer, David G. Long, Wu-Yang Tsai, James E. Graf, Carroll Winn, James N. Huddleston, R. Scott Dunbar, Michael H. Freilich, Frank J. Wentz, W. Linwood Jones
Postlaunch Sensor Verification And Calibration Of The Nasa Scatterometer, David G. Long, Wu-Yang Tsai, James E. Graf, Carroll Winn, James N. Huddleston, R. Scott Dunbar, Michael H. Freilich, Frank J. Wentz, W. Linwood Jones
Faculty Publications
Scatterometer instruments are active microwave sensors that transmit a series of microwave pulses and measure the returned echo power to determine the normalized radar backscattering cross section (sigma-0) of the ocean surface from which the speed and direction of near-surface ocean winds are derived. The NASA Scatterometer (NSCAT) was launched on board the ADEOS spacecraft in August 1996 and returned ten months of high-quality data before the failure of the ADEOS spacecraft terminated the data stream in June 1997. The purpose of this paper is to provide an overview of the NSCAT instrument and sigma-0 computation and to describe the …
Cryosphere Applications Of Nscat Data, David G. Long, Mark R. Drinkwater
Cryosphere Applications Of Nscat Data, David G. Long, Mark R. Drinkwater
Faculty Publications
Though designed to measure vector winds over the ocean, new imaging techniques facilitate the use of NASA scatterometer data (NSCAT) in cryosphere studies. NSCAT provides data of unprecedented coverage, resolution, and quality which, when coupled with the scatterometer image reconstruction with filtering (SIRF) algorithm, enables images of O at resolutions approaching 8 km over stationary targets. Such images are useful in ice mapping and classification, and multidecadal studies are possible by comparison with Seasat Scatterometer (SASS) data. The utility of NSCAT data in polar ice studies is illustrated through a review of two cryosphere applications of NSCAT data: (1) sea-ice …
Azimuthal Modulation Of C-Band Scatterometer Over Southern Ocean Sea Ice, David G. Long, David S. Early
Azimuthal Modulation Of C-Band Scatterometer Over Southern Ocean Sea Ice, David G. Long, David S. Early
Faculty Publications
In a continuing evaluation of the ERS-1 C-band scatterometer as a tool for studying polar sea ice, the authors evaluate the azimuthal modulation characteristics of Antarctic sea ice. ERS-1 AMI scatterometer mode data sets from several study regions dispersed in the Antarctic seasonal sea ice pack are evaluated for azimuthal modulation. When appropriate, the incidence angle dependence is estimated and removed in a study region before determining whether azimuthal modulation is present in the data. Other comparisons are made using the fore and aft beam measurement difference. The results show that over the ice pack, azimuthal modulation is less than …
Tradeoffs In The Design Of A Spaceborne Scanning Pencil Beam Scatterometer: Application To Seawinds, David G. Long, Chialin T. Wu, Michael W. Spencer
Tradeoffs In The Design Of A Spaceborne Scanning Pencil Beam Scatterometer: Application To Seawinds, David G. Long, Chialin T. Wu, Michael W. Spencer
Faculty Publications
SeaWinds is a spaceborne wind scatterometer to be flown on the second Japanese Advanced Earth Observing Satellite (ADEOS-II) in 1999. An important international element of NASA's Earth Observing System (EOS), SeaWinds is an advanced follow-on to the NASA scatterometer (NSCAT) on the first ADEOS platform. Unlike previous operational spaceborne scatterometer systems. SeaWinds employs a scanning "pencil-beam" antenna rather than a "fan-beam" antenna, making the instrument more compact and yielding greater ocean coverage. The goals of this paper are twofold. First, the overall SeaWinds functional design and backscatter measurement approach are described, and the relative advantages of the pencil-beam technique are …
Radar Backscatter Measurement Accuracy For A Spaceborne Pencil-Beam Wind Scatterometer With Transmit Modulation, David G. Long, Michael W. Spencer
Radar Backscatter Measurement Accuracy For A Spaceborne Pencil-Beam Wind Scatterometer With Transmit Modulation, David G. Long, Michael W. Spencer
Faculty Publications
Scatterometers are remote sensing radars designed to measure near-surface winds over the ocean. The difficulties of accommodating traditional fan-beam scatterometers on spacecraft has lead to the development of a scanning pencil-beam instrument known as SeaWinds. SeaWinds will be part of the Japanese Advanced Earth Observing Satellite II (ADEOS-II) to be launched in 1999. To analyze the performance of the SeaWinds design, a new expression for the measurement accuracy of a pencil-beam system is required. In this paper the authors derive a general expression for the backscatter measurement accuracy for a pencil-beam scatterometer which includes the effects of transmit signal modulation …
Calibration Of Spaceborne Scatterometers Using Tropical Rain Forests, David G. Long, Gary B. Skouson
Calibration Of Spaceborne Scatterometers Using Tropical Rain Forests, David G. Long, Gary B. Skouson
Faculty Publications
