Copernicus: Sentinel-5 (Atmospheric Monitoring Mission) in LEO
Sentinel-5 is an atmospheric monitoring mission within the European Copernicus program, formerly the GMES (Global Monitoring for Environment and Security) program, jointly implemented by ESA and the EC (European Commission). The main objective of the mission is the operational monitoring of trace gas concentrations for atmospheric chemistry and climate applications. It will provide accurate measurements of key atmospheric constituents such as ozone, nitrogen dioxide, sulphur dioxide, carbon monoxide, methane, formaldehyde, and aerosol properties. 1) 2)
Table 1: Copernicus is the new name of the former GMES program 3)
The space segment will be implemented as an imaging spectrometer to be flown on EUMETSAT's MetOp -SG (Second Generation) satellites. From a sun-synchronous LEO orbit, Sentinel-5 measurements will complement the Sentinel-4 GEO data over Europe and provide a daily global coverage at an unprecedented spatial resolution of 7 km x 7 km at nadir. 4)
While Sentinel-1, -2 and -3 are completing their development phases and are scheduled to launch until 2014, Sentinels-4/5 are foreseen to provide a synergetic, complementary data set for atmospheric monitoring starting in 2020.
The Sentinel-5 mission is a payload, consisting of a single instrument named UVNS; it will be hosted as a CFI (Customer Furnished Item) on a post-EPS (MetOp) spacecraft, i.e. MetOp-SG, and will be operated by EUMETSAT. 5) In November 2012, the EUMETSAT Council successfully concluded the approval process for the EPS-SG (EUMETSAT Polar System Second Generation) Preparatory Program with all 26 Member States having now firmly committed themselves. 6) 7)
A two-satellite architecture has been selected for MetOp-SG by ESA and EUMETSAT, namely MetOp-SG A and B, flying in the same sun-synchronous orbit. Unlike the current MetOp system of identical satellites operating in a relay, the MetOp-SG system envisages a pair of different satellites, each carrying a different but complementary suite of instruments. This will comprise a mix of instruments offering data continuity with improved performance and new instruments to meet the evolving demands of the meteorological community.
Altogether, the new MetOp-SG system concept features six satellites; the launch of the first one is planned for 2020. The overall system lifetime is 21 years, with each satellite designed to exceed an eight and a half year lifetime. - On March 28, 2014, ESA signed a contract with Airbus Defence and Space in Ottobrunn, Germany for the Sentinel-5 instrument of Europe's Copernicus program. 8)
Table 2: Preliminary concept of the MetOp-SG program with candidate sensor complement 9)
Both types of satellites (A and B) will be designed for launch on a Soyuz-class launcher and to be technically compatible with three potential launch vehicles (Soyuz, Ariane and Falcon 9).
Table 3: MetOp-SG A satellite platform main characteristics
Figure 1: Illustration of the MetOp-SG A satellite (image credit: ESA)
Launch: A launch of Sentinel-5 A on MetOp-SG-A is planned for 2021.
Orbit: Sun-synchronous orbit, altitude = 817 km, inclination = 98.5º, LTDN (Local Time on Descending Node) = 9:30 hours. The orbital cycle is 29 days (14 orbits per day, 412 orbits per cycle). The orbit cycle is the time taken for the satellite to return over the same geographical point on the ground with the same viewing geometry.
Sensor complement: (Sentinel-5 UVNS)
Sentinel-5 UVNS (Ultra-Violet /Visible/Near Infrared/SWIR) Spectrometer
The MetOp-SG A satellite(s) will carry the Sentinel-5 instrument as the Sentinel-5 mission as part of the EU's Copernicus Program. The development of the SENTINEL-5 instrument is done by Airbus Defence & Space, Ottobrunn, under contract with ESA. Many European industries have contributed to the subsystems. 11)
The main characteristics of SENTINEL-5 UVNS instrument are:
- Type: passive grating imaging spectrometer
- Configuration: Push broom staring (non-scanning) in nadir viewing
- Swath width: 2 670 km
- Spatial sampling: 50 x 50 km2 (UV1), 7.5 x 7.5 km2 (all other channels)
- Spectral: 5 spectrometers (1 in UV1, 1 in UV2VIS, 1 in NIR, 2 in SWIR)
- Radiometric accuracy (absolute): 3%, 6%(SWIR) of the measured earth spectral reflectance.
