David A. Yocky

Image merging and data fusion by means of the discrete two-dimensional wavelet transform

A new technique is developed for the merging and data fusion of two images. Two spatially registered images with differing spatial resolutions and color content are merged by combining multiresolution wavelet-decomposition components from each and then reconstructing the merged image by means of the inverse wavelet transform. The wavelet merger can employ a variety of wavelet bases, but in presentation of the concept, simple orthonormal sets—Haar and Daubechies wavelets—are explored. The wavelet technique is compared with the intensity–hue–saturation merging technique by means of multispectral and panchromatic test images. The results of the comparison show the wavelet merger performing better in combining and preserving spectral–spatial information for the test images.

ARTIFACTS IN WAVELET IMAGE MERGING

Intensity modulation (IM), intensity-hue-saturation (IHS), and multiresolution wavelet decomposition (MWD) are {ital ad} {ital hoc} image merging techniques that enhance low-spatial-resolution color imagery with high-resolution black and white imagery. While the MWD is a relatively new technique, both IM and IHS can be found in commercial software. These three merging techniques are compared to find which best retains spectral fidelity while adding spatial information. With this criterion, merging artifacts are identified and characterized. Specifically, the three techniques were tested with respect to resampling, misregistration, intensity variance, and complexity problems that arise in multiresolution image merging. Test images were used allowing the mean difference, standard deviation difference, entropy difference, and affected area to be calculated. {copyright} {ital 1996 Society of Photo{minus}Optical Instrumentation Engineers.}

An implementation of a fast backprojection image formation algorithm for spotlight-mode SAR

In this paper we describe an algorithm for fast spotlight-mode synthetic aperture radar (SAR) image formation that employs backprojection as the core, but is implemented such that its compute time is comparable to the often-used Polar Format Algorithm (PFA). (Standard backprojection is so much slower than PFA that it is impractical to use in many operational scenarios.) We demonstrate the feasibility of the algorithm on real SAR phase history data sets and show some advantages in the SAR image formed by this technique.

Terrain elevation mapping results from airborne spotlight-mode coherent cross-track SAR stereo

Coherent cross-track synthetic aperture radar (SAR) stereo is shown to produce high-resolution three-dimensional maps of the Earth surface. This mode utilizes image pairs with common synthetic apertures but different squint angles allowing automated stereo correspondence and disparity estimation using complex correlation calculations. This paper presents two Ku-band, coherent cross-track stereo collects over rolling and rugged terrain. The first collect generates a digital elevation map (DEM) with 1-m posts over rolling terrain using complex SAR imagery with spatial resolution of 0.125 m and a stereo convergence angle of 13.8/spl deg/. The second collect produces multiple DEMs with 3-m posts over rugged terrain utilizing complex SAR imagery with spatial resolutions better than 0.5 m and stereo convergence angles greater than 40/spl deg/. The resulting DEMs are compared to ground-truth DEMs and relative height root-mean-square, linear error 90-percent confidence, and maximum height error are reported.

The effect of scattering from buildings on interferometric SAR measurements

The determination of elevation models of buildings using interferometric synthetic aperture radar (IFSAR) is an important area of active research. The focus of this paper is on some of the unique scattering mechanisms that occur with buildings and how they affect the IFSAR height measurement and the coherence. The authors show by theory and examples that the various data products obtained from IFSAR can be used to aid in interpreting building height results. They also present a method that they have used successfully in mapping buildings in Washington D.C.

Comparison of algorithms for use in real-time spotlight-mode SAR image formation

This paper compares three algorithms for potential use in a real-time, on-board implementation of spotlight-mode SAR image formation. These include: the polar formatting algorithm (PFA), the range migration algorithm (RMA), and the overlapped subapertures algorithm (OSA). We conclude that for any reasonable spotlight-mode imaging scenario, PFA is easily the algorithm of choice because its computational efficiency is significantly higher than that of either RMA or OSA. This comparison specifically includes cases in which wavefront curvature is sufficient to cause image defocus in conventional PFA, because a post-processing refocus step can be performed with PFA to yield excellent image quality for only a minimal increase in computation time. We demonstrate that real-time image formation for many imaging scenarios is achievable using PFA implemented on a single Pentium M processor. OSA is quite slow compared to PFA, especially for the case of moderate to high resolution (9 inches and better). RMA is not competitive with PFA for situations that do not require wavefront curvature correction. For those cases in which PFA requires post-processing to correct for wavefront curvature, RMA comes closer in efficiency to PFA, but is still outperformed by the modified PFA.

Shift–Scale Complex Correlation for Wide-Angle Coherent Cross-Track SAR Stereo Processing

Automated synthetic aperture radar (SAR) stereo correspondence becomes increasingly difficult when imaging high-relief terrain utilizing large stereo crossing-angle geometries because high-relief SAR image features can undergo significant spatial distortions, causing a failure of traditional correlation matching. This paper presents eight coherent spotlight-mode cross-track stereo pairs with stereo crossing angles averaging 93.7deg collected over a terrain with slopes greater than 20deg. These stereo pairs suffer from terrain-induced distortions, resulting in a decrease in complex correlation (coherence) when utilizing scanning-window correlation calculations. The search to maximize complex correlation is changed from a shift-only (disparity) search to a shift-and-scale search using the downhill simplex method. This approach is tested against complex imagery with simulated distortions and then employed on the eight wide-angle stereo collects. The resulting digital terrain maps (DTMs) are compared to ground truth. Using a shift-and-scale correlation approach to estimate disparity, the relative height errors decrease, and the number of reliable DTM posts increase

Minimum-Latency Polar Format Algorithm

The polar format algorithm (PFA) is a computa- tionally efficient image formation method for high-resolution spotlight-mode SAR data collections. Yet, its usual real-time implementation waits until the entire synthetic aperture has been collected before beginning image formation. This results in a latency time. This paper presents a new approach to PFA that performs range and azimuth interpolation and range compres- sion as the aperture is collected. Thus, the remaining latency is approximately the azimuth compression time. Performing image formation during aperture collection makes minimum-latency PFA a viable real-time image formation algorithm.

Two-target height effects on interferometric synthetic aperture radar coherence

Useful products generated from interferometric synthetic aperture radar (IFSAR) complex data include height measurement, coherent change detection, and classification. The IFSAR coherence is a spatial measure of complex correlation between two collects, a product of IFSAR signal processing. A tacit assumption in such IFSAR signal processing is that the terrain height is constant across an averaging box used in the process of correlating the two images. This paper presents simulations of IFSAR coherence if two target with different heights exist in a given correlation cell, a condition in IFSAR collections produced by layover. It also includes airborne IFSAR data confirming the simulation results. The paper concludes by exploring the implications of the results on IFSAR height measurements and classification.

Interferometric SAR coherence classification utility assessment

The classification utility of a dual-antenna interferometric synthetic aperture radar (IFSAR) is explored by comparison of maximum likelihood classification results for synthetic aperture radar (SAR) intensity images and IFSAR intensity and coherence images. The addition of IFSAR coherence improves the overall classification accuracy for classes of trees, water, and fields. A threshold intensity-coherence classifier is also compared to the intensity-only classification results.