4.1

Critical Subunits qualification

The EM FPCA and interferometer qualification has been already presented in ref[2]. We will just recall the main outcomes.

EM FPCA allowed validating the overall concept with regard to thermal and mechanical environment as well as the cooling capacities. After alignment, specific sequence with EM detectors cooled validated also the preliminary radiometric performances and the radiated field susceptibility (auto-compatibility).

The interferometer show very stable optical and dynamic characteristics after environment tests (thermal and mechanical). During thermal environment simulating worst non-operational case, the thermal gradient and slopes inside the KBr mounts (prisms and beam spliter) were as predicted and well in the limit fixed to prevent from any material injury. The mechanical tests confirmed the integrity of KBr prisms up to ~20g.

4.2

Microvibration campaign

An end-to-end microvibration campaign was performed mainly on EM Instrument and EM interferometer, as illustrated on Figure 3.

Figure 3:

EM Instrument transfer function measurement (1), wavefront tilt evaluation on EM interferometer (2), correction efficiency verification on PFM Interferometer (3)

First, the transfer function was fully characterized from the internal contributors to the interferometer interface on EM Instrument. Then, the optical wave-front tilt generated by perturbations at interferometer interface was assessed on EM Interferometer. To finish, the correction efficiency of wavefront tilt has been validated during thermal vacuum campaign on PFM interferometer.

After this campaign, the overall instrument internal microvibration budget has been significantly reduced compared to the CDR. It allowed thus to reduce the risk and to give a good confidence that final levels will be acceptable with regard to the performances. It was also the occasion to highlight some contributors like the scan mechanism. During the tests on EM Instrument, Satellite Velocity Compensation (SVC) was activated and was responsible of visible level of perturbation. The control loop of the SVC mechanism has been consequently modified giving negligible residual levels as shown on Figure 4.

Figure 4:

microvibration induced at interferometer interface by scan mirror mechanism before (in blue) and after (in red) optimization of control loop.

As during tests at instrument and satellite levels the SVC is not activated, these effects would have been detected very late. This kind of earlier derisking campaign is therefore essential to overcome the limitation of the models with regard to the microvibrations.

4.3

Conducted EMC qualification, functional tests and delivery to Satellite

EM Instrument was also the opportunity to perform the conducted EMC qualification, a first campaign of functional test, and procedures validation at satellite level on a simulator.

During EMC qualification, the results were consistent with the ones obtained at subunit level and thus compliant to the specification, except few exceedances that have been already accepted at IF Satellite level. In particular, the detection chain filtering have shown a great efficiency to reject perturbation, no susceptibility could be evidenced, promising no parasitic spikes from METOP-SG electronics or other instruments.

First functional tests with representative SW allowed identifying some minor anomalies and corrections/adaptations to be applied on final PFM SW. After delivery to Satellite team end of 2021, the procedure validation campaign on a Satellite simulator was a success, allowing preparing the PFM activities.

5.1

Critical PFM Subunits verification

PFM FPCA and Interferometer validation campaign have been described in ref[2].

Concerning FPCA, the radiometric performances were compliant with the expectations. A first set of IPSF figures was also measured (see Figure 5).

Figure 5:

B1 noise measurement (left) and B4 IPSF (right) on the 16 FPCA sounding pixels.

At interferometer level, the spectral performances were assessed under thermal vacuum in Gaïa vacuum chamber with different sources including a laser @ 3.76 μm and a gas cell (see Figure 6). In particular, the absolute spectral calibration and its stability were excellent and well in line with the predictions. It is already confirmed in B4 that the relative stability will be better than 5.10^-7 and the a posteriori relative spectral error below 1,4.10^-6 (limitation of the measurement accuracy being around 1,5. 10^-6).

Figure 6:

PFM interferometer in Gaïa chamber (left) and ISRF measurement (right).

After this thermal vacuum test, +/-1g measurements allowed confirming the order of magnitude of necessary beam splitter adjustment with regard to orientation at satellite level and with no gravity (less than 200μrad of wavefront tilt variation).

5.2

Instrument integration and alignment

After integration and fine alignment of optical units (telescopes, FPCA, Interferometer), they are integrated one by one on the I-OH optical bench. First, the FPCA, then the interferometer, the ITA, the ATA, the scan mirror, the imager, and to finish the calibration sources. The overall alignment is performed through dedicated reference cubes. The Figure 7 illustrates an intermediate step just before integrating the ATA.

