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The 44 MODIS data sets, or products, as they
are called, are somewhat like a biological food web. Products
fall into one of four categories, with lower-level products
being fed as input to the higher-level, more complex products.
The base of the food web is formed from the minimally processed
raw radiance counts provided in the Level
1A product. The Level 1A data product is a collection
of the radiances MODIS detected reflecting back from the earth
and its atmosphere – a stream of numbers, one after
another, all day, every day. To make these raw radiance numbers
useful for earth science research, two things have to happen:
calibration and geolocation.
Calibration is the process by which scientists adjust the
raw output of a measuring device--in this case, the MODIS
sensor--for known sources of error or interference. Geolocation
is the process by which scientists specify where on the Earth's
surface or in the atmosphere a specific radiance signal was
detected. These processes take place in the MOD 02 Level 1B
and MOD03 data sets. Together, MOD 02 and 03 form the basis
for every other MODIS product. So, if MOD 02 or 03 have any
errors, all subsequent products that "feed" on these
data will have the same errors.
It is precisely for this reason that calibration is so vitally
important to MODIS. There are many factors that can affect
the radiance signals that MODIS is detecting and can cause
them to contain errors, for example broken or malfunctioning
detectors, as well as normal degradation of the sensor over
time as it is exposed to the harsh environment of space. Unless
we take into account how these things affect the instrument,
the data will be inaccurate. To counteract this, MODIS was
designed with a number of built-in calibration instruments
– the Black Body, the Solar Diffuser, and the Spectroradiometric
Calibration Assembly.
The Black Body (BB) is designed to monitor the temperature
inside the instrument and provide calibration data for the
thermal (heat) bands. This is important because in the infrared
(heat) wavelengths, every object emits photons (particles
of light) according to temperature. Because the BB is black
(a color that absorbs all light), it will have a known temperature
against which the instrument can be calibrated. When MODIS
looks at the BB and records a temperature, the difference
between what is recorded and what the temperature should be
will tell MODIS scientists how much they have to adjust the
data to make them accurate.
Instead of calibrating the thermal bands, the Solar Diffuser
(SD) provides calibration for the reflective bands. The white
color of the SD reflects all light from the Sun. Because the
Sun has a constant light output, MODIS scientists know exactly
what MODIS should see when it looks at the SD. Differences
in what MODIS actually sees and what MODIS should see will
tell scientists that MODIS instrument and/or the SD are degrading,
and that they have to adjust the data to make them accurate.
The Spectroradiometric Calibration Assembly (SRCA) is like
a little self-contained laboratory inside of MODIS that monitors
visible, near-infrared, and short wave infrared detection
within the instrument, but it also monitors and calibrates
itself to ensure that the calibration data it provides are
not themselves inaccurate. The SRCA is quite important, because
it monitors not just one region of light, but three. The data
that the SRCA gathers are sent to MODIS scientists, who then
adjust MODIS data accordingly.
Not only do these components allow MODIS scientists to ensure
that the instrument itself doesn’t interfere with the
data, but one calibration component, the Solar Diffuser, even
has its own calibration monitor, the Solar Diffuser Stability
Monitor (SDSM). The SDSM monitors a critical calibration component,
the Solar Diffuser, and ensures that the MODIS data are that
much more reliable. (For more information on MODIS’
calibration components, please refer to the components
page.) We must also take into account that MODIS is on a spacecraft.
The spacecraft occasionally has problems or experiences events
that affect MODIS’ data, and those phenomena must also
be taken into account in the calibration process.
On the ground, MODIS scientists incorporate the calibration
data that they receive from the instrument into the data set
production process. Calibration, then, allows scientists to
produce useful products such as radiance, reflection, or temperature
from the sensor's raw output after monitoring the instrument
and its components for degradation or other events and phenomena
that affect the data, and then adjusting for those factors.
Calibration, therefore, is a process of both data conversion
and quality control.
The more we learn about the technology of the instrument and
the planet it observes, the more refined the data will become.
Thus, to keep the data web up to date and increasingly reliable,
there will be a major reprocessing effort every year or two.
By ensuring that the base MODIS products are as high quality
as possible, we ensure that all subsequent products and understanding
drawn from them are dependable for the scientific community
and the world.
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