In the last couple of years, the use of weak signal propagation reporter (WSPR) has grown significantly in the radio amateur community and beyond. This protocol allows to probe potential propagation paths between radio transceivers, operating at a low-power level. The protocol decodes the received signals and translates them into appropriate signal-to-noise ratio levels, which reveal the possible propagation paths between the transmitter and receiver using ionospheric reflections. In this article, specifically the 160-m radio amateur band is addressed. This band used less intensity for WSPR communication, compared to the other radio amateur bands (80 m and 40 m). Additionally, the 160-m band has specific features such as the link between propagation performance and the Earth’s electron gyro-effect. The aim of this article is to address these features experimentally. First, two identical 160-m band WSPR receiver stations are conditioned to compare the performance of different 160-m band antennas. Each setup, separated by a limited distance, generates almost identical SNR reports, allowing the comparison between the two antennas. Second, a more extended experimental investigation of the propagation path performance on the 160-m band reveals information on the radio wave behaviour between the transmitter and receiver. The first experiment allowed the identification of the most optimal antenna, specifically in the 160-m band. The second experiment shows that the SNR values can vary depending on the polarization shift of the received signal. Possibly, this can be linked to the effect of the magnetic field of the Earth via the electron gyro-frequency.@en

Currently, the amount of space debris is increasing rapidly due to the tremendous amount of satellite launches. Having an autonomous re-entry system aboard satellites at their end-of life, creates possibilities in the frame of lowering space debris, as well as for sample return from space. This paper aims at describing two possible deployment systems for the Thermal Protection System. Also, the integration of an onboard Fibre Bragg Grating-based shape monitoring system, coupled to an adaptive control system is described. Additionally, several re-entry trajectory steering systems are investigated, combined with an inflatable concept analysis. The combination of a deployable Thermal Protection System, an onboard closed feedback loop for shape monitoring, and the ability to adapt the shape of the Thermal Protection System, creates the possibility to design an autonomous dynamically steered re-entry system.@en

An instrument capable of imaging the field of NO2 in various open-air situations has been designed, manufactured, and tested. It is an improved version of the NO2 camera relying on an AOTF (acousto-optical tunable filter) which has demonstrated, amongst other things, its capability to quantify the NO2 released by power plant smokestacks. The improved version which is presented has a larger field of view, a higher frame rate, and better spectral registration performance. The working principle of the instrument has been preserved: by driving the AOTF with the appropriate acoustic frequency, a spectral image of the scene captured by the camera is recorded at a particular wavelength. The recording of a number of spectral images allows to form an hypercube: two spatial dimensions, and a spectral one. While the earlier instrument was relying on a handful of wavelengths to quantify the slant column density of NO2 observed in each pixel line of sight, the new instrument can now record "continuous" portions of the visible-light spectrum, typically between 440, and 460nm, where the NO2 exhibits some of its largest absorption lines. When the target is stable, like the air observed above a city skyline, the NO2 camera has enough time to build a large hypercube, and the spectrum measured in each pixel can be processed by the DOAS (differential optical absorption spectroscopy) method. This approach is better suited when NO2 is expected across the entire scene, not just in the plume of a smokestack for instance. The new instrument will be presented, and results of measurements performed in an urban context will be shown. The performance of the NO2 camera will be discussed based on the results of an intercomparison with the MAX-DOAS of Uccle, Brussels, and other air quality stations. @en

