No calibration is applied to the normalized, raw visible-channel data (Gibson 1984), and the data must be corrected with the aid of external validation measurements.
On-board infrared calibration is done by viewing a temperature monitored blackbody and space, and then using a two-point calibration scheme. NOAA originally updated these calibrations of the infrared data weekly. These revisions resulted in a look-up table used to replace the original infrared measurements with calibrated radiances that can be related to brightness temperatures through another table distributed by NOAA (Gibson 1984). For more accurate measurements, as are required for the sounding algorithms, a more rigorous approach is needed, one that makes use of the calibration target measurements and the cold space data at each revolution of the satellite (i.e., every 0.6 s). The space measurement is made each scan on both sides of the Earth's disk. The lower of the two values, chosen to avoid the risk of contamination by radiation from the Sun or Moon, is used together with the blackbody measurement. (Personal communication with W. P. Menzel, 1991.) As the blackbody calibration target is not external to the instrument and its radiation does not pass through the telescope foreoptics, these influences have to be modeled. Following Menzel et al. (1981), the calibration equations are:
where V is the output voltage of the detector, N is the input radiance, a is the responsivity of the detector, 
is the transmissivity of the telescope foreoptics,
and 
is the detector offset voltage; z denotes space, i denotes the internal blackbody, t denotes the measurement of the Earth pixel, and v denotes the contributions
to the background radiation of the telescope foreoptics. Taking the space radiance measurement to be zero, equations (2) to (4) can be rewritten as
Ni and Nv are calculated from the temperatures of the blackbody target and eight selected components of the telescope foreoptics; is calculated from the
emissivities, reflectivities and obscuration fractions of the telescope foreoptics, which are determined before launch.
The measurements from the infrared channels are thus converted to calibrated radiance, and these can be related to thermodynamic temperature through the Planck function modulated by the normalized spectral transfer function of each channel.
