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morning and afternoon observations
- Provide highly consistent time series of observations used to improve our understanding
of climate change at seasonal-to-decadal time scales
The first prototype of MODIS was launched on December 18, 1999 aboard the Terra
EOS-AM-1 platform. The MODIS on the Aqua EOS-PM1 spacecraft was launched on
May 4, 2002.
2.5.2 Technical specifications
MODIS was initially conceptualized as a double instrument MODIS-N (Nadir) and
MODIS-T (Tilt). Aiming improved ocean colour capabilities, MODIS-T was designed with
the ability to tilt away from specular reflection directions and avoid sun glint effects.
Despite its success on SeaWiFS, a tilting mechanism was not retained for the MODIS
project because the combination of data collected from Terra and Aqua MODIS was proven
to provide almost similar spatial coverage [Gregg, 1992], [Gregg and Woodward, 2007],
with the additional advantage of both morning and afternoon observations. MODIS is
composed of 36 spectral bands ranging from the visible (0.4µm) to the far infrared (14µm)
and centered at wavelenghts dedicated to three major applications. Bands 1-7 are devoted
to the study of land remote sensing, cloud detection and aerosol estimation. These bands
are centered at wavelenghts similar to Landsat TM and measure data at spatial resolutions
of 250 m for bands 1-2 and 500 m for bands 3-7. Stringent requirements associated with
ocean color monitoring and studies conducted on CZCS and SeaWiFS instruments lead to
nine spectral bands on MODIS (8-16). Compared to SeaWiFS, MODIS ocean color bands
are narrower (average of 10 nm width compared to 20 nm on SeaWiFS), and, therefore
allow more reliable atmospheric correction with higher signal-to-noise ratio values. Most
of the remaining bands 17-26 were spectrally positioned with respect to HIRS, AVHRR
and ATRS. To provide accurate Sea Surface Temperatures (SST), two split-windows at
mid-wave infrared (MWIR) (bands 23-24) and long-wave infrared (LWIR) (bands 31 and
32) were included. The split-window composed of bands 31-32 enables the derivation of
day time SST measurements, because channels 23 and 24 are contaminated with sun glint
effects, still persisting in the MWIR portion of the electromagnetic spectrum. MODIS
was designed as a whiskbroom sensor ; it uses the obital motion of the satellite to acquire
successive lines using a scanning mirror that rotates at ± 55˚. MODIS swath reaches 2330
km and allows a global coverage of the entire earth every one to two days.
2.5.3 Components
Compared to its predecessors, MODIS includes many components (figure 2.10), each
playing a specific role in the acquisition process. As we shall see, emphasis was given to the
calibration of the instrument. We present here a brief description of the main subsytems.