Auroras can be observed at the north and south pole of the Earth. At these locations, the Earth magnetic field enters the atmosphere, causing ionization of the atmospheric constituents. Due to this phenomenon, different optical wavelengths are emitted from these constituents, causing the typical spectacular displays. Several optical wavelengths are representative for the auroral emission. The purpose of the described instrument is to measure the degree of linear polarization of the northern lights, in order to detect and confirm previous observations in which only a low level of polarization was seen. The instrument is capable of resolving the polarization measured at any wavelength in the visible region, using two optical chains, each containing an acousto-optical tunable filter (AOTF). The latter is responsible for the selection of the desired optical wavelength and projects the two polarized beams onto an optical detection system. The driving of the AOTF, using a radio-frequency (RF) signal, is a key-element in establishing trustworthy results. The design of this double RF chain, consisting of a generator, an amplifier, and the necessary circumjacent electronics, is described in the study. In addition , the optical setup of the instrument, together with the detection system is explained. The instrument will be used in the harsh environment of the pole regions, so it must be able to withstand these weather conditions. Hence, a thermal compensation system is necessary and is also described.
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Introduction: VenSpec-H (Venus Spectrometer with High resolution), part of ESA’s EnVision M5 mission to Venus, is currently in Phase A (started in January 2019) in which the feasibility of the instrument is studied. The instrument successfully passed the mid-term mission consolidation review (May 2020). The mission selection review by ESA is planned beginning 2021. Scientific objectives: EnVision aims at the investigation of the geo-logical activity on Venus and its relationship with the atmosphere, in order to understand how Venus and Earth could have evolved so differently. The VenSpec suite consists of VenSpec-H and two other instuments, VenSpec-M, an IR mapper, and VenSpec-U, a UV spectrometer. The overall scientific objective of this suite is to search for temporal variations in surface temperatures and tropospheric concentrations of gases indicative for volcanic eruptions. Additionally, the study of surface-atmosphere interactions and weathering by mapping surface emissivity and tropospheric gas abundances is also part of the mission. The goal of the VenSpec-H instrument is to target different molecular species related to volcanism and surface changes on Venus. The measurement of the H2O and HDO constituents of the Venus’ atmosphere, as well as the probing of H2O, HDO, CO, OCS, SO2 in a 30 – 40 km altitude range is planned on the nightside and above the cloud top on the dayside. Several spectral windows are used to monitor these constituents in the range of 1 to 2.5 μm. VenSpec-H works in a nadir viewing geometry. Instrument design: VenSpec-H is a spectrometer, hence, the heart of the instrument consists of a spectrometer section where the incoming uniform light is diffracted into its spectral components. This section is cooled down to approximately -45 °C in order to suppress as much as possible the thermal background. This is achieved by means of a large dedicated radiator. The warm baseplate is kept around 0 °C (by means of a small warm section radiator). The warm baseplate is mounted to the spacecraft deck using three kinematic feet. The diffracting element in VenSpec-H is an echelle grating. The incoming light is first rendered parallel on a parabolic mirror before it hits the grating. The parallel light diffracted from the grating is focused by a dedicated optical system towards the detector. The cold spectrometer section is preceded by a warmer band selection section. It is a combination of a filter wheel and a butcher’s block of two horizontal filters (on top of the spectrometer entrance slit). The spectrometer section is followed by the detector section. The detector is an Integrated Dewar Detector Cooler Assembly (IDDCA). The dewar window sits in the exit aperture of the spectrometer section. The Focal Plane Array (FPA) is located a bit further in the focal plane of the spectrometer exit optics. The FPA is cooled by means of a cryocooler. Preliminary optical and thermal analysis and modelling has been performed. The instrument design relies partly on heritage from the NOMAD instrument, which is a spectrometer suite on board ESA’s Trace Gas Orbiter (TGO) satellite, a part of the ExoMars 2016 mission. Ongoing tests: Currently, thermo-mechanical prototyping work is performed using the facilities at BIRA-IASB, in order to prove the feasibility of some critical systems. One of the most critical elements in the VenSpec-H design is the cold section and its interconnection to a warm support structure. Several tests are planned. In a first series of tests, the thermal conductivity is investigated by using different types of feet. Using a fixed cold temperature at one side, and knowing heat injection at other side, a ΔT can be determined. In a second series of tests, the mounting mechanism of the cold section onto the warm section using a fixed connection on one side and a gliding pin on the other side, is tested. For this, an instrument prototype (including mass dummies for optical elements) is used, keeping the cold section at -25°C, while the warm base plate is at +20°C, the retraction and dilatation of the cold section is measured using capacitive sensors. The different results obtained during these tests will be presented at the conference. Acknowledgements: This work has been performed with the support of the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office.@en

