The AVHRR/2
The AVHRR images are constructed by using a scan mirror inclined at 455 rotating about an axis along the flight direction. The ground speed of the sub-satellite
point is ~6.67 km 
and the spatial resolution of the AVHRR is ~1.1 km at nadir, but becomes poorer towards the edge of the swath because of geometrical
effects. Thus, successive scans provide contiguous, or overlapping, data. Each scan of the swath is divided into 2048 pixels, although these are not truly
independent (e.g., Breaker 1990). The swath width is defined as 
55.45 of nadir and this results in wide images (>3000 km) which provide complete global coverage
twice daily. The scan direction is from West to East on ascending arcs.
Included in each rotation of the scan mirror are measurements of an on-board blackbody calibration target, the temperature of which is monitored by four platinum
resistance thermometers (PRTs). The digital readout of these thermometers is also included in the telemetry stream. Additionally, "cold space" data, i.e.,
measurements of space radiation from the direction away from the sun, are taken.
The channel spectral responses are defined by beam-splitters and multilayer interference filters. The characteristics of these change from satellite to satellite;
this information is distributed by NOAA (e.g., Kidwell 1988). The detectors for Channels 1 and 2 are made of silicon, for Channel 3 indium antimonide, and
for Channels 4 and 5 mercury cadmium telluride. The detectors are chilled to a low temperature of ~105 K by passive radiative coolers. The sampling time of
each pixel is ~25 
s.
The detectors measure a segment, 
, of the electromagnetic spectrum, R(
), weighted by the transfer function, 
(), of Channel i :
If the radiation is emanating from a blackbody source, R() is given by the Planck function. This is the radiation at the height of the satellite, which is
often referred to as being at the top of the atmosphere (TOA).
The signals from each channel are digitized on-board the spacecraft to 10-bit resolution (0-1023).
