Calibration of Microwave Radiometers from GPM to Cubesats

While spaceborne microwave radiometers have been providing Earth observations for many decades, more recently, applications have focused on the use of constellations of well-calibrated sensors. Perhaps the best known of these is the Global Precipitation Measurement (GPM) mission, which involves state-of-the-art radars and a radiometer to measure precipitation, but also acts as the calibration reference for a constellation of research and operational radiometers. This involves challenges related to the development of a high-quality stable calibration reference, intercalibration of a diverse suite of radiometers, and revisiting calibration issues in older instruments that are no longer operational. Lessons from this effort include the importance of instrument design, on-orbit calibration maneuvers, and detailed knowledge of factors such as instrument pointing, thermal conditions, spectral response functions, antenna patterns, RFI, etc. Even more recently, the development of miniaturized radiometer components to be deployed in constellations of low-cost CubeSat satellites has raised new calibration challenges.