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Narrow field-of view spectral measurements of twilight sky brightness as a function of solar zenith angle in the 89°-95° range allow to retrieve lower stratospheric and tropospheric aerosol extinction profiles. The measurements were carried out over Tbilisi, Georgia, South Caucasus during 2021-2022 in the 700-800 nm wavelength range using a SBIG ST9 CCD camera and a SBIG SGS spectrograph. The Monte Carlo code Siro, developed in the Finnish Meteorological Institute was used to design a forward model. Aerosol extinction profiles at 780 nm were retrieved using the Levenberg–Marquardt algorithm.Stratospheric aerosol loading was low in the considered period whereas the upper (about 9-10 km) and lower troposphere (about 4-5 km) were sometimes disturbed due to small volcanic eruptions and dust transport events. Particularly, ash clouds from Etna eruptions on 10 and 21 February were observed at about 9 km altitude.

ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere) is an atmospheric limb mission being implemented in ESA's Earth Watch programme and planned for launch in early 2026. The primary objective of the mission is to measure high-resolution stratospheric ozone concentration profiles. Secondary objectives are the retrievals of stratospheric aerosols particle density, NO2, water vapor and other minor species concentrations.This innovative instrument consists of three spectral high-resolution imagers: UV (250-355 nm), VIS (440-675 nm) and NIR (600-1040 nm) channels. The UV channel uses a stack of four Fabry-Pérot interferometers, while the VIS and NIR channels each rely on an AOTF (Acousto-Optical Tunable Filter). Each channel can image scenes independently of the others at given wavelengths and with a moderate spectral resolution, and high spatial sampling. The agility of ALTIUS allows for series of observations at desired wavelengths carefully chosen to retrieve the vertical profiles of species of interest.The instrument will perform measurements in different geometries to maximize global coverage: observing limb-scattered solar light in the dayside, solar occultations at the terminator, and stellar, lunar, and planetary occultations in the nightside.The status of the ALTIUS mission will be presented as well as the foreseen quality of the Level-1 observations. The quality of the retrieved profile densities will be discussed with a particular focus on the high vertical resolution that can be achieved using this instrument. The added-value of the native imaging capabilities of ALTIUS in terms of observations, and in-flight calibrations, will be highlighted.

The AOTF-based NO2 camera is a remote sensing instrument primarily aimed at imaging and quantifying the NO2 field above cities or in industrial plumes. The measurement principle consists in acquiring a number of spectral images of the scene at selected wavelengths. Each pixel is therefore recording a discrete spectrum of the radiance collected in its acceptance cone, enabling the retrieval of the NO2 column density in its optical path by application of the DOAS method on the measured spectrum.The core element of the instrument principle is the acousto-optical tunable filter (AOTF). This device works under the principle of the acousto-optical interaction, the coupling of the light electric field with the modulation of the crystal lattice by a shear acoustic wave created by a transducer. The coupling takes place at a single wavelength, and diffracts that part of the spectrum into another direction. By blocking the undiffracted light beam, and imaging the diffracted order, one can capture a monochromatic image of the scene.We propose to expand the capabilities of the NO2 camera by exploiting another aspect of the acousto-optic interaction. The coupling between light and sound actually takes place in a birefringent crystal (TeO2), and one usually works with a single linear polarization of the incoming light (e-light, or o-light). The two polarization components are diffracted in different directions. If the current design is modified such that the two components can be imaged, then an information on the degree of linear polarization of the light can be obtained.In the atmosphere, the scattering of light by air (Rayleigh), and particles (Mie) is controlling the state of polarization of the scattered solar light. Hence, aerosols not only introduce a smooth spectral signatures, but also a change of the state of polarization. The proposed modification of the NO2 camera design can provide some sensitivity on this, potentially enhancing the scientific return of the instrument with aerosol retrievals capabilities. The new instrumental design will be presented, and vector radiative transfer simulations will be produced to estimate the benefit of this change.

ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere) is the upcoming stratospheric ozone monitoring limb sounder from ESA's Earth Watch programme. Measuring in the ultraviolet–visible–near-infrared (UV–VIS–NIR) spectral regions, ALTIUS will retrieve vertical profiles of ozone, aerosol extinction coefficients, nitrogen dioxide and other trace gases from the upper troposphere to the mesosphere. In order to maximize the geographical coverage, the instrument will observe limb-scattered solar light during daytime (i.e. bright limb observations), solar occultations at the terminator and stellar/lunar/planetary occultations during nighttime. This paper evaluates the constraint of ALTIUS ozone profiles on modelled stratospheric ozone by means of an observing system simulation experiment (OSSE). In this effort, a reference atmosphere has been built and used to generate ALTIUS ozone profiles, along with an instrument simulator. These profiles are then assimilated to provide ozone analyses. A good agreement is found between the analyses and the reference atmosphere in the stratosphere and in the extra-tropical upper troposphere. In the tropical upper troposphere, although providing significant information in the analyses, the assimilation of ozone profiles does not completely eliminate the bias with respect to the reference atmosphere. The impacts of the different modes of observations have also been evaluated, showing that all of them are necessary to constrain ozone during polar winters where solar/stellar occultations are the most important during the polar night and bright limb data are the most important during the development of the ozone hole in the polar spring.

A comprehensive inter-comparison of seven radiative transfer models in the limb scattering geometry has been performed. Every model is capable of accounting for polarization within a spherical atmosphere. Three models (GSLS, SASKTRAN-HR, and SCIATRAN) are deterministic, and four models (MYSTIC, SASKTRAN-MC, Siro, and SMART-G) are statistical using the Monte Carlo technique. A wide variety of test cases encompassing different atmospheric conditions, solar geometries, wavelengths, tangent altitudes, and Lambertian surface reflectances have been defined and executed for every model. For the majority of conditions it was found that the models agree to better than 0.2 % in the single-scatter test cases and better than 1 % in the scalar and vectorial test cases with multiple scattering included, with some larger differences noted at high values of surface reflectance. For the first time in limb geometry, the effect of atmospheric refraction was compared among four models that support it (GSLS, SASKTRAN-HR, SCIATRAN, and SMART-G). Differences among most models with multiple scattering and refraction enabled were less than 1 %, with larger differences observed for some models. Overall the agreement among the models with and without refraction is better than has been previously reported in both scalar and vectorial modes.

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...

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.

Twilight sky brightness spectral measurements are an inexpensive and effective way to observe enhancements of stratospheric aerosols. In this work, we present our observations of the volcanic cloud produced by the eruption of Raikoke volcano (Kuril Islands, 48°N, 153°E) above two distinct sites in South Caucasus and Western Europe, respectively: Tbilisi, Georgia (41° 43’ N, 44° 47° E) and Halle, Belgium (50° 44′ N, 4° 14′ E).We present our dataset, which describes the evolution of the stratospheric aerosol in the period July 2019-December 2020. Stratospheric aerosol vertical extinction profiles were retrieved at 780 nm from spectral measurements of twilight sky brightness above both sites.The first aerosols originating from Raikoke were observed in the beginning of July above Halle and in August above Georgia. The layer maximum was mostly observed at 17 km above Georgia and at 10-17 km above Belgium until April-May 2020. Later, the volcanic cloud was observed sporadically until the end of 2020.

ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere) is the upcoming stratospheric ozone monitoring limb sounder from ESA's Earth Watch programme. Measuring in the ultraviolet-visible-near infrared spectral regions, ALTIUS will retrieve vertical profiles of ozone, aerosol extinction coefficients, nitrogen dioxide and other trace gases from the upper troposphere to the mesosphere. In order to maximize the geographical coverage, the instrument will observe limb- scattered solar light during daytime, solar occultation at the terminator and stellar/lunar/planetary occultations during nighttime. This paper evaluates the constraint of ALTIUS ozone profiles on modelled stratospheric ozone by the means of an Observing System Simulation Experiment (OSSE). In this effort, a reference atmosphere has been built and used to generate ALTIUS ozone profiles, along with an instrument simulator. These profiles are then assimilated to provide ozone analyses. A good agreement is found between the analyses and the reference atmosphere in the stratosphere and in the extra-tropical upper troposphere. In the tropical upper troposphere, although providing a significant weight in the analyses, the assimilation of ozone profiles does not allow to completely eliminate the bias with the reference atmosphere. The weight of the different modes of observations have also been evaluated, showing that all of them are necessary to constrain ozone during polar winters where solar/stellar occultations are the most important during the polar night and limb data are the most important during the development of the ozone hole in the polar spring.