The Airborne ROmanian Measurements of Aerosols and Trace gases (AROMAT) campaigns took place in Romania in September 2014 and August 2015. They focused on two sites: the Bucharest urban area and large power plants in the Jiu Valley. The main objectives of the campaigns were to test recently developed airborne observation systems dedicated to air quality studies and to verify their applicability for the validation of space-borne atmospheric missions such as the TROPOspheric Monitoring Instrument (TROPOMI)/Sentinel-5 Precursor (S5P). We present the AROMAT campaigns from the perspective of findings related to the validation of tropospheric NO2, SO2, and H2CO. We also quantify the emissions of NOx and SO2 at both measurement sites. We show that tropospheric NO2 vertical column density (VCD) measurements using airborne mapping instruments are well suited for satellite validation in principle. The signalto- noise ratio of the airborne NO2 measurements is an order of magnitude higher than its space-borne counterpart when the airborne measurements are averaged at the TROPOMI pixel scale. However, we show that the temporal variation of the NO2 VCDs during a flight might be a significant source of comparison error. Considering the random error of the TROPOMI tropospheric NO2 VCD, the dynamic range of the NO2 VCDs field extends from detection limit up to 37 (2:61016 molec. cm-2) and 29 (21016 molec. cm-2) for Bucharest and the Jiu Valley, respectively. For both areas, we simulate validation exercises applied to the TROPOMI tropospheric NO2 product. These simulations indicate that a comparison error budget closely matching the TROPOMI optimal target accuracy of 25% can be obtained by adding NO2 and aerosol profile information to the airborne mapping observations, which constrains the investigated accuracy to within 28 %. In addition to NO2, our study also addresses the measurements of SO2 emissions from power plants in the Jiu Valley and an urban hotspot of H2CO in the centre of Bucharest. For these two species, we conclude that the best validation strategy would consist of deploying ground-based measurement systems at well-identified locations.

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Acousto-optical tunable filters (AOTFs) are still little known to the Earth atmosphere remote sensing community. The bulk of passive atmospheric remote sensing instruments remains divided into two families: those relying on interferometric techniques (mostly for the long-wave absorbing species), and those based on diffraction gratings (better suited for UV-VIS absorbing species). Still, AOTFs have some unique features which should deserve more attention, in particular their angular acceptance, and their polarization sensitivity. The first one because it allows to work in an imaging setup, the second because many atmospheric processes have a polarizing effect. In this paper, we will present different AOTF-based instrument concepts (or even prototypes) which take advantage of these features in order to improve the sate-of-the-art of measurement techniques in several fields of atmospheric science. We will first present the improved NO2 camera: its new capabilites, the subsystems which have been changed, and some preliminary results. Then, we will discuss two other potential applications: the study of the solar spectral irradiance variability in the UV, and the detection of auroral polarized emissions. For each concept, we will discuss the current challenges faced by the existing instruments, and analyze how the use of AOTFs could overcome them. A suggestion for the AOTF selection will be made, and the expected instrument performance will be estimated.@en

The performance of an acousto-optical tunable filter (AOTF) depends on several key parameters, such as the diffraction efficiency and the applied RF signal at the external network of the transducer. The latter has a specific bandwidth in which its efficiency is high. The goal of this paper is to experimentally investigate the relationship between the electrically matched RF bandwidth and the achieved diffraction efficiency, especially at the edges of the RF frequency band. The experiment revealed that the impedance matching is not completely correlated to the diffraction efficiency. Temperature measurements on the AOTF under test show that, at one edge of the tuning range, the RF signal is converted into a noninteracting acoustic mode.@en

The ALTIUS Mission (Atmospheric Limb Tracker for Investigation of the Upcoming Stratosphere) aims at the development of a limb sounder based on a small satellite concept (i.e. PROBA-Next small platform). ALTIUS will monitor the distribution and evolution of stratospheric ozone at high vertical resolution in support of operational services and long term trend monitoring. It will provide detailed stratospheric profile information at high vertical resolution, which is a valuable addition to ozone total column for data assimilation systems based on nadir sounders used by operational centers. In addition, some secondary scientific mission objectives are targeted, including measurements of vertical concentration profiles of other atmospheric species. The ALTIUS Mission was first proposed by the Belgian Institute for Space Aeronomy and, after several studies in different programmes within ESA, is now being developed as an element of ESA's Earth Watch Programme currently with the participation of Belgium, Canada, Luxembourg and Romania. The ALTIUS Mission concept consists of three spectral imagers flying at an altitude of approximately 700 km Sun- Synchronous Orbit on-board the next generation of PROBA platform. The ALTIUS Instrument shall allow observation of the Earth's atmospheric bright limb in an extended spectral region from the Ultraviolet to the Short Wave Infrared (SWIR). In addition, the ALTIUS instrument shall perform solar and stellar occultation observations in the dark limb. The ALTIUS Instrument three hyperspectral channels are based respectively on Acousto-Optic Tuneable Filters (AOTFs) in the Visible (440-800nm) and SWIR (900-1800nm) range, and a cascade of Fabry Perot Interferometers (FPI) in the UV (250-370nm). The use of tuneable active spectral filter shall allow the ALTIUS Instrument to perform observations with a spectral resolution ranging between 1nm and 10nm in an extremely versatile operational concept. The use of the AOTFs and Fabry Perot technologies constitutes a novelty in the limb sounding missions and its development critical aspects as well as its performances have been proven in the frame of an extensive Instrument predevelopment phase. In this phase the critical technologies necessary to ensure the performance and functionalities of the ALTIUS Instrument have been designed, developed and qualified. In particular the following pre-developments were undertaken to pave the way towards Instrument Preliminary Design Review: Design, manufacturing and qualification of the VIS AOTF Assembly and RF control electronics; Design, manufacturing and qualification of the UV FPI assembly stack and control electronics; Development and qualification of UV filters and coatings. Design and manufacturing of sensor electronics based on selected CMOS sensor for UV and VIS channels; Design, manufacturing and partial qualification of the Instrument mechanism; Bread-boarding of the full VIS channel for functionality and end-to-end performances verification. This paper presents the status and main results of the above-mentioned ALTIUS Instrument pre-development activities. In particular: major outcomes during the development process, achieved performances and lessons learned for the continuation of the ALTIUS Instrument flight model design are being highlighted.

