The satellite sensor is the "Advanced Very High Resolution Radiometer" (AVHRR),
which is carried onboard Polar-orbiting Operational Environmental Satellites (POES) like NOAA-15 and NOAA-17.
The sensor measures in several spectral bands (AVHRR/3 measures in
In the APT system an analog signal is transmitted continuously on VHF and can be received in real time.
The nominal resolution is 4 km/pixel and the line rate is 120 lines/minute (2 lines/second). This system provides a geometrically-corrected
(to reduce the perspective effect due to the Earth's curvature and the satellite altitude)
and reduced resolution data stream from the AVHRR/3 instrument. Only two spectral bands are transmitted to the ground at any time:
a visible channel (Ch. 2) is used to provide visible APT imagery during
daylight, and an IR channel (Ch. 4) is used constantly (day and night). A second IR channel can be scheduled to replace Ch. 2 during the
night time portion of the orbit.
An APT line, consisting of one line of Video A and one line of Video B, is output every third AVHRR scan. Ancillary AVHRR data appear
on one edge of each line and their 64 second repetition period defines the APT frame lenght. The resulting line rate is 2 per second.
Click on the next picture for a real size and unprocessed NOAA satellite image, where the different parts are described. Only six minutes
from an entire satellite pass are shown.