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 signal-to-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 (σ), the dynamic range of the NO2 VCDs field extends from detection limit up to 37 σ (2.6×1016 molec. cm−2) and 29 σ (2×1016 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.

ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere) is a satellite mission dedicated to continue Earth limb measurements for atmospheric sciences (Fussen et al., JQSRT, 2019). It is an element of the ESA Earth Watch programme and is expected to be launched in 2024 on a low earth polar orbit. The instrument is based on three spectral imagers that will measure in UV-vis-NIR wavelength range and will operate in different viewing geometry: limb scattering and occultation of the sun, the moon, the planets and the stars. ALTIUS will retrieve vertical profiles of ozone, nitrogen dioxide, aerosol extinction, among others.In this study, we present an Observing System Simulation Experiment (OSSE) of ALTIUS ozone profiles that we have compared with the existing observations from Aura Microwave Limb Sounder (MLS). For this purpose, we have created a stratospheric ozone reference dataset between June and September 2008 based on the assimilation of MLS data with the Belgian Assimilation System for Chemical Observations (BASCOE). During the MLS assimilation experiment, the ozone state is saved in the space of ALTIUS previously determined with the ALTIUS orbit simulator, then perturbed according to the ALTIUS error budget, which creates ALTIUS synthetic observations. The assimilation of these ALTIUS ozone profiles agrees well with those of MLS. The assimilation of the different modes of ALTIUS reveals that all modes are necessary to constrain ozone during the polar night: solar and stellar occultations are the most constraining during the June-August period while limb scattering profiles are the most constraining from September onward.

The recent surge in the development of small satellite platforms could offer the opportunity to significantly decrease the overall cost of science missions, if suitable instruments can be operated from such platforms. PICASSO is a CubeSat demonstration mission initiated by the Belgian Institute for Space Aeronomy and implemented by the European Space Agency (ESA). Its objective is to assess the ability of very low-cost satellites to carry out atmospheric measurements. PICASSO focuses on retrieving the ozone distribution in the stratosphere, the temperature profile up to the mesosphere, and the electronic density and temperature in the ionosphere.Following its launch on a VEGA rocket in March 2020, PICASSO will fly for two years. The overall demonstration mission includes the end-to-end development of the satellite, launch, operation, and analysis of the scientific data. PICASSO follows a polar orbit at an altitude ranging from 475 to 500 km and an inclination of 98°. Its payload consists of a miniaturised hyper-spectral imager (VISION) and a four needle-like Sweeping Langmuir Probe (SLP). It utilizes four deployable solar panels with an average power generation of 8.7W, two on-board computers (OBC and PLC), and a high-performance ADCS with a pointing accuracy of around 1°.The scientific objectives of VISION are to demonstrate the retrieval of polar and mid-latitude stratospheric ozone vertical profiles from multispectral transmittances observed in solar occultations. For SLP, the main goals are to study the ionosphere-plasmasphere coupling and aurora structures, and to monitor the density irregularities in the polar cap ionosphere.

Raikoke volcano (Kuril Islands, Russia) eruption on 21 June 2019 sent an ash plume at 10-13 km altitude, which is higher than the local tropopause. Volcanic aerosols were transported around the globe, causing spectacular purple twilights. We will present ground-based measurements of monochromatic twilight sky brightnesses at 780 nm wavelength in two geographical points: Tbilisi, Georgia (41° 43’ N, 44° 47° E) and Halle, Belgium (50° 44′ N, 4° 14′ E). Aerosol extinction vertical profiles in the lower stratosphere-upper troposphere were retrieved with the help of the Levenberg–Marquardt algorithm. Monte Carlo code Siro was used to design a forward model. Raikoke aerosols observed above the both sites have shown essentially cloudy and variable structure. Multiple layers were observed between 10 and 17 km with extinction up to 0.01 km-1. We will present Raikoke aerosol cloud evolution in the period July 2019 –January 2020.

