Fluorescence Imaging Spectrometer (FLORIS) for ESA FLEX Mission

remote sensing

Article Fluorescence Imaging Spectrometer (FLORIS) for ESA FLEX Mission

Peter Coppo 1, Alessio Taiti 1, Lucia Pettinato 1, Michael Francois 2,*, Matteo Taccola 2 and Matthias Drusch 2 1 Leonardo, Via A. Einstein, 35, Campi Bisenzio, 50013 Florence, Italy; [email protected] (P.C.); [email protected] (A.T.); [email protected] (L.P.) 2 ESA ESTEC, Keplerlaan 1, P.O. Box 299, 2200 AG Noordwijk ZH, The Netherlands; [email protected] (M.T.); [email protected] (M.D.) * Correspondence: [email protected]

Received: 10 May 2017; Accepted: 18 June 2017; Published: 23 June 2017

Abstract: The Fluorescence Explorer (FLEX) mission has been selected as ESA’s 8th Earth Explorer mission. The primary objectives of the mission are to provide global estimates of vegetation fluorescence, actual photosynthetic activity, and vegetation stress. FLEX will fly in tandem formation with Sentinel-3 providing ancillary data for atmospheric characterization and correction, vegetation related spectral indices, and land surface temperature. The purpose of this manuscript is to present its scientific payload, FLORIS, which is a push-broom hyperspectral imager, flying on a medium size platform. FLORIS will measure the vegetation fluorescence in the spectral range between 500 nm and 780 nm at medium spatial resolution (300 m) and over a swath of 150 km. It accommodates an imaging spectrometer with a very high spectral resolution (0.3 nm), to measure the fluorescence spectrum within two oxygen absorption bands (O2A and O2B), and a second spectrometer with lower spectral resolution to derive additional atmospheric and vegetation parameters. A compact opto-mechanical solution is the current instrument baseline. A polarization scrambler is placed in front of a common dioptric telescope serving both spectrometers to minimize the polarization sensitivity. The telescope images the ground scene onto a double slit assembly. The radiation is spectrally dispersed onto the focal planes of the grating spectrometers. Special attention has been given to the mitigation of stray-light which is a key factor to reach good accuracy of the fluorescence measurement. The absolute radiometric calibration is achieved by observing a dedicated Sun illuminated Lambertian diffuser, while the spectral calibration in flight is performed by means of vicarious techniques. The thermal stabilization is achieved by using two passive radiators looking directly to the cold space, counterbalanced by heaters in a closed loop system. The focal planes are based on custom developed CCDs. The opto-mechanical design is robust, stable vs. temperature and easy to align. The optical quality is very good as recently demonstrated by the latest tests of an elegant breadboard. The scientific data products comprise the Top Of Atmosphere (TOA) radiance measurements as well as fluorescence estimates and higher-level products related to the health status of the vegetation addressing a wide range of applications from agriculture to forestry and climate.

Keywords: ﬂuorescence imager; FLEX mission; Sentinel-3

1. Introduction and Objectives Observing the plant functional status from space represents a major interest for farming, forest management, and assessment of the terrestrial carbon budget. The measurement of solar induced chlorophyll fluorescence [1] directly addresses the photosynthetic efficiency of the terrestrial vegetation layer and complements traditional reflectance measurements used to infer parameters like Leaf Area

Remote Sens. 2017, 9, 649; doi:10.3390/rs9070649 www.mdpi.com/journal/remotesensing Remote Sens. 2017, 9, 649 2 of 18

Index (LAI) or chlorophyll absorption [2]. Fluorescence estimates provide an early and more direct approach for diagnosis of the functioning and health status of vegetation. In fact the fluorescence emission is in competition with the photochemical conversion and may allow more accurate carbon assimilation estimation and earlier stress detection than is possible from only reflectance data [3]. The FLuorescence EXplorer (FLEX) mission has been selected for the ESA Earth Explorer EE8 program [4]. FLEX will fly in tandem with Sentinel 3 mission, making use of data synergy with other visible reflectance (from OLCI, [5]) and surface temperature data (from SLSTR, [6,7]). The FLuORescence Imaging Spectrometer (FLORIS) is the FLEX payload. It will be mounted on a medium/small size satellite and fly in a Sun synchronous orbit at a height of about 815 Km. Two different spectral channels with high spectral sampling (0.1–2 nm) have been implemented by means of two spectrometers with spatial co-registration enhanced by design using a common fore-optics with ground spatial sampling of 300 m and swath of 150 km. High efficient anti-reflection coatings, super-polished surfaces, tight particle contamination control and holographic gratings are essential to control the spatial and the spectral stray-light, which have a direct impact on the fluorescence measurement accuracy within the O2 absorption bands. The operative spectral region is selected by a band pass filter, placed in front of the scrambler. Out of band stray-light is also mitigated by using two additional filters, one for each channel, coated on two folding mirrors placed near the double slit assembly. The developed design is robust, stable versus temperature, easy to align and with good optical quality for the whole field of view including excellent corrections of transverse chromatic aberration and distortions (keystone and smile). A thermally stabilized optical bench and three cooled back-side illuminated high speed CCDs detectors are used to guarantee the required Signal to Noise Ratio (SNR) and stability accuracy (spectral and radiometric) between two consecutive on board calibrations. For the absolute radiometric calibration a dedicated Lambertian diffuser illuminated by the sun can be inserted as first element in the optical path by means of a rotating wheel, while spectral calibration is achieved by observing the atmospheric and/or the sun absorption lines (vicarious technique). The thermo-mechanical layout on a very thermal stable Al6061 alloy Optical Bench (OB) has been chosen in order to minimize volume and mass, thus reducing costs and impact on the satellite interfaces and simplifying the Assembly, Integration and Tests (AIT). All optics, detectors and Front End Electronics (FEEs) are attached with titanium/aluminum supports on the optical bench and there are two radiators in the upper position of the instrument completely in view of the cold space for an effective thermal control. These radiators are needed to dissipate heat from detector units and FEEs. The purpose of this paper is to present the state of art of the FLORIS instrument at the start of the phase B2/C/D. The principal requirements are discussed in Section2 in comparison with the foreseen achievable performance. In Section3 the instrument baseline design is presented, including the optical and the detector architecture. The mechanical layout and in-flight calibration architecture are reported respectively in Sections4 and5, while the on ground calibration/validation activites and the principal performance are described respectively in Sections6 and7. The paper will end with a brief discussion of the elegant breadboard activities (Section7), conclusions (Section8) and outlook (Section9).

2. Main Instrument Requirements The key requirements for the instrument design and sizing are summarized in Tables1 and2. Four spectral bands can be clearly identiﬁed in the requirements (Table2):

- Low Resolution (LR): (500 nm–677 nm) & (697 nm–740 nm)

-O 2-B: (677 nm–697 nm) -O 2-A: (740 nm–780 nm)

Two spectrometers have been implemented to cover the required spectral range: the LR (Low Resolution) spectrometer for the Low Resolution bands (range 500–758 nm) with un-binned Spectral Remote Sens. 2017, 9, 649 3 of 18

Sampling Interval (SSI) of 0.6 nm/px and the HR (High Resolution) spectrometer for the two oxygen bands (range 677–697, 740–780 nm) with un-binned SSI of 0.0933 nm/px. A spectral binning ×3 and ×5 can be selected in some speciﬁc bands to meet the SSI requirement (Table2). The spectral range 677–697 nm and 740–758 nm is also acquired by the LR spectrometer for HR-LR spectral inter-band co-registration.

Table 1. Principal FLORIS speciﬁcation and performance. SSD = Spatial Sampling Distance, ARA = Absolute Radiometric Accuracy, RSRA = Relative Spectral Radiometric Accuracy, RXRA = Relative Spatial Radiometric Accuracy, ISRF = Instrument Spectral Response Function, FWHM = Full Width Half Maximum, ALT/ACT = ALong/ACross-Track, DOP = Degree Of Polarisation, SEDF = System Energy Distribution Function, p = ACT detector pitch, F = focal length, ts = sampling time, H = satellite altitude, GTO = Ground To Orbit, BOL = Begin Of Life, EOL = End Of Life., TBC = To Be Conﬁrmed.

