Calibrating a System of Satellite Instruments - Satellite-based Applications on Climate Change - page 16

Fig. 2.1 Three areas of
calibration: radiometric,
spectral, and spatial
Fig. 2.2 Basic concept of
radiometric calibration
common to most satellite
systems
Nevertheless, the word “calibration” often refers to radiometric calibration, unless
specified otherwise.
In radiometric calibration, a satellite instrument calibration system often uses
two relatively well-known calibration points, one is typically the space view, or SP 0
in Fig. 2.2 , and the other is an onboard calibration device or calibration point CP 0 .
This device is typically a blackbody for the infrared and microwave instruments,
while it is a solar diffuser or lamp for the solar bands. Other calibration systems are
also used for satellite radiometers, such as noise diodes for some microwave
radiometers as a calibration source. Assuming that the system has a linear response
to incoming radiance, this two-point calibration is sufficient to calibrate all
observations to sensor data records, or Level 1b data. For a nonlinear system, a
quadratic equation is typically used where the quadratic term is determined
prelaunch and additional uncertainties may be introduced.
While the deep space SP 0 in the two-point calibration is believed to be stable, the
stability of neither a solar diffuser nor the blackbody for CP 0 can be assumed. In
fact, the stability of this calibration point has become a critical issue in the on-orbit