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 the power plants in the Jiu Valley. Their main objectives were to test recently developed airborne observation systems dedicated to air quality studies and to verify the concept of such campaigns in support of the validation of spaceborne atmospheric missions such as TROPOspheric Monitoring Instrument (TROPOMI)/Sentinel-5 Precursor (S5P). We show that tropospheric NO2 vertical column density (VCD) measurements using airborne mapping instruments are valuable for satellite validation. The signal to noise ratio of the airborne NO2 measurements is one order of magnitude higher than its spaceborne counterpart when the airborne measurements are averaged at the TROPOMI pixel scale. A significant source of comparison error appears to be the time variation of the NO2 VCDs during a flight, which we estimated at about 4 x 1015 molec cm-2 in the AROMAT conditions. Considering the random error of the TROPOMI tropospheric NO2 VCD (σ), the dynamic range of the NO2 VCDs field extends from detection limit up to 37σ (2.6 x 1016 molec cm-2) or 29σ (2 x 1016 molec cm-2) for Bucharest and the Jiu Valley, respectively. For the two areas, we simulate validation exercises of the TROPOMI tropospheric NO2 product using airborne measurements. These simulations indicate that we can closely approach the TROPOMI optimal target accuracy of 25 % by adding NO2 and aerosol profile information to the mapping data, which constrains the investigated accuracy within 28 %. In addition to NO2, we also measured significant amounts of SO2 in the Jiu Valley, as well as a hotspot of H2CO in the center of Bucharest. For these two species, we conclude that the best validation strategy would consist in deploying ground-based measurement systems at key locations which the AROMAT observations help identify.

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.

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.

© 1963-2012 IEEE. 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. ispartof: IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT vol:68 issue:3 pages:874-881 status: published

AerGOM is a retrieval algorithm developed for the GOMOS instrument onboard Envisat as an alternative to the operational retrieval (IPF). AerGOM enhances the quality of the stratospheric aerosol extinction retrieval due to the extension of the spectral range used, refines the aerosol spectral parameterization, the simultaneous inversion of all atmospheric species as well as an improvement of the Rayleigh scattering correction. The retrieval algorithm allows for a good characterization of the stratospheric aerosol extinction for a wide range of wavelengths.In this work, we present the results of stratospheric aerosol extinction comparisons between AerGOM and various spaceborne instruments (SAGE II, SAGE III, POAM III, ACE-MAESTRO and OSIRIS) for different wavelengths. The aerosol extinction intercomparisons for λ λ λ > 700 nm.Due to the unique observational technique of GOMOS, some of the results appear to be dependent on the star occultation parameters such as star apparent temperature and magnitude, solar zenith angle and latitude of observation. A systematic analysis is carried out to identify biases in the dataset, using the various spaceborne instruments as references. The quality of the aerosol retrieval is mainly influenced by the star magnitude, as well as star temperature to a lesser degree. To ensure good-quality profiles, we suggest to select occultations performed with star magnitude M T > 6 × 103 K. Stray-light contamination is negligible for extinction coefficients below 700 nm using occultations performed with a solar zenith angle > 110° but becomes important at larger wavelengths. Comparison of AerGOM results in the tropics shows an enhanced bias below 20 km that seem to confirm cirrus clouds as its cause. There are also differences between mid-latitude and tropical observations that cannot yet be explained, with a bias difference of up to 25 %.This bias characterization is extremely important for data users and might prove valuable for the production of unbiased long-term merged dataset.

The GOMOS instrument on Envisat has successfully demonstrated that a UV–Vis–NIR spaceborne stellar occultation instrument is capable of delivering quality data on the gaseous and particulate composition of Earth's atmosphere. Still, some problems related to data inversion remained to be examined. In the past, it was found that the aerosol extinction profile retrievals in the upper troposphere and stratosphere are of good quality at a reference wavelength of 500 nm but suffer from anomalous, retrieval-related perturbations at other wavelengths. Identification of algorithmic problems and subsequent improvement was therefore necessary. This work has been carried out; the resulting AerGOM Level 2 retrieval algorithm together with the first data version AerGOMv1.0 forms the subject of this paper. The AerGOM algorithm differs from the standard GOMOS IPF processor in a number of important ways: more accurate physical laws have been implemented, all retrieval-related covariances are taken into account, and the aerosol extinction spectral model is strongly improved. Retrieval examples demonstrate that the previously observed profile perturbations have disappeared, and the obtained extinction spectra look in general more consistent. We present a detailed validation study in a companion paper; here, to give a first idea of the data quality, a worst-case comparison at 386 nm shows SAGE II–AerGOM correlation coefficients that are up to 1 order of magnitude larger than the ones obtained with the GOMOS IPFv6.01 data set.

