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Design and implementation of a Synthetic Aperture Radar for Open Skies (SAROS) aboard a C-135 aircraft

Description: NATO and former Warsaw Pact nations have agreed to allow overflights of their countries in the interest of easing world tension. The United States has decided to implement two C-135 aircraft with a Synthetic Aperture Radar (SAR) that has a 3-meter resolution. This work is being sponsored by the Defense Nuclear Agency (DNA) and will be operational in Fall 1995. Since the SAR equipment must be exportable to foreign nations, a 20-year-old UPD-8 analog SAR system was selected as the front-end and refurbished for this application by Loral Defense Systems. Data processing is being upgraded to a currently exportable digital design by Sandia National Laboratories. Amplitude and phase histories will be collected during these overflights and digitized on VHS cassettes. Ground stations will use reduction algorithms to process the data and convert it to magnitude-detected images for member nations. System Planning Corporation is presently developing a portable ground station for use on the demonstration flights. Aircraft integration into the C-135 aircraft is being done by the Air Force at Wright-Patterson AFB, Ohio.
Date: August 1, 1994
Creator: Cooper, D. W.; Murphy, M. & Rimmel, G.
Partner: UNT Libraries Government Documents Department

Developments in ground-penetrating radar at LLNL

Description: Lawrence Livermore National Laboratory (LLNL) is developing a side-looking, ground-penetrating impulse radar system that will eventually be mounted on an airborne platform to locate buried minefields. Presently, the radar system is mounted on top of a 60-foot adjustable boom. Several unique as well as commercial antennas having bandwidths in the 200 to 2000 MHz range are being experimented with. Also, LLNL-developed monocycle pulse generators are tailored to be most efficient over this frequency range. A technical description of the system will be presented with details about the video pulser, the wideband antennas, the receiver hardware, and the data acquisition system. The receiver and data acquisition hardware consist of off-the-shelf components. Testing of this system is conducted on a minefield located at the Nevada Test Site (NTS). The minefield contains real and surrogate mines of various sizes placed in natural vegetation. Some areas of the minefield have been cleared for non-cluttered studies. In addition, both metal and plastic mines are buried in the minefield. There is room in the NTS minefield for burying additional objects, such as unexploded ordnance, and this is expected to be done in the future. Recent results indicate success in imaging the NTS minefield using the GPR system. The data has been processed using in-house image reconstruction software, and has been registered with the ground truth data. Images showing clearly visible mines, surface reference markers, and ground clutter will be presented.
Date: May 1, 1994
Creator: Sargis, P. D.
Partner: UNT Libraries Government Documents Department

Three-dimensional ground penetrating radar imaging using multi-frequency diffraction tomography

Description: In this paper we present results from a three-dimensional image reconstruction algorithm for impulse radar operating in monostatic pulse-echo mode. The application of interest to us is the nondestructive evaluation of civil structures such as bridge decks. We use a multi-frequency diffraction tomography imaging technique in which coherent backward propagations of the received reflected wavefield form a spatial image of the scattering interfaces within the region of interest. This imaging technique provides high-resolution range and azimuthal visualization of the subsurface region. We incorporate the ability to image in planarly layered conductive media and apply the algorithm to experimental data from an offset radar system in which the radar antenna is not directly coupled to the surface of the region. We present a rendering in three-dimensions of the resulting image data which provides high-detail visualization.
Date: July 1, 1994
Creator: Mast, J. E. & Johansson, E. M.
Partner: UNT Libraries Government Documents Department

Subsurface object position and image correction for standoff Ground Penetrating Radar

Description: Present applications of standoff (airborne) Ground Penetrating SAR (Synthetic Aperture Radar) allows objects near the surface to be detected but only provides an approximation for the actual location and image. When single media models are employed the lack of correction for the phase velocity and refractive changes at the air/soil interface result in object distortions. Positional errors and image distortions comparable to the size of the object are possible. Correction is possible, if the media properties are known, by modeling the scene as a two-layer medium and accounting for the propagation effects. The propagation parameters for the lower media are estimated in the migration of observable responses for surface and subsurface objects. This approach allows for corrected images to subsurface objects to be produced after data collection. Surface objects will be distorted as a result of this process. The modeling process, simulations, and results with field data will be discussed. An improvement by a factor of two would enable standoff radar to detect objects at depths of on meter or more benefiting Unexploded Ordnance (UXO) and hazardous waste site survey activities.
Date: May 1, 1994
Creator: Kane, R. J.
Partner: UNT Libraries Government Documents Department

