The MODIS instrument
has a full complement of On-Board Calibrators (OBCs) that
generate various stimuli to provide radiometric, spectral,
and spatial calibration for various MODIS components. The
four OBCs are the Blackbody
(BB), the Solar
Diffuser (SD), the Solar
Diffuser Stability Monitor (SDSM), and the Spectroradiometric
Calibration Assembly (SRCA). There are also two other
calibration techniques, explained below.
Technical Description
The Blackbody is the prime calibration
source for the mid- and long-wave infrared bands (located
from 3.5 µm to 14.4 µm), while the Solar Diffuser
provides a diffuse, solar-illuminated calibration source for
the visible, near infrared, and short-wave infrared bands
(0.4 µm < = lambda < = 2.2 µm). The SDSM
tracks changes in the reflectance of the SD via reference
to the sun so that potential instrument changes are not incorrectly
attributed to changes in the calibration source. The SRCA
is a complex, multi-function calibration instrument that provides
in-flight spectral, radiometric, and spatial calibration.
In addition to the dedicated calibration
components above, MODIS has two additional calibration techniques:
looking at the Moon and deep space. The advantage of looking
at the Moon is that it enables MODIS to view an object that
is roughly as bright as the Earth. Like the on-board Solar
Diffuser, the Moon is illuminated by the Sun; however, unlike
the Solar Diffuser or the Earth, the Moon is not expected
to change over the lifetime of the MODIS mission. Looking
at the moon provides a second method for tracking degradation
of the Solar Diffuser. Meanwhile, looking at deep space provides
a photon input signal of zero, which will be used as an additional
point of reference for calibration. The difference between
the actual value of zero and what MODIS “sees”
will tell scientists how much the data need to be adjusted
in compensation.
