Blackbody Calibration Targets for the Microwave Sounder Instruments: Control Methods and Modelling Techniques

RAL Space, part of UK Science and Technology Facilities Council (STFC), has developed blackbody sources for flight and ground based calibration of radiometers. This paper will present the thermal design and modelling aspects of the calibration targets being designed for the prelaunch calibration of the Microwave Sounder (MWS) instruments, to fly on the MetOp-SG-A spacecraft.

The 500mm diameter blackbody has to be controlled to set-points 80-315 K, and stable to 0.05 K/5 min, which is achieved using a combination of liquid nitrogen cooling and heaters. A variable pressure helium gas-gap is used to change the conductive link between the blackbody cavity and baffle, and the LN2 cooled exterior. The thermal simulation indicates that by optimizing the gas pressure and applied heater power, temperature transitions can be controlled, and the thermal uniformity requirement of the blackbody can be achieved.

The radiating surface of the cavity requires millikelvin levels of temperature uniformity across its baseplate to reduce calibration uncertainty. Modelling this is further complicated by its surface geometry of approximately 2500 high aspect ratio pyramids. An additional challenge is posed by the epoxy absorber material coating the aluminium pyramids, which has a low thermal conductivity and so drives temperature non-uniformity through the pyramids. Hence, careful consideration has been required in the thermal modelling to ensure that it represents the true temperatures of the baseplate. Through the combination of an ESATAN-TMS model, used to understand the overall system, and an ANSYS model, for a single detailed pyramid, the time-efficiency of the modelling has been improved. Finally, this paper will discuss the link between the brightness temperature, which drives the calibration uncertainty, and the thermal uniformity, both radially and up the 40 mm pyramids. The understanding of this has allowed the calibration requirements to be fulfilled.