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A deeper insight into the performance of a Hilbert transform and PLL-based generation technique for space missions. The use of an RF generator is quite common in lab testing environments as well as in commercial applications. Many different RF generation techniques exist, both analog and digital. The frequency-domain sets the complexity of the circuit. It is commonly accepted that the higher the frequency, the more complex the design becomes. Designing for space applications change things quite drastically. Elements as survivability, radiation, and redundancy also come into play. Taking into account that adaptations are not any longer possible after launch, implies that the implemented design has to be one hundred percent robust. The complexity rises when designing for scientific missions like for NASA or ESA because of the more severe imposed constraints. This article tries to give an idea about how to design a pure sinewave RF generator at rather low frequencies (< 500 MHz) for a scientific mission capable of surviving under the harsh environmental conditions of space, dealing with the space agency restrictions and limited availability of approved electrical components.@en

The measurement of NO 2 abundance in the air is of interest for monitoring pollution sources and air quality. A new instrumental concept has been recently introduced, which makes use of an acoustooptical tunable filter (AOTF) as the main component of a spectral imager. With the capability of resolving the absorption spectrum of NO 2 , and a high spatial and temporal sampling, this instrument can be seen as a NO 2 camera. In this instrument, the selection of different optical wavelengths by the AOTF is done by applying a radio frequency (RF) signal to the AOTF. The selected frequency and the applied amplitude are the key parameters for the quality of the measurements. The design and assembly of an RF chain, consisting of an RF generator and RF amplifier, will lead to an electronics bench capable of driving the NO 2 camera. In view of a potential spaceborne application of this instrumental concept, the design of the RF generator is only making use of space-qualified components. This paper focuses on the development of this part of the electronics bench. @en

In the frame of an ESA-mission (European Space Agency) a RF-generator which can withstand the harsh space environment is being developed. This RF-generator is a part of the ALTIUS-instrument1 (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere). The goal of the mission is to make hyper spectral images of the limb of the earth in different wavelength domains. For this, a three-channel spectral imager is implemented, measuring in the ultraviolet (250 nm to 450 nm), visible (440 nm to 800 nm) and near infrared (900 nm to 1800 nm) wavelength domains, that is bound to fly aboard a PROBA-satellite. The RF-generator will apply a RF-signal to an AOTF (Acousto-Optical Tunable Filter). The AOTF manages to convert RF-power into acoustic power which implies a diffracted beam at a different angle at the output of the AOTF as a function of the applied frequency and power2. This beam is focused onto a detector which performs the imaging. In this paper a high frequency RF-generator is discussed based on a PLL (Phase Locked Loop) for the three channels (Figure). Other solutions applicable for space were investigated by the author as well3, 4. A merged PCB (Printed Circuit Board) is designed which houses the frequency synthesizer (the ADF4208), an in-house designed loop filter, a VCO (Voltage Controlled Oscillator) and a divide-by-8 prescaler. The selected components are compatible with the ESA standards for space-qualified components. Tests are carried out on the setup for the UV-channel because for this a relatively high frequency has to be generated in comparison with the available RF-generating techniques for space applications. Different design parameters were checked such as the spectral output, spectral resolution, output power, frequency stability and phase noise. The levels of these parameters are based on the harsh environment which the electronics has to withstand. Based on the results it can be stated that the spectral output (130 – 260 MHz) as well as the spectral resolution of 5 kHz is achieved within the requested tolerances. The output power level varies between +2.5 dBm and +3.3 dBm depending on the applied frequency. A long term frequency stability of 5 kHz within 100 seconds is achieved as well. The phase noise at 10 kHz offset is -101 dBc/Hz which is well within specification. Currently only the UV-channel setup is breadboarded as a merged PCB. For the other channels an additional prescaler is needed (Figure). This will have an impact on the performance of the RF-generator and is under investigation.@en