In preparation for the operations of the ALTIUS mission, research is carried out to assess the accuracy of the SGP4 orbital propagator in predicting stellar occultation events. The quantification of the accuracy and its consequent improvement will enable reliable measurement planning and, therefore, maximize the number of measurements. To this end, predictions are made for the timing of occultations for the GOMOS instrument on-board the ENVISAT, which are then compared to actual occultation occurrences. It is found that the error is substantial but follows a trend that can be interpolated. This enables devising a method for highly accurate predictions given a sufficient number of data points. Statistically significant results for the accuracy of the propagator and a calibration method are presented. Recommendations for a measurement planning procedure of ALTIUS are formulated.

We present a passive method for the retrieval of atmospheric pressure profiles based on the measurement of the apparent flattening of the solar disk as observed through the atmosphere by a spaceborne imager. This method was applied to simulated sunsets. It relies on accurate representation of the solar disk, including its limb darkening, and how its image is affected by atmospheric refraction. The Zernike polynomials are used to quantify the flattening in the Sun images. The inversion algorithm relies on a transfer matrix providing the link between the atmospheric pressure profile and a sequence of Zernike moments computed on the sunset frames. The transfer matrix is determined by a training dataset of pressure profiles generated from a standard climatology. The performance and limitations of the method are assessed by two test cases. Pressure profiles similar to the training dataset show that retrieval error can be up to 10 times smaller than the natural variability in the lower mesosphere, and up to 500 times smaller in the upper troposphere. Tests with other independent profiles emphasize the need for better representativeness of the training dataset.

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. ispartof: URSI GA 2017 vol:2017-January issue:1 ispartof: URSI GA 2017 location:Montréal date:19 Aug - 26 Aug 2017 status: published

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-instrument (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 power. This beam is focused onto a detector which performs the imaging. ispartof: pages:1-1 ispartof: URSI Benelux Forum 2017 pages:1-1 ispartof: URSI Benelux Forum 2017 location:Brussels, Belgium date:16 Feb - 16 Feb 2017 status: published

This article describes the objectives, concept and expected performance of the ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere) mission in view of the continuation of earth limb measurements for atmospheric science. The instrument, consisting of three independent spectral imagers in the UV–vis–NIR range, will be operated from several observation geometries with a global coverage in a 3-days revisit cycle. ALTIUS will retrieve vertical concentration profiles of ozone with a required 3–10% total error in the stratosphere. Other important scientific objectives will address NO2, H2O, CH4, various aerosol and cloud extinction profiles as well as other minor absorbers. Tomography, temperature retrievals and self-calibrating observations will be also implemented. This type of measurement became rare with the failure of the European ENVISAT mission in 2012 and the number will further decrease when several Canadian, Swedish and US limb missions will terminate within the next few years. It is a small mission initiative based on a micro-satellite platform of the PROBA (Project for On-Board Autonomy) class, with a high agility allowing for atmospheric limb observations in different remote sensing geometries from a low earth orbit. The instrument consists of three independent spectral imagers covering the UV–vis–NIR ranges. The ALTIUS mission will be implemented as an element of the ESA Earth Watch programme. The paper identifies the general scientific context of the project and derives the mission, instrument and scientific products requirements. The general design of the payload and platform systems is discussed. The preliminary data processing chain is presented, from telemetry data to retrieved geophysical profiles, with a complementary data assimilation level. A preliminary assessment of the mission performance is discussed with focus on ozone profile retrievals, which are the main objective of the mission.

