3.1.3 Subsystem description

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)

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  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)

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  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)

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  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)

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  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

3.1.3.1.2.1 Interferometer optics (INO)

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  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)

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)

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  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:

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)

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  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)

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  The FCE is responsible for controlling the two synchronised Stirling coolers that keep the detectors at 70 K.