Observation is performed over the complete sunlit (with respect to the subsatellite point) part of the orbit while the instrument calibrations are done during the dark part of the orbit. The expected instrument operation is shown schematically in Figure 2.
Figure 2: SENTINEL-5 UVNS instrument operations scenario (image credit: ESA)
Airbus Defence and Space is the prime contractor for the design, development, assembly, integration, test and verification of the Sentinel-5/UVNS instrument, which is a customer furnished item for the MetOp-SG-A platforms. 12)
Figure 3: Illustration of the UVNS instrument (image credit: Airbus DS)
Table 4: UVNS parameter specification
Table 5: Key features of the UVNS
Figure 4: Optical modules of the UVNS instrument for Sentinel-5 (image credit: Airbus DS)
The UVNS instrument is a pushbroom imaging spectrometer, directly imaging the entrance slit onto the Earth's surface. The principle is depicted in Figure 5. The entrance slit is providing the spatial coverage or swath in the ACT (Across-Track) direction, where the projected detector pixels define the spatial samples. The ALT (Along-Track) dimension is acquired by the satellite motion. The light from the slit is spectrally dispersed by a diffraction grating and imaged onto a two-dimensional detector. The spatial sampling along the track is defined by the detector timing. The dwell time, over which typically several exposures are co-added, defines the ALT spatial sampling distance (SSD). The mission will generate Level-1b data consisting of TOA (Top Of Atmosphere) Earth spectral radiance and extra-terrestrial spectral solar irradiance. Division of the Earth radiance spectra by the regularly measured solar irradiance yields the reflectance spectra, which feature the spectral signature of absorbing gases and scattering aerosols. Dedicated retrieval techniques are employed to infer the vertical column amounts of the absorbing and scattering species from these reflectance measurements (Ref. 4).
Figure 5: Principle of the Sentinel-5 pushbroom imaging dispersive spectrometer (image credit: ESA)
The Sentinel-5 mission is targeting a wide range of products for the atmospheric applications. It will provide vertical column amounts of O3 (stratospheric and tropospheric), NO2, SO2, CO, CH4, CH2O, as well as some profile information on O3 and aerosols. Due to the broad spectral range needed to observe all targeted molecular species, the instrument is a complex assembly of various spectrometers dedicated to subbands. In Figure 6, a sketch of an optical concept, developed during the phase 0 study, is depicted, showing a possible implementation with six spectrometers (UV-1, UV-2, VIS, NIR, SWIR-1, SWIR-3) sharing a common telescope.
Figure 6: Sketch of a proposed phase 0 concept of the Sentinel-5 UVNS instrument, showing various individual spectrometers sharing a common telescope (image credit: ESA)
1) Geometric requirements:
One of the most outstanding characteristics of the Sentinel-5 mission are manifested by an unprecedented spatial coverage and sampling. The scientific requirements demand full coverage of the entire globe with a daily revisit frequency. Figure 7 depicts the observation geometry and coverage. For a LEO mission at MetOp SG's envisaged orbit altitude of 817 km, daily coverage requires an average swath width of 2715 km (slightly varying due to the oblateness of Earth and orbit eccentricity). As can be seen from upper right panel, small gaps occur at the equator resulting in a revisit period of about two days, whereas daily revisit is reached beyond 14° latitude (see lower panel). The enormous swath width translates at instrument level into a large cross-track FOV of 108.4° (see upper left panel).
Figure 7: Sentinel-5 geographical coverage requirements (image credit: ESA)
Legend to Figure 7: Upper left: Daily global coverage requires a wide swath (2715 km) and across-track field of view of 108.4°. Upper right: Swaths of two consecutive orbits, indicating small gaps at the equator. Below: Resulting revisit frequency map, showing the daily coverage within less than one day above 14° latitude.
