A.

MTG Context

The MTG Programme includes MTG-I and MTG-S satellites series [1]. This programme is being realised through the well-established and successful cooperation between EUMETSAT and ESA. It will ensure the future continuity with, and enhancement of, operational meteorological (and climate) data from geostationary orbit as currently provided by the Meteosat Second Generation (MSG) system, the last of which MSG4/MET11 has been successfully launched and commissioned in 2015.

The MTG space segment activities are now entering the manufacturing and testing phases from equipment levels to the platform and instrument assemblies. For the platform and instruments, the Structural and Thermal Models (STM) hardware is being manufactured, with initial integration activities of the Platform core structure on-going. At equipment level, most of Engineering Model (EM) units are available and platform EM integration will start in Q3/2016.

MTG-I mission is a continuity of the MSG satellite series carrying as a main payload the SEVIRI radiometric imager. The FCI as a MTG main payload is an improvement of SEVIRI and it is designed to provide images of the Earth every 10 to 2.5 minutes in 16 spectral channels between 0.44 and 13.3 μm, with a ground resolution ranging from 0.5 km to 2 km. It is to be noted that SEVIRI IR channels’ calibration is based on an on-board blackbody (absolute accuracy of 0.7K at end of life) [2] and a vicarious calibration is used for the solar channels (with no specification). In the contrary to the SEVIRI instrument, the FCI solar channels’ calibration is based on an innovative end-to-end full entrance aperture on-board VIS/NIR calibration. It is performed by inserting a

Metallic Neutral Density (MND) at the telescope exit pupil. As a consequence the on-board calibration requirements are also more demanding than the SEVIRI ones, and especially in the VIS/NIR domain for which an absolute calibration is required to fulfill climatology needs. Overall, the instrument calibration will be performed by inserting in the useful beam path a black body for IR channels in addition to the deep space for offset correction and a diffuser target plate illuminated by the Sun through the front optic for VIS/NIR channels. Both are full pupil coverage.

Overall, the performance predictions for the satellites/instruments are very encouraging, and are confirmed in several critical areas by good early test results (particularly with respect to the detector chain and related cryogenic performances). A very high level of compliance, to the very stringent satellite system/mission requirements, can be confidently expected for both imaging (MTG-I) and sounding (MTG-S) missions promising state of the art performances when the first satellites are launched in the 2020 timeframe.

B.

The FCI: an absolute VNIR radiometer

The FCI being an absolute radiometer, it must be able to perform in-flight radiometric calibration both for solar channels grouped in 5 VIS channels and 3 NIR channels (VNIR or VIS/NIR) and thermal channels grouped in three focal planes (IR1, IR2, IR3). The detailed design of the FCI and its calibration are discussed elsewhere [3]. Tab. 2 indicates the calibration improvement from SEVIRI to FCI instruments as specified by the end users.

Tab. 1.

Absolute calibration & stability requirements comparative between SEVIRI (MSG) and FCI (MTG) instruments.

Tab. 2.

Complete absolute calibration flowchart from primary laboratory to the MOTA under its final operational environment.

The FCI radiometric accuracy is defined as the mean radiometric error associated with a spectra channel, within a temporal repeat cycle. It is specified below 5 % for the VNIR channels and below 0.7 K for the IR channels. When the FCI operates in the High Resolution Fast Imagery, the absolute accuracy is relaxed to 10 % and 1 K for VNIR and IR respectively.

Besides, the FCI must maintain its radiometric stability over a day below 0.1 % for the VNIR channels and below 0.1 K for the IR channels. The radiometric stability over the entire instrument lifetime amounts to 2 % for the VNIR channels and to 0.3 K for the IR ones.

For the IR channels an approach similar to the one implemented for SEVIRI is followed: the signal offset, including stray-light, instrument thermal background and detection dark signal is measured at each scan line during deep space imaging and subtracted to the Earth acquisitions. A well characterized radiance source is introduced to calibrate the gain fluctuation which is due to instrument ageing and thermal fluctuations. This source is a black body which is periodically inserted in the optical path between M2 and M3, near the intermediate focal plane. The black body is cross calibrated with a large black body on-ground and only its drifts contribute to the calibration budget. To cope with the fact that the entrance mirrors M0, M1 and M2 are not directly calibrated, the temperature of the black body can be adjusted.

The on-board calibration equipment benefits from field-stop and exit pupil proximity and both MND and black-body are carried on the same calibration wheel mounted on the optical bench. The switching between different positions will be done by rotation of the multifunction wheel. The rotation axis is parallel to the chief ray direction between M3 and M4 to ensure a parallel placement of the metallic neutral density filters into the optical beam.

The VIS/NIR calibration principle is based on 3 steps:

The IR calibration principle is based on 3 steps:

C.

The MOTA OGSE: an initial on-ground spectroradiometric absolute reference

The MOTA (Fig. 1) is an OGSE, designed and developed by Bertin Technologies which will be positioned in front of the FCI. The whole set-up can work either in air or in vacuum. The MOTA allows to simulate objects at infinite under FCI representative environment. The measurements performed from the MOTA emission allows to adjust the FCI and to characterize its optical and radiometric performances. Among the main mission of the MOTA (detailed description of MOTA missions are presented in [4]) one can list:

Fig. 1.

