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AVHRR/3

avhrr_photo

The AVHRR/3 is a six-channel imaging radiometer which detects energy in the visible and infrared (IR) portions of the electromagnetic spectrum. The instrument measures reflected solar (visible and near-IR) energy and radiated thermal energy from land, sea, clouds, and the intervening atmosphere.

The instrument has an instantaneous field-of-view (IFOV) of 1.3 milliradians providing a nominal spatial resolution of 1.1 km (0.69 mi) at nadir. A continuously rotating elliptical scan mirror provides the cross-track scan, scanning the Earth from 55.4 from nadir. The mirror scans at six revolutions per second to provide continuous coverage.

The AVHRR/3 provides spectral and gain improvements to the solar visible channels that provide low light energy detection. Channel 3A, at 1.6 microns, provides snow, ice, and cloud discrimination. Channel 3A will be time-shared with the 3.7- micron channel, designated 3B, to provide five channels of continuous data. An external sun shield and an internal baffle have been added to reduce sunlight impingement into the instrument's optical cavity and detectors.

For the NOAA-N Spacecraft, and later missions, the hysteris scan mirror has been replaced with a 360 RPM Brushless DC motor. This motor is more reliable and operates with significantly reduced jitter.

Characteristics


Parameter
Performance
Telescope
8 in. diameter afocal Cassegrain
Scan Motor
360 rpm hysterisis - synchronous
Scan Mirror
8.25 in. x 11.6 in. Elliptical ribbed beryllium
Cooler
Two stage radiant cooler controlled @ 105 K
Data Output
10 bit parallel words
Video Sample Rate
40 kHz simultaneous sample of all channels
Output Data Rate
200 k word/sec max
Line Sync Pulse Out
100 microseconds @ 6 pps
Input Clock
0.9984 MHz
Overall Dimensions
31.33 in. x 14.35 in. x 11.5 in.
Weight
73 lbs.
Line to Line Scan Jitter
+/- 17 microseconds
Scan Sync Drift/24 hours
< 3.0 microseconds

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Goddard Space Flight Center