Requirement Speciﬁcation Performance Comments Mission lifetime 3.5/5 years 5 years - Sun zenith < 75◦, Coverage −56◦ < latitude < 75◦ −56◦ < latitude < 75◦ Observ. zenith < 15◦ Satellite height 805–830 km 805–830 km Sentinel 3 orbit. Swath width >150 km >151 km H > 805 km. <288 m (along-track) H < 830 km, p = 84 µm, SSD <300 m (nadir) <300 m (across-track) ts = 43.5 ms Image Quality <1.2 SSD 1.1 SSD FWHM of SEDF. Spatial co-registration Keystone negligible wrt telescope <0.15 SSD <0.15 SSD (HR or LR) lateral color. HR versus LR spatial <0.1 SSD 0.08 SSD At instrument level. co-registration knowledge Geo-location accuracy <0.4 SSD 0.24 SSD At instrument level. Smile (0.4 SSI) negligible wrt to Spectral co-registration <2 SSI 1.2 SSI O2A-O2B detectors alignment. <±0.1 SSI (orbit) <±0.1 SSI Stabilized optical bench (±1 ◦C) Spectral stability <±1 SSI (lifetime) <±1 SSI and detector (±0.1 ◦C). Incl. on-ground characterization, ISRF knowledge <2% 1.6% GTO and BOL-EOL variations. HR Straylight 0.2 (L0) <0.8 (L0) At 40 SSD from the edge of a bright (mW/m2/sr/nm) 0.04 (L1b) 0.04 (L1b) zone. L1b performance TBC. Absolute and Relative ARA/RSRA/RXRA 5%/1%/0.5% 5%/1%/0.5% Radiometric Accuracy. <2% (LR) <2.0% (LR) Polarisation Sensitivity Scrambler DOP = 3% (HR), 7% (LR). <1% (HR) <0.9% (HR) Inter-channel temporal HR-LR on ground separation of <2.5 s 2.4 s co-registration 15.9 km

Table 2. FLORIS spectral requirements & performance. SR = Spectral Resolution = FWHM of ISRF, SSI = Spectral Sampling Interval, SNR = Signal to Noise Ratio. LR = Low Resolution, HR = High Resolution, req. = requirement, per. = performance. SNR req. at reference radiance (Lref) and SSI req.

Band Name PRI Chlorophyll O2BO2B Red Edge O2AO2AO2AO2A Spectrometer LR LR HR HR LR HR HR HR HR Band (nm) 500–600 600–677 677–686 686–697 697–740 740–755 755–759 759–769 769–780 SSI req. (nm) 2 2 0.5 0.1 1 0.5 0.5 0.1 0.5 SSI per. (nm) 1.8 1.8 0.467 0.093 0.6 0.467 0.467 0.093 0.467 SR req. (nm) 3 3 0.7 0.3 2 0.7 0.7 0.3 0.7 SR per. (nm) 2 2 0.474 0.28 1.8 0.474 0.474 0.28 0.474 115 (759–762 nm), Linear from 510 Linear from 115 SNR req. 245 340 175 425 (@740 nm) to 1015 1015 (@762 nm) to 455 1015 (@755 nm) (@769 nm) Remote Sens. 2017, 9, 649 4 of 18

RadiometryRemote Sens. 2017 is, 9, a 649 key driver to measure the small ﬂuorescence signal variation with an accuracy4 of 18 of 10%. ThereforeRadiometry a high is Signala key driver to Noise to measure Ratio (SNR the small > 100) fluorescence is needed signal for each variation spectral with channel an accuracy with low 2 referenceof 10%. radiance Therefore (e.g., a high Lref Signal = 7.5 to W/m Noise/sr/ Ratioµm (SNR at 761 > 100) nm) is withinneeded thefor each narrow spectral O2 absorption channel with bands (Spectrallow Samplingreference radiance Interval (e.g., < 0.1 Lref nm) = (Table7.5 W/m2).2/sr/μm at 761 nm) within the narrow O2 absorption Anotherbands (Spectral fundamental Sampling requirement Interval < 0.1 nm) is straylight, (Table 2). which shall be limited (and further reduced with on groundAnother correction fundamental algorithms) requirement to is avoid straylight, pollution which of shall the ﬂuorescencebe limited (and signal. further The reduced stray-light with on ground correction algorithms) to avoid pollution of the fluorescence signal. The stray-light signal within the O2A and O2B bands at a distance larger than 40 Spatial Sampling Distance (SSD) fromsignal the edge within of the a transition O2A and O between2B bands at a brighta distance (e.g., larger a cloud) than 40 and Spatial a dark Sampling zone (e.g.,Distance the (SSD) reference from the edge of a transition between a bright (e.g., a cloud) and a dark zone (e.g., the reference vegetation) (Figure1a) shall be less than 0.04 mW/m 2/sr/nm at level 1b (after straylight correction) vegetation) (Figure 1a) shall be less than 0.04 mW/m2/sr/nm at level 1b (after straylight correction) and 0.2 mW/m2/sr/nm at level L0 (without correction). An important contribution is the spectral and 0.2 mW/m2/sr/nm at level L0 (without correction). An important contribution is the spectral stray-light,stray-light, which which comes comes from from the the light light scatteredscattered by the the spectrometer spectrometer optical optical elements elements (and (andthe the gratingsgratings in particular) in particular) from from the the continuum continuum bands bands into the the oxygen oxygen bands bands (Figure (Figure 1b).1 b).This This is affecting is affecting measurementsmeasurements even even without without the the presence presence of of bright bright targetstargets in in the the scene. scene.

Input Radiances 700

600

500

400 Lcloud

40 SSD 300

200 Radiance (W/m2/sr/mic) Radiance 100 Lref

0 740 750 760 770 780 Wavelength (nm)

(a) (b)