The MOTA configuration. (a) General view: with the SA (Source Assembly) on the bottom and the HRC (High Resolution Collimator) on the top. The SA, itself, is made of a (b) Pattern Plate carriage which can move in X,Y and Z directions and of (c) VNIR (adjustable integrating sphere) and IR sources (cryogenic Black Bodies) to cover FCI 16 spectral channels. The pattern plate is imaged by highly stable HRC onto a collimated 300 mm diameter pupil and 9x9 mrad square angular field in order to match with the FCI.

As indicated above, one of the main missions of the MOTA is to provide an adjustable and calibrated radiance in order to be used as a reference for the FCI before its first calibrations in flight. Here are the MOTA requirements regarding absolute calibration:

The complexity of the MOTA calibration is thus due to the combination of cutting edge performances and flexible operability.

A.

Overview

The global logic of the calibrations steps with description of the different environments and configurations is presented in Tab. 2.

As indicated in Tab. 2, the main steps of the calibration are:

As it can be noticed, this procedure has been optimized in order to provide both high radiometric performances and high operability.

B.

Calibration transfer tools

In order to be able to carry out with the best performances the complete calibration procedure described in Tab. 2, it was necessary to design or carefully select commercial tools. Here is the description of these key elements:

Fig. 2.

Setup for calibration transfer from NIST calibrated secondary standard to the MOTA. (a) Measurement of the MOTA output radiance and (b) comparison to calibrated secondary standard output radiance. (c) Picture of the homemade radiometer during measurement of integrating sphere behaviour under vacuum.

C.

Hygrometry management

When measuring under atmospheric pressure, specific attention has to be paid on the 1380 nm channel as it is impacted by gas water attenuation. This phenomenon is ruled by Beer-Lambert law:

In (1), T is the transmission through a propagation distance L (m) under an hygrometry level of H (%) at 20°C. α(λ) is the attenuation coefficient given in m^–1.%^–1 which may depend on the wavelength λ. The issue with

calibration procedure is that, depending on the configurations and moment of measurement both the hygrometry level and the propagation distance can change which results in attenuation varying up to several tens of %. This value is not in line with required calibration accuracy of 3% (which must be given under vacuum), correction is thus necessary.

The final selected strategy is to measure the attenuation spectrum α(λ) with both high accuracy and spectral resolution. This spectrum was obtained by measuring, at a known hygrometry level, the radiance from a source with the high resolution spectrometer at two different distances. For this last measurement, a specific care was required in the selection of hygrometer which needed to be absolutely calibrated down to 0.5% at 3σ, this was done directly at the Swiss primary laboratory. Using this attenuation spectrum, the exact propagation distances for each measurement configuration (at NIST and at TAS/Bertin: Fig. 2), the hygrometry level in room during measurements, and the attenuation law (1), it was then possible to eliminate the impact of water attenuation down to residual error of 1.7% at 3σ.

D.

Extreme channels specific optimization

Despite the optimization of the hardware and of the process, the channel at higher wavelength (2250 nm at center) is suffering of bad SNR when used at the lowest radiance levels. This is both due to poor transmission of silica fibers, important noise of NIR 2 InGaAs linear detector of SR4500 spectrometer and very low specified radiance level. In order to enhance the SNR, the implemented solutions were to:

The 3 channels situated in the 400-700 nm range are strongly impacted by the color temperature drift of the QTH lamp which causes large variations of the blue channels of the secondary standard integration sphere with time. This specific sensitivity in blue is due to the slope of the spectrum in this region (radiance of QTH lamp is approximately a black body at 3000 K). In order to avoid too often unacceptable recalibrations at NIST, the radiance drift is monitored using a comparison with measurements at 1000 nm which is a much more stable region (maximum radiance spectrum) and corrected. Regular calibrations of the sphere every 50 hours of use are however still necessary to preserve calibration performances (limitations of the drifts to 1% at 3σ).

A.

Theoretical budget

The final theoretical performance has been calculated from the complete calibration procedure, it is presented in a simplified way on Tab. 2.

From Tab. 3 it can be noticed that the most relevant contributors to the final calibration accuracy are CMC, stabilities & accuracies of the different tools and hardwares, hygrometry (for 1380 nm channel only) and the SNR at the lowest radiances levels (mostly for NIR channels).

Tab. 3.

Final expected absolute calibration performances (given in % at 3σ) for complete radiance dynamic. Final performance for restricted radiance dynamics (10% to 100% of maximum radiances) is given too.

Compliance is expected for all channels except for 1380 and 2250 nm. The non-conformances are mainly due to hygrometry and spectrometer noise respectively. It is however important to underline that for the 1380 nm channel, performance is very close to the specification and that for 2250 nm channel, bad figure is expected for the lowest radiances only.

B.

Experimental qualification

Beside this pure theoretical analysis, important experimental validation work at subsystem level has been carried out to assess the figures in Tab. 3. More precisely it has been checked that all most critical parts (except CMC which is based on NIST statement, regularly validated by BIPM) are within theoretical expectations, this includes:

This experimental work confirms that theoretical analysis is correct and that the expected final performances will be met.