The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the European platform ENVISAT (ENVironment SATellite) was dedicated to the study of the of Earth's atmosphere using the stellar occultation technique. The spectral range of the GOMOS spectrometer extends from the UV (ultra violet) to the near infrared, allowing for the retrieval of species such as O3, NO2, NO3, H2O, O2, air density, aerosol extinction and OClO. Nevertheless, OClO cannot be retrieved using a single GOMOS measurement because of the weak signal-to-noise ratio and the small optical thickness associated with this molecule. We present here the method used to detect this molecule by using several GOMOS measurements. It is based on a two-step approach. First, several co-located measurements are combined in a statistical way to build an averaged measurement with a higher signal-to-noise ratio; then, a differential optical absorption spectroscopy (DOAS) method is applied to retrieve OClO slant column densities (SCD). The statistics of the sets of GOMOS measurements used to build the averaged measurement and the spectral window selection are analyzed. The obtained retrievals are compared to results from two balloon-borne instruments. It appears that the inter-comparisons of OClO are generally satisfying (relative differences are about 15–60%). Two nighttime climatologies of OClO based on GOMOS averaged measurements are presented. The first depicts annual global pictures of OClO from 2003 to 2011. From this climatology, the presence of an OClO SCD peak in the equatorial region at about 35 km is confirmed and strong OClO SCD in both polar regions are observed (more than 1016 cm−2 in the Antarctic region and slightly less in the Arctic region), a sign of chlorine activation. The second climatology is a monthly time series. It clearly shows the chlorine activation of the lower stratosphere during winter. Moreover the equatorial OClO SCD peak is observed during all years without any significant variations. This very promising method, applied on GOMOS measurements, allowed us to build the first nighttime climatology of OClO using limb-viewing instruments.

we examined acousto-optic interaction in the KH 2 PO 4 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.

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 × 250 m2 area). The detection limit was close to 5 × 1016 molecules cm−2, with a maximum detected SCD of 4 × 1017 molecules 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 100 m upwards, the observed NO2 plume concentration increased at a rate of 0.75–1.25 g 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.

AerGOM is a retrieval algorithm developed for the GOMOS instrument onboard Envisat as an alternative to the operational retrieval (IPF). AerGOM enhances the quality of the stratospheric aerosol extinction retrieval due to the extension of the spectral range used, a refinement of the aerosol spectral parameterisation, the simultaneous inversion of all atmospheric species as well as an improvement of the Rayleigh scattering correction. The retrieval algorithm allows for a good characterisation of the stratospheric aerosol extinction for a wide range of wavelengths. In this work, we present the results of stratospheric aerosol extinction comparisons between AerGOM and various spaceborne instruments (SAGE II, SAGE III, POAM III, ACE-MAESTRO and OSIRIS) for different wavelengths. Due to the unique observational technique of GOMOS, some of the results appear to be dependent on the star occultation parameters such as star apparent temperature and magnitude, solar zenith angle, latitude and obliquity. A systematic analysis is carried out to identify biases in the dataset, using the various spaceborne instruments as references. This bias characterization is extremely important for data users and might prove valuable for the production of unbiased long-term merged dataset.

The GOMOS instrument on EnviSat has succesfully demonstrated that a UV/Vis/NIR spaceborne stellar occultation instrument is capable of delivering quality data on the gaseous and particulate composition of Earth's atmosphere. Still, some problems related to data inversion remained to be treated. In the past, it was found that the aerosol extinction profile retrievals in the upper troposphere and stratosphere are of good quality at a reference wavelength of 500 nm, but suffer from anomalous, retrieval-related perturbations at other wavelengths. Identification of algorithmic problems and subsequent improvement was therefore necessary. This work has been carried out; the resulting AerGOM Level 2 retrieval algorithm together with the first data version AerGOMv1.0 forms the subject of this paper. First, a brief overview of the operational IPFv6.01 GOMOS algorithm is given, since the AerGOM algorithm is to a certain extent similar. Then, the discussion on the AerGOM algorithm specifically focuses on the new aspects that were implemented to tackle the aerosol retrieval problems. Finally, a first assess- ment of the obtained aerosol extinction data quality is presented, clearly showing significant improvement of aerosol profile shape, spectral behaviour and similarity to SAGE II data.

Although the retrieval of aerosol extinction coefficients from satellite remote measurements is notoriously difficult (in comparison with gaseous species) due to the lack of typical spectral signatures, important information can be obtained. In this paper we present an overview of the current operational nighttime UV/Vis aerosol extinction profile results for the GOMOS star occultation instrument, spanning the period from August 2002 to May 2008. Some problems still remain, such as the ones associated with incomplete scintillation correction and the aerosol spectral law implementation, but good quality extinction values are obtained at a wavelength of 500 nm. Typical phenomena associated with atmospheric particulate matter in the Upper Troposphere/Lower Stratosphere (UTLS) are easily identified: Polar Stratospheric Clouds, tropical subvisual cirrus clouds, background stratospheric aerosols, and post-eruption volcanic aerosols (with their subsequent dispersion around the globe). For the first time, we show comparisons of GOMOS 500 nm particle extinction profiles with the ones of other satellite occultation instruments (SAGE II, SAGE III and POAM III), of which the good agreement lends credibility to the GOMOS data set. Yearly zonal statistics are presented for the entire period considered. Time series furthermore convincingly show an important new finding: the sensitivity of GOMOS to the sulfate input by moderate volcanic eruptions such as Manam (2005) and Soufrière Hills (2006). Finally, PSCs are well observed by GOMOS and a first qualitative analysis of the data agrees well with the theoretical PSC formation temperature. Therefore, the importance of the GOMOS aerosol/cloud extinction profile data set is clear: a long-term data record of PSCs, subvisual cirrus, and background and volcanic aerosols in the UTLS region, consisting of hundreds of thousands of altitude profiles with near-global coverage, with the potential to fill the aerosol/cloud extinction data gap left behind after the discontinuation of occultation instruments such as SAGE II, SAGE III and POAM III.