Figure 1. Non uniform scene (a) for spatial stray-light and non-uniform spectra (b) for spectral Figure 1. Non uniform scene (a) for spatial stray-light and non-uniform spectra (b) for spectral stray-light evaluation. Lbright = Lcloud, Ldark = Lref. stray-light evaluation. Lbright = Lcloud, Ldark = Lref. 3. Instrument Design 3. Instrument Design 3.1. Instrument Architecture and Optical Design 3.1. Instrument Architecture and Optical Design The instrument architecture is based on a push-broom hyperspectral imager with a common Thetelescope instrument and two architecturespectrometers is in based field separated on a push-broom [8,9]. hyperspectral imager with a common telescopeThe and operational two spectrometers temperature in fieldrange separated is 293.15 K [8 ,±9 ].2 K. The pupil diameter to comply the SNR Therequirement operational is 75.6 temperature mm for HR and range 36.1 ismm 293.15 for LR. K ± 2 K. The pupil diameter to comply the SNR A single telescope (234.5 mm of focal length and pupil diameter of 75.6 mm, f-number 3.1) requirement is 75.6 mm for HR and 36.1 mm for LR. (Figure 2) is implemented to collect the radiation coming from two single strips of 300 m × 150 Km A single telescope (234.5 mm of focal length and pupil diameter of 75.6 mm, f-number 3.1) onto 2 identical slits (84 μm × 44.1 mm) placed in its focal plane and separated by a distance of 4.5 (Figuremm2) (Figure is implemented 3), corresponding to collect to an the on-ground radiation separation coming fromof 15.9 two km. single strips of 300 m × 150 Km onto 2 identicalThe co-registration slits (84 µm ×between44.1 mm) the placedHR and in LR its bands focal planeand the and Sentinel separated 3 data by is a distanceperformed of on 4.5 mm (Figureground.3), corresponding In front of the to telescope an on-ground an external separation baffle ofis 15.9placed km. in order to reduce the out of field Thestraylight. co-registration A dual Babinet between scrambler the HR is andneeded LR to bands meet andthe polarization the Sentinel sensitivity 3 data is performedrequirements. on The ground. In frontwedge of theangle telescope of the scrambler an external plates baffle is the result is placed of a trade-off in order between to reduce the thestrength out ofof fielddepolarization straylight. A dual Babinetand the scramblerimage quality is needed degradation. to meet The the required polarization spectral sensitivity resolution requirements. is obtained by The using wedge two angle of themodified scrambler Offnër plates spectrometers is the result (Figure of a 2). trade-off Holographi betweenc gratings the are strength used due of to depolarization the relatively high and the groove density and because they can guarantee low grating imperfections (residual roughness, image quality degradation. The required spectral resolution is obtained by using two modified Offnër profile errors, etc.). This is very important because the gratings are the major contributors of spectral spectrometers (Figure2). Holographic gratings are used due to the relatively high groove density and straylight, which directly impact on the fluorescence measurement accuracy within the O2 because they can guarantee low grating imperfections (residual roughness, profile errors, etc.). This is very important because the gratings are the major contributors of spectral straylight, which directly Remote Sens. 2017, 9, 649 5 of 18 Remote Sens. 2017, 9, 649 5 of 18 impact on the fluorescence measurement accuracy within the O absorption band. Three identical low absorptionRemote Sens. 2017band., 9, 649Three identical low noise backside illuminated2 frame transfer 1060 × 450 5(42 of 18 μ m noisespatial backside ×28 illuminatedμm spectral) frameCCD detector transfer units, 1060 cooled× 450 ( (42−35°C)µm by spatial a radiator,×28 µallowm spectral) to cover CCD the three detector ◦ units,spectral cooledabsorption (range−35 band. (OC)2B Three by, O a2A radiator, identicaland LR low allowbands). noise to The coverbackside required the illuminated three SNR spectral is frameachieved range transfer by (O means10602B,O ×2A 450ofand a(42 42.8 LRμm ms bands). The requiredintegrationspatial ×28 SNR time μm is (300spectral) achieved m on CCD ground by detector means) without units, of a saturation 42.8cooled ms (−35°C) integrationduring by cloud a radiator, timeobservations (300allow mto (full oncover ground)well the capacity three without saturationofspectral 1.25 during Me- range for cloud each(O2B , observationspixel),O2A and 1.7 LRMHz bands). (full readout well The freq capacityrequireduency, ofSNR four 1.25 isports Me-achieved and for each ×2by pixel means pixel), binning of 1.7 a 42.8 MHzalong ms readoutthe integration time (300 m on ground) without saturation during cloud observations (full well capacity frequency,spatial four direction ports at and the ×CCD2 pixel level. binning A further along binning the along spatial the direction spectral direction at the CCD (×3 or level. ×5) can A furtherbe providedof 1.25 Me- by thefor Fronteach pixel), End Electronics 1.7 MHz readout (FEE). frequency, four ports and ×2 pixel binning along the binningspatial along direction the spectral at the direction CCD level. (× A3 further or ×5) binning can be along provided the spectral by the direction Front End (×3 Electronicsor ×5) can be (FEE). provided by the Front End Electronics (FEE).

FigureFigure 2. 2.Instrument Instrument opticaloptical lay-out. lay-out. Figure 2. Instrument optical lay-out.

FigureFigure 3. 3. DoubleDouble slitslit assembly: HRHR andand LR LR slits slits separated separated by by 4.5 4.5 mm. mm. Figure 3. Double slit assembly: HR and LR slits separated by 4.5 mm.

The design approach foresees no aberration compensation between the telescope and the spectrometers to simplify the separate procurement and test of each subsystem. A stabilization Remote Sens. 2017, 9, 649 6 of 18

Remote Sens. 2017, 9, 649 6 of 18 ± ◦ of the opticalThe benchdesign ofapproach1 C isforesees implemented no aberration in order compensation to achieve the between required the spectral telescope stability. and the All the materialsspectrometers selected to are simplify common the forseparate space procurement projects: and test of each subsystem. A stabilization of the optical bench of ±1 °C is implemented in order to achieve the required spectral stability. All the • Optics: F2G12, Lak9G18, CaF2, BK7G18 for lenses, N-BK7, S-FPL51 for mirrors and Fused Silica materials selected are common for space projects: for gratings, • • Mechanics:Optics: F2G12, Aluminum Lak9G18, 6061, CaF2, Titanium BK7G18 and for Invar.lenses, N-BK7, S-FPL51 for mirrors and Fused Silica for gratings, • 3.2. TelescopeMechanics: Aluminum 6061, Titanium and Invar. A3.2. Petzval Telescope objective with 5 lenses (2 aspherical lenses) is used as fore optics (Figure4). It has a real entranceA Petzval pupil objective located with 75 5 mm lenses in front(2 aspherical of the ﬁrstlenses) lens, is used where as fore the instrumentoptics (Figure aperture 4). It has stopa is accommodatedreal entrance and pupil it islocated telecentric 75 mmin in thefront image of the focal first lens, plane. where It guarantees the instrument good aperture image stop quality is up to F#accommodated = 3.1 in the HR and spectral it is telecentric band (FOVin the image = ±5.4 focal◦ × plane.0.0◦) It and guarantees to F# = good 6.5 in image the LRquality spectral up to band (FOVF# = =± 3.15.4 in◦ ×the1.1 HR◦). spectral A smaller band F# (FOV is needed = ±5.4° × in 0. the0°) and HR to spectral F#=6.5 in band the LR for spectral radiometric band (FOV constraints = (lower±5.4° input × 1.1°). radiance A smaller and F# higher is needed spectral in the resolution HR spectral with band respect for radiometric to the LR constraints spectrometer). (lower Chromaticinput aberrationradiance correction and higher is achievedspectral resolution with only with three resp radiationect to the resistantLR spectrometer). glasses (lateralChromatic color aberration <4.7 micron correction is achieved with only three radiation resistant glasses (lateral color <4.7 micron in the HR in the HR range and <10.5 micron in the LR one). Radial distortion is less than 0.2% and WFE rms is range and <10.5 micron in the LR one). Radial distortion is less than 0.2% and WFE rms is less than less than 0.23 waves in HR (0.14 waves in LR) channel by design. 0.23 waves in HR (0.14 waves in LR) channel by design.

Figure 4. Telescope optical layout. Figure 4. Telescope optical layout. 3.3. High Resolution Spectrometer 3.3. High Resolution Spectrometer The proposed solution is based on the Lobb’s theory of concentric designs for grating Thespectrometer proposed [10]. solution It works in is the based spectral on range the 670–780 Lobb’s nm theory with aof pixel concentric size of 28 μ designsm in the spectral for grating spectrometerdirection [and10]. 84 It μ worksm in the in thespatial spectral one. The range F# is 670–780 the same nm of withthe telescope a pixel sizeand ofthe 28 magnificationµm in the spectral is direction+1. and 84 µm in the spatial one. The F# is the same of the telescope and the magnification is +1. The starting point is the Offnër relay. Performances of the relay are improved by the addition of The starting point is the Offnër relay. Performances of the relay are improved by the addition of a concentric spherical lens through which the light passes twice. The spectrometer is obtained from a concentric spherical lens through which the light passes twice. The spectrometer is obtained from the relay design by changing the convex spherical mirror with a convex spherical grating [8]. An the relayadditional design flat by mirror changing (High theResolution convex Flat spherical mirror) mirrorplaced withafter the a convex HR entrance spherical slit is grating needed [to8]. An additionalmake flateasier mirror the HR (High spectrometer Resolution mechanical Flat mirror) accommodation placed after the and HR in entrance particular slit the is neededseparation to make easierbetween the HR the spectrometer slit assembly mechanical and the HR detector. accommodation and in particular the separation between the slit assemblyThe andconvex the holographic HR detector. grating has a groove density of 1450 grooves/mm and the average Theincidence convex angle holographic on the grating grating is approximately has a groove 38.2°. density The −1 of order 1450 ofgrooves/mm the diffracted beam and theis back average incidencereflected angle close on to the the grating incident isbeam, approximately with diffraction 38.2 angles◦. The between−1 order 21.3° of the(at 677 diffracted nm) and beam30.9° (at is back reflected780 nm). close This to beam the incident is then guided beam, towards with diffraction the focal plane angles by betweenmeans of 21.3a second◦ (at reflection 677 nm) of and the 30.9◦ concave mirror and the second transmission through the meniscus (Figure 5). Apart from the (at 780 nm). This beam is then guided towards the focal plane by means of a second reflection of zero-order (specular direction) beam which is stopped by a light trap, all other diffraction orders are the concave mirror and the second transmission through the meniscus (Figure5). Apart from the evanescent [10]. The use of the high frequency in the grating has two main advantages: zero-order (specular direction) beam which is stopped by a light trap, all other diffraction orders are evanescent [10]. The use of the high frequency in the grating has two main advantages: Remote Sens. 2017, 9, 649 7 of 18

