i Detection of Smoke and Dust Aerosols Using Multi-sensor Satellite ...

3.1.2 MODIS data
The 55 0 scan angle of MODIS enables it to achieve almost daily global coverage
except for some small gaps in equatorial areas. Each MODIS sensor passes the same site
twice a day: one in the daytime orbit for all thirty-six bands and the other in the nighttime
orbit only for the TEBs (bands 20-25 and 27-36). Totally, four measurements can be
collected every day with two MODIS sensors. The data used in this dissertation is the
version 5 L1B measurements issued by NASA. Each MODIS file (referred to as a
“granule”) collects consecutive measurements within five minutes, typically 203 scans,
covering an area with 2030 km (along-track) by 2330 km (cross-track) (Nishihama et al.,
2000; Isaacman et al., 2003). In each scan, 1,354 data frames are recorded for each
detector. For 500 m resolution band, 2 sub-frames or samples per frame (1 km) are
recorded for its 20 detectors, and 4 sub-frames for each 250 m band and its 40 detectors
(Xiong et al., 2003). Thus, each granule typically contains 2030 rows (along-track) ×
1354 columns (along-scan) for 1 km resolution bands, 8120 rows × 5416 columns for 250
m resolution bands, and 4060 rows × 2708 columns for 500 m resolution bands. The
geolocation data (latitude and longitude) in L1A are used to find the selected site
automatically. The land/sea mask dataset assists users to quickly separate land from
ocean (water). In L1B, the stored data format is digital number, which is generally
converted into the reflectance or the radiance for RSBs and the radiance for TEBs with
carefully calibrated scale and offset. Moreover, 250 m (band 1-2) and 500 m (band 3-7)
resolution measurements can be aggregated to match 1 km resolution band for further
spectral arithmetic (Nishihama et al., 1997).
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