Wind scatterometers are radar systems designed specifically to measure the normalized radar backscatter coefficient (O) of the ocean's surface in order to determine the near-surface wind vector. Postlaunch calibration of a wind scatterometer can be performed with an extended-area natural target such as the Amazon tropical rain forest. Rain forests exhibit a remarkably high degree of homogeneity in their radar response over a very large area though some spatial and temporal variability exist. The authors present a simple technique for calibrating scatterometer data using tropical rain forests, Using a polynomial model for the rolloff of O with incidence angle, the …
Vegetation Studies Of The Amazon Basin Using Enhanced Resolution Seasat Scatterometer Data, David G. Long, Perry J. Hardin, Peter T. Whiting
Vegetation Studies Of The Amazon Basin Using Enhanced Resolution Seasat Scatterometer Data, David G. Long, Perry J. Hardin, Peter T. Whiting
Faculty Publications
The Seasat-A scatterometer (SASS) was designed to measure the near-surface wind field over the ocean by inferring the wind from measurements of the surface radar backscatter. While backscatter measurements were also made over land, they have been primarily used for the calibration of the instrument. This has been due in part to the low resolution of the scatterometer measurements (nominally 50 km). In a separate paper the present authors introduced a new method for generating enhanced resolution radar measurements of the Earth's surface using spaceborne scatterometry. In the present paper, the method is used with SASS data to study vegetation …
Resolution Enhancement Of Spaceborne Scatterometer Data, David G. Long, Perry J. Hardin, Peter T. Whiting
Resolution Enhancement Of Spaceborne Scatterometer Data, David G. Long, Perry J. Hardin, Peter T. Whiting
Faculty Publications
A method for generating enhanced resolution radar images of the Earth's surface using spaceborne scatterometry is presented. The technique is based on an image reconstruction technique that takes advantage of the spatial overlap in scatterometer measurements made at different times to provide enhanced imaging resolution. The reconstruction algorithm is described, and the technique is demonstrated using both simulated and actual Seasat-A Scatterometer (SASS) measurements. The technique can also be used with ERS-1 scatterometer data. The SASS-derived images, which have approximately 4-km resolution, illustrate the resolution enhancement capability of the technique, which permits utilization of both historic and contemporary scatterometer data …
Spaceborne Radar Measurement Of Wind Velocity Over The Ocean-An Overview Of The Nscat Scatterometer System, David G. Long, F. M. Naderi, Michael H. Freilich
Spaceborne Radar Measurement Of Wind Velocity Over The Ocean-An Overview Of The Nscat Scatterometer System, David G. Long, F. M. Naderi, Michael H. Freilich
Faculty Publications
Scatterometry and scatterometer design issues are reviewed. The design of the NASA Scatterometer (NSCAT) to be flown on the Japanese ADEOS mission is presented. Building on Seasat experience, the NSCAT system includes several enhancements, such as three antenna azimuths in each of two swaths, and an onboard digital Doppler processor to allow backscatter measurements to be colocated everywhere within the orbit. These enhancements will greatly increase the quality of the NSCAT wind data. The ground processing of data is discussed, and scatterometers of the next decade are briefly described.
Identifiability In Wind Estimation From Scatterometer Measurements, David G. Long, Jerry M. Mendel
Identifiability In Wind Estimation From Scatterometer Measurements, David G. Long, Jerry M. Mendel
Faculty Publications
The problem of identifiability of a wind vector that is estimated from wind scatterometer measurements of the radar backscatter of the ocean's surface is addressed. The traditional wind estimation approach produces multiple estimates of the wind direction. A second processing step, known as dealiasing or ambiguity removal, is used to select a single wind estimate from these multiple solutions. Dealiasing is typically based on various ad hoc considerations. The traditional wind estimation approach results in multiple solutions associated with local minima in an objective function formed from the noisy backscatter measurements. The authors discuss the question of the uniqueness of …
A Median-Filter-Based Ambiguity Removal Algorithm For Nscat, David G. Long, Scott J. Shaffer, R. Scott Dunbar, S. Vincent Hsiao
A Median-Filter-Based Ambiguity Removal Algorithm For Nscat, David G. Long, Scott J. Shaffer, R. Scott Dunbar, S. Vincent Hsiao
Faculty Publications
A description is given of the baseline NSCAT (the NASA scatterometer) ambiguity removal algorithm and the method used to select the set of optimum parameter values. An extensive simulation of the NSCAT instrument and ground data processor provides a means of testing the resulting tuned algorithm. This simulation generates the ambiguous wind-field vectors expected from the instrument as it orbits over a set of realistic mesoscale wind fields. The ambiguous wind field is then de-aliased using the median-filter-based ambiguity removal algorithm. Performance is measured by comparison of the selected wind fields with the true wind fields. Results have shown that …