• Spectrometer compactness: the larger are the dispersion angles the smaller is the required grating radiusRemote ofSens. curvature 2017, 9, 649 (it corresponds to a lower focal length); 7 of 18 • High grating efficiency. Average efficiency greater than 60% can be achieved with an optimized • Spectrometer compactness: the larger are the dispersion angles the smaller is the required saw-tooth groove profile. grating radius of curvature (it corresponds to a lower focal length); Grating• High polarization grating efficiency. sensitivity Average is less efficiency than 20% greater and than the required60% can be grating achieved Bidirectional with an optimized Reflectance saw-tooth groove profile. Distribution Function (BRDF), describing the angular distribution of the scattered signal, shall be better than the oneGrating of a mirror polarization with asensitivity surface roughness is less than of 220% nm rms.and the required grating Bidirectional TheReflectance system givesDistribution excellent Function correction (BRDF), for describing chromatic the aberrationsangular distribution and distortions. of the scattered This signal, means that shall be better than the one of a mirror with a surface roughness of 2 nm rms. the slit images on detector are straight and parallel, falling on detector columns without significant The system gives excellent correction for chromatic aberrations and distortions. This means that µ µ spatialthe and slit spectral images on co-registration detector are straight errors and (keystone parallel, falling less thanon detector 0.4 mcolumns and smilewithout less significant than 0.5 m by design).spatial and spectral co-registration errors (keystone less than 0.4 μm and smile less than 0.5 μm by design).

Figure 5. High Resolution Spectrometer lay-out (HRF = High resolution Flat mirror, HRL = High FigureResolution 5. High ResolutionLens, HRM Spectrometer= High Resolution lay-out curved (HRF Mirror, = HighHRG resolution= High Resolution Flat mirror, Grating, HRL HR = High ResolutionHigh Lens,Resolution HRM Detector). = High Resolution curved Mirror, HRG = High Resolution Grating, HR = High Resolution Detector). 3.4. Low Resolution Spectrometer 3.4. Low ResolutionThe LR spectrometer Spectrometer optical design is of the same family of the HR one but with a different design of the corrector lens. It works in the spectral range 500–758 nm with a pixel size of 28 μm in The LR spectrometer optical design is of the same family of the HR one but with a different design the spectral direction and 84 μm in the spatial one. The LR spectrometer works with magnification of the corrector−1 and its lens.aperture It works is reduced in the down spectral to 36.1 range mm (f/6.5) 500–758 by means nm with of a apupil pixel stop size placed of 28 atµ mthe in grating the spectral directionplane. and 84 µm in the spatial one. The LR spectrometer works with magnification −1 and its aperture isStarting reduced from down a standard to 36.1 mm 2 mirrors (f/6.5) Offnër by means spectrometer, of a pupil a stopgrating placed grooves at the density grating of 500 plane. Startinglines/mm from has a been standard selected 2 mirrors in order Offnër to achieve spectrometer, the required a grating spectral grooves sampling density minimizing of 500 lines/mmthe has beenspectrometer selected in size order and toghosts achieve due theto grating required non- spectraloperational sampling orders. In minimizing order to improve the spectrometer the image size and ghostsquality, due a spherical to grating lens non-operational has been added. Due orders. to the In envelope order to constraints improve the the design image approach quality, of a this spherical lens is different from the one used for the HR spectrometer. lens has been added. Due to the envelope constraints the design approach of this lens is different from The incidence angle on the grating is approximately 17.4°. The beam is diffracted by the first the oneorder used between for the HR33.2° spectrometer. (at 500 nm) and 41.9° (at 740 nm). The grating is holographic and with a The incidence angle on the grating is approximately 17.4◦. The beam is diffracted by the first order between 33.2◦ (at 500 nm) and 41.9◦ (at 740 nm). The grating is holographic and with a saw-tooth Remote Sens. 2017, 9, 649 8 of 18 Remote Sens. 2017, 9, 649 8 of 18 saw-toothprofile. After profile. a second After reflection a second on reflection the primary on mirrorthe primary (M2 has mirr theor same (M2 radius has the of curvaturesame radius of M1)of curvaturethe beam isof foldedM1) the by beam M3 mirror is folded towards by M3 the mirror lens L2 towards before reaching the lens theL2 focalbefore plane reaching (Figure the6). focal The planeoptical (Figure quality 6). is almostThe optical diffraction quality limited is almost (WFErms diffraction < 0.13 limited waves) and(WFErms the correction < 0.13 waves) of distortions and the is correctionexcellent (keystoneof distortions less thanis excellent 0.5 µm (keystone and smile less less than than 0.5 0.1 μµmm). and smile less than 0.1 μm).

Figure 6. Low Resolution Spectrometer lay-out. Figure 6. Low Resolution Spectrometer lay-out. 3.5. Detector & Front End Electronics (FEE) 3.5. Detector & Front End Electronics (FEE) Three identical CCDs (2 for HR and 1 for LR) (Figure 7a,b), backside illuminated, split frame Three identical CCDs (2 for HR and 1 for LR) (Figure7a,b), backside illuminated, split frame transfer, radiation tolerant, 4 output ports, 1072 × 460 format with 42 μm × 28 μm (spatial × spectral transfer, radiation tolerant, 4 output ports, 1072 × 460 format with 42 µm × 28 µm (spatial × spectral direction) pixel size and ×2 spatial on-chip binning have been selected. 1050 × 430 (rows × col) will direction) pixel size and ×2 spatial on-chip binning have been selected. 1050 × 430 (rows × col) will allow the acquisition of 150 km swath and of 40 nm/20 nm spectral range for the O2A/O2B bands of allow the acquisition of 150 km swath and of 40 nm/20 nm spectral range for the O /O bands HR spectrometer (258 nm for LR). Further 10 × 2 columns and 5 × 2 rows are used as2A margin2B for of HR spectrometer (258 nm for LR). Further 10 × 2 columns and 5 × 2 rows are used as margin for alignment, while 5 × 2 columns and 6 × 2 rows respectively for dark current and smearing signal alignment, while 5 × 2 columns and 6 × 2 rows respectively for dark current and smearing signal corrections. The selected format (Figure 7) with a shorter device width along the spectral direction is corrections. The selected format (Figure7) with a shorter device width along the spectral direction is selected for a fast row driver (row transfer time < 1.25 μs) in order to reduce the smearing. A 35 selected for a fast row driver (row transfer time < 1.25 µs) in order to reduce the smearing. A 35 micron micron active silicon thickness with a HfOx 97.9 nm anti-reflection coating has been selected in order active silicon thickness with a HfOx 97.9 nm anti-reflection coating has been selected in order to to maximize quantum efficiency and minimize reflectivity in the O2A-O2B bands, with a max 0.65% of maximize quantum efficiency and minimize reflectivity in the O -O bands, with a max 0.65% of etalon effect (fringing) at 780 nm, which can be corrected by means2A 2B of a flat field measurement etalon effect (fringing) at 780 nm, which can be corrected by means of a flat field measurement during during calibration. 1.7 MHz readout frequency for each port allows download of all pixels at 43.5 ms calibration. 1.7 MHz readout frequency for each port allows download of all pixels at 43.5 ms pixel pixel reading time. The selected gain (0.75 μV/e- for 2.5 Me- Charge Handling Capacitance (CHC) for reading time. The selected gain (0.75 µV/e- for 2.5 Me- Charge Handling Capacitance (CHC) for the ×2 binning pixels) allows measurement of clouds signals both in HR and LR bands for possible the ×2 binning pixels) allows measurement of clouds signals both in HR and LR bands for possible in-flight straylight estimation [11,12]. The readout noise is reduced with a Correlated Double in-flight straylight estimation [11,12]. The readout noise is reduced with a Correlated Double Sampling Sampling (CDS) device in the Front End Electronics (FEE). A dumping gate has been implemented (CDS) device in the Front End Electronics (FEE). A dumping gate has been implemented to skip part to skip part of swath during the diagnostic mode. In this case all pixels of the swath for a of swath during the diagnostic mode. In this case all pixels of the swath for a homogeneous target homogeneous target are acquired without binning at different successive times, by using the same are acquired without binning at different successive times, by using the same CCD timing as the CCD timing as the operative mode. The operative detector temperature of 238 K ± 0.10 K (to mitigate operative mode. The operative detector temperature of 238 K ± 0.10 K (to mitigate the Charge Transfer the Charge Transfer Efficiency-CTE—degradation due to traps and to increase radiometric stability) Efficiency-CTE—degradation due to traps and to increase radiometric stability) is achieved by using is achieved by using an external radiator & heater/sensor near the package. No window is foreseen an external radiator & heater/sensor near the package. No window is foreseen in order to avoid in order to avoid multi-reflections with the anti-reflection coating placed on the detector sensible area. multi-reflections with the anti-reflection coating placed on the detector sensible area. Two adjacent Two adjacent detectors (Figure 7b) are placed in the HR focal plane along the spectral direction and detectors (Figure7b) are placed in the HR focal plane along the spectral direction and separated separated of a distance less than 12.06 mm in order to cover the two O2 absorption bands (677–697 nm of a distance less than 12.06 mm in order to cover the two O absorption bands (677–697 nm and and 740–780 nm). 2 740–780 nm). The Front End Electronic (FEE) is split into two parts: the Focal Plane Proximity Electronics (FPPE) devoted to pre-amplification and CCD bias conditioning, and the Video Acquisition Unit

