3.1.2 Instrument Functionality
Figure3.2 below shows the
functions of AATSR. In summary, AATSR
operates by reflecting infrared and
visible energy from either the
Earth or calibration targets off the rotating Scan Mirror onto a Paraboloid Mirror. The
energy is then focused and reflected
into the infrared and visible Focal Plane Assemblies
(FPA) where detectors convert
the radiant energy into electrical
signals. The low level signals from the FPA
are then amplified by a Signal Pre-Amplifier
(SPA) before undergoing signal
processing (including digitisation) and
data formatting. The data are then passed onto
other systems on ENVISAT for
storage and transmission back to the
Earth. The next sections describe how these
functions are mapped to the
individual units in AATSR and section 3.1.3. discusses the
instrument's internal data flow.
|Figure 3.2 AATSR Functional Schematic
The functions are assigned to units and
assemblies as shown in figure3.3 below.
|Figure 3.3 AATSR Hardware Architecture
The components of AATSR
Figure3.4 shows the major
components of AATSR in their
relative positions on ENVISAT. If ENVISAT is
in flight, this is looking from the
right hand side of the satellite,
with ENVISAT moving from left to right in
|Figure 3.4 AATSR's Major Components and their Locations
The AATSR instrument consists of the following
- The instrument itself, known as the Infrared and Visible
Radiometer (IVR) 18.104.22.168.
- The Instrument Electronics
Unit (IEU) 22.214.171.124. ,
providing the signal channel
processing function, the
scan mirror drive control and temperature
sensor conditioning. The Black Body Electronics
Unit (BBU) 126.96.36.199. is
mounted on top, and provides the
control of the black body
heaters and collects temperature sensor data.
- The Digital Electronics Unit
Conditioning and Switching Unit
Unit (DBU) 188.8.131.52. for instrument control,
data formatting and power
- The Cooler Control Unit 184.108.40.206.
which controls the Stirling
cycle coolers (SCC).
- The Instrument Harness 220.127.116.11. ,
which electrically connects
the above items.
These are described in more detail in the
sections which follow.
18.104.22.168 Infrared and Visible Radiometer Assembly
The Flight Model IVR can be seen in figure3.5 below. This
photograph shows the IVR with the thermal blankets present.
|Figure 3.5 AATSR Flight Model IVR
This view is looking up the along-track
(forward view) baffle. The nadir view baffle
is to the left and the two cylinders at the
top and bottom of the picture between
the baffles are the Black Body
cavities. The baffle on the right face
of the IVR is for the VISCAL.
The main features of the IVR are shown in figure1.2 in the user guide.
Figure3.6 below shows an
alternative view of the IVR, looking
from the Earth-viewing side of the instrument.
|Figure 3.6 AATSR IVR
The Infrared and Visible Radiometer is made
up of a number of units:
The IVR structure provides physical
support and mechanical interfaces
for other AATSR components and
ensures stable alignment for
AATSR's optics. Baffles keep
direct sunlight out of the IVR
during normal instrument operation.
The Scan Mirror Unit
(SMU) provides the required
positioning and rotation of the Scan
Mirror. The plane inclined Scan
Mirror reflects the radiation from
the Earth's surface or
calibration sources onto the
Paraboloid Mirror (figure3.8 shows the
scan cycle). The control of the
mechanism is performed by the Instrument
Electronics Unit 22.214.171.124.1. .
Mirror Assembly (PMA) provides the
support for the off-axis paraboloid
mirror and the aperture stop. The
mirror focuses the reflected
radiation from the Scan Mirror into
the Focal Plane Assembly.
The Focal Plane
Assembly (FPA) includes the
detectors which convert the
radiation into electrical
signals and the optical components
necessary to select the seven spectral
ranges of interest.
The FPA consists of two parts, one,
the Infrared FPA (IRFPA), for the
four infrared detectors, which are
cooled to 80K, and the other,
the Visible FPA (VFPA), for the
three visible detectors. The VFPA is
not cooled. The IRFPA contains the
single field stop, which
defines AATSR's Instantaneous
Field of View (IFOV) for all the
detectors. Details of the IFOV and
AATSR's sampling around the
scan are contained in section 126.96.36.199.1. .
Preamplifier (SPA)amplifies each of
the detector signals from the FPA,
before they are passed on to
Electronics Unit 188.8.131.52.1. for processing.
A pair of Black Body
cavities (BBCs) provide a hot (at
approximately 305K) and cold (at
approximately 265K) calibration
reference for the infrared detectors
on every scan, as well as a zero
radiance reference for the
visible detectors. They are
controlled by the Black Body
Electronics unit (BBU) 184.108.40.206.2. . On
AATSR, there are 6 temperature
sensors on each of the BBCs,
compared to the 7 on each for ATSR-1 and ATSR-2. Five of
the temperature sensors are mounted
on the base of the cavity, with the
sixth on the wall.
The VISCAL unit is
the "bright" calibration
source for the visible
channels on AATSR (one of the BBCs
is used as the "dark"
calibration source). The VISCAL
provides an approximately 16%
radiance calibration reference
signal when illuminated by the sun
for approximately 34 seconds once
per orbit near sunrise.