Another characteristic of Sentinel-5 is the high spatial resolution of the trace gas measurements. At nadir, the area over which all targeted species are determined, will have a maximum size of 7 km x 7 km. Figure 8 shows a comparison between the ground pixel size of Sentinel-5 and its predecessor missions GOME, SCIAMACHY, and OMI. The high spatial resolution will enable more accurate detection of emission sources and provide an increased number of cloud-free ground pixels. The ground samples will be observed under equal sampling angles, which means that the across-track SSD (Spatial Sampling Distance) will vary over the swath and reach 35 km at its edges. The along-track (ALT) component of the SSD is determined by the dwell time and remains constant at 7 km. The MetOp-SG platform will perform continuous yaw steering maneuvers over the orbit to compensate for image distortion and misregistrations induced by Earth rotation.
Figure 8: Comparison of spatial resolution (ground pixel size) of Sentinel-5 (S5) with heritage missions (image credit: ESA)
Some atmospheric products are inferred from simultaneous measurements in different spectral bands, which have to be acquired over identical spatial samples (or ground pixels). This results in stringent co-registration requirements, allowing for inter-band deviation of only 10% of the ground pixel size (< 700 m at nadir) and driving optical as well as mechanical designs to minimize such errors. Apart from optimizing image quality (minimize keystone errors) this involves a careful design of the polarization scrambler, which can induce coregistration errors depending on the polarization state of the measured radiance.
2) Spectral requirements:
The large number of targeted atmospheric constituents can only be measured over an extremely broad spectral bandwidth, spanning from the UV (starting at 270 nm) to the SWIR spectral regions (up to 2385 nm). The simultaneously measured spectral bands are depicted in Figure 9. The spectral resolution varies from 1 nm in the UV1 (270-300 nm), used for retrieval of stratospheric O3 profiles, over 0.5-0.4 nm for the visible and NIR range, respectively, to 0.25 nm in the two SWIR bands. A spectral resolution element is sampled by 2.5-3.0 detector pixels in order to avoid spectral aliasing.
The broad and partially discontinuous spectral range dictates, that the light collected by the instrument's telescope must be spectrally split and distributed to a number of individual spectrometers. For each of them, a spectral knowledge of 1.5 pm (picometer) is demanded; the stability requirements (15 pm) call for a very robust design and an accurate on-ground calibration. The shape of the ISRF (Instrument Spectral Response Function) has to be known within 1% throughout the mission lifetime (7.5 years), which calls for an in-flight monitoring utilizing a spectral stimulus device. Technology solutions for meeting these demanding specifications are currently under investigation.
Figure 9: Sentinel-5 spectral band definition (spectral radiance of SWIR bands not in scale), image credit: ESA
3) Radiometric requirements:
The signal-to-noise (SNR) requirements are driving the instrument pupil size, and have been derived from sensitivity analyses for the retrieval of each individual molecular species. Which spectral ranges are driving the aperture size of the instrument depends on the chosen concept. A notoriously challenging region, especially in terms of straylight, is the UV2 region due to a variation of radiance levels over 3 orders of magnitude within a short spectral range (indicated in Figure 9). Special techniques, like graded filters may be necessary to cope with the enormous dynamic range. The high accuracy requirement for retrieval of NO2 columns is driving the SNR in the VIS region to 1500. In the SWIR regions, where CO, CO2, CH4 and H2O provide strong absorption signals, a known limiting factor is the readout noise of the MCT (Mercury Cadmium Telluride) CMOS detectors.
While SNR limits are in principle always achievable by increasing the pupil size (and with it size and mass) of the instrument, the demanding requirements for relative and absolute radiometric accuracy are pushing technology to the limits of feasibility. The relative spectral radiometric accuracy (RγRA) describes spurious features in the measured spectra, which propagate into the retrieval error. There are many contributors to this error, including speckles from the sun calibration diffuser, spectral straylight and polarization scrambler effects. One major contributor is also the instrument's sensitivity to the strongly varying polarization of the signal.
