The Spectroradiometric Calibration Assembly (SRCA) is a calibration
device designed to monitor the visible (VIS), near infrared
(NIR), and short-wave infrared (SWIR) bands on the MODIS instrument.
The SRCA can also generate band-to-band registration (i.e.
spatial) information on-orbit for all 36 spectral bands. The
instrument has internal sources, optics, mechanisms, and electronics
that enable it to generate and modify input stimuli to MODIS
in three modes without interfering with the normal operation
of the main sensor. This means that the SRCA can generate
calibration information at the same time that MODIS is collecting
and recording spectral data, which allows long stretches of
continuous data (which in turn is valuable to scientific pursuits).
The three modes that the SRCA can operate in are spectral,
radiometric, and spatial.
In the spectral calibration
mode, a light source from the integrating sphere (define)
provides illumination for the visible (VIS), near infrared
(NIR), and short wave infrared (SWIR) bands. The light coming
from the integrating sphere is bounced off of a toroidal (3-D
doughnut shape) relay mirror into the entrance of a modified
Czerny-Turner monochromator (an instrument that obtains the
light of one wavelength or very narrow band of the light spectrum).
The monochromator contains a motor-driven grating/mirror assembly,
which is used in conjunction with a filter wheel (to block
out-of-band energy) to diffract the light into one of the
VIS, NIR, or SWIR bands. The diffracted light coming out of
the monochromator is then sent into a Cassegrain telescope
that moves the light to the right elevation and angle (called
collimating) and then bounces it off of a fixed-fold mirror
onto the Scan Mirror. The light from this point moves through
MODIS’ main optical system and onto the detectors where
the resultant calibration data is integrated into the general
At minimum, one SRCA spectral wavelength measurement is taken
per revolution of the Scan Mirror. Computer modeling has shown
that the accuracy of this calibration method is within 1 micrometer.
The SRCA system is also able to perform self-calibration by
inserting didymium glass near the light source and sensing
the SRCA’s response profile using a photodiode at the
exit of the monochromator.
In the radiometric calibration mode, the entrance and exit
slits to the SRCA are open and a silvered mirror replaces
the grating. This difference makes the monochromator into
a relay, where light from the integrating sphere is sent to
the entrance aperture for the calibration of MODIS’
reflective bands. In this mode, the SRCA is designed to provide
six radiance levels that are stable to within one percent
over an orbital period of 100 minutes. This stability is achieved
by a radiance feedback loop that is based on input from a
In the spatial registration mode, the entrance to the monochromator
is open, and a reticle pattern (a grid or pattern that establishes
scale or position) is placed at the exit of the monochromator.
In addition to the VIS-NIR-SWIR illumination, the radiation
from a resistive heater is coupled into the system via an
ITO dichroic beamsplitter (splits the light into two colors),
which provides energy to support the spatial registration
for the thermal bands. The specially designed reticle patterns
at the exit slit of the monochromator are then projected into
the MODIS optical system, which are then re-imaged and scanned
by the various FPAs to generate the data for the spatial registration
algorithm. The result is a record of the FPA spatial registration
over the life of the instrument, which is very important to
demonstrating MODIS’ reliability.