2.1

Mertz Interferometer

The detailed design of Mertz interferometer has been exposed in [4] and is sketched in Fig 2.

Figure 2.

(left) sketch of the Mertz Interferometer (right) Mertz Interferometer design with Dual Swing Mechanism for IASI NG instrument

The Mertz Interferometer is composed by two pairs of prisms: the internals and the externals prims that have a reflecting face. As in a Michelson interferometer, the incoming optical flux is separated in amplitude by the separating plate, associated with a compensating plate for symmetry. Moving the two pairs of prism with the translations indicated by the arrows, allows to tune in the same time the difference of the mirrors positions and the difference of the blade (formed by the Internal-external prisms) thickness between the two arms.

The optical path difference OPD can be written as

Where Δe_glass stands for the difference of glass thickness between the two arms of the interferometer, Δe_air the difference of the air thickness, n the refractive index of the prism material : KBr for IASI-NG design, and θ the field angle inside the interferometer.

This equation becomes quite independent of the field angle θ when the Mertz compensation condition is fulfilled : Δe_air = . This condition is a proportionality between differential air thickness and differential glass thickness, including the glass refraction index. It will be exactly fulfilled only for a specific wavelength, but the residues over the wide IASI-NG spectral band are negligible. This compensation allows to widen the field and reduce the pupil diameter inside the interferometer as explained in Fig 1 (right).

In practice the prisms are moved by a single mechanism called the dual swing mechanism, patented by ADS. This mechanism introduces two rotating translations with a fixed proportionality in order respect the compensation condition. It is represented on Fig 2 (right).

The benefits of this design is that the interferogram envelop (called Modulation Efficiency or ME in the paper) and phase (and so the effective Optical Path Difference OPD) are quite independent of the field, reducing drastically the auto-apodisation effect that limits the Michelson interferometer in similar conditions of field and arm length. Field effects being negligible, the pupil’s effects are predominant on the shape of the ME and OPD and become a performance driver. The differential wave front errors in the pupil plane is usually decomposed between tilt (first order Zernike polynomial decomposition) and higher orders named WFE in this paper. Let us detail the consequences of both effects.

A tilt leads at first order to a loss of contrast driven by the well-known formula

with σ the wavenumber, α the differential tilt, D_pup the pupil diameter inside the Interferometer. In order to optimize the contrast, the beam splitter assembly can be realigned on ground and in flight over 500 μrad. This realignment ensures a very good spectral performance, but concern only long terms drifts. Short term variation, especially microvibrations inside the interferometer are critical.

The WFE sensitivity is approximated by the formula

with δWFE_rms the differential surface error cumulated by the science beam crossing the blades and the prisms surfaces. As the prisms are moving, this quantity is time-dependent and cannot be ignored in the instrument modelization.

As can be noticed in the OPD simplified formula (1), the OPD is deeply linked to the refractive index of the prisms and so to the wavenumber. As the consequence the spectral calibration is wavenumber dependent with a variation linked to the refractive index difference between metrology wavenumber and science wavenumber (Fig 6). This instrument requires a specific ground and flight calibration strategy in order to comply with the mission requirements.

2.2

Instrumental design

The instrumental design is detailed in Fig 3 and reference [2]. The main performances drivers are recalled hereafter.

Figure 3.

(left) sketch of the Mertz Interferometer (right) Mertz Interferometer design with Dual Swing Mechanism for IASI NG instrument

We just have seen that the Mertz interferometer allows to acquire 4x4 sounder Pixel at the same time, improving acquisition duration up to 0.74s that benefits to the SNR as well as the spectral performances.

Inside the interferometer, a 5-beams metrology at 1.5μm allows to monitor precisely and in real time the OPD and the prisms differential rigid body behavior, with a decomposition on the 4 first Zernike polynomials: tilt_—u, tilt_—v, focus and astigmatism.

Around the interferometer, the optical flux from 3.6μm to 15.5μm is split in 4 bands in the focal plane assembly and is detected with high performance cooled IR detectors (PC for B1, PV, for B2 B3 B4). The B1-detector temperature of 75 K is ensured with active cryo-cooling.

The geometric performances are optimized at both side of the optical chain: on one side the scan mirror and its mechanism pointing performances, on the other side, the optical design of the Cold Optics optimized for IPSF geometric performances.

Regarding radiometric calibration as for IASI a high performance warm blackbody is measured at each end of swath as well as a cold space view.

The new instrumental concept includes two calibration sources that where not present for IASI: a Fabry-Perot view for spectral calibration monitoring and a WFS source at 3.6μm for spectral performances and model long term monitoring.

4.1

Ground processing correction

Spectral performances aim at quantifying the difference between an ideal ISRF (Gaussian) and the measured ISRF: these differences are expressed in terms of centroid shift, shape variations and parasites for microvibration. As the expected performances are really stringent, a ground processing correction can be taken into account, and the performance is estimated after correction.

