The wind mission Aeolus of the European Space Agency was a seminal achievement in Earth observation and space laser technology. During its nearly five-year lifetime, the space-borne Doppler wind lidar instrument ALADIN onboard the Aeolus satellite employed two switchable ultraviolet laser transmitters to measure atmospheric wind profiles with global coverage, contributing to improving the accuracy of numerical weather prediction. Despite the excellent performance of the nominal and redundant laser which was optimized during the mission through thermal adjustments, the atmospheric return signal levels declined between 2019 and 2022 due to decreasing transmission of the optics between the redundant laser and the telescope. The root cause analysis of the signal loss was supported by the Pierre Auger Observatory in Argentina whose fluorescence detector registered the ultraviolet laser pulses emitted from the instrument in space, thereby offering an independent measurement of the laser energy.

In the three years after the launch of ESA's wind lidar mission Aeolus in 2018, DLR conducted four airborne campaigns to validate its wind products. The DLR Falcon aircraft was equipped with two Doppler wind lidar (DWL) instruments, the ALADIN Airborne Demonstrator (A2D), the prototype of the Aeolus instrument, and a scanning heterodyne detection 2-µm DWL as a reference system. These instruments were operated concurrently for over 26,000 km along the Aeolus measurement track. Complementary and synergistic observations from both DWLs were used to characterize systematic and random wind errors of the Aeolus products, and to recommend algorithm improvements. Recently, the data obtained from campaigns in Iceland (2019) and Cape Verde (2021) was applied to validate reprocessed Aeolus wind products (Baseline 16). This paper provides an overview of the airborne validation campaigns and ongoing contributions to the Aeolus performance validation and processor evolution.

Aeolus was the first Doppler wind lidar in space. During its almost 5 years lifetime after launch in August 2018 it made use of two lasers operating at a wavelength of 354.8 nm. In contrast to coherent wind lidars that use heterodyning, the direct-detection approach applies high-resolution interferometry and hence requires accurate and regular calibration, but also allows measurements outside aerosol-loaded areas. The Instrument Response Calibration (IRC) plays an essential role in the Aeolus processing chain for L1B wind data, and helps to ensure the precision and accuracy of L2B wind data disseminated to the scientific community. The IRC is used to determine spectral characteristics of the receiver and the relationship between the instrument measurement output data (the response) and the actual frequency shift. We explain fundamental aspects of IRC, such as the monitoring of IRC-related parameters, the quality assessment of the receiver channels' calibration curves and the utilization of IRC-based parameters in the wind retrieval algorithm.

The Aeolus mission by ESA was operational from August 2018 to July 2023. Aeolus carried the direct-detection Atmospheric LAser Doppler INstrument (ALADIN). To support Aeolus, the ALADIN Airborne Demonstrator (A2D) was developed. Both ALADIN and A2D consist of so-called Rayleigh and Mie channels to measure wind from molecular and particulate backscatter signals, respectively. The Mie channel relies on determining the spatial location of a fringe being imaged on the detector. The accuracy of the retrieved winds depends on the analytic algorithm used for determining the fringe location. In this paper, the performance of two non-linear fit-based algorithms is investigated by applying them to airborne A2D data. For performance validation, the data of a heterodyne-detection wind lidar are used as a reference. In addition, a fast and non-fit-based algorithm relying on a four-pixel intensity ratio approach (R4) has been developed and yielded similar accuracy, but at a much faster computation time.

