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
