An Electromagnetic Simulator for Sentinel-3 SAR Altimeter Waveforms Over Land—Part I: Bare Soil

ALtimetry for BIOMass (ALBIOM) is a Permanent Open Call Project funded by the European Space Agency (ESA) to explore the possibility of forest biomass retrieval by using Copernicus Sentinel-3 (S-3) Synthetic aperture Radar ALtimeter (SRAL) in low- and high-resolution mode at Ku- and C-bands. It represents an original work in the research of new techniques for vegetation observation using altimetry data. Because of the complexity of the land surfaces, no algorithm has been developed for a specific retracking of the altimetric land waveform. This calls for the development of a model able to reproduce the acquisition system and the target scattering phenomena to simulate the interaction of the radar pulse with the land. In this first work, we present the electromagnetic simulator of S-3 SRAL altimeter measurements over bare soil scenarios realized through a modification of the Soil and Vegetation Reflection Simulator (SAVERS) simulator developed by the team for Global Navigation Satellite System Reflectometry (GNSS-R) over land. The impact of topography has also been taken into account. We demonstrate that SAVERS for S-3 SRAL proved its capability to reproduce altimeter waveforms’ main attributes for both flat and topography scenarios.

I. Introduction

Satellite radar altimetry, originally designed for the open ocean domain, involves today new applications related to inland water, ice sheets, and coastal zones, as on-board instrumentation and signal processing improve. However, modeling the altimeter echo and, consequently, enabling the range estimation with high precision strictly depend on the studied surface. Few attempts have been made to investigate the nature of altimetric signal from land and its modeling because of the huge variability in surface height and land backscatter over short distances, especially in the presence of vegetation. As a consequence, by looking at the magnitude and shape of the altimeter echoes, it can be difficult to determine which bin corresponds to the terrain nadir return in the altimeter waveform and what kind of cover is. Most of the effort considers ground-based processing (i.e., retracking) optimized for ocean application in order to obtain some results over land. For this reason, developing an electromagnetic simulator of land waveforms is of paramount importance, and it may clarify the role of the different geometrical and dielectric properties of the scenarios observed by the altimeter. In this context, the European Space Agency (ESA)-funded ALtimetry for BIOMass (ALBIOM, https://eo4society.esa.int/projects/albiom/) project, kicked off in November 2019, aims at offering to the scientific community insights into the mechanisms underlying land altimetric observations. The main goal of ALBIOM was to investigate the possibility of retrieving forest biomass using Synthetic aperture Radar ALtimeter (SRAL) data from the Copernicus Sentinel-3 (S-3) Mission [1]. In the present study, we examine the development, the validation, and the challenging results offered by the electromagnetic simulator of S-3 SRAL altimeter measurements, focusing on bare soil. A companion paper has also been published about forested surfaces [2], proving the model’s capability to reproduce the main characteristics of the altimetric waveforms over land.