In the NIR region, the degree of polarization of the TOA Earth radiance can vary rapidly with wavelength from nearly 0 (in the continuum) to almost 100% (in the center of the O2 A-band). The Sentinel-5 requirements limit the polarization sensitivity of the instrument to below 0.5% in the UV, VIS and NIR, which is only achievable by utilizing a spatial pseudo-depolarizer as an optical component, which de-polarizes the light collected by the telescope.
Another contributor to spectral errors analyzed within the S5 feasibility studies are radiometric artifacts arising from the spatially heterogeneous nature of the radiance emanating from the Earth surface (due to irregular cloud cover or albedo variations within a spatial sample). The varying radiance levels across the slit (in ALT direction) result in an inhomogeneous illumination of the entrance slit, and consequently in a distortion of the ISRF (Instrument Spectral Response Function).
The upper right panel of Figure 10 depicts a simulated ISRF, which is deformed by realistic scene heterogeneity as inferred from MODIS imager data. The radiometric error resulting from the absence of knowledge in the ISRF distortions is referred to as PN (Pseudo Noise), because it behaves like random noise in the spatial direction whereas it correlates strongly with the measured signal along the spectral direction. The simulated PN in the NIR-2 for the analyzed scene is also plotted in Figure 10. Error levels of several percent are incompatible with the requirement on RγRA, which allow maximum errors of only 0.05% in the VIS and NIR bands.
Figure 10: Pseudo noise and distorted ISRF due to across-slit scene heterogeneity (image credit: ESA)
Therefore, dedicated correction techniques have to be developed. A significant part of the PN error results from a spectral shift of the ISRF barycenter, seen in Figure 10, and can largely be corrected by suitable spectral calibration techniques. However, the distortion of the ISRF shape still exceeds the 1% required for Sentinel-5 and additional measures may be necessary for PN mitigation. A prediction of the distorted ISRF is possible by means of temporally highly sampled radiance measurements, which provide sub-SSD spatial resolution in the ALT direction. The resulting possible software correction is supported by dedicated requirements for Sentinel-5, requesting so-called "small-pixel" data. Alternatively, it is possible to correct the non-uniform slit illumination by means of a slit homogenizer.
Table 6: Summary of S5 instrument characteristics 13)
Special hardware developments:
SH (Slit Homogenizer):
Special components, referred to as slit homogenizers, are being developed in order to mitigate radiometric errors arising from naturally occurring (across-slit) scene heterogeneity. One possible realization of a slit homogenizer, shown in Figure 11, is a three-dimensional slit, consisting of two parallel, highly reflective mirrors. The incident beam at the SH entrance (coming from the telescope) is reflected several times in ALT (across slit) direction between the mirror surfaces, which "scrambles" the spatial information in this direction. In order to maintain the ACT spatial resolution, an anamorphosis has to be introduced, focusing the ALT component of the beam from the telescope on the entrance of the SH and the perpendicular ACT component on its exit.
Figure 11: Sketch of a slit homogenizer for mitigation of across-slit (ALT) scene heterogeneity (image credit: ESA)
A perfect SH would equally distribute the radiation incident at the entrance across the exit of the device, resulting in a uniform illumination pattern at the exit. The homogenizing effect can be quantified by transfer functions, which represent the across-slit intensity distribution resulting from a Dirac-type stimulus at the entrance of the SH. For an ideal device, the transfer functions are boxcar shaped for any across-slit position of the incident beam. In reality, they deviate from this behavior due to interference between mirror reflections. The left panel of Figure 12 shows a 2D color representation of the across-slit energy distribution (vertical) as a function of the position of the stimulus at the SH entrance (horizontal). A cross section of this plot depicted in the right panel of Figure 12 illustrates the non-ideal transfer functions. Nevertheless, first results indicate that SH devices reduce PN levels by roughly an order of magnitude.
