AVHRR description

Scanning mechanism

The AVHRR/3, a six channel scanning radiometer, provides three solar channels in the visible-near infrared region and three thermal infrared channels. The instrument utilises an 8-inch diameter-collecting telescope of the reflective Mersenne type. Cross-track scanning is accomplished by a continuously rotating mirror direct driven by a hysteresis synchronous motor. The three thermal infrared detectors are cooled to 105K by a two-stage passive radiant cooler.

The Sun angle is defined as the angle between the satellite-to-Sun line and the normal-to-orbital plane with the spacecraft in its operating orientation. The AVHRR/3 instrument, with the exception of the radiant cooler, operates within specification when exposed to any Sun angle from 0° to 80°. The radiant cooler operates within specification for solar angles between 0° to 68°. The spacecraft provides a Sun shield to protect the radiant cooler.

The AVHRR/3 uses an internal rotational scanning mirror that also views deep space and an internal calibration source. Those facilities (view to space and on-board black body targets) are used for the primary calibration of the IR channels (3B, 4 and 5), that occurs at each scan cycle.

  Central wavelength
(mm)
Half power points
(mm)
Channel noise specifications Time availability
1 0.630 0.580 / 0.680 S/N 9:1 @ 0.5% albedo 24 hours
2 0.862 0.725 / 1.000 S/N 9:1 @ 0.5% albedo 24 hours
3a 1.1610 1.580 / 1.640 S/N 20:1 @ 0.5% albedo Day (Selectable)
3b 3.740 3.550 / 3.930 NEDT
0.12K @ 300K
Night (Selectable)
4 10.800 10.300 / 11.300 NEDT
0.12K @ 300K
24 hours
5 12.000 11.500 / 12.500 NEDT
0.12K @ 300K
24 hours

AVHRR/3 Channel Characteristics

Last update: 28 June 2006

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