MODIS Instrument Characteristics, Performance, and Data for Climate Studies - Satellite-based Applications on Climate Change - page 32

by the very successful SeaWiFS instrument that begin operation in 1997 and ceased
operating in December 2010.
The MODIS is a cross-track, “whisk broom” radiometer that provides observations
in 36 spectral bands (see Table 3.1 and Fig. 3.1 ) covering wavelengths from 0.41 to
14.4

μ

m and at three nadir spatial resolutions: 250 m, 500 m, and 1 km. Together
Terra and Aqua MODIS have produced more than 10 years of global data sets that
have significantly helped scientists worldwide to better understand the Earth as an
interacting system and the impacts on this system due to human-related activities.
The calibration performance requirements for the instrument (at a typical radi-
ance and within scan angles of 45 ) are 5% radiance absolute accuracy and 2%
reflectance accuracy relative to the sun in the reflected solar radiation bands (RSB).
For the thermal infrared bands (TIR), the requirements are

1% radiance accuracy

for the majority of the bands, but 0.75% for band 20, 10% for band 21, and

0.5% for bands 31 and 31. A more detailed description of the MODIS instrument
and on-orbit performance is provided by Xiong et al. ( 2009 ). Additionally the
location of the observations (“pixels”) on the Earth surface was required to be
150 m (rms) with a goal of 50 m (rms).
In order to maintain on-orbit calibration and data product quality expressed as
the requirements noted previously, MODIS was built with a very capable set of
onboard calibrators (OBCs), including a solar diffuser (SD), a solar diffuser stabil-
ity monitor (SDSM), a blackbody (BB), a deep space view (SV) port, and a
spectroradiometric calibration assembly (SRCA). These calibrators along with the
monitoring of selected ground-based target areas, plus deep space and the moon as
seen through the space view port, have been used to monitor and assess the
performance of the instrument since launch. These calibration devices are very
important and necessary because one cannot simply rely on the prelaunch instru-
ment characterization (which was extensive) due to on-orbit degradation or change
in the reflective optical surfaces, the emissive properties of instrument components,
and the change in the properties of detectors and the calibration sources themselves
(e.g., the solar diffuser), plus the possibility as seen prior to the launch of the Terra
and Aqua missions that changes that could occur in the registration of the bands and
included detectors relative to each other and spectral shifts in the bands on-orbit.
Figures 3.1 and 3.2 depict the various calibration devices.
Both instruments have been shown to be operationally stable over the 10 or more
years of operation (i.e., through to early 2012). The temperature variations of the
warm focal plane assemblies of both instruments have been no more than 3.5 in the
case of the Terra MODIS and 2 for the Aqua MODIS. The cold focal plane
assemblies have shown a small temperature increase in recent years of less than
0.1 K/year. The temperature of the blackbody is nominally set at 285 K and has
been relatively constant. The blackbody temperature of the Terra MODIS has only
changed 30 m-K over 12 years and the Aqua MODIS effectively zero degrees over
10 years.
There, however, have been significant changes in other components that had to
be accommodated. The visible spectral bands (VIS) have shown relatively large
changes in detector responses (or gains). The changes are different at the varying