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Instrument: VNIR : Visible and Near Infrared Radiometer
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Description
The Visible and Near Infrared Radiometer (VNIR) is one of
ASTER's three different subsystems.

The VNIR subsystem consists of two independent telescope
assemblies to minimize image distortion in the backward and
nadir looking telescopes. The detectors for each of the bands
consist of 5000 element silicon charge coupled detectors
(CCD's). Only 4000 of these detectors are used at any one
time. A time lag occurs between the acquisition of the backward
image and the nadir image. During this time earth rotation
displaces the image center. The VNIR subsystem automatically
extracts the correct 4000 pixels based on orbit position
information supplied by the EOS platform.

The VNIR optical system is a reflecting-refracting improved
Schmidt design. The backward looking telescope focal plane
contains only a single detector array and uses an interference
filter for wavelength discrimination. The focal plane of the
nadir telescope contains 3 line arrays and uses a dichroic
prism and interference filters for spectral separation
allowing all three bands to view the same area
simultaneously. The telescope and detectors are maintained at
296 + 3K using thermal control and cooling from a platform
provided cold plate. On-board calibration of the two VNIR
telescopes is accomplished with either of two independent
calibration devices for each telescope. The radiation source
is a halogen lamp. A diverging beam from the lamp filament is
input to the first optical element (Schmidt corrector) of the
telescope subsystem filling part of the aperture. The
detector elements are uniformly irradiated by this beam. In
each calibration device, two silicon photo-diodes are used to
monitor the radiance of the lamp. One photo-diode monitors the
filament directly and the second monitors the calibration beam
just in front of the first optical element of the
telescope. The temperature of the lamp base and the
photo-diodes is also monitored. Provision for electrical
calibration of the electronic components is also provided.

The system signal-to-noise is controlled by specifying the NE
delta rho to be < 0.5% referenced to a diffuse target with a
70% albedo at the equator during equinox. The absolute
radiometric accuracy is to be + 4% or better. The VNIR
subsystem produces by far the highest data rate of the three
ASTER imaging subsystems. With all four bands operating (3 nadir
and 1 backward) the data rate including image data, supplemental
information and subsystem engineering data is 62 Mbps.

[Summary provided by NASA]