|Cryocooler (Yellow) - The ten Mercury-Cadmium-Telluride detectors in
each of the five TIR channels are cooled to 80 K using a
mechanical split Stirling cycle cooler of long life and low
Reference Plate (Black Body) (Green) - A high emissivity reference
plate is used as the on-board calibration reference for the TIR
subsystem. This reference plate is viewed before and after each
observation to provide an estimate of instrument drift and
periodically this plate is heated through a range of temperature
to provide an estimate for both instrument gain and offset.
Scan Mirror (Red) - The scan mirror is used for both scanning and
pointing. In the scanning mode the mirror oscillates across the
ground track at about 7 Hz. This mirror can point +/- 8.54
degrees from the nadir direction to allow coverage of any point
on the earth over the spacecraft's 16 day mapping cycle. This
mirror can also rotate 180 degrees from the nadir direction to
provide a view of the reference plate for calibration.
Telescope (Blue) - The TIR subsytem uses a
Newtonian catadioptric system with an aspheric primary mirror and
lens for aberration correction. Unlike the VNIR telescope, the
telescope of the TIR subsystem is fixed and both pointing and
scanning is done by the mirror.
The TIR subsystem uses a Newtonian catadioptric
system with an aspheric primary mirror and lenses for aberration
correction. Unlike the VNIR and SWIR telescopes, the telescope of
the TIR subsystem is fixed with pointing and scanning done by a
mirror. Each band uses 10 Mercury-Cadmium-Telluride (HgCdTe)
detectors in a staggered array with optical band-pass filters
(Table II) over each detector element. Each detector has its own
pre-and post-amplifier for a total of 50. Performance of the
system will be improved if photovoltaic detectors can be used.
Development of such detectors is a technical challenge.
As with the SWIR subsystem, the TIR subsystem will use a
mechanical split Stirling cycle cooler for maintaining the
detectors at 80K. In this case, since the cooler is fixed, the
waste heat it generates will be removed using a platform supplied
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 one 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. Another major technical
challenge facing the ASTER team is to establish before flight
that the elements of the inflight calibration and subsystem
design will permit high quality accurate thermal radiometry.
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 has been set at 16%. The
cryocooler, like that of the SWIR subsystem, will operate with a
100% duty cycle.