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

Matthew Garthwaite

doi:10.3390/rs9070648

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.

Highlights

The Persistent Scatterer Interferometric Synthetic Aperture Radar (PSInSAR) technique [1,2,3,4,5] has become a popular remote sensing method for monitoring ground or infrastructure displacements induced by wide-ranging natural and anthropogenic phenomena
Radar Cross Section (RCS), Signal-to-Clutter ratio (SCR) and derived line of sight (LOS) displacement errors detected at the deployment sites
There is a larger variation in clutter values between TCR sites the RSAT-2 imagery, which may be because of the coarser pixel resolution resulting in greater speckle in the RSAT-2 imagery, which may be because of the coarser pixel resolution resulting in greater variation compared with the higher resolution X-band imagery

Summary

Introduction

The Persistent Scatterer Interferometric Synthetic Aperture Radar (PSInSAR) technique [1,2,3,4,5] has become a popular remote sensing method for monitoring ground or infrastructure displacements induced by wide-ranging natural and anthropogenic phenomena. The technique uses a stack of SAR interferograms and determines the motion history for pixels that are identified to have temporal phase stability (i.e., pixels whose overall response is dominated by a strong back-scatterer) The distribution of these “persistent scatterers” (PS) can be quite dense in urban areas (e.g., several hundred PS/km2 ), where there are many man-made angular structures and corners to reflect incident radar energy back to the observing SAR sensor. Recent advances have seen the development of algorithms for absolute positioning of SAR scatterers using stereo SAR images of targets acquired using multiple imaging geometries [17,18] Common to all these applications is the need for an artificial target with a geodetically known position that has been designed to have a bright and stable response in SAR imagery.

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