The SPICAM instrument onboard Mars Express has successfully performed two Martian years (MY 27 and MY28) of observations. Water ice cloud optical depths spatial and temporal distribution was retrieved from nadir measurements in the wavelength range 300–320 nm. During the northern spring the cloud hazes complex distribution was monitored. The clouds in the southern hemisphere formed a zonal belt in the latitude range 30–60°S. The edge of the retreating north polar hood merged with the northern tropical clouds in the range 250–350°E. The development of the aphelion cloud belt (ACB) started with the weak hazes formation (cloud optical thickness 0.1–0.3) in the equatorial region. At the end of the northern spring, the ACB cloud optical thickness reached already values of 0.3–1. The ACB decay in the end of the northern summer was accompanied with a presence of clouds in the north mid-latitudes. The expanded north polar hood merged with the north mid-latitude clouds in the eastern hemisphere. The interannual comparison indicates a decrease in cloud activity immediately after a strong dust storm in southern summer of MY28. The strong dust storms of the MY28 may also be a reason of the observed north polar hood edge shifting northward by 5°.

In this paper, we consider occultations of celestial bodies through the atmospheric limb from low Earth orbit satellites and we show how the usual change of tangent altitude associated with atmospheric refraction is inseparably connected to a variation of the observed apparent intensity, for extended and pointlike sources. We demonstrate, in the regime of weak refraction angles, that atmospheric optical dilution and image deformation are strictly concomitant. The approach leads to the integration of a simple differential equation related to the observed transmittance in the absence of other absorbing molecules along the optical path. The algorithm does not rely on the absolute knowledge of the radiometer pointing angle that is related to the accurate knowledge of the satellite attitude. We successfully applied the proposed method to the measurements performed by two past occultation experiments: GOMOS for stellar and ORA for solar occultations. The developed algorithm (named ARID) will be applied to the imaging of solar occultations in a forthcoming pico-satellite mission.

PICASSO - A PICo-satellite for Atmospheric and Space Science Observations is an ESA project led by the Belgian Institute for Space Aeronomy, in collaboration with VTT, Clyde Space Ltd. (UK), and the Centre Spatial de Liege (BE). VTT Technical Research Centre of Finland Ltd. will deliver the Visible Spectral Imager for Occultation and Nightglow (VISION) for the PICASSO mission. The VISION targets primarily the observation of the Earth's atmospheric limb during orbital Sun occultation. By assessing the radiation absorption in the Chappuis band for different tangent altitudes, the vertical profile of the ozone is retrieved. A secondary objective is to measure the deformation of the solar disk so that stratospheric and mesospheric temperature profiles are retrieved by inversion of the refractive raytracing problem. Finally, occasional full spectral observations of polar auroras are also foreseen. The VISION design realized with commercial of the shelf (CoTS) parts is described. The VISION instrument is small, lightweight (~500 g), Piezo-actuated Fabry-Perot Interferometer (PFPI) tunable spectral imager operating in the visible and near-infrared (430 – 800 nm). The spectral resolution over the whole wavelength range will be better than 10 nm @ FWHM. VISION has is 2.5° x 2.5° total field of view and it delivers maximum 2048 x 2048 pixel spectral images. The sun image size is around 0.5° i.e. ~500 pixels. To enable fast spectral data image acquisition VISION can be operated with programmable image sizes. VTT has previously developed PFPI tunable filter based AaSI Spectral Imager for the Aalto-1 Finnish CubeSat. In VISION the requirements of the spectral resolution and stability are tighter than in AaSI. Therefore the optimization of the of the PFPI gap control loop for the operating temperature range and vacuum conditions has to be improved. VISION optical, mechanical and electrical design is described.

