How can I know the exact satellite overpass time for each pixel?
A. The AIRS Level 3 Daily Product is produced through binning and gridding the concatonation of AIRS Level 2 products for a day. Thus, if there is more than a single datum in the 1 deg x 1 deg bin, as happens when polar orbital swaths overlap (happens beginning at |lat| > 35 deg), the time associated with that bin is a range (which becomes greater with increasing latitude). The 1:30 PM local time for ascending is the nominal Equator crossing time going from South to North. The only correct way to determine the exact time an observation included in the bin was made is to look at the Level 2 Data Product. Contained therein is the start time of a granule, and it is a simple calculation to calculate the exact time a particular footprint (MW, IR, vis/NIR) was observed from its position in the granule (scanline and footprint within scanline). The duration of a granule is exactly 6 minutes (45 AMSU scans – 8 seconds/scan; 135 AIRS scans – 2.6667 seconds/scan; 30 AMSU footprints/scan; 90 AIRS
