Photons emitted by any object – such as the Earth’s surfaces and clouds – will tell its temperature when the photons are viewed in the far infrared (mid- and long-wave infrared wavelengths). MODIS takes advantage of this by using the Blackbody (BB) to help calibrate its thermal bands. The Blackbody is designed to be non-reflective (black) and kept at a precise temperature so that when MODIS “sees” a photon when looking at the BB, the photon’s temperature can be determined very precisely.
The Blackbody is located in front of and slightly above the Scan Mirror, which views the BB with every revolution. The BB assembly provides a full-aperture radiometric calibration source of the MWIR and LWIR bands to within 1 percent absolute accuracy. It provides known radiance levels and is also used in the DC restore operation (a space-view signal level provides the second level for all bands in the two-point calibration). The requirement for absolute calibration imposes the need for excellent temperature uniformity and a high-level effective emissivity of >0.992. Temperature knowledge of the BB is also critical; it is designed with v-grooves cut at an included angle of approximately 45 degrees, which offers exceptional performance in a compact package. The surface of the BB was finished with a proprietary process that minimizes scatter and ensures the effective cavity emissivity.
In normal operation the BB is kept at the instrument’s ambient temperature (nominally 273K), though it is possible to heat and control the BB to 315K. Twelve sensors below the assembly’s surface monitor its temperature. Each sensor is calibrated to National Institute of Standards & Technology (NIST) traceable standards, and can determine the temperature of the assembly to within ± 0.1K.