Remote Sens. 2017, 9, 649 9 of 18

(VAU) for gain/offset drift compensation, bias and clocks generation/driving, FEE Telemetry (TM)/Telecommand (TC) and digital processing/serializing. The FPPE is placed at 5 cm from the detector while the VAU is at a distance of about 20–30 cm from FPPE. A 3 MHz Analog to Digital Converter (ADC) working at 16 bit with about 1.6 LSB total noise and three space-wire links versus nominal/redundantRemote Sens. 2017, 9, 649 Main Electronics (ME) are used. 9 of 18

(a)

(b)

Figure 7. Detector configuration: (a) 460 columns and 536 × 2 rows are used in the Image Areas IA I-II Figure 7. Detector conﬁguration: (a) 460 columns and 536 × 2 rows are used in the Image Areas IA I-II (incl. alignment pixels), 5 × 2 columns are used for blind pixels (dark estimation) while 6 × 2 rows for (incl. alignment pixels), 5 × 2 columns are used for blind pixels (dark estimation) while 6 × 2 rows for smearing corrections. Two different Storage Areas (SA I-II) are used for the reading-while smearing corrections. Two different Storage Areas (SA I-II) are used for the reading-while integrating integrating process though 2 output ports for each one. (b) geometry of the HR1 (O2B) and HR2 (O2A) process though 2 output ports for each one. (b) geometry of the HR1 (O ) and HR2 (O ) detectors detectors with 12 mm of separation between the image pixels. 2B 2A with 12 mm of separation between the image pixels.

The Front End Electronic (FEE) is split into two parts: the Focal Plane Proximity Electronics (FPPE) devoted to pre-ampliﬁcation and CCD bias conditioning, and the Video Acquisition Unit (VAU) for gain/offset drift compensation, bias and clocks generation/driving, FEE Telemetry (TM)/Telecommand (TC) and digital processing/serializing. The FPPE is placed at 5 cm from the detector while the VAU is at a distance of about 20–30 cm from FPPE. A 3 MHz Analog to Digital Remote Sens. 2017, 9, 649 10 of 18

Remote Sens. 2017, 9, 649 10 of 18 Converter (ADC) working at 16 bit with about 1.6 LSB total noise and three space-wire links versus nominal/redundant4. Mechanical Layout Main Electronics (ME) are used.

4. MechanicalThe optical Layout Bench is in Al6061 T6 with optics holders in Titanium or Aluminium for high thermal insulation & stability (Figure 8a). The Earth nadir baffle is also in aluminum and it is The optical Bench is in Al6061 T6 with optics holders in Titanium or Aluminium for high thermal divided in three parts: an external fixed one, an internal one within the calibration unit and another insulation & stability (Figure8a). The Earth nadir baffle is also in aluminum and it is divided in three internal fixed one in front of the scrambler/Band Pass (BP) filter device. Three white painted parts: an external fixed one, an internal one within the calibration unit and another internal fixed radiators cooling the detectors and the VAU’s are mounted on the upper side of instrument (Figure one in front of the scrambler/Band Pass (BP) filter device. Three white painted radiators cooling the 8b), with a full view to the cold space for optimum SpaceCraft (S/C) accommodation & thermal detectors and the VAU’s are mounted on the upper side of instrument (Figure8b), with a full view efficiency. The isostatic mount concept with three titanium bipods at 120 degrees assures the to the cold space for optimum SpaceCraft (S/C) accommodation & thermal efficiency. The isostatic mechanical stability (Figure 8a). The ME is separated by the instrument and is mounted on the S/C mount concept with three titanium bipods at 120 degrees assures the mechanical stability (Figure8a). payload module. The ME is separated by the instrument and is mounted on the S/C payload module.

(a)

(b)

FigureFigure 8.8. ((a)) FLORISFLORIS MechanicalMechanical layout:layout: internal view;view; ((b)) FLORISFLORIS MechanicalMechanical layout:layout: external view.

Remote Sens. 2017, 9, 649 11 of 18 Remote Sens. 2017, 9, 649 11 of 18 5. Instrument Calibration 5.The Instrument spectral Calibrationcalibration is performed on ground during pre-launch activities and will be checked/updatedThe spectral in-flight calibration by means is performed of vicari onous groundtechniques during using pre-launch the measurement activities and of willthe be atmosphericchecked/updated gas and/or in-flight the sun by Fraunhofer means of vicarious lines. In techniques flight radiometric using the calibration measurement is obtained of the atmospheric with a twogas points and/or calibration the sun scheme. Fraunhofer A Sun lines. illuminat In flighted radiometric reflection diffuser calibration (of Spectralon is obtained with with ρ a two=100%, points θinc.calibration = 65°) is observed scheme. every A Sun 7–15 illuminated days at South reflection Pole diffuser, while a (of dark Spectralon reference, with usefulρ =100%, for darkθinc. current = 65◦ ) is andobserved electronic every offset 7–15 data, days is meas at Southured Pole,every while 2–3 orbits. a dark A reference, satellite sl usefulew of for30° dark is planned current for and the electronic sun observation.offset data, The is measuredcalibration every unit 2–3is composed orbits. A satellite by a carousel slew of 30(Figure◦ is planned 9a) which for the can sun rotate observation. in three The differentcalibration positions. unit isThe composed first one, by relevant a carousel to the (Figure nominal9a) which observation can rotate (Nadir), in three allows different telescope positions. pupil The illuminationfirst one, relevantby the light to the coming nominal from observation the front (Nadir),aperture allows through telescope the nadir pupil baffle illumination (Figure 9b), by the the light secondcoming one frompresents the frontblack aperture flat target through at the thetelescope nadir baffle input (Figure for instrument9b), the second dark calibration one presents (Figure black flat 9c), targetand the at thelast telescope one allows input the for sun instrument calibration dark by calibration observing (Figure a Sun-illuminated9c), and the last reference one allows stable the sun reflectioncalibration diffuser by observingwith the Sun a Sun-illuminated entering into the reference instrument stable by reflection the solar diffuserport placed with on the the Sun lateral entering sideinto (Figure the instrument9d). by the solar port placed on the lateral side (Figure9d). DuringDuring almost almost all operations all operations after after instrument instrument delivery delivery (calibration (calibration and andsatellite satellite Assembly, Assembly, IntegrationIntegration and and Verification-AIV—activities), Verification-AIV—activities), thethe launch,launch, the the Earth Earth observation observation and theand dark the calibration, dark calibration,the diffuser the diffuser remains remains enclosed enclosed in a small in a room,small room, protected protected from possiblefrom possible external external and internaland internalcontaminations. contaminations. Only Only during during the solarthe solar calibration calibration the diffuser the diffuser is placed is placed in the in calibration the calibration position, position,with sunwith entering sun entering through through the solar the port.solar port.