Since the recent losses of several atmospheric instruments with good vertical sampling capabilities (SAGE II, SAGE III, GOMOS, SCIAMACHY,. . . ), the scientific community is left with very few sounders delivering concentration pro les of key atmospheric species for understanding atmospheric processes and monitoring the radiative balance of the Earth. The situation is so critical that at the horizon 2020, less than five such instruments will be on duty (most probably only 2 or 3), whereas their number topped at 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 vertically-resolved and accurate data at all latitudes. In this frame, the Belgian Institute for Space Aeronomy (IASB-BIRA) proposed a gap-filler 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, on-board processing ressources, and agility, the ALTIUS concept relies on a hyperspectral imager observing limb-scattered radiance and solar/stellar occultations every orbit. The objective is twofold: the imaging feature allows to better assess the tangent height of the sounded air masses (through easier star tracker information validation by scene details recognition), 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 in one channel. In each channel, the spectral filter will be an acousto-optical tunable filter (AOTF). Such devices offer reasonable etendue with good spectral resolution and excellent robustness and compactness. 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 progress towards UV AOTF's have been made with the advent of KDP-based 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 ALTIUS will be discussed

Ground-based spectral measurements of twilight sky brightness were carried out between October 2009 and August 2011 in Georgia, South Caucasus. The algorithm which allowed to retrieve the lower stratospheric and upper tropospheric aerosol extinction profiles was developed. The Monte-Carlo technique was used to correctly represent multiple scattering in a spherical atmosphere. The estimated stratospheric aerosol optical depths at a wavelength of 780 nm were: 3.0 × 10−3 ± 1 × 10 −3 (31 August 2009–15 January 2011) and 1.1 × 10−2 ± 3 × 10−3 (18 July 2011–03 August 2011, 10 observations). The first optical depth value corresponds to the background stratospheric aerosol level, the last one to the volcanically disturbed one after the Nabro eruption in June 2011. Reconsideration of measurements acquired soon after the Pinatubo eruption in 1991 allowed to model the phenomenon of the "second purple light", a twilight sky brightness enhancement at large solar zenith angles (97–102°). Monte-Carlo modeling reveals that the second purple light is caused by multiple scattering in the stratospheric aerosol layer. The modeling also shows that, assuming a hypothetical mesospheric aerosol layer with optical extinction comparable to typical noctilucent cloud values, a measurable twilight sky brightness increase at wavelength 440 nm follows at solar zenith angles 98–99&deg.

The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the European platform ENVISAT was dedicated to the study of the atmosphere of the Earth using the stellar occultation technique. The spectral range of the GOMOS spectrometer extends from the UV to the near infrared, allowing for the retrieval of species such as O3, NO2, NO3, H2O, O2, air density, aerosol extinction and OClO. Nevertheless, OClO can not be retrieved using a single GOMOS measurement because of the weak signal-to-noise ratio and the small optical thickness associated with this molecule. We present here the method used to detect this molecule by using several GOMOS measurements. It is based on a two-step approach. First, several co-located measurements are combined in a statistical way to build an averaged measurement with a higher signal-to-noise ratio. Then, a Differential Optical Absorption Spectroscopy (DOAS) method is applied to retrieve OClO slant column densities. The statistics of the sets of GOMOS measurements used to build the averaged measurement and the spectral window selection are analyzed. The obtained retrievals are compared to results from two balloon-borne instruments. It appears that the inter-comparisons of OClO are generally satisfying. Then, two nighttime climatologies of OClO slant column densities based on GOMOS averaged measurements are presented. The first depicts annual global pictures of OClO from 2003 to 2011. From this climatology, the presence of an OClO layer in the equatorial region at about 35 km is confirmed and strong concentrations of OClO in both polar regions are observed, a sign of chlorine activation. The second climatology is a monthly time series. It clearly shows the chlorine activation of the lower stratosphere during winter. Moreover the equatorial OClO layer is observed during all the years without any significant variations. Finally, the anti-correlation between OClO and NO2 is highlighted. This very promising method, applied on GOMOS measurements, allowed us to build the first nighttime climatology of OClO.