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28 2. Remote Sensing with MODIS Figure 2.9 – The issue of limited spatial coverage illustrated on Aqua MODIS Level 3 nightime 11 µm Sea Surface Temperature (SST) (Left) Daily product with missing measurements due to clouds, sun glint and distance between swaths (Right) 8 day composite product showing a substantial increase in spatial coverage. regional weather prediction models. The methodology tackles the issue of cloud contamination using a temporal compositing approach, able to estimate SST missing values while ensuring global spatial continuity of SST gradients. 2.5 The Moderate Resolution Imaging Spectroradiometer (MODIS) 2.5.1 Context and objectives In 1978, the Coastal Zone Color Scanner (CZCS) was launched aboard the spacecraft Nimbus-7. Designed as a proof-of-concept instrument with a mission duration of one year, CZCS primary goal was to determine the potential of satellite imagery for the quantification of ocean chlorophyll concentration and other dissoved and suspended organic matter. Despite many limitations related to its prelaunch characterisation and on-orbit calibration, CZCS imposed the basis of modern ocean color satellite sensors. The CZCS operational period (1978-1986) was followed by the launch of several instruments dedicated to the study of oceans, MOS, OCTS, POLDER and SeaWiFS, each including specific technological innovations with increased dynamic range, signal-to-noise ratio and number of spectral bands. Motivated by the demanding requirements of remote sensing scientists, the improvement in satellite sensors design reached its climax with the Moderate Resolution Imaging Spectrometer (MODIS). As a multipurpose mission, MODIS primary goals are : - Ensure the continuity of data collection provided by heritage sensors such as AVHRR, CZCS, SeaWiFS and HIRS - Deliver products in a variety of disciplines with improved radiometric quality - Monitor changes that occur at short time-scales combining Terra and Aqua MODIS
29 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.
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134 BIBLIOGRAPHIE P. Perona and J.
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