AIRS Level 1B Visible/Near-Infrared Channels ATBD - NASA's Earth ...

AIRS Level 1b ATBD Part 2: Vis/Near IR Channels
Most of the parameters in Table 4.1 are self-explanatory. To allow for non-linearities in the
instrument scan rate across-track, a polynomial can be determined relating across track
column number to the column number that would result from a perfectly uniform scan.
(Non-linearities can arise in either the scan mirror motion or the detector readout process.)
The array indicated holds coefficients for a third order polynomial: shown is the nominal case
of uniform sampling. The “Time offset between channels” parameter allows for a systematic
time delay between readouts of each Vis/NIR channel: its nominal value is zero because all
channels are supposed to be sampled simultaneously. Each of the parameters in Table 4.1
has been measured on the ground, and a subset will be verified/refined using in-flight
calibration.
Table 4.1. Vis/NIR Instrument Geometric Model
Geometric Parameters
Nominal Value
Effective Focal Length
77.427 mm
Number of pixels across-track in one scan line 720
Number of pixels along-track in one scan line 9
Coefficients to account for non-linearity of scan rate [0, 1, 0, 0]
Scan mirror tilt 45°
Total across-track angular coverage in one scan line 99.0°
Total along-track angular coverage in one scan line 1.665°
Misalignment between instrument coordinate system and
spacecraft coordinate system [roll, pitch, yaw]
Scan mirror rotation period
Interval between reading consecutive pixels in an array
Interval between consecutive readouts of the same pixel
Time offset between channels
[0°, 0°, 0°]
2.667 sec
6.0E-5 sec
2.801E-3 sec
0 sec
The instrument model, combined with absolute time information, spacecraft ephemeris data,
and an Earth model, allows any pixel in an image to be assigned to an Earth longitude and
latitude. The second step in spatial calibration is to assemble a data base of Ground Control
Points (GCP’s), which are easily located features for which extremely accurate geocentric
positions are known. (Small islands make excellent GCP’s, and they can be recognized both
by the two-dimensional shape of their land/ocean interface and by the albedo statistics
associated with a given radius from their center. These two recognition schemes are called
feature-based and area-based matching, respectively.) By locating the GCP’s in flight data,
and comparing their calculated geocentric positions with their known locations, the
instrument pointing can be validated or updated. An update is performed by using a least-
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