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

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-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.@en

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

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

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