This paper presents an extensive investigation of the signal fringe profile for the Fizeau interferometer used in the first spaceborne wind lidar Aeolus and considers the fundamental implications for the wind measurement accuracy in Aeolus and future systems. The early Aeolus design phase considered that the basic fringe would be made up of a Fizeau instrumental component of ≈100 MHz (full width at half maximum, FWHM), folded with the laser pulse spectral width of ≈50 MHz (FWHM), both of Lorentzian form. Fringe anomalies observed before the mission and related to surface defects in the interferometer plates triggered the development of wave-optic methods for analysis of the fringe formation. These methods, herein described in an instructional appendix, were subsequently found to be essential for rigorous modelling of complex fringes for different physical and optical arrangements. Initial signal returns from Aeolus suggested that the Fizeau fringe profile was in fact broadened with a large Gaussian component. The laser pulse was subsequently shown to have a profile close to Gaussian of ≈45 MHz (FWHM) and thus provided a partial contribution. However, detailed examination of experimental Aeolus fringes constructed from ground return signals showed a large Gaussian component up to ≈130 MHz (FWHM). Wave-optic modelling established that Fizeau “aperture broadening”, of this form and magnitude, would be generated for the input signal beam of 500 µrad field of view (FOV) set at a large angle of incidence (AOI) of 300 µrad. These findings have strong implications for fringe shift and wind measurement accuracy, as given in the quantum-limited Cramér–Rao expression and the paramount importance of minimizing line width. Extensive modelling and simulation for the broadened profiles calculated above shows good agreement with measured Aeolus global wind measurement accuracies and indicates that loss of signal could be due to beam clipping at the field stop for such a large AOI. It is established that optimization of the present Aeolus Fizeau parameters could lead to a factor of 2.5 improvement in wind measurement precision. Future upgrades of the Fizeau interferometer and the laser within reasonable parameters suggest the potential for an factor of 7.6 improvement on the in-orbit performance.

The detection of side-scattered ultraviolet light from spaceborne lasers with fluorescence telescopes of cosmic ray observatories offers unique opportunities for systematic studies of the aerosol content of the local atmosphere. It also enables the validation of the optical calibration of the telescopes. Additionally, these observations provide valuable ground-based monitoring of the performance of the scientific instruments aboard satellites used for Earth climate observation. Here, we report on results from the reconstruction of laser shots from the spaceborne lidar instrument ALADIN aboard the Aeolus satellite in 2019, 2020 and 2021. Furthermore, we present initial observations of laser shots from ATLID, the atmospheric lidar of the EarthCARE satellite, launched in 2024. EarthCARE's orbit is particularly well-suited for enabling laser detection within a few days at both the Pierre Auger Observatory and the Telescope Array Experiment, facilitating a relative calibration of the energy scales of these observatories.

The Aeolus mission, launched by the European Space Agency in August 2018, was a landmark in Earth observation by providing global wind profiles in near-real time using the first Doppler wind lidar in space: the Atmospheric Laser Doppler Instrument (ALADIN). Despite challenges such as systematic errors affecting data quality at the beginning of the mission, Aeolus surpassed its planned lifetime of three years and proved invaluable for weather prediction and scientific research until its conclusion in July 2023. A permanent challenge throughout the mission involved mitigating the impact of hot pixels on the ALADIN charge-coupled device (CCD) detectors on the wind data. The related dark current anomalies, which manifested as random telegraph signal noise and sporadic shifts in median dark current signal, necessitated the development of dedicated calibration techniques to minimize the induced systematic wind speed errors. The regular dark current calibrations of up to eight times per day yielded a comprehensive dataset that was used to categorize the hot pixels according to their characteristics and to derive statistical parameters that are of relevance for the reprocessing of the Aeolus data products. Following the end of the operational mission in April 2023, a series of specialized in-orbit tests, referred to as end-of-life (EOL) activities, provided valuable insights into the temperature dependence of the dark currents, shedding light on potential root causes of the hot pixels. Additionally, the EOL tests revealed other detector anomalies that had caused significant wind biases in certain altitudes following strong cosmic ray events in 2022. This work summarizes the performance of the ALADIN detectors during the Aeolus mission, with a focus on hot pixel characterization and mitigation strategies. Furthermore, it highlights findings from the EOL activities that are relevant for future space lidar missions and other satellite missions using CCD detectors.

The wind mission Aeolus of the European Space Agency was a groundbreaking achievement for Earth observation. Between 2018 and 2023, the space-borne lidar instrument ALADIN onboard the Aeolus satellite measured atmospheric wind profiles with global coverage which contributed to improving the accuracy of numerical weather prediction. The precision of the wind observations, however, declined over the course of the mission due to a progressive loss of the atmospheric backscatter signal. The analysis of the root cause was supported by the Pierre Auger Observatory in Argentina whose fluorescence detector registered the ultraviolet laser pulses emitted from the instrument in space, thereby offering an estimation of the laser energy at the exit of the instrument for several days in 2019, 2020 and 2021. The reconstruction of the laser beam not only allowed for an independent assessment of the Aeolus performance, but also helped to improve the accuracy in the determination of the laser beam's ground track on single pulse level. The results presented in this paper set a precedent for the monitoring of space lasers by ground-based telescopes and open new possibilities for the calibration of cosmic-ray observatories.