In the frame of a mission of the European Space Agency (ESA) a space qualified RF-generator needs to be developed to drive an AOTF (Acousto-Optical Tunable Filter) to make hyper-spectral images of the limb of the Earth. Custom off-the-shelf electronics can not fulfil the requirements for this scientific mission, hence alternative solutions for the RF-generation are investigated. This paper describes one of the possible solutions.@en

The abundance of NO₂ in the boundary layer relates to air quality and pollution source monitoring. Observing the spatiotemporal distribution of NO₂ above well-delimited (flue gas stacks, volcanoes, ships) or more extended sources (cities) allows for applications such as monitoring emission fluxes or studying the plume dynamic chemistry and its transport. So far, most attempts to map the NO₂ field from the ground have been made with visible-light scanning grating spectrometers. Benefiting from a high retrieval accuracy, they only achieve a relatively low spatiotemporal resolution that hampers the detection of dynamic features. We present a new type of passive remote sensing instrument aiming at the measurement of the 2-D distributions of NO₂ slant column densities (SCDs) with a high spatiotemporal resolution. The measurement principle has strong similarities with the popular filter-based SO₂ camera as it relies on spectral images taken at wavelengths where the molecule absorption cross section is different. Contrary to the SO₂ camera, the spectral selection is performed by an acousto-optical tunable filter (AOTF) capable of resolving the target molecule's spectral features. The NO₂ camera capabilities are demonstrated by imaging the NO₂ abundance in the plume of a coal-fired power plant. During this experiment, the 2-D distribution of the NO₂ SCD was retrieved with a temporal resolution of 3 min and a spatial sampling of 50 cm (over a 250 × 250m² area). The detection limit was close to 5 × 10¹⁶ molecules cm^-2, with a maximum detected SCD of 4 × 10¹⁷molecules cm^-2. Illustrating the added value of the NO₂ camera measurements, the data reveal the dynamics of the NO to NO₂ conversion in the early plume with an unprecedent resolution: from its release in the air, and for 100m upwards, the observed NO₂ plume concentration increased at a rate of 0.75-1.25g s^-1. In joint campaigns with SO₂ cameras, the NO2 camera could also help in removing the bias introduced by the NO₂ interference with the SO₂ spectrum.

