MISR's Spatial Resolution and Coverage on the Ground
How does MISR view the ground?
Each MISR camera sees instantaneously a single row of pixels at right angles
to the ground track, -- we call it a "pushbroom" imager because of the way it
sweeps along the ground track. MISR collects data only on the daylit side of the
Earth. During each orbit, MISR obtains a swath of imagery that is
360 km wide by about 20,000 km long.
What is the reason for the swath width?
Changes in the reflectance of Earth's surface during the vegetation
growing season are observable on weekly time scales. In accordance with this,
the selected MISR viewing swath width of 360 km allows viewing the entire
Earth's surface in a period of 9 days, -- very close to one week.
What is the spatial resolution of the MISR images?
Since many physical phenomena, such as the lateral diffusion of radiation
within aerosol layers, have characteristic horizontal scales on the order of 1
km, sub-kilometer resolution is required to resolve these scales. The intrinsic
crosstrack dimensions of the MISR pixels was therefore chosen to be 275 meters
at all off-nadir angles. For reasons of economy, the nadir camera makes use of
the A camera design, resulting in a slightly higher crosstrack resolution of 250
meters, and this allows it to provide a slightly better ground locating
reference that is passed on to the observations from the other cameras.
The time between acquisition of successive lines of MISR data (the line
repeat time) is the same for all cameras, with an average of 275 meters of
downtrack displacement. For the 705 km orbit, this requires a repeat time of
40.8 milliseconds.
What is Local Mode?
MISR is capable of taking image data in two different spatial resolution
modes. In Local Mode, selected targets 300 km long are observed at the maximum
resolution of 275 meters (pixel to pixel) in all cameras (250 meters across
track for the nadir camera.)
However, the data transmission rate would be excessive if the instrument
worked continuously at this maximum resolution. Therefore, away from these
selected targets (there will typically be only 6 of these each day) the
instrument operates in what is termed Global Mode, where Earth is observed
continuously at lower resolutions. This is achieved by averaging the adjacent
samples (in both the cross-track and along-track directions on the ground.) This
averaging can be either 4 x 4, 1 x 4, or 2 x 2 pixels, and can be individually
selected for each camera and spectral band.
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