Engineering topics

Vesecky, John F.

Publications and source records attributed to Vesecky, John F..

14 records

NASA NTRS1992

New approaches in interferometric SAR data processing

It is well established that interferometric synthetic aperture radar (SAR) images can be inverted to perform surface elevation mapping. Among the factors critical to the mapping accuracy are registration of the interfering SAR images and phase unwrapping. A novel registration algorithm is presented that determines the registration parameters through optimization. A new figure of merit is proposed that evaluates the registration result during the optimization. The phase unwrapping problem is approached through a new method involving fringe line detection. The algorithms are tested with two SEASAT SAR images of terrain near Yellowstone National Park. These images were collected on Seasat orbits 1334 and 1420, which were very close together in space, i.e., less than 100 m. The resultant elevation map is compared with the USGS digital terrain elevation model.

Lin, Qian↗

NASA NTRS1992

Robustness of Spann-Wilson segmentation on SAR imagery

The performances of the Spann-Wilson algorithm on simulated synthetic aperture radar (SAR) images with varying degrees of speckle (one to four looks and varying amounts of white noise) is described. One hundred forty-eight test images are considered, of which the algorithm segmented most without any adjustment to the algorithm's parameters. The effect of speckle on fractal boundaries is studied. The effect of varying multiplicative and additive noise distributions for a fixed set of segmentation parameters is examined. The modified Spann-Wilson algorithm on four-look imagery is evaluated.

Daida, Jason M.↗

NASA NTRS1992

Registration of interferometric SAR images

Interferometric synthetic aperture radar (INSAR) is a new way of performing topography mapping. Among the factors critical to mapping accuracy is the registration of the complex SAR images from repeated orbits. A new algorithm for registering interferometric SAR images is presented. A new figure of merit, the average fluctuation function of the phase difference image, is proposed to evaluate the fringe pattern quality. The process of adjusting the registration parameters according to the fringe pattern quality is optimized through a downhill simplex minimization algorithm. The results of applying the proposed algorithm to register two pairs of Seasat SAR images with a short baseline (75 m) and a long baseline (500 m) are shown. It is found that the average fluctuation function is a very stable measure of fringe pattern quality allowing very accurate registration.

Lin, Qian↗

NASA NTRS1992

Diffraction from sharply peaked waves as an ocean surface scattering mechanism

The role of sharply peaked waves as a major ocean scattering mechanism for radar is investigated. A prototype three-dimensional wedgelike wave shape was constructed, and its scattering properties were analyzed. Using results from the theory of the statistical geometry of the ocean surface, it is estimated how many such wedges there are per unit area, as a function of sea conditions. Taking into account a directional distribution of the wedges, the total radar cross section due to wedge diffraction effects is estimated. At large incidence angles, wedge diffraction appears to account for a significant amount of the radar cross section on the ocean surface. The wedgelike wave shape used is a more realistic representation of sharplypeaked waves. The scale-size and spatial density of the wedgelike waves are computed directly from the wave-height spectrum.

Jensen, Glenn A.↗

NASA NTRS1991

Mathematical morphology for automated analysis of remotely sensed objects in radar images

A symbiosis of pyramidal segmentation and morphological transmission is described. The pyramidal segmentation portion of the symbiosis has resulted in low (2.6 percent) misclassification error rate for a one-look simulation. Other simulations indicate lower error rates (1.8 percent for a four-look image). The morphological transformation portion has resulted in meaningful partitions with a minimal loss of fractal boundary information. An unpublished version of Thicken, suitable for watersheds transformations of fractal objects, is also presented. It is demonstrated that the proposed symbiosis works with SAR (synthetic aperture radar) images: in this case, a four-look Seasat image of sea ice. It is concluded that the symbiotic forms of both segmentation and morphological transformation seem well suited for unsupervised geophysical analysis.

Daida, Jason M.↗

NASA NTRS1991

A finite mixtures algorithm for finding classes in images

The authors describe an algorithm for estimating the proportions of classes in a SAR (synthetic aperture radar) image without any user interaction. The method assumes that the image is a mixture of a known number of different pixel types. A maximum likelihood estimate of the parameters of the resulting mixture distribution is then used to find the proportions for the various classes. The technique was successfully applied to aircraft SAR images of sea ice. Computer simulations were used to determine the relative errors of the technique. The technique performs well even with extremely noisy images.

Samadani, Ramin↗

NASA NTRS1991

Topography estimation with interferometric synthetic aperture radar using fringe detection

Methods are presented for using Synthetic Aperture Radar (SAR) interferometry data to estimate surface topography. An expression is given to relate the elevation of a ground point to the phase difference of SAR images received from two spatially separated antennas. An iterative algorithm which solves for the position and elevation of each point in the image simultaneously is developed. One of the critical issues that determines the accuracy of the terrain mapping is the phase unwrapping. An approach to the problem by fringe line detection is proposed. The algorithms are tested with two Seasat SAR images of terrain near Yellowstone National Park. The resultant elevation map is compared with a USGS terrain elevation model. The error of the SAR elevation with respect to the digital terrain map is about 8.2 percent of the total terrain variation.

