3.1.3 Subsystem description The space segment of MIPAS is divided into two modules.
These modules are further divided into the
following subsystems and assemblies (see index)
3.1.3.1 MIPAS Optics (MIO) module
This module includes the Front End Optics 3.1.3.1.1.
subsystem, the interferometer and the
focal plane subsystem, mounted at the
anti-sunward end of Envisat-1. The MIO, is about 1.36 m
in the flight direction, 1.46 m high in the
nadir direction and
0.74 m in the deep space direction. It has a
mass of about 170 kg.
The outside of the MIO is covered in
multi-layer insulation to minimize heating
from the Sun and the Earth shine. MIPAS
is an infrared sensor. It is necessary to
reduce the amount of infrared radiation
emitted by the instrument itself so that
this radiation does not mask the infrared
radiation of the atmosphere. To reduce the
thermal self-emission of the optical
components, the MIO is cooled using
passive cooling. A large radiator is used
to cool all optical components to about 210
K and two smaller radiators are used to cool
the compressor of the Stirling cycle coolers
of the detectors and to pre-cool the focal plane subsystem.
All radiators are tilted away from nadir by 20 degrees top
reduce the Earth shine and thus improve
their efficiency.Below the
MIO are two baffles that reduce the
amount of stray light that may enters MIPAS: one for the rear
view and one for the side view. The baffle
for the rear view extends sufficiently
far from the first optical component to
prevent the direct entry of sunlight when
the instrument is observing the south
pole region in summer. In this situation,
the minimum angle between the Sun and the LOS of the instrument
could be as small as 8 degrees. The size of
the baffle is reduced on the side
illuminated by the Sun to reduce heat
input. Further reduction of the
baffles' temperature is achieved by
using a coating that is reflecting in the
visible but absorbing in the thermal infrared.
|
Figure 3.3 3-D view of the MIO |
|
Figure 3.4 Picture of the MIO on its back from the rear side. Note the characters on the back. |
3.1.3.1.1 Front End Optics (FEO) subsystem
This subsystem houses the
azimuth scan unit and elevation scan
unit, the anamorphic
telescope and the internal
calibration blackbody assembly.
3.1.3.1.1.1 Azimuth Scan Unit (ASU)
-
The azimuth
scan unit allows the
selection of the line of
sight within the two field
of view regions, and also to
access an internal
calibration blackbody source
for gain calibration. A flat
steering mirror is rotated
about an axis parallel to
nadir to direct the
light into the instrument.
This steering mirror has a
dimension of about 295 mm in
height and 109 mm in width
and thus forms the
largest optical component of
MIPAS.
A second function of the ASU
is the protection of the
interior of the optics
module from contamination; a
shield is mounted behind
the steering mirror and
rotates with it. When the
mirror is turned to an end
stop, the shield closes the
input aperture to the ASU
and thus the ASU
mirror from contamination
during ground handling and
the early flight phase.
3.1.3.1.1.2 Elevation Scan Unit (ESU)
-
The elevation
scan unit determines the
actual limb height of a
particular measurement, and
thus requires a very high
pointing accuracy
over a limited angular
range. It comprises a flat
steering mirror rotating
about an axis that is
orthogonal to nadir and
flight direction.
The angle covered by this
mirror is less than 3 °
which is sufficient to reach
limb heights between 5 km
and 250 km; the high
value will be used for
measurements of cold space
to determine the instrument
self emission for offset
calibration.
3.1.3.1.1.3 Calibration Blackbody
Assembly (CBA)
-
This is a blackbody
mounted in the azimuth scan
unit is the
calibration blackbody, used
for the in-flight
calibration of the
instrument responsivity. To
fill the IFOV, it
needs a rather large clear
aperture (55.165
mm2). Its design
is derived from the
blackbody design for the
along-track scanning
radiometer (ATSR), presently
flying on the ERS-1 and
-2 satellites. Its emissivity
is above 99.6 %, so that a
high accuracy for the gain
calibration becomes
achievable. For precision
gain calibration
measurements, it can be
heated to about 40 K above
the ambient instrument
temperature to increase its
radiance emission.
