On the Use of Transponders as Coherent Radar Targets for SAR Interferometry

Monitoring ground deformation using SAR interferometry (InSAR) sometimes requires the introduction of coherent radar targets, especially in vegetated nonurbanized areas. Passive devices such as corner reflectors were used in such areas in the past. However, they suffer from drawbacks related to their large size and weight, conspicuousness, and loss of reliability because of geometric variations as well as material and maintenance-related degradation over several years of deployment. The viability of smaller, lighter, and less conspicuous radar transponders as an alternative is demonstrated via two field experiments: validation tests in a controlled environment, and operational performance for monitoring landslides in a heavily vegetated area. Comparison of 113 transponder-InSAR observations with independent validation measurements such as leveling and the global positioning system yields an empirical precision range of 1.8-4.6 mm, after outlier removal, for double-difference (spatial and temporal) transponder phase measurements in the radar line of sight, for Envisat and ERS-2.