(a) (b)

Figure 9. Rotating Calibration Unit internal view (a) and in three different positions: (b) Earth Nadir Figure 9. Rotating Calibration Unit internal view (a) and in three different positions: (b) Earth Nadir Observation,Observation, (c) Black (c) Black target target observation, observation, (d) Sun (d) Sundiffuser diffuser observation observation through through the solar the solar port. port.

Remote Sens. 2017, 9, 649 12 of 18

6. On Ground Calibration/Validation Activities The methodology used to verify instrument performance on-ground encompasses the characterization of components and subsystems during the instrument build/integration and the calibration/characterization/validation of the instrument. The key performance criteria of components, items, and subsystems is established at various stages of instrument development so that the relevant geometric, radiometric, spectral, polarization and straylight properties are determined to verify all governing requirements during the pre-flight assembly, integration, test and calibration of the instrument. On-ground instrument calibration/ characterization/validation will be performed in facilities designed to simulate the on-orbit environment using specific Ground Support Equipments (GSEs). Instrument level alignment and a reduced test campaign before and after instrument qualification will be performed at Leonardo site. Full geometric calibration as well as spectral, polarimetric, radiometric and straylight test and calibration will be performed at the calibrator facility. The calibration data, generated during on-ground calibration, will be used as the reference calibration of FLORIS when it begins its on-orbit mission. Subsequent in-flight calibration campaigns will be used to monitor and update the calibration parameters throughout the mission. The geometric calibration includes the Instrument Spatial Response (called System Energy Distribution Function, SEDF) characterization and the determination of alignment parameters relevant for instrument pointing knowledge when integrated on S/C and then in flight with respect to earth/sun target. Also spatial co-registration measurements for each and between HR and LR channels will be performed. The spectral calibration includes the measurement of Instrument Spectral Response Function (ISRF) shape, Spectral Sampling Interval (SSI), Spectral Resolution (SR), Smile, Calibration Unit (CU) spectral features. Radiometric Calibration measures the instrument radiometric response in all spectral bands. It includes radiance calibration coefficients, response linearity, noise, detector Pixel-to-pixel Response Non-Uniformity (PRNU), detector Dark Signal Non-Uniformity (DSNU), etc. Radiometric calibration shall also include a characterization of the instrument response through the sun calibration port versus sun angle at instrument level. The radiometric calibration is performed using fully and partial linearly polarized light and fully unpolarized light. Straylight is an important characterization and calibration parameter as the levels of straylight at various scene/radiance conditions contribute to errors in the radiometric accuracy of the instrument and in the fluorescence retrieval algorithm. Straylight assessment is one of the most challenging tasks of calibration and requires a combination of methods including point source and/or extended target stimulus in order to simulate extended non-uniform scenes.

7. Performance The more stringent requirement for the Spectral Resolution (SR = 0.3 nm) is relevant to the two HR O2 absorption bands 686–697 nm and 759–769 nm (Table2). This has been achieved by using a slit width of three times (84 µm) the pixel sampling interval (28 µm). A similar concept has been applied for the LR 697–758 nm band. For these HR and LR bands the Spectral Resolution is principally driven by the slit width (Figure 10a). The other HR bands (677–686 nm, 740–759 nm and 769–780 nm) have been obtained by means of ×5 spectral binning performed at VAU level, with a spectral resolution principally driven from the aggregated pixel size (140 µm) (Figure 10b, Table2). The remaining LR bands (500–697 nm) have been obtained by means of x3 spectral binning at VAU level, with a spectral sampling equal to the slit with (84 µm) (Figure 10c). Finally the Instrument Spectral Response Function (ISRF) is a quasi-trapezoidal function for almost all bands and a quasi-triangular function for the LR 500–697 nm bands (Figure 10a–c). In fact, considering the achieved optical quality, the ISRF can be principally obtained by a convolution between two rectangular functions, representing the slit width Remote Sens. 2017, 9, 649 13 of 18 of 84 µm and the spectral pixel size after binning ×1, ×3 or ×5, corresponding respectively to 28 µm (FigureRemote Sens. 10a), 2017 84, 9µ, m649(Figure 10c) and 140 µm (Figure 10b). 13 of 18

(a) (b)

(c)

Figure 10. ISRF at focal plane (x-scale: 84 μm = 0.28 nm for HR and 1.8 nm for LR) for a slit width of Figure 10. ISRF at focal plane (x-scale: 84 µm = 0.28 nm for HR and 1.8 nm for LR) for a slit width 84 μm and (a) a spectral pixel of 28 μm (LR = 697−758 nm, HR = 686−697, 759−769 nm) with SR = 0.28 of 84 µm and (a) a spectral pixel of 28 µm (LR = 697−758 nm, HR = 686−697, 759−769 nm) with nm (HR) and SR = 1.8 nm (LR), (b) a ×5 binned spectral pixel of 140 μm (HR = 677–686, 740–759, SR = 0.28 nm (HR) and SR = 1.8 nm (LR), (b) a ×5 binned spectral pixel of 140 µm (HR = 677–686, 769−780 nm) with SR = 0.47 nm, (c) a ×3 binned spectral pixel of 84 μm (LR = 500−697 nm) with SR = 740–759, 769−780 nm) with SR = 0.47 nm, (c) a ×3 binned spectral pixel of 84 µm (LR = 500−697 nm) 2.03 nm. with SR = 2.03 nm. The Image quality has been evaluated from the Full Width Half Maximum (FWHM) of the SystemThe Energy Image quality Distribution has been Function evaluated (SEDF), from thewhich Full represents Width Half the Maximum spatial response (FWHM) of of the Systemoptical Energysystem Distributionto a spectrally Function uniform (SEDF), point source. which representsA system Modulation the spatial responseTransfer Function of the optical (MTF) system budget to ahas spectrally been performed uniform taking point source.into account A system the followi Modulationng End-Of-Life Transfer (EOL) Function contributions (MTF) budget for the has across been performedand/or the taking along intotrack account directions: the following scrambler, End-Of-Life telescope, (EOL) slit, spectrometer, contributions fordetector the across pixel and/or response the along(incl. trackcross-talk directions: and scrambler,Charge Transfer telescope, Efficiency), slit, spectrometer, pixel integration detector pixel along response motion (incl. and cross-talk jitter and(instrument ChargeTransfer and satellite). Efﬁciency), Then pixelthe SEDF integration has been along obtained motion (Figure and jitter 11a)(instrument from the inverse and satellite). Fourier ThenTransform the SEDF of the has system been obtained MTF. The (Figure FWHM 11 a)of fromSEDF the in inversethe across Fourier and along Transform track ofdirections the system (Figure MTF. The11b) FWHM are respectively of SEDF in95.6 the μm across (along-track) and along and track 88.7 directionsμm (across-track) (Figure 11correspondingb) are respectively respectively 95.6 µ tom 1.14 SSD and 1.06 SSD (with SSD = Spatial Sampling Distance = 84 μm).