Figure 12: Transfer function of a slit homogenizer (image credit: ESA)
Legend to Figure 12: The 2D color plot on the left panel shows the slit exit illumination (vertical axis) for a Dirac input illumination as a function of its position across the input of the homogenizer (horizontal axis). The right panel shows a few the slit exit energy distributions, which are cross sections of the left plot. To obtain the ISRF from these illumination patterns they have to be convolved with the spectrometer and detector point spread functions.
The stringent requirements on polarization sensitivity and RγRA necessitate the deployment of a pseudo-depolarizer or scrambler. The most suitable type is the DBCP (Dual Babinet Compensator Pseudo-depolarizer), consisting of four wedges of birefringent material arranged in pairs (Figure 13). The wedges of a pair are bonded with crossed crystal axes, and the first pair is rotated by 45° w.r.t. the second one. Light beams passing through this device experience polarization dependent phase delays, which vary over the pupil and the resulting polarization states at the exit largely average out.
The ideal position for the scrambler would be in front of the telescope so that all optical components are illuminated with de-polarized light. However, this is impossible for the Sentinel-5 instrument, because of the large ACT FOV and the corresponding large incidence angles (up to 54°) resulting in non-acceptable spectral oscillations in the residual polarization sensitivity. The telescope mirrors placed before the scrambler will therefore contribute to the instrument's polarization sensitivity.
A known feature of DBCP scramblers is that polarized light passing through the four wedges is split into four beams due to birefringence. This effect is a direct consequence of the depolarization and cannot be avoided. Due to the geometry of the device, the four beams are arranged in a parallelogram, resulting in a "diamond spot pattern" at the detector. The stronger the de-polarization power of the DBCP, the greater is the separation of the four spots. As a consequence of the beam separation, a detector element traced through the instrument is imaged four-fold onto the Earth surface, which effectively reduces the spatial resolution. The latter is described in terms of the integrated (or ensquared) energy, representing the fraction of photons originating from a given ground pixel. The requirements for polarization sensitivity and integrated energy are in conflict, one calling for high and the other for low depolarization power, and a balance between them has to be found by careful design.
Figure 13: Sketch of a Dual Babinet Compensator Pseudo-depolarizer (polarization scrambler), image credit: ESA
The energy distribution between the four spots of a classical DBCP scrambler's diamond pattern depends on the polarization state of the incoming light, which varies strongly not only with the viewing geometry, but also with wavelength. This gives rise to a spectrally dependent effective pointing, inducing a co-registration error between spatial samples probed at different wavelength. In order to comply with the stringent co-registration specifications for single- (10% of SSD, 700 m at nadir) and multi-band (20% of SSD) retrievals, modifications of the DBCP scrambler design are being developed by the Sentinel-5 industrial primes to reduce polarization independent pointing.
Table 7: Instruments using a polarization scrambler 14)
Diffraction gratings are the preferred dispersing components for wide-swath push-broom spectrometers and for atmospheric applications requiring a spectral resolution around 0.5 nm. However, they are also a significant source of straylight and polarization sensitivity. Therefore special care has to be taken regarding the choice of grating technology for each individual spectral band. In the UV, VIS and NIR, a number of solutions are available, including convex blazed holographic, binary transmission, Echelle and prism gratings. For the two SWIR regions, the resolution of 0.25 nm in combination with the stringent limitations on volume and mass make the use of immersed gratings mandatory. These devices are reflective gratings formed on a prism, where the incident light is dispersed inside the substrate material.
Refraction at the exit surface further increases the angular dispersion, which allows reaching high spectral resolution with a much smaller grating than in a conventional setup. As a rule of thumb, the reduction in volume resulting from using an immersed grating w.r.t. a conventional one, is on the order of n3, with n being the refractive index of the substrate material. This advantage is essential for Sentinel-5 and its stringent envelope limits (1.2 m x 1.1 m x 1.0 m). Among possible substrate materials with high refractive index in the SWIR, silicon (n=3.4) offers the most mature technology and is the baseline for the Sentinel-5 Precursor mission.
