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.