1.4.2

Entrance Telescope

The FCI entrance telescope is “pupil-and-field” off-axis with a well-adapted 5.5 aperture number for channels grouping optimization. This optical layout has been designed to be diffraction limited for the whole instrument field-of-view, with no central occultation for straylight reasons, mainly sensitive in the infrared part of the spectrum.

The angular off-axis has been introduced to obtain an accessible intermediary focal plane after the secondary mirror to simplify the calibration device and diaphragm implementation (solar input baffling). This off-axis angle allows keeping the chief ray of the FOV center parallel to the optical axis.

Figure 4:

The FCI telescope is designed and developed by OHB Systems in Oberpfaffenhofen under an ESA/Thales Alenia Space contract. On the left CAD view without baffles, and on the right the FCI Telescope first flight model, after delivery to Thales Alenia Space Cannes facilities

The design of the telescope has reached full maturity: qualification close-out planned early 2021.

1.4.3

Spectral Separation

The 16 spectral channels, extended from the visible part of the spectrum to the thermal infra-red, have been split in five groups of spectral bands with a pure spectral separation performed by dichroic beam-splitters. The separation type minimizes the in-field separation which contributes significantly to the scanning law error budget. The selected polarization-oriented design leads to the current three dimensional arrangement offering a perfect polarization balancing between in-plane and out-of-plane polarization states.

Figure 5:

Spectral Separation Assembly is designed and developed by Safran-Reosc, under an ESA/Thales Alenia Space Contract. Picture on the right side: : credit Romain Berruée – Safran Reosc

SSA qualification level has made a large step forward in 2020 after the successful test of the coating robustness under sun intrusion. Formal qualification closure is expected by the first quarter of 2021.

1.4.4

Cold-Optics

There are 1 NIR and 3 IR spectral groups, each of them having its camera. These cameras offer at the same time high image quality at cryogenic temperature, low distortion and high throughput.

Figure 6:

schematic view and as-built picture (Cover image © 2018 Thales SESO^®) of the CO-I elements These elements are designed and manufactured by Thales SESO^®, under an ESA/Thales Alenia Space contract.

All qualification activities are now finalized, and the closure is planned Q1 2021

1.4.5

Detector Assemblies

The 16 spectral channels have been grouped in 5 Detector Assemblies (VIS, NIR, IR1,IR2,IR3)

Each Detector Assembly is based on a CMOS image sensor die mounted in a package that incorporates optical filters, a permanent window and a ribbon flex terminated with a custom 61 way Deutsch connector.

In order to ensure full image over lifetime and be robust to any pixel operability evolution, a four pixel redundancy is implemented on each sensor.

Figure 7:

The VIS Detector Assembly (on the left) is developed by Teledyne e2V, while the NIR/IR Das (on the right) are developed by Lynred, both under ESA/Thales Alenia Space contracts.

The VIS detector Assembly is located at the Telescope focal plane, and operated at ambient.

The NIR/IR detector Assemblies are located at Cold Optics foci, inside the cryostat. They are operated at 58K.

This new generation offers a wide field of view of about 200km, which, associated to the scan speed, offers the MTG short revisit time of a Full Earth Disc in 10min.

The VIS detector Assembly design is qualified.

The NIR/IR Detectors qualification will be pursued all along 2021. This late qualification has been properly anticipated in the development plan, with risks covered by early validation tests on representative samples.

1.4.6

Video Chain Unit (VCU)

MTG-FCI Video Chain Unit function is developed by Thales Alenia Space. It is composed of 5 Front-End Electronics (FEE), one for each detector assembly, and one Video Acquisition Electronics (VAE), which performs the Analogic to digital conversion. The time tagged video data are sent to the Instrument Control Unit.

Figure 8:

VCU Protoflight Model, during its EMC tests.

The qualification process is under finalization, with formal closure planned early 2021.

1.4.7

Cryo-Cooler Unit (CCU)

The Large Pulse Tube Cooler brings the 4 FCI NIR/IR detectors down to 58K with limited microvibration generation.

Figure 9:

The MTG Large pulse-Tube cooler is developed Air-Liquide Advanced Technologies under an ESA/Thales Alenia Space contract

This cooler has already a strong flight heritage and its qualification for use on MTG is closed.

1.4.8

Elastomer Suspension Elements

In order to filter the remaining microvibration, the cryo-cooler is suspended on elastomer elements. This technology has been developed by Thales Alenia Space under MTG contract. Its design, and in particular its cut-off frequency, is tuned to both

Figure 10:

FCI elastomer Suspension Elements (ESE), a Thales Alenia Space technology

ESE have recently successfully passed ageing test, and qualification close-out is planned to be formally close early 2021.

1.4.9

Instrument Control Unit (ICU)

The ICU is the central computer in charge of instrument operating modes management and merges the main FCI interfaces with the platform. It is responsible for:

Beyond the classical instrument management functions (Modes Management, Telecommand/Telemetry and Failure Detection and Isolation, Power Distribution and Thermal Control), this ICU also includes the mechanisms drive electronics.

On top of its multiple functions, one of its key performances is to monitor the internal blackbody (used for IR channels gain in-flight calibration) to a few mK accuracy.

Figure 11:

Instrument Control Unit is developed by Thales Alenia Space.