The final key component encountered by the light is the focal plane assembly housing the detector. The broad targeted spectral range requires individual technology solutions for each spectrometer. In the UV, VIS and NIR regions, frame transfer CCDs (Charge-Coupled Devices) based on silicon are usually the technology of choice. Scientific CCDs feature high quantum efficiency and full well capacity, and good linearity over a wide dynamic range. The technology is mature and available in a variety of formats, which can be shaped according to the needs. From the principle of the pushbroom spectrometer concept, it is clear that detectors with fast readout capability are required, to maximize the detection time and avoiding gaps in spatial coverage. In case of CCD detectors, this is achieved by a frame transfer, in which the image acquired over the dwell time is transferred into a storage region.
From the storage region the image is successively read out while the next image is already acquired in the exposure region. The readout can be performed both in spectral (across-slit) and spatial (along-slit) direction, which in turn has implications on the video chain layout. Since the detector pixels are continuously illuminated during the charge transfer, the frame transfer has the disadvantage to create a smear effect on the signal, which needs to be corrected.
Since silicon is not responsive for wavelengths beyond 1000 nm, alternative detection media have to be considered in the SWIR spectral regions, like InGaAs or HgCdTe (MCT). MCT CMOS devices are a viable solution due to their high industrial maturity. While U.S. companies have accumulated a considerable advance in this technology, European detectors are only available in relatively small formats (256 x 1024 pixels) and with large pixel pitch (30 µm), which imposes constraints on the optical design. ESA has initiated pre-development activities with European companies on new generation SWIR detectors offering formats of 1024 x 1024 pixels with pixel sizes < 20 µm. Although these pre-developments are not yet completed, they are currently baselined as the detector solution for the Sentinel-5 mission.
UV2VIS spectrometer module of UVNS:
SODERN (Societe Anonyme d'Etudes et Realisations Nucleaires (French instrument company, Limeil-Brévannes, France) is developing the optics of the UV2VIS spectrometer (UV2VIS SO). It is the optical part of the UV2VIS spectrometer, linking the slit – attached to the telescope – to the CCD array. It operates from 300 to 500 nm. The Sodern instrumental participation in Sentinel programs at a glance is depicted Table 8. 15)
Table 8: Sodern Sentinel instrumental participation
Note: As of 2016, UV2VIS SO is in its PDR (Preliminary Design Review) phase. The outcome of the study, using the competence and know-how on optical modules is well on its way and continues through completion and delivery of the EQM (Engineering and Qualification Model) in late 2017.
The UV2VIS Spectrometer optics (SO) is one of the seven optical modules of the UVNS currently under development. In contrast with OLCI optomechanical design that benefits of the close heritage of MERIS, UV2VIS Spectro-Optic (SO) presents a completely new approach and challenge in direct response to Airbus DS needs. This includes extensive design, modelling and analysis of optical diffractive and refractive optical elements and optomechanical mounting and adjusting with kinematic configurations.
How it works: After the fore optics plus scrambler, the working principle is that the collimator brings the light from the slit to the grating with its pupil on the grating or disperser surface. Then the grating disperses the incoming light. Finally the camera re-images the light onto a two-dimensional CCD array. The classical block diagram of instruments is provided in Figure 14 showing Sodern project's scope. The width of the UVNS total spectral range to be covered and the differencing requirements and technologies necessitate an implementation using a few separate dispersive elements not shown.
Figure 14: Quick review on basically convention "building block" (image credit: Sodern)
Figure 15: COSA optical design and ray-trace (image credit: Sodern)
Figure 16: Photo of the COSA flight model (image credit: Sodern)
The major features relevant for Sodern activities are provided in Table 9.
Table 9: Major features facilitating comparison
UV2VIS SO (Spectro Optic) overview
The specification allocates a certain volume to the UV2VIS SO and also sets the position of the input and output ports. It is adapted to a refractive design with mirrors present inside the collimator only to redirect the light from the input port (the slit) to the output port (the detector). Due to the spectral range (300 nm – 500 nm) there are only two optical materials available: fused silica and calcium fluoride (CaF2). These two optical materials are used for chromatic correction.
