On the Design of Radar Corner Reflectors for Deformation Monitoring in Multi-Frequency InSAR

Abstract

Trihedral corner reflectors are being increasingly used as point targets in deformation monitoring studies using interferometric synthetic aperture radar (InSAR) techniques. The frequency and size dependence of the corner reflector Radar Cross Section (RCS) means that no single design can perform equally in all the possible imaging modes and radar frequencies available on the currently orbiting Synthetic Aperture Radar (SAR) satellites. Therefore, either a corner reflector design tailored to a specific data type or a compromise design for multiple data types is required. In this paper, I outline the practical and theoretical considerations that need to be made when designing appropriate radar targets, with a focus on supporting multi-frequency SAR data. These considerations are tested by performing field experiments on targets of different size using SAR images from TerraSAR-X, COSMO-SkyMed and RADARSAT-2. Phase noise behaviour in SAR images can be estimated by measuring the Signal-to-Clutter ratio (SCR) in individual SAR images. The measured SCR of a point target is dependent on its RCS performance and the influence of clutter near to the deployed target. The SCR is used as a metric to estimate the expected InSAR displacement error incurred by the design of each target and to validate these observations against theoretical expectations. I find that triangular trihedral corner reflectors as small as 1 m in dimension can achieve a displacement error magnitude of a tenth of a millimetre or less in medium-resolution X-band data. Much larger corner reflectors (2.5 m or greater) are required to achieve the same displacement error magnitude in medium-resolution C-band data. Compromise designs should aim to satisfy the requirements of the lowest SAR frequency to be used, providing that these targets will not saturate the sensor of the highest frequency to be used. Finally, accurate boresight alignment of the corner reflector can be critical to the overall target performance. Alignment accuracies better than 4 • in azimuth and elevation will incur a minimal impact on the displacement error in X and C-band data.

Integration of Corner Reflectors for the Monitoring of Mountain Glacier Areas with Sentinel-1 Time Series

“…As expected, the measured RCS are smaller than the theoretical ones. Several factors which can introduce a loss of RCS compared to the theoretical value are listed in [4]. A RCS loss exceeding 10 dB can result from a 5 mm plate curvature in a 1.0 m trihedral CR in X-band [27].…”

Multi-Temporal X-Band Radar Interferometry Using Corner Reflectors: Application and Validation at the Corvara Landslide (Dolomites, Italy)

“…MTI techniques can be grouped in two main classes: Persistent Scatterers (PS) [39] or Small BAseline Subsets (SBAS) [40] to deliver displacements along the Line Of Sight (LOS) direction with accuracy in the order of a few millimeters per year [41]. Whilst building or infrastructure is taken as PS in urban areas [42], artificial Corner Reflectors (CRs) were installed to investigate areas characterized by low interferometric coherence or when a point wise analysis is foreseen [43][44][45][46][47][48][49]. Notti et al [50] demonstrated the potential of X-band SAR data using TerraSAR-X for landslide mapping and monitoring but a few natural PS were found in mountainous areas.…”

Editorial for Special Issue “Advances in SAR: Sensors, Methodologies, and Applications”

“…Corner reflectors are important for various applications in a SAR sensor's lifecycle. Consequently, Garthwaite discusses design considerations for corner reflectors used for deformation monitoring [2]. In an interesting piece on interferences, Monti-Guarnieri et al [3] present their work on the radio frequency interferences in C-band based on an analysis of the first 8-10 echo measurements per burst to provide a first radio frequency interference map over Europe.…”