The Cooler Mechanism
Assembly provides mechanical support
for, and includes, the pair of
Stirling cycle coolers (SCC) which
cool the IRFPA detectors. The
structural attachment to the FPA is
decoupled by a pair of bellows,
while the thermal attachment is via
a pair of flexible braids. The
coolers are controlled by the Cooler Control Unit 220.127.116.11. .
The heat dissipated by the coolers is
radiated into space by a thermal
radiator, which is connected to the
Cooler Mechanism Assembly by
The IVR also has
thermal hardware (i.e. multi-layer
temperature sensors), to ensure that
the component temperatures are
maintained within safe limits and
that performance of the
radiometer is not compromised.
The electrical connections between
items on the IVR are provided by the
18.104.22.168 Instrument Electronics Unit/Black
Body Electronics Unit
The radiometer electronics are contained in
the Instrument Electronics Unit (IEU). The
Black Body electronics (BBU) are in a
separate unit which is mounted on the
top of the IEU. Power and the
instrument's Command and Telemetry Bus
are routed through the IEU to the BBU.
22.214.171.124.1 Instrument Electronics Unit
The IEU forms a part of the electronics
subsystem for the AATSR Instrument. As
shown in figure3.3 , it interfaces
with the Digital Electronics
Unit (DEU) and Power Conditioning
and Switching Unit (PCSU) 126.96.36.199. ,
the Signal Preamplifier
(SPA), the Scan Mirror Unit
(SMU), the BBU 188.8.131.52.2. , and the Infrared Visible
Radiometer (IVR) 184.108.40.206. itself.
The IEU houses the electronics for
processing the analogue signals from the
detectors in the Focal Plane Assembly
(FPA) after they have been amplified
by the SPA, for temperature sensors
and analogue monitors and for driving
The interfaces to the DEU/PCSU are for
the transfer of command, telemetry and
science data and for the supply to the
IEU of regulated power.
The electronic system in the IEU are
subdivided into the sub-systems listed below:
(1) Signal Channel Processing
- There are seven Signal Channel
Processors (SCP) which receive the
analogue inputs from the FPA's
detectors after they have been
amplified by the SPA. The SCPs
scale and convert the analogue
inputs to serial digital
words. Scaling is performed under
control of the DEU 220.127.116.11. 's Auto Gain/Offset
and Auto Offset software.
(2) Scan Mirror Drive
- The IEU includes the control system
for the SMU. The performance of the
SMU and its control system is
described in section 18.104.22.168.2. .
(3) Temperature Sensors
- The IEU conditions and digitises
integrated circuit temperature
sensors mounted on the instrument
structure, SMU, VFPA and visible
calibration target assembly. It also
provides these functions for the
cryogenic temperature sensors in the IRFPA.
(4) Visible Calibration Monitor
- The VISCAL includes a
monitor circuit which measures
the radiance of the VISCAL source.
The monitor output is conditioned
and digitised by the IEU.
22.214.171.124.2 Black Body Electronics Unit
The Black Body electronics Unit (BBU) has
the following main functions:
(1) Heater control
- The BBU selects the BBC to be heated
and the heater level under
command of the DEU 126.96.36.199. . Only one BBC
can be heated at a time and
the main heater has three power
settings, of which the intermediate
level is normally used.
(2) Temperature Sensors
- The temperature sensors (6 platinum
resistance thermomeneters on each
CBB and a temperature sensor in the
BBU) are sampled by the BBU on
request of the DEU 188.8.131.52. .
184.108.40.206 Digital Electronics Unit/Power
Conditioning and Switching Unit
The Digital Electronics Unit/Power
Conditioning and Switching Unit
(DEU/PCSU) assembly consists of two units:
The DEU contains the Instrument
Control Unit (ICU)
function and the function to format
the detector and
other data from the IEU. The ICU
from the ground via the Payload
and sends telemetry back. The ICU
software to control the
instrument as well
as for monitoring housekeeping
parameters for out of
limits conditions. The ICU software
is stored in
Electrically Eraseable Programmable
(EEPROM) and can be updated in flight.
the pixel selection function based
programmable pixel map and the
hardware to assemble
the science data and auxiliary data
to make up the
instrument source packets for
the spacecraft's High Speed
AATSR (unlike ATSR-1 and ATSR-2), the source packet
contains all 12
bits for each pixel in the Earth and
calibration targets for all channels
all the time.
Therefore, the pixel map will not be
routinely varied in flight after it
optimised during commissioning (see
section 220.127.116.11. ). The
pixel map is
discussed in more detail in
- The assembly includes the PCSU, which
provides all secondary
regulated and unregulated supplies to
the AATSR instrument.
The DEU interfaces to the spacecraft's
Onboard Data Handling bus via
the Digital Bus Unit, supplied by ESA.
18.104.22.168 Cooler Control Unit
The Cooler Control Unit (CCU) controls the
two Stirling cycle
coolers in response to commands from the
DEU 22.214.171.124. . The CCU commands
the coolers' operation and continuously
monitors appropriate parameters to monitor
their operation. The cooler subsystem
ensures that the IRFPA detectors are
maintained at a stable cryogenic
temperature (80K nominally), around the
orbit and over longer time periods.
126.96.36.199 Instrument Harness
The instrument harness electrically connects
the AATSR assemblies. It also
includes electrical bonding straps from
the IVR and electronics units to the
ENVISAT ground reference rail.