Remote Sens. 2017, 9, 649 14 of 18

Remote Sens. 2017, 9, 649 14 of 18 (along-track) and 88.7 µm (across-track) corresponding respectively to 1.14 SSD and 1.06 SSD (with µ RemoteSSD = Sens. Spatial 2017, Sampling9, 649 Distance = 84 m). 14 of 18

(a) (b) Figure 11. (a) 2D System(a) Energy Distribution Function (SEDF) at the detector(b) focal plane; (b) Along and across-track sections of the SEDF (SSD = 84 μm = 288 m at 805 km altitude). FigureFigure 11. ((a) 2D System EnergyEnergy DistributionDistribution FunctionFunction (SEDF)(SEDF) atat the the detector detector focal focal plane; plane; ( b(b) Along) Along and across-track sections of the SEDF (SSD = 84 μm = 288 m at 805 km altitude). Theand combination across-track sections of a high of the SNR SEDF and (SSD a high = 84 µ specm = 288tral m sampling at 805 km altitude).requirement asks for a demanding instrumentThe combination in terms of of the a high smearing SNR and noise a high (row spec transfertral sampling frequency requirement = 0.8 MHz), asks pupil for a diameterdemanding (75.6 The combination of a high SNR and a high spectral sampling requirement asks for a demanding mm)instrument and optics in terms transmission of the smearing (37%). noiseThe pupil (row transferdiameter frequency is about =three 0.8 MHz), times pupilhigher diameter with respect (75.6 to instrument in terms of the smearing noise (row transfer frequency = 0.8 MHz), pupil diameter (75.6 mm) thatmm) of and the optics OLCI transmission spectrometer (37%). embarked The pupil on diamet Sentineler is 3 about platform three andtimes the higher previous with respectMERIS to (on and optics transmission (37%). The pupil diameter is about three times higher with respect to that Envisat),that of thewhich OLCI has spectrometer a similar on embarkedground sampling on Sentinel (300 3m) platform and integration and the timeprevious (45 ms) MERIS but a(on more of the OLCI spectrometer embarked on Sentinel 3 platform and the previous MERIS (on Envisat), relaxedEnvisat), spectral which hassampling a similar interval on ground and samplingspectral (300resolution m) and (about integration 10–20 time times). (45 ms) The but comparison a more which has a similar on ground sampling (300 m) and integration time (45 ms) but a more relaxed betweenrelaxed spectralSNR requirement sampling interval and performance and spectral arresolutione reported (about in Figure10–20 times).12. The The minimum comparison SNR spectral sampling interval and spectral resolution (about 10–20 times). The comparison between SNR performancebetween SNR is 125requirement at 760.7 nm and with performance SSI of 0.093 arnm,e reportedcompliant in with Figure requirement 12. The (SNRminimum = 111) SNR with a requirement and performance are reported in Figure 12. The minimum SNR performance is 125 at marginperformance of +12%. is 125 at 760.7 nm with SSI of 0.093 nm, compliant with requirement (SNR = 111) with a 760.7 nm with SSI of 0.093 nm, compliant with requirement (SNR = 111) with a margin of +12%. margin of +12%.

SNR Requirements & Performance SNR Requirements & Performance 1600 1600 SNR Requirements & Performance SNR Requirements & Performance 14001600 16001400

12001400 14001200

10001200 12001000

8001000 1000800 L,SNR 600800 L,SNR 800600 L,SNR L,SNR 400600 600400

200400 400200

200 0 200 0 500 550 600 650 700 750 800 750 755 760 765 770 775 780 0 0 Wavelength (nm) Wavelength (nm) 500 550 600 650 700 750 800 750 755 760 765 770 775 780 SNR SNR requiredWavelength (nm)Lref resampled (W/m2/sr/mic) SNR WavelengthSNR required (nm) Lref resampled (W/m2/sr/mic) SNR SNR required Lref resampled (W/m2/sr/mic) SNR SNR required Lref resampled (W/m2/sr/mic) (a) (b) (a) (b) Figure 12. Comparison between SNR requirements and performance: blue curve = SNR requirement, FigureFigure 12. 12. ComparisonComparison between SNR requirements andand performance:performance:blue blue curve curve = = SNRSNR requirement, requirement, green curve = SNR performance, pink curve = Reference input radiance (W/m2 2/sr/μm). (a) Total greengreen curve = SNRSNR performance,performance, pinkpink curve curve = = Reference Reference input input radiance radiance (W/m (W/m/sr/2/sr/µμm).m). (a(a)) Total Total Band,Band, (b (b) )OO2A band.band. Band, (b) O2A2A band. Considering that the slit represents an intermediate field stop of the system, the total Considering that the slit represents an intermediate field stop of the system, the total instrumentinstrument stray-lightstray-light can be divideddivided inin twotwo term terms,s, one one coming coming from from the the telescope telescope and and the the other other comingcoming fromfrom thethe spectrometer.spectrometer. InIn bothboth casescases thethe main main stray-light stray-light contributions contributions are are the the scattering scattering comingcoming from from thethe surfacesurface roughness ofof thethe opto-mec opto-mechanicalhanical elements, elements, the the scattering scattering coming coming from from the the distributeddistributed PArticlePArticle Contamination (PAC)(PAC) andand thethe ghosts.ghosts. Optical Optical design design is isbased based on onvery very demandingdemanding roughnessroughness values (0.5(0.5 ÷÷ 1.51.5 nmnm rms rms for for lenses lenses and and mirrors, mirrors, 2.5 2.5 nm nm for for scrambler scrambler and and 2 2 nmnm for for grating),grating), lowlow AR coatings reflectivityreflectivity (0.5 (0.5−−1.5%),1.5%), including including the the detector detector retina, retina, high high grating grating efficiencyefficiency (>60%)(>60%) andand high mirror reflectivityreflectivity (98. (98.5%).5%). Moreover Moreover contamination contamination will will be betaken taken under under controlcontrol forfor thethe overalloverall Assembly, IntegrationIntegration andand TestTest (AIT) (AIT) and and Assembly, Assembly, Integration Integration and and VerificationVerification (AIV)(AIV) activitiesactivities in orderorder toto achieve achieve ch challengingallenging values values of of EOL EOL PAC PAC (250 (250 ppm ppm for for the the first first

Remote Sens. 2017, 9, 649 15 of 18

Considering that the slit represents an intermediate field stop of the system, the total instrument stray-light can be divided in two terms, one coming from the telescope and the other coming from the spectrometer. In both cases the main stray-light contributions are the scattering coming from the surface roughness of the opto-mechanical elements, the scattering coming from the distributed PArticle Contamination (PAC) and the ghosts. Optical design is based on very demanding roughness values (0.5 ÷ 1.5 nm rms for lenses and mirrors, 2.5 nm for scrambler and 2 nm for grating), low RemoteAR coatings Sens. 2017 reflectivity, 9, 649 (0.5−1.5%), including the detector retina, high grating efficiency (>60%)15 and of 18 high mirror reflectivity (98.5%). Moreover contamination will be taken under control for the overall surface,Assembly, 35 Integrationppm for scrambler and Test (AIT)assembly and Assembly,and telescope Integration inner surfaces and Verification and 150 (AIV)ppm activitiesfor all others in components).order to achieve challenging values of EOL PAC (250 ppm for the first surface, 35 ppm for scrambler assemblyMitigation and telescope actions innerwill be surfaces adopted and by 150 preventing ppm for all the others increase components). of contaminants: operations in ISO5 classroomMitigation with actions contamination will be adopted control by preventing and cleaning theincrease of exposed of contaminants: surfaces (if necessary) operations for in all ISO the main5 classroom assemblies with contaminationup to the delivery control to and satellite cleaning prime, of exposed use of surfacesa quasi-hermetic (if necessary) cover for alland the closer main of carouselassemblies during up to almost the delivery all AIT/AIV to satellite activities prime, useat satellite of a quasi-hermetic site, purging cover with and dry closer nitrogen of carousel during activitiesduring almost in which all AIT/AIV the instrument activities will at satellitenot be in site, a purgingcontrolled with environment dry nitrogen (long during storage activities periods, in vibrationwhich the tests, instrument route willfrom not a becleanroom in a controlled to another, environment and other (long activities storage periods,at satellite vibration site), tests,use of dedicatedroute from asnorkels cleanroom (cold to another, space and oriented) other activities in th ate satellitedesign site), in useorder of dedicated to facilitate snorkels on-orbit (cold de-contamination.space oriented) in the design in order to facilitate on-orbit de-contamination. ActualActual performance performance (Figure (Figure 13)13 )are are higher higher wi withth respect respect to tolevel level L0 L0requirement requirement (0.2 mW/m(0.2 mW/m2/sr/nm)2 /sr/nm)of about a of factor about 4, meaning a factor that 4, meaningthe requirement that the at requirementlevel 1b (0.04 atmW/m level2/sr/nm) 1b (0.04 will bemW/m obtained2/sr/nm) only will after be obtaineda dedicated only afteron-ground a dedicated stray-light on-ground measurement stray-light measurement campaign campaignand model correctionand model activity correction (similar activity to (similar that performed to that performed for OLCI for OLCIon Sentinel on Sentinel 3). 3).In-flight In-flight verification verification by measuringby measuring the thetransition transition between between a acloud cloud and and a a vege vegetatedtated target and/orand/or bybyobserving observing the the transition transition betweenbetween moon moon and cold space areare alsoalso planned.planned.

