International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004
MULTI-SPECTRAL IMAGER
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Figure 1: Alsat-1 structure
Alsat-1 structure is mainly Aluminium. The solar panels, the
Earth and Space Facing Panels are honeycomb structure. The
satellite size is 600x600x600 mm with a mass of 98Kg.
2.1 Payload description
The Alsat-1 payload is a multispectral camera which works in a
push-broom mode (forward scan is provided by the spacecraft
motion). It is in fact a couple of two imagers (of three channels
each) which work separately or both together. This adds
flexibility to program images (satellite operations).
For each spectral band (green, red, near-infrared), two channels
(from both banks) provide a 600 km swath with (5% overlap
between them) from its 686km altitude.
Figure 2: Alsat-1 Imager
2.2 Camera
It is a line scan camera system designed to provide medium
resolution (32m GSD), high dynamic range and low noise
imagery from orbit. It consists of 6 channels split into two
banks and fixed on a V plate.
Due to the extremely wide swath, each bank acquires half of the
scene (with 5% overlap).
2.3 Channel
Each channel is in fact an independent camera and contains a
complete optic-system plus a PCB supporting the sensor &
linked to the main PCB board in the stack (one main board per
bank) thanks to a flexi-rigid PCB cable.
Sensor: lt is a KLI-10203 linear CCD sensor, designed for
color scanning applications. This is an RGB, 3 channels, 10k
pixels. Due to the mission requirements, this COTS element
was requested from Kodak without the RGB organic dye filters
(applied to the light sensitive areas) but with a multilayer AR
coated cover glass.
Figure 3: Camera head
Optics: The lens is a Schneider Apo-Componon HM 150mm
FL (aperture f/4). Is a flight proven lens (flown onboard
Tsinghua-l satellite in 2000). This commercial lens has been
subject to a set of test prior to use.
Filters: They are high quality dichroic filters in the optical
assembly provided by BARR Associates Inc.. Due to the
mission requirement, the spectral band is red [523-605nm],
Green [629-690nm] and Near Infrared [774-900nm]. The
spectral characteristics are identical to the Landsat ETM+ bands
(2,3 and 4)
Electronic modules: Two boards shares the same PCB module
(one for each bank of three channels).
Each board are fully independent and drives the
three channels (red, green, infrared). It encloses:
_ An FPGA circuit which is the heart of the module as it plays
the main role: receives imaging parameters and generates
signals for CCD sensor, LVDS, ADC...ctc.
— Three programmable ADC which convert the image signal
into digital value and send the data via an LVDS to the
storage modules.
— A local microcontroller interfaces the board with the OBC
via a CAN node.
Each module can be operated individually or synchronised
together to work in a full 6 channels mode.
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