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Wide aperture acousto-optical tunable filters (AOTF) appeared in the 70’s and found applications in various areas such as agriculture (crop stress monitoring), food industry (product quality), biology (fluorescence spectroscopy), etc. Their main advantages are the robustness, compactness, low power consumption, high filtering efficiency, tunability, potentially high spectral resolution (<1nm) and good image quality. In general, they offer interesting features to meet the needs in hyperspectral imaging applications. The physical process behind the wide aperture AOTF is the interaction of light and sound inside a birefringent crystal. For a given acoustic frequency, only photons of particular energy (i.e. wavelength) will couple with the acoustic wave, then leave the crystal in a slightly different direction than the rest of the light. By focusing the diverted beam onto a detector, one effectively performs a spectral image of the scene. Selecting another window of the light spectrum only requires the tuning of the sound frequency. A piezoelectric transducer bonded to the crystal is responsible for converting the electrical RF-signal into an acoustic wave. As most AOTFs operating in the visible and near-infrared domains are driven with frequencies ranging a few MHz to several hundreds of MHz, they do not necessitate particular electronics equipment. This statement does not hold when it comes to operating AOTFs in space environment: the RF-driving chain must be made from the limited catalogue of space-qualified parts. The problem gets even bigger when frequencies of several tens of MHz or higher must be generated. The work reported in this paper addresses different solutions to the problem of building a space-qualified RF-chain for an AOTF. This research was undertaken as part of the development of the ALTIUS space mission (atmospheric limb tracker for the investigation of the upcoming stratosphere) which aims at the measurement of atmospheric trace species (ozone, nitrogen dioxide, methane, water vapor,…) concentration profiles with a high spatial resolution. The measurement concept relies on the acquisition of spectral images of the bright atmospheric limb at well-chosen wavelengths. The imager concept allows to discope the need for scanning the atmosphere, as it was done by previous remote sensing missions. The instrument will be mounted onboard a PROBA-satellite (Project for On-Board Autonomy). The PROBA-satellite is a platform containing all the essential subsystems like a GPS, an attitude and orbit control system etc. to facilitate the payload which is placed on top of the platform. The complete project (platform and payload) is developed under the supervision of ESA (European Space Agency) and with funding of the Belgian Science Policy Office (BELSPO). The original ALTIUS concept made use of three independent spectral imagers (channels), each of them relying on an AOTF capable of isolating narrow passbands across the channel spectral range (ultraviolet (UV): from 250 to 400nm, Visible: from 440 to 800nm, near-infrared (NIR): from 900 to 1800nm). For the Visible and NIR channels, the two AOTFs will be made of a paratellurite crystal. For the UV channel, the AOTF would have been made from a KDP crystal (potassium dihydrogen phosphate). Unfortunately, the latter did not reach the necessary level of maturity for a space mission, and the KDP-based AOTF was replaced by a stack of Fabry-Pérot interferometers (FPI). Nevertheless, for technological interest, the AOTF approach will also be developed and matured for the UV-channel up to flight level. The focus of this paper is to design for each of the different channels a dedicated RF-chain, containing a RF-generator and a RF-amplifier. The output of the RF-amplifier is tunable to a specific frequency and a specific power level and is injected in the AOTF’s transducer via an impedance matching network. Different architectures are possible to generate the frequency range needed for the different wavelength domains. While the Hilbert Transform solution will be only summarized here and not further investigated, this paper focuses on the Phase Locked Loop (PLL) solution. PLL-solutions are breadboarded for the three wavelength domains. A detailed study is done on the achievable power levels in the Infrared, Visible and the Ultraviolet.@en

we examined acousto-optic interaction in the KH₂PO₄ crystal used in devices providing control of optical radiation parameters. The crystal was applied in a wide-angle tunable acousto-optic filter capable of image processing in the ultraviolet region of spectrum. The filter operated at different temperature conditions because of changes of environmental temperature and also due to heating of the crystal by driving electric and acoustic power. As found experimentally and proved theoretically, changes in the temperature of the instrument and appearance of thermal gradients in the crystal volume sufficiently influenced on operation characteristics of the acousto-optic device.

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The ALTIUS-instrument is a three-channel spectral imager, measuring in the ultraviolet, visible and near infrared wavelength domains, that is bound to fly aboard a PROBA-satellite. The goal of the ALTIUS-instrument is to make hyper spectral images of the limb of the earth. In each of ALTIUS' three channels an AOTF (Acousto-Optical Tunable Filter) will be used. For each channel an RF-generator will be developed and subjected to a suite of environmental tests such as thermal-vacuum, vibration, radiation, shock and Electromagnetic Compatibility (EMC). For the AOTF RF-generator different solutions are possible. Currently the design of an analog Hilbert transform and a high frequency phase-locked loop (PLL) are proposed as possible solutions. The choice of components, the PCB-design and the manufacturing of both designs has to be done in accordance to space qualified standards. This limits the choice of design and lifts the design challenge to a higher level. Because of the limited choice of space qualified components, general commercial approaches are not possible. For this reason specific designs have to be investigated and research has to be done to select the appropriate solution.

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Spectral imaging is a powerful tool for remote sensing applications. Among other valuable options, acousto-optical tunable filters (AOTF) offer interesting properties to this class of instruments. When sensitivity to ultra-violet (UV) light is required, KH₂PO₄ crystal (KDP) is a good choice for the acousto-optic (AO) interaction medium as its AO figure of merit is relatively large compared to other materials transparent in UV. However, it generally requires more acoustic power than well-known TeO₂-based devices to achieve the highest diffraction efficiency. We report on measurements performed with a KDP-based AOTF (9-cut) driven with RF signals up to 3 W. It has been found that owing to various effects (Joules, acoustic wave attenuation,...) the heating of the crystal by a few degrees yields severe alteration of the AOTF spectral transmission function (STF): shift of the central wavelength, increase of the bandwidth and decrease of the peak filter efficiency have been observed. These three effects degrade the AOTF overall performance if not taken into account. This paper presents the experimental evidences and shows how a temperature-dependent AOTF model allows to derive the inner temperature of the device. Possible reasons for the flattening of the STF are also discussed.

