Development of Methodologies for Brightness Temperature Evaluation for the MetOp-SG MWI Radiometer

The MicroWave Instrument (MWI) is a conical scanning radiometer, which shall be embarked on the MetOp Second Generation satellite. MWI will provide precipitation monitoring, atmospheric moisture and sea ice extent information. Conical scanning radiometers are characterized by a continuous instrument calibration, with the sensors passing, at every rotation, below two calibration sources: a cold sky reflector providing 3K reference, and an On Board Calibration Target (OBCT) which provides an Hot temperature reference. The OBCT temperature must be known with high accuracy, and the gradients along its surface must suppressed to grant the tight performance. However, gradients are intrinsic to the structure of the OBCT and driven by the Sun-Shadow induced temperature cycles of its environment. The minimization of these gradients by design is a key parameter of the instrument thermal control system. The development of a Brightness Temperature computation method, i.e. the computation of the temperature sensed by the radiometer in the Radio Frequency band, was therefore a necessary step for the instrument thermal control optimization. It allowed to allocate the limited instrument resources in the most efficient way, and to support the Thermal Control System design. In this paper the details of the Brightness Temperature (BT) computation are provided, together with several lessons learned about the analytical simulations parameters to be adopted in such a sensitive thermal design. The OBCT temperature maps are then processed, filtering them with the Feed Horns Patterns. This results in the BT profiles along the orbit, with associated errors. The method is then extended to both High and Low Frequency analysis to assess the influence of each position of the rotation cycle on the BT. Results are shown, demonstrating that a passive thermal control meets the strict performance requirements. An overview of the verification strategy at OBCT and instrument level is provided.