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MISR's Sensitivity to Light
What range of sensitivity does MISR need?
It is anticipated that many scenes imaged by MISR will be quite dark, e.g.
aerosols over water, while others will contain high reflectance contrasts, e.g.
clouds over ocean, inland lakes surrounded by bright terrain. It is therefore
imperative that the instrument provide high sensitivity for a wide range of
scene reflectance (0.02% to 100%) without change in gain.
How much noise can the MISR detectors afford to have?
A signal to noise ratio greater than 100 for dark (2% reflectance) surfaces
is typically required to attain sufficient radiometric precision.
The instrument sensitivity required for ocean color measurements has been
calculated assuming a relatively turbid atmosphere (visibility around 8 km) and
a marine aerosol. In order that uncertainties in the derived pigment
concentrations be limited to factors inherent in the data reduction algorithms
and not instrumental noise sources, a signal-to-noise ratio of around 300 is
required for equivalent scene reflectances of between 4% and 20% for the four
spectral (color) bands. This sensitivity will also be necessary to investigate
aerosol effects over the oceans.
What is special about the way MISR is calibrated?
Because the important parameters for climate research are derived from
measurements taken over times longer than the life of individual satellites, the
only way to ensure that observed trends are the result of changes on Earth, and
not in the instruments, is to provide and maintain an absolute calibration.
Calibration also ensures a synergism between the various sensors operating at
the same time, e.g., other EOS instruments on the Terra spacecraft, Landsat.
To meet these needs, MISR is calibrated using state-of-the-art techniques.
This extends the use of standard detectors, commonly used in national standards
laboratories, to both the preflight testing and the on-orbit environment. A
material that has previously only been used in laboratories as a reflectance
standard, called Spectralon, which is valued for its reflectance uniformity, is
incorporated into the MISR On-Board Calibrator (OBC). The OBC consists of a pair
of deployable Spectralon-coated plates that, when rotated into the field of
view, reflect diffuse light from the sun directly into the instrument, where it
is monitored not only by the main MISR cameras, but also (for
comparison/calibration) by a set of precision photodiodes that measure the
specific light levels. These photodiodes are expected to remain stable for the
life of the instrument.
The OBC data are supplemented with so-called vicarious calibration data sets
when MISR observes specific ground locations at the same time as complementary
in-situ instruments (on Earth's surface). This calibration technique is
desirable from two perspectives. Firstly, the vicarious data are collections of
data taken in the same MISR configuration as all other science data. This
provides assurance that the instrument is performing identically during
calibration as during science data acquisition. Secondly, the independent
methodology from in-situ measurements allows uncertainties to be determined.
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