Multi-Temporal Soil Moisture Retrieval From Spaceborne SAR Missions Operating at Different Frequencies

The new Copernicus Radar Observing System for Europe in L-band (ROSE-L), expected to work in synergy with the C-band Sentinel-1 mission, will create a multi-platform Synthetic Aperture Radar (SAR) facility acquiring data in a systematic and coordinated way. This paper investigates the performance of a novel soil moisture retrieval scheme, extending the capability of a previously proposed multi-temporal and multi-polarization algorithm to the case of multi-frequency SAR data. It relies on a Bayesian statistical criterion to invert a forward electromagnetic model based on the hypothesis that soil moisture can change abruptly, whereas soil roughness remains stable over time. The algorithm is applied to simulated data to compare two possible operational scenarios of ROSE-L and Sentinel-1 observations: L-band and C-band coincident (LC) or alternate (L-C) acquisitions. The case of single frequency (L or C) data is also considered in the analysis. Additionally, quad-polarization (VV, VH, HH) and dual-polarization (VV, VH) data for ROSE-L are compared when combined with dual-polarization (VV, VH) data for Sentinel-1. The simulated multi-polarization C-band and L-band SAR data are generated considering time variant scenarios of bare soil and crop covered fields. The algorithm is also tested on a time-series of non-coincident L-band SAOCOM-1A and C-band Sentinel-1A data to evaluate the improvements of the soil moisture retrieval against in-situ data when the two frequencies are merged in the multi-temporal scheme. For the simulated case, results for bare soils show that the alternate configuration reaches a retrieval accuracy higher than that of single frequency, with an average percentage improvement in RMSE of approximately 18% compared to single C-band and 5% compared to single L-band. In many cases, it approaches the performance of the coincident acquisitions, maintaining a key advantage in terms of revisit time. The experiment on real data further confirms the advantage of alternating the acquisitions from the two frequency bands when exploited within a multi-temporal framework.