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

AIRS Level 1b ATBD Part 2: Vis/Near IR Channels
squares regression to modify the instrument model in such a way as to minimize the
difference between calculated and known positions in a large sampling of GCP’s.
The spatial calibration technique just described can be used to update the relative pointing of
the four Vis/NIR channels as well as their absolute Earth coordinates. With this approach,
geolocation to within 10% of a pixel (~200 m at nadir) is straightforward, and location to 1%
of a pixel (~20 m at nadir) should be possible.
4.2 Spatial Calibration of Visible Channels to the Infrared
Excluding research products, the primary purpose of the visible channels is to provide
statistical information on cloud structures and surface inhomogeneities within an infrared
footprint. For these purposes, it is sufficient that the visible and IR instruments be aligned to
within half of a visible pixel. The previous section pointed out that it is straightforward to
locate the visible channels with respect to the spacecraft boresight to within 0.1 visible pixels
(about 200 meters on the ground at nadir). Section 5.1 of Part 1 of the Level 1b ATBD found
the IR footprint can be determined to ~1% of the IR field of view, which is 0.06 Vis/NIR
pixels. The combined uncertainty between the IR and Vis/NIR boresights is then ~0.12
Vis/NIR pixels, well within the 0.5 pixel requirement.
4.3 Co-Alignment of the MODIS and AIRS Instruments
The MODIS instrument observes in the visible and infrared, with nadir spatial resolution of
about 0.2 km at visible wavelengths, and 1 km in the infrared. (This compares to the 2.3 km
size, at nadir, of a Vis/NIR pixel.) Pre-launch calibration of MODIS allows its orientation
relative to the spacecraft boresight to be known to better than 0.2 MODIS pixels. Combining
this uncertainty with the 0.1 to 0.01 Vis/NIR pixel uncertainty in the AIRS boresight
(discussed in 4.1) yields a pointing alignment between MODIS and AIRS visible channels of
0.10 to 0.02 Vis/NIR pixels. Similarly, the pointing between MODIS IR channels and AIRS
Vis/NIR channels is accurate to 0.13 to 0.09 Vis/NIR pixels.
Some potential uses of MODIS data by AIRS are improved radiometric calibration of the
AIRS visible channels (discussed in Section 2.4), and statistical studies of variability on
scales smaller than an AIRS visible pixel. For these purposes, alignment of AIRS and
MODIS to ~0.1 of an AIRS visible pixel is sufficient. The estimates in the previous
paragraph indicate that this requirement is met (albeit marginally for the MODIS IR channels)
by the pre-launch spatial calibration of MODIS. The on-orbit spatial calibration routine
described in Section 4.1 will be used to correct for any alignment changes resulting from
launch or gravity release, and will eventually result in a spatial calibration more accurate than
that determined pre-launch.
34 Version 2.2 10 November 2000