Qualification tests have been completed end 2020, and qualification review is planned Q1 2021.

1.4.10

Envelope and engineering budgets

The FCI mass is below 470kg, which is well within the platform capability.

Its envelope is about 1.55mx1.4mx2m, and is accommodated to allow, on the same platform side, the accommodation of the Lightening Imager instrument (developed by Leonardo under an ESA/Thales Alenia Space contract).

Figure 12:

MTG-FCI overview. Note that three electronics (Instrument Control Unit, Cryo-Cooler electronics and Scan Electronics) are located inside the platform.

2.1.1

Configuration of the specimen under test

The DCEA Engineering Model is built from the following equipments, having flight representative interfaces and performances:

Figure 13:

Detection Control Electronics Assembly: the heart of the MTG FCI Image acquisition

Together assembled, these units form the heart of the instrument, were the key optical and radiometric performances are generated.

2.1.2

Tests

The DCEA EM has been submitted to the following main tests:

Figure 14:

For these tests, several tests configuration have been developed, including custom made Optical Ground Support Equipments (OGSEs).

2.1.3

Lesson learnt

The DCEA EM demonstrated the suitability of the tests setup to

A side lesson learnt was that the polarization sensitivity measurement of the DCEA was degraded by test configuration straylight. The Proto-Flight model polarization characterization has thus been performed with a more suitable Ground Support Equipment.

Also, DCEA EM performances were measured as predicted, improving confidence in the FCI design:

2.2.1

Configuration of the specimen under test

The STM is composed of representative structure and thermal control (both passive, like multi-layer insulation and active, like the cryo-coolers and heaters). Also, all the mechanically representative mechanisms are included, and in particular the refocus mechanism, the calibration and obturation mechanism, and the scanner.

In order to get an accurate understanding of the behavior of the FCI under the critical space environments, the FCI STM is equipped with up to 400 thermocouple and 100 accelerometers.

Figure 15:

FCI STM finalization, on the shaker.

2.2.2

FCI STM Tests

In addition to the classical mechanical tests, qualifying the design against launch loads, the FCI STM has been submitted to very specific and complex tests: thermal solar vacuum tests at ESA/ESTEC and microvibration tests in Thales Alenia Space Cannes facilities. FCI STM was then delivered for satellite level testing (more details can be found in [2]).

2.2.3

Vibration tests

Mechanical vibration tests are quite classical for space borne instruments. What was particular for the FCI was the deep international cooperation set in place: Thales Alenia Space for the instrument, OHB for the Telescope Assembly and ADS-Spain for the structure. The risk associated to this complex responsibility chain has been mitigated by a modal survey (low level sine), which helped to anticipate, at structure level, the Instrument Finite Element Model correlation.

2.2.4

Thermal tests

As for all earth observation instruments located in geostationary orbit, the sun is illuminating the FCI aperture each day around midnight. Depending upon the day in the year and the phasing with the scan law, this sun intrusion goes more or less deep inside the instrument optical path illuminating, not only the mirrors and lenses, but also the baffles and surrounding structures and thermal control. Only a solar test with a space simulator could bring an environment representative enough to test the FCI.

This is why the FCI STM travelled from Cannes(France) to ESTEC (Netherlands) Large Space Simulator (LSS).

Figure 16:

FCI STM entering the ESTEC Large Space Simulator

The main lessons learnt was the demonstration of the FCI design to withstand its thermal environment in space. More precisely, the temperatures in hot, cold with and without sun confirmed the expectations, validating the instrument thermal control design. This test also drove some design optimization applicable to the flight models: some heaters were added on loop heat pipes, and the thermal mathematical correlation led to adjust some regulation parameters of the thermal control software applicable to flight.

2.2.5

Microvibration and EMC tests

Back to Thales Alenia Space facilities in Cannes (France), the FCI STM completed its test campaign by microvibration and EMC tests.

A microvibration test has been held to quantify the level of FCI LoS jitter that the Cryo-cooler (CCU) Noise Source generates. This test qualified not only the FCI pointing performance (LoS jitter), but demonstrated also the efficiency of the cryo-cooler suspension (ESE).

For this test the FCI STM was attached to a Vibration Test Adaptor (VTA), suspended from the floor with pressurized Air Actuators. Critical Noise positions were excited (CCU & SCAN Noise Source Interface) with suspended μShakers and exported vibrations were measured by accelerometers (placed on optical performance critical points). The generated Transfer Functions were used to verify (and if needed to correlate) the Numerical Model of the Test Setup. Moreover, the CCU Noise Source was activated and exported vibration were measured by accelerometers. To allow qualifying directly the FCI LoS jitter, these accelerometers were positioned in a way to be able to reconstitute the LoS jitter for each optic. The Total FCI LoS jitter could then be reconstituted, by combining all optics, with help of the optical sensitivity matrix.

The FCI STM was finally used to assess the EMC shielding efficiency which is of particular importance for the MTG-I mission. Indeed a Data Collection System & Geosar payload are co-passenger on MTG-I satellites to collect and distribute SAR data (Search & Rescue) and are sensitive to E-Field interference.

Figure 17:

FCI STM in Faraday room for EMC tests(on the left), and during micro-vibration tests (on the right)