Building Earth Observation Cameras

141Optomechanical Scanners

To monitor the radiometric stability of the instrument, ETM+ carries three

calibration devices: an internal calibrator consists of two tungsten lamps, a

black body, and a shutter that oscillates in front of the primary focal plane.

There are two external calibration schemes, which direct sunlight into the

ETM+. In one case—partial aperture solar calibrator—an additional optics

covering a reduced aperture directs sunlight to the instrument. Viewing

this when the Earth underneath is dark is used to monitor the stability of

the reective channels. The other external calibration scheme has a diffuser

panel deployed in front ...