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 scan data.
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 temperature-controlled photodiode.
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.