ASTER User's Guide - Aster - NASA

ASTER Users Handbook
The scanning mirror functions both for scanning and pointing. In the scanning mode the mirror
oscillates at about 7 Hz. For calibration, the scanning mirror rotates 180 degrees from the nadir
position to view an internal black body which can be heated or cooled. The scanning/pointing
mirror design precludes a view of cold space, so at any one time only a single point temperature
calibration can be effected. The system does contain a temperature controlled and monitored
chopper to remove low frequency drift. In flight, a single point calibration can be done
frequently (e.g., every observation) if necessary. On a less frequent interval, the black body may
be cooled or heated (to a maximum temperature of 340K) to provide a multipoint thermal
calibration. Facility for electrical calibration of the post-amplifiers is also provided.
For the TIR subsystem, the signal-to-noise can be expressed in terms of an NE delta T. The
requirement is that the NE delta T be less than 0.3K for all bands with a design goal of less than
0.2K. The signal reference for NE delta T is a blackbody emitter at 300K. The accuracy
requirements on the TIR subsystem are given for each of several brightness temperature ranges
as follows: 200 - 240K, 3K; 240 - 270K, 2K; 270 - 340K, 1K; and 340 - 370K, 2K.
The total data rate for the TIR subsystem, including supplementary telemetry and engineering
telemetry, is 4.2 Mbps. Because the TIR subsystem can return useful data both day and night, the
duty cycle for this subsystem is set at 16%. The cryocooler, like that of the SWIR subsystem,
operates with a 100% duty cycle.
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