Special Topics: LDCM and LDCM Components
The Calibration Subsystem for the Operational Land Imager (OLI) on LDCM uses both the sun and the absence of light to establish the light and dark end points, respectively, of a scale used to interpret the collected reflectance data. It does this with a solar diffuser panel that is periodically rotated into the optical path to reflect the sun’s energy onto the focal plane array (light end of the scale), and with a dark shutter that is closed periodically to create an absence of light (dark end of the scale). The image data is corrected on the ground using this information along with calibration data taken prior to launch. Without accurate calibration the data would not be reliable enough for scientific applications. Calibration ensures a high level of accuracy for each spectral band’s reflectance measurements.
The OLI Calibration Subsystem Assembly has two moving parts. The first is a wheel which holds two solar diffuser panels (i.e. nearly white panels extended in front of the OLI full aperture) that are rotated into place for solar calibration as the satellite ascends over the North Pole from the night side so the Sun’s irradiance can be reflected into the aperture. The second is a separate shutter that allows dark data to be collected when closed. The open position in the diffuser wheel is rotated into place for routine imaging. The nadir (down-looking imaging) port and the solar calibration port are protected from stray light by sunshade baffles.
With information about both the reflectance properties of the diffuser panel surfaces and the sun’s angle when radiation strikes the panels, Landsat scientists know the expected radiance of the diffuser, this expected radiance can be compared to the actual radiance to calibrate the reflective bands.