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

AIRS Level 1b ATBD Part 2: Vis/Near IR Channels
in W/m 2 /count.
2.5.4.3 Gain Smoothing
Because there is noise on the gain calibration views, it is desirable to combine (N) gain
views, thereby obtaining a N decrease in the noise level. Because the gain of each detector
is not expected to vary significantly during the eight minutes when the radiometric calibration
lamp is on, it is acceptable to simply average the observed gain counts for a single eightminute
calibration cycle. This produces (for each spectral channel and for each detector pixel)
a single value of gain which is representative of the eight-minute period. (Information on the
stability of the gain within the eight-minute period is maintained for QA and error purposes,
as described in Section 5.)
Based on experience with other visible detectors, the gain is not expected to vary appreciably
even on a scale of hours. This will be verified during on-orbit checkout. Therefore,
radiometric calibrator target views can be averaged over several eight-minute windows, even
though they are separated by many orbits. A sliding window will be used to average these
infrequent gain views to the time of any observations. (Data numbers, not calculated gains,
will be averaged.) Ground test data (Hofstadter 1999b) indicates that only one eight-minute
period should be used for the gain of Channels 2 through 4 (achieving signal-to-noise ratios
well in excess of 1000), but that two eight-minute periods should be combined for Channel 1
to insure an SNR near 100 when using calibration Lamp 3.
2.5.4.4 Vicarious Gain Correction
Because of the large uncertainty in the absolute value of the calculated gain values (due to the
20% uncertainly in absolute radiance of the radiometric targets), it is desirable to correct these
gain values relying on ground truth (see Section 2.3). For purposes of the operational
calibration algorithm, this correction consists simply of multiplying the calculated smoothed
gain values by factors, γ(g) and γ(M), as shown in Eq. 2-15. These correction factors are
provided by the science team, and are a non-routine (research) product. The default value for
the vicarious gain correction factors is 1.0 for all channels.
2.5.5 Radiometric Calibration Application
2.5.5.1 Two Point Calibration
Given the calculated offsets and gains, calibrated radiances (in units of W/m 2 ) are obtained
directly by subtracting the calculated offsets from the pre-processed observed scene counts,
and multiplying this difference by the calculated instrument gain (Eq. 2-2).
30 Version 2.2 10 November 2000