Granularity of AIRS Data
The Aqua satellite on which AIRS flies makes two passes per day over the globe, orbiting Earth from pole to pole. On the ascending pass, the satellite is moving from the south pole to north pole. On the descending pass, the satellite is moving from the north pole to the south pole.
AIRS collects data as it sweeps along the orbit, and the data is then sectioned into pieces called "granules". Each AIRS granule is roughly 2250 x 1650 kilometers, or 1400 x 1025 miles and contains 6 minutes of data. There are nominally 240 Level 1B and 240 Level 2 granules of 6-minute duration generated each day. The orbital repeat cycle is 16 days, but orbital maintenance maneuvers can shift granules along orbits by a small fraction of a granule. Maps showing the locations of granules are generated daily and available for download. AIRS data users use maps like these when making requests from AIRS data servers.
Level 1 and Level 2 Data Products
The AIRS Level 1 and Level 2 data (with the exception of the AIRXBCAL) are broken into a series of 6-minute granules and each granule is a file. Each file contains all observations of a given type made during a period of exactly 6 minutes. For every day there are 240 granules, numbered 1-240. Over the course of 6 minutes the EOS-Aqua platform travels approximately 1500 km, and the AIRS-suite instruments scan a swath approximately 1650 km wide (extending ±49.5° to either side of nadir).
Start times of granules are keyed to the start of 1958. Because of leap seconds, they do not start at the same time as days do. For data from launch through December-31-2005, granule 1 spans 00:05:26 UTC - 00:11:26 UTC and granule 240 starts at 23:59:26 UTC and ends at 00:05:26 UTC the next day. For data December-31-2005 through the next leap second, granule 1 spans 00:05:25 UTC - 00:11:25 UTC and granule 240 starts at 23:59:25 UTC and ends at 00:05:25 UTC the next day.
Level 3 Data Products
The Level 3 data products are binned into 1 degree bins creating arrays of 180 x 360 (lat x lon). Users of AIRS Level 3 products should be aware that the temporal span of Level 3 daily files is not midnight-to-midnight. The data proceeds in time from left (-180.0°) to right (180.0°) with neighboring cells of data no more than a swath of time apart. This ensures that data points in a grid box are always coincident in time, if the data were gridded using a midnight-to-midnight time scheme, the start of the day and the end of the day would be in the same grid cell, producing an artificial time discontinuity across the grid. The edges of the AIRS Level 3 cells are at the date line (the 180E/W longitude boundary). When plotted, this produces a map with 0 degrees longitude in the center of the image. This method sometimes called a “Data Day” is preferred because the left side of the grid and the right side of the grid contain data farthest apart in time. The method used is analogous to that used to create TOVS Pathfinder level 3 products.
Each Level 3 daily product contains information for a temporal period of 24 hours for either the descending or ascending orbit (rather than midnight-to-midnight) where “ascending or descending” refers to the direction of movement of the sub-satellite point in the satellite track at the equatorial crossing. The ascending direction of movement is from Southern Hemisphere to Northern Hemisphere, with an equatorial crossing time of 1:30 PM local time; the descending direction of movement is from Northern Hemisphere to Southern Hemisphere, with an equatorial crossing time of 1:30 AM local time. Outside of the polar zones, these correspond respectively to daytime and nighttime.
The Monthly Level 3 files follow the same “Data Day” definition as the daily files but over longer time periods.