The EarthCare Thermal IR Imager.

EarthCare is an ESA satellite mission, due to be launched in 2018, that will measure radiative effects of clouds and aerosols, that can have implications for climate change.  Along with Lidar and Radar instruments and a broad-band radiometer, the payload includes multi-spectral imagers operating in the spectral ranges of reflected sunlight and emitted thermal infrared (TIR).  The multi-spectral imagers have quite coarse spatial resolution, covering 300km swath widths at ½ km ground sample distance, but it is important that they are calibrated in flight to provide very good radiometric accuracy.

Dan Lobb produced the optical concepts and designs for the TIR imager.  The TIR system uses a micro-bolometer area-array detector, which has the critical advantage that it does not, like thermal-IR photon detectors, require cryogenic cooling.  The detector is relatively insensitive, but good signal to noise ratios are achieved by binning signal from multiple rows of the area array, and using reference rows and columns to measure and correct some of the larger noise components.

The TIR optical system forms a primary image of Earth through dichroic beam splitters, which split the image between three spectral filters.  The beams are then re-imaged at short f/number onto separate rows of the detector.  Relative calibration includes use of an isolated internal surface that provides a stable background signal.  Absolute calibration is performed using a rotatable mirror outside the primary image optics: this is used to select between the view to Earth (shown in the diagram), a view to cold space and a view to a black body.  An original feature is use of a concave back surface of the scan mirror.  This makes the calibration system smaller and more accurate by allowing a small, deep-cavity black body to fill the instrument aperture and field.