Its nominal temperature will
then reach up to 250 K. The
temperature of the
calibration blackbody is
monitored by a platinum
resistance
temperature (PRT)
sensor. The
electrical signal of the PRT is
included in the data packet
downlinked to the
ground station.
3.1.3.1.1.4 Receiving Telescope (TEL)
-
The front-end
telescope collects the
incident radiation to
collimate it so as to match
it to the input dimensions
of the
interferometer, and defines
the IFOV of MIPAS.
Driven by the demand for an
atmospheric object size with
a large edge ratio (30 km
horizontal to 3 km
vertical dimension), the
overall volume of telescope
and interferometer resulted
in a design with a
magnification of 6 in
elevation and 1 in
azimuth. The input aperture
of the telescope is 55.165
mm2, and thus the
entrance aperture of the
interferometer is
55.27 mm2. A
further reduction of the
free aperture to 135.35
mm2 by two Lyot
stops is necessary to reduce
the stray light
contribution. A field
stop in the focal plane of
the front-end telescope
defines the instrument IFOV.
Thus, the view geometry of
all following components is
uniquely determined by
this component and not by
the position of the cold
stops in front of the
detector elements, thereby
ensuring that all detection
channels view the same
atmospheric volume at the
Earth's limb.
3.1.3.1.2 Interferometer (INT) subsystem
To meet
the radiometric and spectrometric
performance requirements, as well as the
lifetime requirement of four years of
continuous operation in space, a
symmetrical dual slide interferometer
with dual input and output ports has
been selected. This provides
highest detectable signal at the
outputs, the least uncertainties in
design, the highest degree of
redundancy, and the most compact
dimensions. It has a folded path to
allow a more compact arrangement of the
interferometer and to allow better
compensation of the momentum
generated by the cube corners during the
reversal of their motion. The incident
angle of the radiation onto the
beamsplitter is 30 ° to
reduce polarization effects by the
beamsplitter. The MIPAS
interferometer is over 0.58 m long and
about 0.36 m wide, and has a mass of
about 30 kg. It has the following
major subassemblies:
Interferometer Optics (INO),
Interferometer Mechanism Assembly (IMA),
and Optical-path Difference Sensor. Click here for details
on how the interferometer works 1.1.3.1. .
|
Figure 3.5 The MIPAS interferometer and its main parts |
3.1.3.1.2.1 Interferometer optics (INO)
-
The interferometer optics
comprises the beamsplitter
assembly, flat
steering mirrors, and the
cube
corners on the slides.
The beamsplitter coatings
themselves are quite critical,
as they have to provide a
reflectance near 50 % throughout
the broad spectral
range. More difficult to
manufacture are the broadband
antireflection coatings on the
other surfaces that are
essential to reduce
undesired interferometer effects
that would modulate the
transmission of the substrate
and could result in ghost
spectra. The
beamsplitter assembly also
has to compensate the phase
delays caused by the varying
refractive index throughout the
spectral range. This is
done with a second substrate of
same thickness as the
beamsplitter itself and mounted
with a narrow gap to the
beamsplitter coatings. Both
substrates have a slight wedge
angle to reduce the residual
Etalon effects.
3.1.3.1.2.2 Interferometer mechanism
assembly (IMA)
The two
identical interferometer drive
units perform the actual
translation of the cube corners.
Linear motors behind the cube
corners generate the
drive force. The slides are
guided by mechanical bearings.
The lifetime requirement of four
years' continuous
operation corresponds to about
20 million motion cycles for
each of the bearings. Lifetime
tests have shown that dry
lubricated ballbearings
operating with a light preload
can well achieve this
lifetime. The difference
velocity between the two slides
has to be controlled
with less than 1% rms error. A
drive control loop processes the
inputs from linear optical
encoders in each of the drive
arms for a coarse control and
for centering of the slides, and
from a built-in laser
interferometer (called the
optical-path difference
sensor or ODS) for fine velocity
control. The laser
interferometer is also required
to trigger the sampling of the
detector output at very
precise intervals of optical
path values.
