A Study of Minimal Site Conditions for Singular Corner Reflectors used as Ground Control Points for JERS-1 and ERS-2 SAR Images

Corner reflectors are suitable as ground control points when they are placed in spaces that are very large, flat, and empty. Since their reflection points are known, they can be detected and identified with relative ease in SAR images through comparisons of images before and after their installation. However, in practical applications, large flat empty spaces are not often availble near the locations necessary for a particular application. Even when these corner reflectors are placed in a flat empty lot closest to the desired location, they are not always easily identifiable in SAR images due to insufficient size or influences from surrounding areas. Using existing singular corner reflectors which were installed as ground control points for JERS-1 SAR images, we examined the minimal conditions required for empty lots to be used as installation sites, considering factors such as the number of observations, identifiability of the area surrounding the installation site, identifiability of the installation site in the images, and existence of any confounding point targets surrounding the corner reflector. We also investigated whether the same corner reflectors could be used as ground control points for the C-band ERS-2 SAR, which has an incident angle relatively similar to that of the JERS-1 SAR. We found that the most crucial condition necessary for easy detection and identification of corner reflectors in both JERS-1 SAR images and ERS-2 SAR images was that the installation site be located with adequate accuracy in the JERS-1 SAR image by visually locating the site or estimating the location from nearby distinct features. It was not necessary for the site to be very large. As long as they were larger than 10 x 10 pixels (roughly 150 m in the range direction and 50-60 m in the Azimuth direction), we could detect these sites, including any adjacent areas that appeared very dark, such as agricultural fields, rice paddies, tea plantations, water bodies, grassland and athletic fields, from single look JERS-1 SAR images.