FigureFigure 13. 13. ComparisonComparison between StraylightStraylight radianceradiance evaluation evaluation and and requirement requirement at at level level L0. L0.

8.8. FLORIS FLORIS Elegant Elegant Breadboard Breadboard TheThe FLEX FLEX pre-development pre-development has beenbeen addressedaddressed fromfrom thethe very very beginning beginning by by ESA ESA toward toward an an experimentalexperimental validation validation of the optical concept,concept, thethe relatedrelatedalignment alignment and and test test procedures procedures and and the the earlyearly risk risk retirement retirement of thethe mostmost criticalcritical technologiestechnologies as as for for instance instance detectors detectors and and gratings. gratings. An An extensiveextensive breadboard breadboard development development has has been been initiate initiatedd since since the the phase phase A/B1 A/B1 study study of ofthe the mission, mission, and itsand refurbishment its refurbishment is currently is currently in progress in progress for foran Elegant an Elegant BreadBoard BreadBoard (EBB). (EBB). The The current current status status of of the breadboard includes the optical bench, the common telescope and the HR spectrometer. Service detectors with a pixel size comparable to the flight ones are implemented in the breadboard (Figure 14). The breadboard has been recently tested confirming the good optical quality of the design [13]. A new test campaign is currently in preparation in cleanroom ISO 5 to perform a better straylight characterization. The breadboard will be then refurbished with the inclusion of the polarization scrambler and the LR spectrometer and a new measurement campaign is planned.

Remote Sens. 2017, 9, 649 16 of 18 the breadboard includes the optical bench, the common telescope and the HR spectrometer. Service detectors with a pixel size comparable to the ﬂight ones are implemented in the breadboard (Figure 14). The breadboard has been recently tested conﬁrming the good optical quality of the design [13]. A new test campaign is currently in preparation in cleanroom ISO 5 to perform a better straylight characterization. The breadboard will be then refurbished with the inclusion of the polarization scrambler and the LR spectrometer and a new measurement campaign is planned. Remote Sens. 2017, 9, 649 16 of 18

(a) (b)

Figure 14. (a) Optical Ground Support Equipment (OGSE) used for the phase A-B1 tests of the Figure 14. (a) Optical Ground Support Equipment (OGSE) used for the phase A-B1 tests of the FLORIS FLORIS BreadBoard (BB), (b) FLORIS BB for telescope and HR spectrometer. BreadBoard (BB), (b) FLORIS BB for telescope and HR spectrometer.

9. Conclusions 9. Conclusions The optical design, the required and the foreseen performance of the FLORIS instrument have The optical design, the required and the foreseen performance of the FLORIS instrument have been presented. The baseline is a very compact, robust, stable versus temperature and easy to align been presented. The baseline is a very compact, robust, stable versus temperature and easy to align optical design. Good performance is achieved in terms of optical quality, distortions (keystone and optical design. Good performance is achieved in terms of optical quality, distortions (keystone and smile) and Signal to Noise Ratio in the whole field of view and spectral range. The common smile) and Signal to Noise Ratio in the whole field of view and spectral range. The common fore-optics fore-optics and the double slit assembly assure an optimum spatial co-registration stability by and the double slit assembly assure an optimum spatial co-registration stability by design between design between the spectral channels. Particular attention has been also devoted to the reduction of the spectral channels. Particular attention has been also devoted to the reduction of the stray-light the stray-light effects by using efficient coatings, holographic gratings, low roughness for each effects by using efficient coatings, holographic gratings, low roughness for each optical elements optical elements and particular attention to the instrument contamination level. The adopted Offnёr and particular attention to the instrument contamination level. The adopted Offnër solutions for solutions for the spectrometers have a large heritage in Leonardo [14–16]. The CCD detector design the spectrometers have a large heritage in Leonardo [14–16]. The CCD detector design is not critical is not critical from a technological point of view and a breadboard activity is on-going. The same from a technological point of view and a breadboard activity is on-going. The same format has been format has been chosen for all the three focal planes in order to reduce smearing and achieve an high chosen for all the three focal planes in order to reduce smearing and achieve an high pixel capacitance pixel capacitance with low noise, no Peltier stage and pixels devoted to smearing and dark current with low noise, no Peltier stage and pixels devoted to smearing and dark current corrections in flight. corrections in flight. A simple mechanical layout in a monolithic aluminum optical bench allows the A simple mechanical layout in a monolithic aluminum optical bench allows the use of isostatic mounts use of isostatic mounts and a good thermal stabilization by means of radiators looking at the cold and a good thermal stabilization by means of radiators looking at the cold space view and heaters. space view and heaters. 10. Outlook 10. Outlook The FLEX tandem mission concept with its highly innovative FLORIS payload will provide unprecedentedThe FLEX tandem information mission on theconcept functioning with its of high vegetation,ly innovative actual FLORIS photosynthetic payload efficiency will provide and unprecedentedhealth status. Theinformation primary dataon the products functioning contain of ve qualitygetation, controlled, actual photosynthetic spectrally and efficiency geometrically and healthcharacterized status. andThe radiometricallyprimary data products calibrated contain TOA radiancesquality controlled, that will bespectrally made available and geometrically to the users characterizedwithin 24 h of sensing.and radiometrically Higher-level calibrated scientific data TOA products radiances comprise that will TOA be synergymade available data sets to from the OLCI,users withinSLSTR 24 and h FLORISof sensing. as wellHigher-level as fluorescence scientific emission data products in the oxygencomprise absorption TOA synergy lines, data the spectrallysets from OLCI,integrated SLSTR total and fluorescence FLORIS as emission well as fluorescence and the peak emission values around in the 680oxygen nm andabsorption 740 nm lines, [4]. Thethe spectrallyvalidation integrated of these products total fluorescence follows the emission recommendations and the peak of values the Committee around 680 on nm Earth and Observation 740 nm [4]. TheSatellites validation (CEOS) of inthese that groundproducts truth follows measurements the recommendations at top of canopy of the will Committee be up-scaled on toEarth the ObservationFLEX spatial Satellites resolution. (CEOS) The correspondingin that ground infrastructuretruth measurements at ground at top will of becanopy developed, will be tested, up-scaled and toimplemented the FLEX spatial during resolution. the coming The years. corresponding infrastructure at ground will be developed, tested, and implemented during the coming years. As an Earth Explorer mission, FLEX can serve as a vanguard for a future operational mission supporting a large variety of services. Potential application areas comprise forest health monitoring, early detection and warning of stress-induced strain in perennial food crops, evaluation of land-use management strategies, or phenotyping assessments of food and feed crops. In coastal areas, there is the potential for using the observations to detect and track toxic algal blooms.

Remote Sens. 2017, 9, 649 17 of 18

As an Earth Explorer mission, FLEX can serve as a vanguard for a future operational mission supporting a large variety of services. Potential application areas comprise forest health monitoring, early detection and warning of stress-induced strain in perennial food crops, evaluation of land-use management strategies, or phenotyping assessments of food and feed crops. In coastal areas, there is the potential for using the observations to detect and track toxic algal blooms.

Acknowledgments: The instrument design and breadboard activities have been funded by ESA. Phase B2-C-D activities, currently in progress, principally regard the assessment of the design of the instrument baseline and the performance budget, the final definition of the system and subsystems specifications and the organisation within the industrial core team (Leonardo, OHB and Thales) of the Invitation to Tenders (ITT) for the selection of sub-contractors. Author Contributions: Peter Coppo contributed as system engineer and performance manager of the FLEX project. Alessio Taiti defined the optical system architecture and performed optical engineering tasks. Lucia Pettinato performed the alignment and tests of the instrument elegant breadboard and the planning of on ground calibration/validation activities for the space instrument. Michael Francois is the ESA responsible for the overall instrument development and supervised the study. Matthias Drusch is the ESA responsible for the FLEX science applications and products. Matteo Taccola contributed to the instrument pre-development and breadboard activities as ESA instrument engineer. All the authors participated to the preparation of the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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