3.1.3.1.2.3 Optical-path difference
sensor (ODS)
-
The
built-in laser
interferometer makes use of
a single-mode 1.3 micron
diode laser which is located
in the optics module near
the Stirling
coolers. The output from the
diode laser is guided by a
single mode polarizing
optical fibre to the
interferometer. Although
the individual components
are proven in many
communication systems, their
use in a spaceborne
instrument with operation
over a wide
temperature range is new and
requires space
qualification. The 1.3
micron radiation from the ODS laser
and its fibre optics are
circularly polarised and
injected to the
interferometer via dedicated
filter coatings on
the beamsplitter. The
circular polarisation allows
to retrieve both sine- and
cosine components of the
superimposed
beams, and thus to determine
the direction of the cube
corner motion. This
direction information will
be important as the
interference fringes of the
optical path difference
system will provide an
absolute position reference
between two gain calibration
sequences, that must be
accurately maintained. The
laser diode is stabilised in
temperature to limits its
frequency drift to less
than 50 MHz for periods of
200 seconds. No absolute
frequency control is used
since the spectra acquired
by MIPAS can
easily be spectrally
calibrated using known
atmospheric lines.
3.1.3.1.3 Focal Plane Subsystem (FPS)
The two
output beams from the interferometer are
reduced in size by two small off-axis
Newton telescopes, and directed into the
cold focal plane subsystem, which
houses the signal detectors with their
interfaces to the active coolers, as
well as the associated optics
required for spectral separation and
beam shaping. It is smaller than the
interferometer (0.36 m wide and 0.45 m
high, including the precooler
radiator on top) and has a mass of 16
kg. To achieve the best
radiometric sensitivity, a set of four
detectors in each output port (thus a
total of eight detectors) are used,
each optimized for highest sensitivity
in a spectral band. A set of
beamsplitters and steering mirrors
separate the input from the two
interferometer ports to the different
spectral bands, and the optics required
to illuminate each detector element. All
optical elements are mounted and
aligned in a very tight package.
All optics and the detectors are cooled
to 70 K to reduce their thermal
emission. Cooling is performed by
a pair of active Stirling cycle coolers.
Thus, although the focal plane subsystem
is conceptually a simple design,
the numerous interfaces between the
optics, the detectors and the coolers
under the constraints of good thermal
insulation and high alignment
stability of the optical components
result in very demanding
requirements. The focal plane
subsystem has the following
elements: detector/preamplifier unit
(DPU) and focal-plane cooler assembly
(FCA) (see below).
3.1.3.1.3.1 Detector/preamplifier unit (DPU)
To achieve the
specified radiometric sensitivity,
detectors have to be optimized for a
specific spectral band. An analysis
has shown that four spectral
bands in each interferometer output
port are required to achieve the low
instrument noise contribution and to
provide some redundancy at the
long wavelength region. Thus a total
of eight detector elements are
needed in MIPAS. In the long
wave spectral region (14.6 to
about 7 microns), only
photoconductive HgCdTe detectors
(PC-CMT) are able to meet the
specifications on low noise
contribution and electronics
bandwidth. At the shorter
wavelengths (7 to 4 microns),
photovoltaic HgCdTe detectors
(PV-CMT) are the best choice.
The detector elements are cooled to
about 70 K to reduce their internal
noise contribution. The
preamplifiers are individually
optimized for each detector to
fulfill stringent requirements on
noise, phase distortions and
linearity. The cold part of
the preamplifiers are mounted in the
detector housing, while final
amplification is performed in an
externally mounted package at room
temperature.