Synthetic aperture radar processing with tiered subapertures

Description: Synthetic Aperture Radar (SAR) is used to form images that are maps of radar reflectivity of some scene of interest, from range soundings taken over some spatial aperture. Additionally, the range soundings are typically synthesized from a sampled frequency aperture. Efficient processing of the collected data necessitates using efficient digital signal processing techniques such as vector multiplies and fast implementations of the Discrete Fourier Transform. Inherent in image formation algorithms that use these is a trade-off between the size of the scene that can be acceptably imaged, and the resolution with which the image can be made. These limits arise from migration errors and spatially variant phase errors, and different algorithms mitigate these to varying degrees. Two fairly successful algorithms for airborne SARs are Polar Format processing, and Overlapped Subaperture (OSA) processing. This report introduces and summarizes the analysis of generalized Tiered Subaperture (TSA) techniques that are a superset of both Polar Format processing and OSA processing. It is shown how tiers of subapertures in both azimuth and range can effectively mitigate both migration errors and spatially variant phase errors to allow virtually arbitrary scene sizes, even in a dynamic motion environment.
Date: June 1, 1994
Creator: Doerry, A. W.
Partner: UNT Libraries Government Documents Department

Interferometric synthetic aperture radar terrain elevation mapping from multiple observations

Description: All prior interferometric SAR imaging experiments to date dealt with pairwise processing. Simultaneous image collections from two antenna systems or two-pass single antenna collections are processed as interferometric pairs to extract corresponding pixel by pixel phase differences which encode terrain elevation height. The phase differences are wrapped values which must be unwrapped and scaled to yield terrain height. We propose two major classes of techniques that hold promise for robust multibaseline (multiple pair) interferometric SAR terrain elevation mapping. The first builds on the capability of a recently published method for robust weighted and unweighted least-squares phase unwrapping, while the second attacks the problem directly in a maximum likelihood (ML) formulation. We will provide several examples (actual and simulated SAR imagery) that illustrate the advantages and disadvantages of each method.
Date: July 1, 1994
Creator: Ghiglia, D. C. & Wahl, D. E.
Partner: UNT Libraries Government Documents Department

Ground-penetrating radar for buried mine detection

Description: Lawrence Livermore National Laboratory (LLNL) is developing an ultra-wideband, side-looking, ground-penetrating impulse radar system that can be mounted on an airborne platform for the purpose of locating buried mines. The radar system is presently mounted on an 18-meter boom. The authors have successfully imaged a minefield located at the Nevada Test Site. The minefield consists of real and surrogate mines of various materials and sizes placed in natural vegetation. Some areas have been cleared for non-cluttered studies. A technical description of the system is presented, describing the wideband antennas, the video pulser, the receiver hardware, and the data acquisition system. The receiver and data acquisition hardware are off-the-shelf components. The data was processed using LLNL-developed image reconstruction software, and has been registered against the ground truth data. Images showing clearly visible mines, surface reference markers, and ground clutter are presented.
Date: April 1, 1994
Creator: Sargis, P. D.; Lee, F. D.; Fulkerson, E. S.; McKinley, B. J. & Aimonetti, W. D.
Partner: UNT Libraries Government Documents Department