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Since the recent losses of several atmospheric instruments with good vertical sampling capabilities (SAGE II, SAGE III, GOMOS, SCIAMACHY, MIPAS), the scientific community is left with very few sounders delivering concentration profiles of key atmospheric species for understanding atmospheric processes and monitoring the Earth's radiative balance. The situation is so critical that, less than five such instruments will be on duty (most probably only 2 or 3) at the horizon 2020, whereas their number topped more than 15 in the years 2000. In parallel, recent inter-comparison exercises among the climate chemistry models (CCM) and instrument datasets have shown large differences in vertical distribution of constituents (SPARC CCMVal and Data Initiative), stressing the need for more accurate vertically-resolved data at all latitudes. In this frame, the Belgian Institute for Space Aeronomy (IASB-BIRA) proposed a small mission called ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere), which is currently in preliminary design phase (phase B according to ESA standards). Taking advantage of the good performances of the PROBA platform (PRoject for On-Board Autonomy) in terms of pointing precision and accuracy, onboard processing ressources, and agility, the ALTIUS concept relies on a hyperspectral imager observing limbscattered radiance and solar/stellar occultations every orbit. The objective is twofold: compared to scanning instruments, the imaging feature allows to better assess the tangent height of the sounded air masses (through easier star tracker information validation by recognition of scene details), while its spectral capabilities will be good enough to exploit the characteristic signatures of many molecular absorption cross-sections (O3, NO2, CH4, H2O, aerosols,⋯). The payload will be divided in three independent optical channels, associated to separated spectral ranges (UV: 250-450 nm, VIS: 440-800 nm, NIR: 900-1800 nm). This approach also offers better risk mitigation in case of failure of one channel. In each channel, the spectral filter will be an acousto-optical tunable filter (AOTF). Such devices offer reasonable étendue with good spectral resolution and excellent robustness and compactness and TeO2-based AOTF's have already been used in space missions towards Mars and Venus (MEX and VEX, ESA). While such TeO2 crystals are common in VIS-NIR applications, they are not transparent below 350 nm. Recent progresses towards UV AOTF's have been made with the advent of KH2PO4-based (KDP) filters. Through collaboration with the Moscow State University (MSU), several experiments were conducted on a KDP AOTF and gave confidence on this material. Here, we present the general concept of ALTIUS and its optical design with particular attention on the AOTF. Several results obtained with optical breadboards for the UV and VIS ranges will be exposed, such as the O3 and NO2 absorption cross-section measurements, or spectral images. These results illustrate the spectral and optical performances to be expected from an AOTF-based hyperspectral imager. Their implications for the ALTIUS mission will be discussed.@en

There is more and more interest in the understanding and the monitoring of the physics and chemistry of the Earth's atmosphere and its impact on the climate change. There is currently a lack of sounders which provide a high vertical resolution in trace gas detection. ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere) is a remote sounding experiment proposed by the Belgian Institute for Space Aeronomy (BIRA/IASB). The mission scenario includes bright limb observations in basically all directions, solar occultations around the terminator passages and star occultations during eclipse. These observation modes allow imaging the atmosphere with a high vertical resolution. The spacecraft will be operated in a 10:00 sun-synchronous orbit at an altitude of 695 km, allowing a 3-day revisit time. The envisaged payload for the ALTIUS mission is an imaging spectrometer, observing in the UV, the VIS and the NIR spectral ranges. For each spectral range, an AOTF (Acousto-Optical Tunable Filter) will permit to perform observations of selectable small wavelength domains. A typical set of 10 wavelengths will be recorded within 1 second. The different operational modes impose a high agility capability on the platform. Furthermore, the quasicontinuous monitoring by the payload will drive the design of the platform in terms of power and downlink capabilities. ALTIUS is one of the mission which are currently envisaged to use the PROBA-NEXT platform, which is currently under development at QinetiQ Space. This paper will present the ALTIUS mission, the envisaged instrument and the spacecraft concept.@en