High-Resolution Synthetic Aperture Radar Interferometry Validation Using Corner Reflectors, 20-R9791Printer Friendly Version
Inclusive Dates: 02/27/08 06/27/08
Background - Interferometric Synthetic Aperture Radar (InSAR) is a general methodology for measuring ground movements based on phase shift of the signal received from radar sensors mounted on ground, aerial or satellite platforms. Artificial corner reflectors are engineered objects designed to enhance the radar signal return to the source when natural reflectors are not available in an area. This project explored the possibility of developing a radar interferometry (InSAR) method using satellite images and artificial permanent scatterers (i.e., corner reflectors or CR) that can measure ground displacement with a resolution less than 1 cm.
Approach - Different CR sizes and geometries may be applicable to a variety of natural environments and applications dealing with displacement monitoring of natural features and engineered structures (i.e., reservoirs, dams, bridges, pipelines and buildings). Several CRs in two different geometries were designed and placed at locations on SwRI grounds, an ideal environment for such tests because of small to no rate of ground movement, existing infrastructure to mount the CR for easy access, and a vegetated environment. Controlled vertical movement was induced on two CRs by raising the structures using 3/8-inch (0.9-cm) thick aluminum plates.
ENVISAT ASAR C-Band satellite data were used in the InSAR analysis. The analysis of SAR imagery included (1) detection of CR response on SAR amplitude channel and (2) line-of-sight (LOS) displacement extraction from a radar interferogram produced by a pair of SAR images. The method was validated by comparing satellite-based measurements of "fixed" CRs as well as calculating displacement for CRs that were raised a controlled distance.
Accomplishments - SwRI successfully designed artificial CRs and performed related InSAR analyses of ground deformation. A strong reflection is required from CRs so the natural changes in the environment within the same image cell as the reflector will not significantly affect the phase measurement. Tested CRs enabled the phase changes caused by the artificial "ground motion" to be quantified. Their strong reflection suggested that displacements down to a few millimeters could be detected over short (and possibly long) time intervals. Regarding the design geometry, trihedral structures performed better than dihedral structures because of increased mechanical stability and easier positional adjustment in the field.
The InSAR displacement measurements compared well with field data, proving the potential of this method for very accurate measurements of ground movement. Project results demonstrated the applicability of the technique in detecting and assessing natural hazards, such as faults, landslides and subsidence. If enough satellite data are available over areas where such CRs will be installed, persistent scatterer interferometry applications based on PSInSAR®, Interferometric Point Target Analysis (IPTA), Coherent Target Monitoring (CTM), or Stable Point Network (SPN) could be developed to increase long-term measurement accuracies to a few cm/year.