Figure 7:

I-OH before ATA integration

After integration, the metrology emission and receiving beams have also been verified to present sufficient margins to insure robustness to mechanical or thermoelastic moves in order to guarantee instrument operations and nominal post processing corrections.

5.3

Mechanical qualification

Quasi-static, sine and acoustic qualification of the instrument was performed in december 2021 and separately for the 2 modules as shown on Figure 8.

Figure 8:

Instrument I-EM (on left) and I-OH (on right) modules under sine qualification.

During acoustic test, the specified levels were passed with success and the instrument behaviour was as expected. During sine test, the instrument response was very close to the prediction and the maximum authorized level was passed on the most sensitive parts. In particular for the KBr prisms for which this maximum level was confirmed after Interferometer EQM qualification (around 20g).

After mechanical tests, the Hardware integrity was checked, in particular the mechanisms and the overall optical alignment. Scan and Interferometer mechanisms showed very stable functional behaviour.

Overall, optical alignment was confirmed as nominal through reference optical cubes. Interferometer showed a wave front tilt variation of around 150μrad, well below the value derived from EQM interferometer qualification. The beam splitter has been well re-aligned with the on board dedicated actuator. To finish, the metrology beams remain well aligned, confirming the overall robustness of optical design with regard to the mechanical environment.

5.4

Functional tests and ambient tests

Ambient test is the first performance test performed on assembled PFM Instrument. The purpose is to have a full geometric characterization, including: LOS measurement of each Sounding Pixels for coregistration with the imager, Instrument Point Spread Function (IPSF), inter-pixels crosstalk, far field induced signal. Functional tests were performed just before to identify potential SW issues, and did not reveal any blocking point.

The ambient test used dedicated OGSE with 3 mirrors collimator, blackbody source, and different optical masks. Installation of this OGSE is illustrated on Figure 9.

Figure 9:

Ambient test set-up installation in front of the instrument

The test was run successfully in May 2022 and results are still under analysis to compute the different geometric characteristics. Nevertheless, we can highlight the main preliminary encouraging results. First, it is confirmed that the overall alignment is excellent, without any vignetting neither decentering. Then, a first assessment of microvibration was done by analyzing the wavefront tilt variation trough the metrology. It confirmed the very low level of microvibration compared to the CDR predictions with amplitudes less than 1μrad as shown on Figure 10.

Figure 10:

wavefront variation tilt induced by microvibration (in μrad)

The first instrument raw spectrum acquired in clean room over the 4 bands was obtained on this occasion and is presented in Figure 11.

Figure 11:

First IASI-NG raw spectrum

5.5

Thermal vacuum test

The thermal vacuum test occurred during June-July 2022, with the main objectives to proceed to the thermal qualification of the instrument and to assess the full spectral and radiometric performances.

This test required quite impressive thermal and optical installations in the so-called Gaïa vacuum chamber at Airbus Defence and Space premises (see Figure 12). Several targets are used for instrument characterization. Two warm black bodies, a Gaz Cell for the spectral calibration, and 8 laser sources (2 per band) for spectral calibration and ISRF shape. To be noticed that three cold black bodies are also used, one for the cold calibration in front of cold space view, and two others associated to Fabry Perot and Gas Cell views.

Figure 12:

Gaïa chamber with targets (left); with instrument and thermal tent installation (right)

From thermal point of view, this test allowed to qualify the instrument compliance to the interface requirements with hot and cold non-operational configuration and one hot plateau and one cold plateau.

All the measurements done are still under analysis. Fabry Perot and Gas Cell views acquisitions are presented in Figure 13 with the corresponding spectral signatures (respectively lines comb and gas chemical components absorption lines).

Figure 13:

Fabry Perot (left) and Gass cell (right) views measurement

The spectral acquisitions with laser sources do not show any apparent parasitic interferogram, indicating that the optical diopter wedges and tilts have been well specified, as shown on Figure 14.

Figure 14:

B3 laser spectrum (showing the two B3 and B4 laser wavelength peaks)

It is also confirmed that the microvibration levels due to internal contributors are very low.

To finish, fine radiometric performances (NedT and absolute calibration) will be derived from the acquisitions with the hot black bodies at different temperatures. First NedT raw assessment @300K gives very encouraging results, and it is a major milestone as it is the first end-to-end radiometric verification.