The Challenges of Thermal Testing and Correlation of Blackbody Cavities

RAL Space, part of UK Research and Innovation (UKRI), develops blackbody sources for both flight and ground based calibration of infrared satellite instruments used for the measurement of geophysical parameters. An example of this is for the Sea and Land Surface Radiometer (SLSTR) on Sentinel-3. To ensure the traceability of these sources to international standards, with uncertainty <100 mK, the radiometric and thermal properties of these sources must be controlled and well understood.
From a thermal engineering perspective, there are many challenges, in particular the need to achieve small temperature gradients over the baseplate of the cavities whilst working within given mass and power constraints. These challenges continue through the test and correlation process. To achieve the required radiometric performance, the knowledge of the temperature gradients within the SLSTR blackbodies needs to be as low as 10 mK, hence any measurement of the gradient is sensitive to small heat leaks due to the test set up. This means that care must be taken when adding thermometry in testing to reduce sensor wire heat leaks and ensure that the test remains as representative of flight conditions as possible. However, this limits the amount of data available for correlation purposes, so careful consideration is needed when setting the correlation criteria in order to achieve the desired modelling accuracy for the baseplate temperature distribution. This paper will discuss the lessons learnt during the test and correlation process of the SLSTR Model C and D flight blackbodies, with reference to flight data from previous SLSTR blackbodies.