Compact wide field-of-view camera design for remote sensing of the Earth’s emitted thermal radiation

Remote sensing from space has become indispensable for climate change monitoring. In this context, we are defining a space mission with the intention to monitor the Earth's radiation budget and to measure all of its components. In this paper, we focus on how we intend to measure the Outgoing Longwave Radiation (OLR), which is the thermal infrared flux that is reemitted to space by Earth. For that purpose, we will use a wide field-of-view (FOV) thermal camera covering the spectral range from 8 to 14m, and featuring a FOV of 140°. This allows observing the Earth from limb to limb from a nominal altitude of 700 km, while accounting for altitude and pointing errors. In addition, we target to achieve high resolution (better than 5 km at nadir) to enable scene identification, while fitting the optics and the detector within 1 CubseSat Unit (1U ∼ 1 dm³). Wide FOV thermal camera systems are not widely available, and no commercially available camera can fulfill our target specifications, indicating the need for the development of a custom imaging system design. This paper describes our thermal camera design, evaluates its performance, and discusses the stray-light and tolerancing analysis. Our design combines 3 Germanium lenses with a commercial-off-the-shelf uncooled microbolometer array. To limit the cost and to ease the fabrication, we traded off between the number of elements, the number of aspherical surfaces and the required performance. At all wavelengths and all fields, the optical design performs close to the diffraction-limit. Consequently, we are confident that the wide FOV thermal camera achieves adequate optical performance, featuring a nadir resolution of 4.455 km. As a result, we can safely state that our novel developed thermal camera design goes well beyond the state-of-the-art in the field of thermal cameras, while being specifically dedicated to monitoring Earth's OLR, and thus improving climate change monitoring.