Introduction, purpose and scope

The purpose of this Algorithm Theoretical Baseline Document (ATBD) is to present a prototype Soil Moisture (SM) retrieval algorithm for the Copernicus Imaging Microwave Radiometer (CIMR) mission. The proposed algorithm is based on the heritage of retrieval algorithms developed for the SMOS and SMAP missions. The algorithm design has been adapted to the particular characteristics of CIMR.

The document is structured as follows. Initially, the historical background of passive microwave remote sensing for soil moisture is provided, along with the justification for the chosen algorithm. The Level-2 soil moisture product is planned to be provided on a geographical grid (EASE2, Equal Area Cylindrical projection) and at two distinct spatial resolutions: a hydroclimatological scale based on inversion of L-band TBs (<60 km) and an hydrometeorological scale based on inversion of L-band TBs sharpened with C-/X-band TBs at an enhanced spatial resolution (~15 km). In the “Baseline Algorithm Definition” section, the forward model and CIMR protoype retrieval algorithm is presented, including a flow diagram that outlines the steps involved in the retrieval of L-band soil moisture and vegetation optical depth. The Algorithm Input and Output Data Definition section outlines the necessary input and output data for the algorithm. A performance assessment of the proposed algorithm based on synthetic reference data is presented in the “Algorithm Performance Assessment” section. Finally, a roadmap for further developments of the CIMR soil moisture algorithm is proposed.