Imaging targets embedded in a lossy half space with Synthetic Aperture Radar

Description: This paper addresses theoretical aspects of forming images from an airborne Synthetic Aperture Radar (SAR) of targets buried below the earth`s surface. Soil is generally a lossy, dispersive medium, with wide ranging variability in these attributes depending on soil type, moisture content, and a host of other physical properties. Focussing a SAR subsurface image presents new dimensions of complexity relative to its surface-image counterpart, even when the soil`s properties are known. This paper treats the soil as a lossy, dispersive half space, and presents a practical model for the radar echo-delay time to point scatterers within it. This model is then used to illustrate effects of refraction, dispersion, and attenuation on a SAR`s phase histories, and the resulting image. Various data collection geometries and processing strategies are examined for both 2-Dimensional and 3-Dimensional SAR images. The conclusions from this work are that (1) focussing a SAR image must generally take into account both refraction and dispersion, (2) resolving targets at different depths in lossy soils requires perhaps unprecedented sidelobe attenuation, that for some soils may only be achievable with specialized window functions, (3) the impulse response of the soil itself places a practical limit on the usable bandwidth of the radar, and (4) dynamic ranges and sensitivities will need to be orders of magnitude greater than typical surface-imaging SARs, leading to significant impact on SAR parameters, for example compressing the usable range of pulse repetition frequencies (PRFs).
Date: May 1, 1994
Creator: Doerry, A. W.; Brock, B. C.; Boverie, B. & Cress, D.
Partner: UNT Libraries Government Documents Department

New formulation for interferometric synthetic aperture radar for terrain mapping

Description: The subject of interferometric synthetic aperture radar (IFSAR) for high-accuracy terrain elevation mapping continues to gain importance in the arena of radar signal processing. Applications to problems in precision terrain-aided guidance and automatic target recognition, as well as a variety of civil applications, are being studied by a number of researchers. Not unlike many other areas of SAR processing, the subject of IFSAR can at first glance appear to be somewhat mysterious. In this paper we show how the mathematics of IFSAR for terrain elevation mapping using a pair of spotlight mode SAR collections can be derived in a very straightforward manner. Here, we employ an approach that relies entirely on three-dimensional Fourier transforms, and utilizes no reference to range equations or Doppler concepts. The result is a simplified explanation of the fundamentals of interferometry, including an easily-seen link between image domain phase difference and terrain elevation height. The derivation builds upon previous work by the authors in which a framework for spotlight mode SAR image formation based on an analogy to three-dimensional computerized axial tomography (CAT) was developed. After outlining the major steps in the mathematics, we show how a computer simulator which utilizes three-dimensional Fourier transforms can be constructed that demonstrates all of the major aspects of IFSAR from spotlight mode collections.
Date: April 1, 1994
Creator: Jakowatz, C. V. Jr.; Wahl, D. E.; Eichel, P. H. & Thompson, P. A.
Partner: UNT Libraries Government Documents Department

New approach to strip-map SAR autofocus

Description: Means for removing phase errors induced in spotlight mode synthetic aperture radar (SAR) imagery are now well-established. The Phase Gradient Autofocus (PGA) algorithm has been shown to be robust over a wide range of spotlight mode imagery and phase error functions. These phase errors could have their origin either in uncompensated platform motion or random propagation delays incurred, for example, from tropospheric turbulence. The PGA technique, however, cannot be directly applied to imagery formed in the conventional strip-mapping mode. In this paper we show that if the fundamental ideas of PGA are modified in an appropriate way, the phase errors in strip-map imagery can be effectively estimated and compensated.
Date: May 1, 1994
Creator: Wahl, D.; Jakowatz, C.; Thompson, P. & Ghiglia, D.
Partner: UNT Libraries Government Documents Department

A model for forming airborne synthetic aperture radar images of underground targets

Description: Synthetic Aperture Radar (SAR) from an airborne platform has been proposed for imaging targets beneath the earth`s surface. The propagation of the radar`s energy within the ground, however, is much different than in the earth`s atmosphere. The result is signal refraction, echo delay, propagation losses, dispersion, and volumetric scattering. These all combine to make SAR image formation from an airborne platform much more challenging than a surface imaging counterpart. This report treats the ground as a lossy dispersive half-space, and presents a model for the radar echo based on measurable parameters. The model is then used to explore various imaging schemes, and image properties. Dynamic range is discussed, as is the impact of loss on dynamic range. Modified window functions are proposed to mitigate effects of sidelobes of shallow targets overwhelming deeper targets.
Date: January 1, 1994
Creator: Doerry, A. W.
Partner: UNT Libraries Government Documents Department

