Geodetic network design for InSAR: Application to ground deformation monitoring

Abstract

For the past two decades, interferometric synthetic aperture radar (InSAR) has been used to monitor ground deformation with subcentimetric precision from space. But the applicability of this technique is limited in regions with a low density of naturally-occurring phase-coherent radar targets, e.g. vegetated nonurbanized areas. Third-party end-users of InSAR survey results cannot, in a systematic way, determine a priori whether these coherent targets have adequate spatial distribution to estimate the parameters of their interest. Additionally, InSAR deformation estimates are referred to a local datum, meaning that the technique is sensitive only to the relative deformation occurring within the SAR images. This makes it difficult to compare these estimates with those from other techniques, e.g. historical levelling data or changes in the sea level. Here we propose the design of a geodetic network for InSAR, aimed at densifying the naturally-occurring measurement network and converting from a local datum to a global one. A practical solution for improving spatial sampling is to deploy coherent target devices such as corner reflectors or transponders on ground, tailored to the specific monitoring application under consideration. The proposed method (1) provides a generic description of any deformation phenomenon; (2) determines whether the naturally-occurring InSAR measurements are adequate in terms of user-defined criteria; (3) finds the minimum number of additional devices to be deployed (if required); and (4) finds their optimal ground locations. It digests, as inputs, any prior knowledge of the deformation signal, the expected locations and quality of the existing coherent targets, and the quality of the deployed devices. The method is based on comparing different covariance matrices of the final parameters of interest with a criterion matrix (i.e., the ideal desired covariance matrix) using a predefined metric. The resulting measurement network is optimized with respect to precision, reliability and economic criteria; this is demonstrated via synthetic examples and a case of subsidence in the Netherlands. As a basis for the choice and number of deployed devices, we evaluate the measurement precision of compact active transponders and demonstrate their viability as alternatives to passive corner reflectors through three field experiments, using different satellite data and geodetic validation techniques. Transponders are shown to be usable for subcentimetre-precision geodetic applications, while improving upon the drawbacks of corner reflectors in terms of size, shape, weight and conspicuousness. For transforming the spatially-relative InSAR deformation estimates (local datum) to a standard terrestrial reference frame (global datum), we introduce a new concept involving the collocation of transponders with Global Navigation Satellite System (GNSS) measurements. The displacement of such a transponder is consequently determined in the standard reference frame used by GNSS, eliminating the need for any assumptions on reference-point stability in applications where the InSAR deformation estimates are compared with results from other techniques. The considerations, results and practical lessons learnt at several permanent GNSS stations in the Netherlands are described.