Correction for the distortions of the final image, that are usually called "frown" or spatial misregistration and "smile" or spectral misregistration, is mandatory (Figure 17). Frown is a variation in position of the spectra associated with each point on ground, introducing errors in spatial registration of spectral data read from parallel detector columns. The adopted optical design consists of a collimator including cylindrical lenses and a camera including a "knee" to correct the lateral chromaticism and to meet the frown requirement.
Figure 17: Smile and frown parameters (image credit: Sodern)
A 3D view of the optics is shown on Figure 18 . It includes; a collimator group made of two lenses with cylindrical surfaces (light blue) two folding mirrors (yellow), and four lenses (green), a monolithic prism (dark blue) with the grooves etched on the exit surface and finally the camera group (red) made of six lenses. The largest refractive elements are on the order of 75 mm clear aperture diameter. The lenses surfaces are either spherical or aspherical. Within the housing are contained LED sources for calibration purposes. Indeed outward-projected sources are used to verify photo response non-uniformity pattern of the detector.
A spectral variable transmission coating or inverse filter is implemented on one of the collimator cylindrical lens in order to optimize the instrument spectral response; including optical transmission, spectrometer diffraction grating efficiency and CCD responsivity, for radiometric performances.
Figure 18: Schematic of SO optical layout (image credit: Sodern)
UV2VIS Spectro-Optic mounting attributes: The quality of optical systems depends not only on the quality of the optical components, but also strongly on the quality of the assembly and mounting of the different optical components.
Driving requirements for the mounting hardware, lens housing and fasteners are: low mass to support tight overall mass budget, high stiffness to meet minimum launch frequency, adequate strength to ensure that the complete system can withstand extreme environmental conditions, high dimensional stability to maintain optical alignment, sufficient thermal conductivity to minimize thermal gradients during operation which cause thermal distortions. Titanium turned out to be the best choice dictated mainly by thermo-mechanical considerations and cost-effective compromise. Indeed the titanium allows manufacturing of lightweight complex shape, no surface preparation is needed minimizing cleaning and contamination risk and it offers an intermediate thermal expansion matched to silica and CaF2 substrates.
In a structural point of view, the Spectro-Optic consists in a main body supported by three kinematics bi-pods as shown Figure 19.
Figure 19: Illustration of the SO (Spectro Optic) instrument (image credit: Sodern)
Each camera lens element is mounted as a singlet. Lens glass substrate demands a unique mount for space launch. The challenge is to design barrels that presents enough strength and stiffness to hold the lens in an accurate position to face vibrations, or shock loads, but that has simultaneously enough flexibility to filter the thermal expansion differential between the titanium and the glass, and externally induced deformations in order to mitigate impact on WFE (Wave Front Error) and birefringence. To this aim, it is necessary to implement three machined flexures inside each barrel as depicted Figure 20 with one degree of freedom per flexure at each mounting point.
With such a concept an isostatic mount with epoxy bonding is achieved. Nevertheless the flexures will only compensate the radial/global thermal load and stress could be induced through the glue (epoxy glue presents a high elastic modulus in the range of 1 GPa) if the CTE mismatch is too large and cause glass breakage. An efficient solution is in this case to insert a pad whose CTE is closer to the one of the glass between the optics and the flexure. In this way the most fragile element (i.e. the glass) is protected.
The proposed bonding technology influences the accuracy of the positioned component as well as the long-term and temperature stability of the assembly. Therefore the bonding and the pad dimension is optimized for each optical element.
This adopted lens mount design has however a direct consequence on the minimum lens edge thickness of about a few millimeters. Moreover for this application involving tight outgassing specification, due to the 300 nm wavelength, the joining technology with organic bonding remains critical, what requires bake-out prior to assembly the lens plus barrel into its final position.