Interferometric SAR phase difference calibration: Methods and results

Description: This paper addresses the steps necessary to determine and maintain the phase calibration of a two-channel interferometric synthetic aperture radar (IFSAR). The method, setup, and accuracy of four different calibration techniques are compared. The most novel technique involves pointing the interferometric baseline at nadir and imaging a lake surface. The other techniques include measuring various flat surfaces in traditional side-looking IFSAR maps, in-flight closed-loop calibration path measurements, and static laboratory measurements. Initial results indicate that, using combinations of these measurements, it is possible to maintain the interferometric phase calibration of Sandia National Laboratories` K{sub U} Band IFSAR to better than 3 degrees. The time variability of various parts of the calibration and requirements for recalibration are also discussed.
Date: December 31, 1993
Creator: Bickel, D. L. & Hensley, W. H.
Partner: UNT Libraries Government Documents Department

Radially combined 30 W, 14-16 GHz amplifier

Description: The paper describes a highly integrated 30 W power amplifier for a Synthetic Aperture Radar, operating in the 14--16 GHz band. The use of a waveguide radial combiner, a microstrip power divider and direct microstrip to waveguide miniaturized ceramic technology, leads to an unusually compact and accessible structure, well suited for commercial production.
Date: April 1, 1994
Creator: Sechi, F.; Bujatti, M.; Knudson, R. & Bugos, R.
Partner: UNT Libraries Government Documents Department

Factors governing selection of operating frequency for subsurface- imaging synthetic-aperture radar

Description: A subsurface-imaging synthetic-aperture radar (SISAR) has potential for application in areas as diverse as non-proliferation programs for nuclear weapons to environmental monitoring. However, subsurface imaging is complicated by propagation loss in the soil and surface-clutter response. Both the loss and surface-clutter response depend on the operating frequency. This paper examines several factors which provide a basis for determining optimum frequencies and frequency ranges which will allow synthetic-aperture imaging of buried targets. No distinction can be made between objects at different heights when viewed with a conventional imaging radar (which uses a one-dimensional synthetic aperture), and the return from a buried object must compete with the return from the surface clutter. Thus, the signal-to-clutter ratio is an appropriate measure of performance for a SISAR. A parameter-based modeling approach is used to model the complex dielectric constant of the soil from measured data obtained from the literature. Theoretical random-surface scattering models, based on statistical solutions to Maxwell`s equations, are used to model the clutter. These models are combined to estimate the signal-to-clutter ratio for canonical targets buried in several soil configurations. Results indicate that the HF spectrum (3--30), although it could be used to detect certain targets under some conditions, has limited practical value for use with SISAR, while the upper VIHF through UHF spectrum ({approximately}100 MHz - 1 GHz) shows the most promise for a general purpose SISAR system. Recommendations are included for additional research.
Date: December 31, 1993
Creator: Brock, B. C. & Patitz, W. E.
Partner: UNT Libraries Government Documents Department

A SAR image-formation algorithm that compensates for the spatially-variant effects of antenna motion

Description: A synthetic aperture radar (SAR) obtains azimuth resolution by combining data from a number of points along a specified path. Uncompensated antenna motion that deviates significantly from the desired path produces spatially-variant errors in the output image. The algorithm presented in this paper corrects many of these motion-related errors. In this respect, it is similar to time-domain convolution, but it is more computationally efficient. The algorithm uses overlapped subapertures in a three-step image-formation process: coarse-resolution azimuth processing, fine-resolution range processing, and fine-resolution azimuth processing. Range migration is corrected after the first stage, based on coarse azimuth position. Prior to the final azimuth-compression step, data coordinates, are determined to fine resolution in range and coarse resolution in azimuth. This coordinate information is combined with measured motion data to generate a phase correction that removes spatially-variant errors. The algorithm is well-suited for real-time applications, particularly where large flight-path deviations must be tolerated.
Date: March 1, 1994
Creator: Burns, B. L. & Cordaro, J. T.
Partner: UNT Libraries Government Documents Department

Three-dimensional subsurface imaging synthetic aperture radar

Description: The objective of this applied research and development project is to develop a system known as `3-D SISAR`. This system consists of a ground penetrating radar with software algorithms designed for the detection, location, and identification of buried objects in the underground hazardous waste environments found at DOE storage sites. Three-dimensional maps of the object locations will be produced which can assist the development of remediation strategies and the characterization of the digface during remediation operations. It is expected that the 3-D SISAR will also prove useful for monitoring hydrocarbon based contaminant migration after remediation. The underground imaging technique being developed under this contract utilizes a spotlight mode Synthetic Aperture Radar (SAR) approach which, due to its inherent stand-off capability, will permit the rapid survey of a site and achieve a high degree of productivity over large areas. When deployed from an airborne platform, the stand-off techniques is also seen as a way to overcome practical survey limitations encountered at vegetated sites.
Date: March 1, 1995
Creator: Moussally, G. J.
Partner: UNT Libraries Government Documents Department

A comparative evaluation of SAR and SLAR

Description: Synthetic aperture radar (SAR) was evaluated as a potential technological improvement over the Coast Guard`s existing side-looking airborne radar (SLAR) for oil-spill surveillance applications. The US Coast Guard Research and Development Center (R&D Center), Environmental Branch, sponsored a joint experiment including the US Coast Guard, Sandia National Laboratories, and the Naval Oceanographic and Atmospheric Administration (NOAA), Hazardous Materials Division. Radar imaging missions were flown on six days over the coastal waters off Santa Barbara, CA, where there are constant natural seeps of oil. Both the Coast Guard SLAR and the Sandia National Laboratories SAR were employed to acquire simultaneous images of oil slicks and other natural sea surface features that impact oil-spill interpretation. Surface truth and other environmental data were also recorded during the experiment. The experiment data were processed at Sandia National Laboratories and delivered to the R&D Center on a computer workstation for analysis by experiment participants. Issues such as optimal spatial resolution, single-look vs. multi-look SAR imaging, and the utility of SAR for oil-spill analysis were addressed. Finally, conceptual design requirements for a possible future Coast Guard SAR were outlined and evaluated.
Date: November 1, 1993
Creator: Mastin, G. A.; Manson, J. J.; Bradley, J. D.; Axline, R. M. & Hover, G. L.
Partner: UNT Libraries Government Documents Department

FESDIF -- Finite Element Scalar Diffraction theory code

Description: This document describes the theory and use of a powerful scalar diffraction theory based computer code for calculation of intensity fields due to diffraction of optical waves by two-dimensional planar apertures and lenses. This code is called FESDIF (Finite Element Scalar Diffraction). It is based upon both Fraunhofer and Kirchhoff scalar diffraction theories. Simplified routines for circular apertures are included. However, the real power of the code comes from its basis in finite element methods. These methods allow the diffracting aperture to be virtually any geometric shape, including the various secondary aperture obstructions present in telescope systems. Aperture functions, with virtually any phase and amplitude variations, are allowed in the aperture openings. Step change aperture functions are accommodated. The incident waves are considered to be monochromatic. Plane waves, spherical waves, or Gaussian laser beams may be incident upon the apertures. Both area and line integral transformations were developed for the finite element based diffraction transformations. There is some loss of aperture function generality in the line integral transformations which are typically many times more computationally efficient than the area integral transformations when applicable to a particular problem.
Date: September 1, 1992
Creator: Kraus, H. G.
Partner: UNT Libraries Government Documents Department

Data fusion for the detection of buried land mines

Description: The authors conducted experiments to demonstrate the enhanced delectability of buried land mines using sensor fusion techniques. Multiple sensors, including imagery, infrared imagery, and ground penetrating radar, have been used to acquire data on a number of buried mines and mine surrogates. The authors present this data along with a discussion of the application of sensor fusion techniques for this particular detection problem. The authors describe the data fusion architecture and discuss some relevant results of these classification methods.
Date: October 1, 1993
Creator: Clark, G. A.; Sengupta, S. K.; Schaich, P. C.; Sherwood, R. J.; Buhl, M. R.; Hernandez, J. E. et al.
Partner: UNT Libraries Government Documents Department

Soil-penetrating synthetic aperture radar

Description: This report summarizes the results for the first year of a two year Laboratory Directed Research and Development (LDRD) effort. This effort included a system study, preliminary data acquisition, and preliminary algorithm development. The system study determined the optimum frequency and bandwidth, surveyed soil parameters and targets, and defined radar cross section in lossy media. The data acquisition imaged buried objects with a rail-SAR. Algorithm development included a radar echo model, three-dimensional processing, sidelobe optimization, phase history data interpolation, and clutter estimation/cancellation.
Date: December 1, 1994
Creator: Boverie, B.; Brock, B. C. & Doerry, A. W.
Partner: UNT Libraries Government Documents Department

Feature discovery in gray level imagery for one-class object recognition

Description: Feature extraction transforms an object`s image representation to an alternate reduced representation. In one-class object recognition, we would like this alternate representation to give improved discrimination between the object and all possible non-objects and improved generation between different object poses. Feature selection can be time-consuming and difficult to optimize so we have investigated unsupervised neural networks for feature discovery. We first discuss an inherent limitation in competitive type neural networks for discovering features in gray level images. We then show how Sanger`s Generalized Hebbian Algorithm (GHA) removes this limitation and describe a novel GHA application for learning object features that discriminate the object from clutter. Using a specific example, we show how these features are better at distinguishing the target object from other nontarget object with Carpenter`s ART 2-A as the pattern classifier.
Date: December 31, 1993
Creator: Koch, M. W. & Moya, M. M.
Partner: UNT Libraries Government Documents Department

FESDIF -- Finite Element Scalar Diffraction theory code

Description: This document describes the theory and use of a powerful scalar diffraction theory based computer code for calculation of intensity fields due to diffraction of optical waves by two-dimensional planar apertures and lenses. This code is called FESDIF (Finite Element Scalar Diffraction). It is based upon both Fraunhofer and Kirchhoff scalar diffraction theories. Simplified routines for circular apertures are included. However, the real power of the code comes from its basis in finite element methods. These methods allow the diffracting aperture to be virtually any geometric shape, including the various secondary aperture obstructions present in telescope systems. Aperture functions, with virtually any phase and amplitude variations, are allowed in the aperture openings. Step change aperture functions are accommodated. The incident waves are considered to be monochromatic. Plane waves, spherical waves, or Gaussian laser beams may be incident upon the apertures. Both area and line integral transformations were developed for the finite element based diffraction transformations. There is some loss of aperture function generality in the line integral transformations which are typically many times more computationally efficient than the area integral transformations when applicable to a particular problem.
Date: September 1, 1992
Creator: Kraus, H.G.
Partner: UNT Libraries Government Documents Department

The single antenna interferometer

Description: Air and space borne platforms using synthetic aperture radars (SAR) have made interferometric measurements by using either two physical antennas mounted on one air-frame or two passes of one antenna over a scene. In this paper, a new interferometric technique using one pass of a single-antenna SAR system is proposed and demonstrated on data collected by the NASA-JPL AirSAR. Remotely sensed L-band microwave data are used to show the sensitivity of this technique to ocean surface features as well as a baseline for comparison with work by others using two-antenna systems. 7 refs., 3 figs.
Date: January 15, 1990
Creator: Fitch, J.P.
Partner: UNT Libraries Government Documents Department

On the detection of crevasses in glacial ice with synthetic-aperture radar.

Description: The intent of this study is to provide an analysis of the scattering from a crevasse in Antarctic ice, utilizing a physics-based model for the scattering process. Of primary interest is a crevasse covered with a snow bridge, which makes the crevasse undetectable in visible-light images. It is demonstrated that a crevasse covered with a snow bridge can be visible in synthetic-aperture-radar (SAR) images. The model of the crevasse and snow bridge incorporates a complex dielectric permittivity model for dry snow and ice that takes into account the density profile of the glacier. The surface structure is based on a fractal model that can produce sastrugi-like features found on the surface of Antarctic glaciers. Simulated phase histories, computed with the Shooting and Bouncing Ray (SBR) method, are processed into SAR images. The viability of the SBR method for predicting scattering from a crevasse covered with a snow bridge is demonstrated. Some suggestions for improving the model are given.
Date: February 1, 2010
Creator: Brock, Billy C.
Partner: UNT Libraries Government Documents Department