Multi‐angular ground‐based SAR system for soil surface roughness characterisation

Abstract

Developped in the Institut of Electronics and Telecommunication of Rennes (IETR), a Ground Based (GB) SAR system is proposed for soil surface roughness discrimination at C-band. By using this GB-SAR system, the multi-angular SAR measurement is realized at different resolution cell sizes. The specific levels of surface roughness are set deliberately to analyze the roughness contribution on multi-angular SAR signature in a controlled experimental condition. Based on the GB-SAR measurement, the effect of incidence angle on SAR sensitivity is studied under two resolution cell sizes for retrieving soil surface roughness. The results show that, by using our GB-SAR system, high incidence angle configuration (e.g. > 40 •) is optimal to discriminate surface roughness, which is more significant at small resolution cell size (e.g. 30 cm). This study indicates the effectivity of GB-SAR system to validate the SAR backscattering mechanisms and will be helpful for future spatial missions [1]. Introduction: With high spatial and temporal resolution, the remote sensing techniques play an important role in monitoring the soil status. Compared with optical remote sensing, the synthetic aperture radar (SAR) in independent of weather and light constraint, owing to the active microwave generation and the penetration ability of microwave. Nevertheless, the mechanisms of SAR backscattering from bare soils are suffered from the coherent superposition of surface roughness contribution with soil moisture. Thus, discrimination of surface roughness effect on SAR signature is crucial for further soil monitoring. Recently, the multi-angular SAR measurements increase the observing dimension [2], making it possible to improve the robustness of soil parameter retrievals. However, the temporal interval of spaceborne multi-angular image acquisitions induces several problems in the multi-angular SAR study: i) the soil characteristics may vary during the multi-angular measurements; ii) it is challenged to accomplish the consistent ground truth measurements over large areas simultaneously with satellite pass. To solve the aforementioned issues, this study proposes a Ground Based (GB) SAR system for bare soil monitoring. This kind of GB-SAR system was also developed for terrain mapping [3], vegetation monitoring [4] and snow pack imaging [5]. However, our study concentrates on the bare soil characterization under multi-angular configuration and the influence of resolution cell size on the multi-angular SAR sensitivity. GB-SAR system for soil monitoring: Our GB-SAR system is mainly comprised of three components: i) A vector network analyzer; ii) Four wide band horn antennas (2-12 GHz); iii) A three-meter long metal rail with an accurate positioning motor. This GB-SAR system is capable of measuring the backscattering matrix of targets under controlled experimental condition. Repeatable measurements can be realized by the GB-SAR system as the motion of the platform is controlled accurately, which is essential for multi-angular SAR measurements. Furthermore, different soil status can be set deliberately in the scene to analyze the induced response in SAR signature. The SAR back projection algorithm [6] makes possible to obtain the absolute backscattering coefficients through several calibration procedures: i) The antenna patterns are compensated along both azimuth and range axis; ii) The methodology proposed by [7] is used to accomplish the polarimetric calibration; iii) The radiometric calibration is realized by comparing the measured RCS (Radar Cross Section) of a canonical reference target with its theoretical RCS value.