Arthur W. Mansfield

Synthetic aperture radar processing system for search and rescue

Synthetic aperture radar (SAR) is uniquely suited to help solve the search and rescue problem since it can be utilized either day or night and through both dense fog or thick cloud cover. This paper describes the search and rescue data processing system (SARDPS) developed at Goddard Space Flight Center. SARDPS was developed for the Search and Rescue Mission Office in order to conduct research, development, and technology demonstration of SAR to quickly locate small aircraft which have crashed in remote areas. In order to effectively apply SAR to the detection of crashed aircraft several technical challenges needed to be overcome. These include full resolution SAR image formation using low frequency radar appropriate for foliage penetration, the application of autofocusing for SAR motion compensation in the processing system, and the development of sophisticated candidate crash site detection algorithms. In addition, the need to dispatch rescue teams to specific locations requires precise SAR image georectification and map registration techniques. The final end-to-end processing system allows for raw SAR phase history data to be quickly converted to georeferenced map/image products with candidate crash site locations identified.

SAR coherent change detection (CCD) for search and rescue

Recent advances in the areas of phase history processing, interferometry, and radargrammetric adjustment have made possible extremely accurate information extraction from synthetic aperture radar (SAR) image pairs by means of interferometric techniques. The potential gain in accuracy is significant since measurements can theoretically be determined to within a fraction of a wavelength (subcentimeter accuracy) as opposed to a fraction of pixel distance (meter accuracy). One promising application of interferometric SAR (IFSAR) is the use of coherent change detection (CCD) over large areas to locate downed aircraft. This application poses an additional challenge since IFSAR must be processed at longer wavelengths to achieve foliage penetration. In this paper a combination of advanced techniques is described for using airborne SAR imagery to carry out this mission. Performance parameters are derived, and some examples are given from actual data.

Wavenumber shift in search and rescue synthetic aperture radar

The wavenumber shift is an important tool in multiple pass synthetic aperture radar interferometry. In addition to overcoming baseline decorrelation, it has proven to have additional benefits. Chief among these is the ability to filter out much of the decorrelated signal, leaving the coherent portion. In the presence of foliage induced temporal decorrelation, this corresponds to filtering out much of the foliage return while strengthening any coherent ground return. We will examine this and other benefits of the wavenumber shift within the context of the Search and Rescue SAR program. An example based on ERS 1/2 data is provided.

Full-resolution interferometric SAR processing

In this paper we present our approach to full resolution IFSAR processing that retains full azimuth and range resolution for a significant fraction of the pixels. This is accomplished through the use of statistical analysis and nonlinear smoothing in conjunction with limited spectral extrapolation. In our approach, a significant fraction of the pixels retain their original phase values all of the way through the processing. At the same time, we are able to reduce the number of residues by several orders of magnitude. An additional benefit of this approach is the ability to detect and discriminate between complete decorrelation due to large bodies of water and partial decorrelation due to foliage. The approach will be presented along with examples based on ERS tandem pair data.

Interferometric SAR to EO image registration problem

Historically, SAR to EO registration accuracy has been at the multiple pixel level compared to sub-pixel EO to EO registration accuracies. This is due to a variety of factors including the different scattering characteristics of the ground for EO and SAR, SAR speckle, and terrain induced geometric distortion. One approach to improving the SAR to EO registration accuracy is to utilize the full information from multiple SAR surveys using interferometric techniques. In this paper we will examine this problem in detail with an example using ERS SAR imagery. Estimates of the resulting accuracy based on ERS are included.

IFSAR phase unwrapping in foliage and extreme terrain

Recent advances in the areas of phase history processing, interferometric SAR (IFSAR) processing algorithms, and radargrammetric adjustment have made it possible to extract extremely accurate Digital Elevation Model (DEM) information from SAR images. Results of tests using recent improvements by the authors in the phase unwrapping and interferogram conditioning steps show that it might be possible to obtain good elevation accuracy from noisy interferograms resulting from foliage or extreme terrain. Results of ERS-1/ERS-2 Tandem data are presented.

Real-time SAR processing for search and rescue

The most important parameter in Search and Rescue is the time it takes to locate the downed aircraft and rescue the survivors. The resulting requirement for wide-area coverage, fine resolution, and day-night all-weather operation dictates the use of a SAR sensor. The time urgency dictates a real-time or near real-time SAR processor. This paper presents alternative real-time architectures and gives the results of feasibility studies of the enabling technologies, including new work by the authors in the area of SAR data compression.

IFSAR reductions from ERS-1,/2 tandem data

Recent advantages in the areas of phase history processing, interferometric synthetic aperture radar processing algorithms, and the use of photogrammetric techniques have made it possible to extract extremely accurate DEM generation from Synthetic Aperture Radar images. Recent improvements by the authors in the phase unwrapping and interferogram conditioning steps are described which make it possible to obtain good elevation accuracy from noisy interferograms resulting from temporal decorrelation due to foliage or extreme terrain. Results are shown of data reductions from separate passes of the ERS-1,/2 Tandem System over Ft. Irwin, California, and Aschaffenburg, Germany.

IFSAR phase unwrapping in the presence of Dirichlet boundary conditions

Phase unwrapping is one of the key computational elements in digital elevation model generation from interferometric SAR. In this paper we present a reformulation of the weighted least squares phase unwrapping approach that incorporates Dirichlet boundary conditions. The application of this formulation to the incorporation of control points into the solution as well as for unwrapping the interferogram in stages is discussed. The ability of the weighted least squares approach to fully unwrap an interferogram can be very dependent on the weight matrix used. This has led us to develop an adaptive approach to updating the weight matrix to be used in conjunction with our weighted least squares approach. Examples along the preliminary results based on ERS data will be presented.

L-band/P-band SAR comparison for search and rescue

A key question in SAR-aided search is the relative utility of L-Band versus P-Band data. A study has been undertaken using target data collected by the NASA Search and Rescue Mission. Comparisons are made based on the ability to detect downed aircraft by use of several polarimetry-based automatic detection techniques developed by the NASA Search and Rescue Mission. Results obtained so far from this study are presented in the paper.