Lin, Qian↗

NASA NTRS1990

Object-oriented feature-tracking algorithms for SAR images of the marginal ice zone

An unsupervised method that chooses and applies the most appropriate tracking algorithm from among different sea-ice tracking algorithms is reported. In contrast to current unsupervised methods, this method chooses and applies an algorithm by partially examining a sequential image pair to draw inferences about what was examined. Based on these inferences the reported method subsequently chooses which algorithm to apply to specific areas of the image pair where that algorithm should work best.

Daida, Jason↗

NASA NTRS1990

Finding curvilinear features in speckled images

A method for finding curves in digital images with speckle noise is described. The solution method differs from standard linear convolutions followed by thresholds in that it explicitly allows curvature in the features. Maximum a posteriori (MAP) estimation is used, together with statistical models for the speckle noise and for the curve-generation process, to find the most probable estimate of the feature, given the image data. The estimation process is first described in general terms. Then, incorporation of the specific neighborhood system and a multiplicative noise model for speckle allows derivation of the solution, using dynamic programming, of the estimation problem. The detection of curvilinear features is considered separately. The detection results allow the determination of the minimal size of detectable feature. Finally, the estimation of linear features, followed by a detection step, is shown for computer-simulated images and for a SAR image of sea ice.

Samadani, Ramin↗

NASA NTRS1990

Extraction of lead and ridge characteristics from SAR images of sea ice

Image-processing techniques for extracting the characteristics of lead and pressure ridge features in SAR images of sea ice are reported. The methods are applied to a SAR image of the Beaufort Sea collected from the Seasat satellite on October 3, 1978. Estimates of lead and ridge statistics are made, e.g., lead and ridge density (number of lead or ridge pixels per unit area of image) and the distribution of lead area and orientation as well as ridge length and orientation. The information derived is useful in both ice science and polar operations for such applications as albedo and heat and momentum transfer estimates, as well as ship routing and offshore engineering.

Vesecky, John F.↗

NASA NTRS1990

A numerical study of the separation wavenumber in the two-scale scattering approximation

The choice of the separation wavenumber Kd for the two-scale scattering approximation is examined by the comparison of two-scale calculations with exact numerical solutions using the moment method. It was found that the optimal choice of Kd is near k/2 when attempting to optimize for all incidence angles. When optimizing for specific incidence angles, Kd varied from k/5 at low incidence angles to just below k at higher incidence angles. These numerical results are in rough agreement with previous theoretical derivations of the separation wavenumber.

Durden, Stephen L.↗

NASA NTRS1989

On the ability of rough surface scattering approximations to predict hydrodynamic modulation of the ocean radar cross section - A numerical study

Scattering from the ocean surface at microwave frequencies is often described by the Bragg scattering model or the two-scale model. Here, the ability of these models to predict cross section modulation is investigated by comparing Bragg and two-scale cross-sections with cross sections calculated by a numerical electromagnetic approach. The numerical method is used as the standard of comparison. For slightly rough surfaces, the Bragg model is found to be very accurate, but for much rougher surfaces, the two-scale model is superior and should be used for calculating both the absolute cross section and the change in cross section due to spatial or temporal modulation of the ocean wave height spectrum. These results are relevant to the interpretation of SAR images of ocean surface phenomena which perturb the wave height spectrum at ripple wavelengths.

Durden, Stephen L.↗

NASA NTRS1988

Observation of sea-ice dynamics using synthetic aperture radar images: Automated analysis

The European Space Agency's ERS-1 satellite, as well as others planned to follow, is expected to carry synthetic-aperture radars (SARs) over the polar regions beginning in 1989. A key component in utilization of these SAR data is an automated scheme for extracting the sea-ice velocity field from a time sequence of SAR images of the same geographical region. Two techniques for automated sea-ice tracking, image pyramid area correlation (hierarchical correlation) and feature tracking, are described. Each technique is applied to a pair of Seasat SAR sea-ice images. The results compare well with each other and with manually tracked estimates of the ice velocity. The advantages and disadvantages of these automated methods are pointed out. Using these ice velocity field estimates it is possible to construct one sea-ice image from the other member of the pair. Comparing the reconstructed image with the observed image, errors in the estimated velocity field can be recognized and a useful probable error display created automatically to accompany ice velocity estimates. It is suggested that this error display may be useful in segmenting the sea ice observed into regions that move as rigid plates of significant ice velocity shear and distortion.

Vesecky, John F.↗

NASA NTRS1986

A conceptual design for a microwave active phased array SAR

The active phased array radar is well known for its assets of rapid electronic beam steering and adaptability to limited degrading influences. In a spaceborne synthetic aperture radar (SAR) application, therefore, it should be possible to provide for rapid beam control in the cross-track (variable incidence angle) and along-track (spot-light mode) directions, as well as allowing for possible relaxation of antenna structural requirements. A design study has been conducted at Stanford University for a conceptual spaceborne C-band SAR application operating in the early to mid-1990's time frame. This study was intended to identify principal design issues, as well as unique features associated with an active phased array antenna SAR. A hypothetical design is used to illustrate these issues. Some of this work should have generic application to space-based communications activities.

Gilchrist, Brian E.↗