3.1.3.1.3.2 FPS Cooler Assembly (FCA)
-
The complete
inner structure of the focal
plane subsystem (housing,
optics, detectors, and
preamplifiers) is cooled to
70 K. Passive
cooling has been considered
but would require a rather
large cooler, while active
coolers allow to reach these
temperatures under all
operating conditions. Stirling
cycle coolers with a
performance that
satisfies the cooling
requirements of MIPAS (500
mW heat lift at 70 K
temperature) are used in a
twin-cooler arrangement,
comprising two identical
compressor and displacer
units that operate
synchronously to
compensate most vibrations
from the oscillating parts.
|
Figure 3.6 Picture of the FPS Cooler Assembly |
3.1.3.2 MIPAS Electronics (MIE) module
The MIE comprises the electronics support
plate (ESP), the instrument control
electronics (ICE) boxes, the MIPAS power distribution
unit (MPD), the digital bus unit (DBU) and
the signal processing electronics
subsystem (SPE). Most of the MIE is located on the
rear-looking side of the instrument.
Some elements of the MIE (SPE, PAW and FCE) are on the deep
space side of the instrument.
|
Figure 3.7 View of some MIE modules mounted on ENVISAT |
3.1.3.2.1 Electronics Support Plate (ESP)
The plate is the support
on which several of the elements of the
MIE are attached.
The ESP is located on the
rear-looking side of the instrument.
3.1.3.2.2 Instrument Control Electronics (ICE)
The instrument control
electronics ICE contains all
electronics modules to supervise
and to execute macrocommands for MIPAS,
and it also houses the plug-in modules
to drive the FEO and INT subsystems. The
Stirling coolers of the FPS are controlled by
a dedicated electronics box. There are
two ICE boxes (ICE 1 and
ICE 2) for redundancy. Both ICE boxes are attached
to the ESP and placed on the
rearward looking side of the
instrument.
3.1.3.2.3 MIPAS Power Distribution Unit (MPD)
This task of this unit is
to distribute power from ENVISAT to the
various electronical and mechanical
components of MIPAS. It is located
below the ICE on the ESP.
3.1.3.2.4 Signal Processor Electronics (SPE)
The onboard signal
processing electronics (SPE) is in
charge of performing the housekeeping
and the first processing of the raw data
collected by the MIPAS instrument. It
is located on the deep-space side of the
instrument over the MIO. In details, it
performs the following functions:
- analogue anti-alias filtering of
the detector outputs
- digitization (16 bit, 77 kHz) of
each signal
- digital filtering to reduce bandwidth
-
decimation to
reduce the data rate
- combination of some detector
outputs, if appropriate,
downsampling, word length
reduction, and data compressing
to reduce the data rate
- combination of all output data,
formatting and transmission
(nominal data rate is 550
kbit/s) to the platform data
handling and
transmission interface.
Onboard decimation is
used to reduce the data rate. Digital
filtering and decimation can be disabled
by telecommand. However, if they are
disabled, the data rate increases to 8
Mbit/s which can be used only for a
short time. During the
formatting of the data stream, the word
length of the interferogram data is
reduced. As the full dynamic range of ADC is used only
near the zero path difference points,
the remainder of the
interferograms can be coded on a much
smaller number of bits which
significantly reduce the data rate.
The interferograms and pointing data are
downlinked to ground, where the phase
correction, anodisation,
retransformation, and
radiometric/spectral calibration will be
performed to yield the atmospheric
spectra. Further processing of these
spectra to derive concentration
profiles of atmospheric constituents
will also be performed by the ground
segment.
3.1.3.2.5 Detector Preamplifier (PAW)
-
The detector
preamplifiers are responsible
for increasing the signal of the
detectors. They are individually
optimized for each
detector to improve the signal
to noise ratio and the linearity
and reduce phase distortion. The
cold part of the preamplifiers
is mounted in the
detector housing and the final
amplification is performed by
the PAW (preamplifier warm)
which is located close to the
FPS above the
MIO and beside
the SPE. The
preamplifiers gain is
programmable by telecommand.
However, once it is
adjusted to achieve the full
dynamic range of the ADC, it
remains constant during
the interferometer sweep and elevation
scans.
3.1.3.2.6 Focal plane cooler drive
electronics (FCE)
|