The ground processing goal is to calculate in real time with 5 beams metrology OPD telemetry the science error OPD, (met + charac), and the science modulation efficiency, (met + charac). This calculation is not simplified as equations (2) and (3) approximations for the Modulation Efficiency, but take into account the defects presented in paragraph 2: tilts due to mechanisms defects or vibrations, WFE due to prisms surface errors, KBr refraction index as well as science and metrology geometry inside the interferometer. So the model is wavenumber and pixel dependent and leans on precise ground characterizations.

Thanks to this ground model, sort of a deconvolution is made and the final response is very similar to the expected Gaussian response with regular OPD. The spectral performances are estimated on the FFT of the corrected interferogram IFG^°r defined by:

In order to estimate the spectral performances, a set of instrument defects and mis-knowledges has been randomly chosen and interferogram simulations including correction process has been performed by ADS.

4.2

Shape and centroid stability performances

The results of the set of instruments trials are given in Fig 4 for the centroid stability and shape error index.

The centroid shift stability is within the specification of 5.10^-7. The performance is met for the shape error index, in the great majority of the simulated cases.

4.3

Microvibration performances

Considering now the spectral specification linked to the microvibrations inside the interferometer

Microvibrations (20Hz up to 200Hz) inside the interferometer creates ME modulation as well as OPD modulation. For each vibration frequency f, a pair of parasites appears around the main peak of the ISRF at +/- f/Vopd wavenumber. These parasites are specified in relative amplitude to the main peak in the instrument specification.

Estimating the amplitude of these peaks, requires to first evaluate the vibrations frequencies and amplitudes inside the interferometer and then simulate science and metrology ME and OPD inside the interferometer and take into account ground processing. The mechanical analysis has been carried with up-to-date microvibration sources hypothesis for IASI-NG instrument as well as the other satellites contributors (METIMAGE, MWS, 3MI, the wheels). And a mechanical coupled model of the platform and the instruments is used to get realistic mechanical transfer-functions between the sources and the interferometer components. After this mechanical analysis the displacement magnitude is deduced for the prisms and blades inside the interferometer: the vibrations are about 5 μrad in rotation and 100 nm in translation. The most perturbed element is the beam-splitter and the main perturbator is IASI-NG cryo-cooler.

With the mechanical displacements an optical simulation allows to simulate the science acquisition and the metrology ones, and to mimic the correction process.

The results are presented in the table Fig 5 for the OPD vibration and the ME Vibration. The single harmonic budget is compliant over a large spectral domain and becomes non-compliant at short wavenumber. The budget made with an absolute sum on contributors and frequencies is out of specification. The system budget has to take into account each frequency, amplitude couple to assess the impact on the mission performances.

Figure 5.

(left) microvibration impact on ISRF. (right) Microvibration performances estimated at CDR, the impact of the ground processing being partially taken into account.

As the whole process of evaluation includes several margin and hypothesis to be consolidated, this NC leads to specifics verification activities in the phase D in order to reduce the margin errors and confirm the real performances. Moreover, activities on the ground model have to be done in order to optimize the microvibration correction process.

4.4

Spectral calibration performances

As explained in paragraph 2.1., IASI-NG centroid depends strongly of the wavenumber. The spectral “de-calibration” after FFT before any correction process is between 21 cm^-1 at 645 cm^-1 (15.5μm) and 11 cm^-1 at 2760 cm^-1 (3.6μm) see Fig 6. The a-priori knowledge of the spectral law at 3.4 10^-5 is achieved thanks to the refractive index measurements and science and metrology beams geometry inside the interferometer characterization.

Figure 6.

(left) spectral native de-calibration of IASI-NG. (right) Fabry Perot simulated spectra

The final spectral calibration at 10^-6 at system level is achieved in flight thanks to a complex calibration process on atmospheric spectra measured by the instrument (ref [3], [5]). A verification of the smooth behavior of the spectral law can be done with Fabry-Perot acquisitions, that presents regular oscillations over the whole IASI-NG (Fig 7. right). This process lean on the good centroid stability of the instrument as expected in paragraph 4.2.

A tight spectral characterization and verification is foreseen at instrument thermal vacuum with a gas cell as well as internal Fabry Perot (FPI) acquisitions. The whole process will be done and its performances checked.

5.1

Absolute calibration

The Fig 7 represents the absolute calibration error budget over the whole lifetime as a function of the wavenumber. The several colors correspond to the equivalent blackbody scene at several temperatures: from one hundred eighty Kelvin to three hundred and ten Kelvin.

In this budget two kinds of contributors are considered:

This overall budget takes into account different timescale contributors short and long terms. The estimated budget at CDR is compliant with margins.

Figure 7.

absolute radiometric calibration performance budget

5.2

NedT performance

The plot here represents the NedT budget for a blackbody scene at 280 K. The budget includes some ground processing contributors linked to the spectral correction process explained in paragraph 4.1. The sixteen colored plot correspond to the sixteen sounder pixel. The NedT performance is expected be very good in general for all spectral bands, especially the B1 thanks to the good KBr transmission and optimized broad band optical coatings.

The slight non conformances are in line for system budgets.

Figure 8.

radiometric noise performance budget for à Black Body at 280 K, including some ground processing contributors and filtering