Figure 20: Lens barrel assembly with radial flexures and pads (breadboard with a CaF2 lens of about 60 mm diameter), image credit: Sodern
UV2VIS Spectro Optic thermal design overview: This includes all hardware used for the passive and active thermal regulation; heaters and thermistors for the thermal regulation in operation mode and heaters and thermostats used to maintain the SO at non-operational temperatures. Thermal modelling and investigations aim at making Spectro Optic insensitive to temperature influences from the environment or interface temperature fluctuations that occur during one orbit. There are stringent requirements on optical performances and stability with tolerances as shown Table 10 therefore appropriate thermal regulation is needed.
Table 10: Classes of in-orbit stabilities for displacements and tilts conforming to all requirements
The SO operates at room temperature and is decoupled thermally from the baseplate with the low conduction bipods. Low external surface emissivity ~0.17 is chosen so that radiative interfaces are expected to have relatively small effects. The cold detector module is connected to the last camera lens mount with a distance of ~6 mm. And besides this additional load being cantilevered and causing bending of the main SO housing, the boundary module temperature complicates the thermal architecture and sizing of the heaters.
Thermal effects, including heating rates and dissipation paths, temperature gradients and transients, thermal induced stresses, and deformations are considered in the modelling and need a significant amount of nodes due to the complexity of the geometry and the quite low conductivity of titanium structure. In addition during non-operational modes a set of thermostatically controlled survival heaters are mounted on the main housing to protect the hardware from reaching low survival limits.
Figure 21: Location of heaters (green), image credit: Sodern
Figure 22: Example of temperatures visualization analyzed with the help of Finite Element Method (FEM), image credit: Sodern
UV2VIS Spectro Optic stray light and narcissus: Stray light control is the main concern in designing this Spectro Optic. The stray light is specified not to affect the measured radiance in a black & white spatial non-uniform scene with the exception of the transition zone of few spaces sampling distance as depicted in Figure 24. The general target for stray light is most difficult to achieve in the short wavelength part due to the drop of the reference scene spectral radiance shown Figure 23.
Figure 23: Scene spectrum for stray light requirement (image credit: Sodern)
Figure 24: Scene irradiance for stray light requirement (image credit: Sodern)
The dominant contributors are in field ghosts due to double reflections between optical surfaces including CCD sensitive area which corresponds to the narcissus. Other contributors are in field scattering due to effect of imperfect polish of optical surfaces or contamination, out of field scattering from of the mechanical mount structure within the SO and also the contribution coming from the rear face of the entrance slit. Diffracted light is considered negligible at these wavelengths.
The same paradigm is used for all the modeling activities using monochromatic point source illumination, empirical formulation of BSDF (Bidirectional Scattering Distribution Function) properties of surfaces including roughness and contamination and finally superposition and summation obtained by weighting taken into account the instrument spectral response function and scene spectral radiance. The stray light analysis program computes by tracing large numbers of rays. They tend to give very noisy results and a large amount of 2 x 107 starting rays and lengthy computing time up to 2½ days is needed for some detailed calculations.
The proposed approach for stray light mitigation purpose considers oversized of free mechanical aperture of optical surfaces with respect to the minimum clear aperture by at least 2 mm. Two physical stops are implemented with one corresponding to the aperture close to pupil plane near the grating and the other the shadow mask on the rear surface. The adopted lens optical layout also takes into account machining constraints with capability to implement blackened vanes mounted on several lens barrels as shown Figure 25.
Figure 25: Location of blackened vanes (brown), image credit: Sodern
A severe criticality is found for the ghost image contribution with the grating detector ghost path as seen Figure 26 and Figure 27. The incident light hits the detector, where it is reflected back towards the grating. The diffraction in reflection guides the rays again towards the detector. The way forward consists of re-optimizing the optical design in order to increase the marginal ray angle of incidence.
Figure 26: Optical layout with the spectral dispersion on the CCD array (image credit: Sodern
Figure 27: Narcissus position with Littrow condition at 350nm (demonstrating the wavelength separation and the spreading of the final ghost), image credit: Sodern
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.https://www.youtube.com/watch?v=dg3PberzvXo
The Sentinel series: