A low-cost passive corner reflector design for SAR and InSAR applications

Passive corner reflectors, made typically of metal plates, are widely used as point targets in synthetic aperture radar (SAR) and SAR interferometry (InSAR) applications. Drawbacks of such corner reflectors include: (i) high cost, especially when many reflectors are required for monitoring purposes; (ii) creation of localised ground motion in soft or unstable soils; and (iii) attractiveness for thieves. We tested several lower-cost and lighter weight materials for making corner reflectors. A cubic trihedral corner reflector, made of 2 mm thick aluminium plates, served as a standard. It was compared to corner reflectors built from 10 mm thick multiwall polycarbonate sheets covered with: (a) nothing; (b) c. 0.1 mm thick aluminium tape; or (c) aluminium-bearing metallic paint. In Sentinel-1 SAR backscatter imagery, the corner reflectors made of aluminium and polycarbonate sheets covered with aluminium tape (PC-AT) were equally visible. In contrast, the plain polycarbonate sheet and the polycarbonate sheet with aluminium paint were invisible. Scanning electron microscope (SEM) analysis of tape microstructure showed a well-distributed and dense layer of aluminium particles, which provides good dielectric response and high visibility in SAR imagery. Furthermore, a PC-AT corner reflector installed on an active landslide site yielded not only highly coherent pixels, but also displacement time series that were closely correlated with temporal variations of in-situ ground properties, such as soil moisture and matric suction. In terms of material cost and weight, the PC-AT corner reflector is approximately 94% cheaper and 71% lighter than the aluminium corner reflector, based on current market prices in Ireland. These low-cost, light-weight PC-AT corner reflectors are thus suitable for operational use in SAR- and InSAR-based monitoring.