The European Space Agency's Aeolus mission was a groundbreaking achievement in Earth observation and space laser technology. Over its nearly five-year lifetime, the space-borne Doppler wind lidar instrument onboard Aeolus utilized two redundant ultraviolet (UV) lasers to measure atmospheric wind profiles globally, significantly enhancing the accuracy of numerical weather predictions. The laser transmitters were frequency-tripled, injection-seeded Nd:YAG systems, configured in a master oscillator power amplifier arrangement, generating single-longitudinalmode pulses at 354.8 nm with a pulse duration of around 20 ns. Across the mission, both transmitters together generated more than 7 109 UV laser pulses. The performance of both the nominal and redundant lasers was optimized and stabilized by carefully regulating their thermal environment, which influenced the laser energy depending on the emission frequency. At the optimum laser bench temperature, both lasers delivered stable UV output with pulse energies exceeding 60 mJ. The energy of the second laser, operational from June 2019 to October 2022, was further enhanced to over 100 mJ through step-wise increases in master oscillator pump power and adjustments of the amplifier pump phase. Following the mission's operational phase, a series of tests was conducted close before the mission end-of-life (EOL) to address instrument-related questions. During these EOL activities, the laser power was boosted to more than 150 mJ for 20 days and even over 180 mJ for 33 h, setting anew, to the best of our knowledge, record for a UV space laser. Additionally, the frequency stability of the two lasers was evaluated, revealing detrimental impact from micro-vibrations caused by the satellite's reaction wheels. EOL tests showed that adjustments to the master oscillator cavity control sequence significantly mitigated these effects, improving the laser frequency stability by a factor of two to better than 7 MHz (standard deviation over the period of one wind observation of 12 s). This paper provides a comprehensive overview of the ALADIN laser transmitters' architecture, operation, and performance during the Aeolus mission from 2018 to 2023, with a focus on energy and frequency stability improvements relevant to current and future space lidar missions such as EarthCARE and Aeolus-2.

<p>The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt, DLR) conducted four airborne campaigns for the validation of the Aeolus L2B wind product during the first three years of ESA’s wind lidar mission between 2018 and 2021. After three campaigns in Europe, the <em>Aeolus VAlidation Through Airborne LidaRs in the Tropics</em> (AVATAR-T) campaign was performed around the Cabo Verde archipelago in September 2021 as part of the Joint Aeolus Tropical Atlantic Campaign (JATAC). AVATAR-T employed the DLR Falcon aircraft which carried two Doppler wind lidar (DWL) instruments: the heterodyne-detection 2-µm DWL acting as a high-accuracy reference and the ALADIN Airborne Demonstrator (A2D), representing a prototype of the direct-detection DWL on-board Aeolus with a high degree of commonality in terms of design and measurement principle.</p> <p>In the framework of AVATAR-T, 11 coordinated flights along the Aeolus track were performed covering nearly 11,000 km of the satellite's measurement swath. The research flights yielded a comprehensive set of A2D and 2-µm DWL wind observations to validate the Aeolus wind product under the influence of the Saharan Air Layer (SAL), the African Easterly Jet, the Subtropical Jet and the Intertropical Convergence Zone. In particular, the campaign results give insight into the impact of atmospheric aerosols onto the operational Rayleigh-clear and Mie-cloudy horizontal line-of-sight (HLOS) winds regarding potential errors that arise from crosstalk between the two complementary receiver channels and their respective wind data coverage in the troposphere.</p> <p>Validation of the Aeolus wind product based on 2-µm DWL data shows that the systematic error almost fulfills the mission requirement of being below 0.7 m/s (HLOS) for both Rayleigh-clear and Mie-cloudy winds. The random error, however, is larger than specified (2.5 m/s HLOS), being close to 3 m/s for Mie-cloudy winds and as high as 7 m/s for Rayleigh-clear winds. A more detailed investigation reveals that the Rayleigh-clear random error is increased at lower altitudes in case of signal extinction due to aerosols.</p> <p>The collocated A2D wind observations provide valuable information on the potential optimization of the Aeolus wind retrieval and related quality control (QC) algorithms. For instance, the A2D, unlike ALADIN, delivered a broad vertical and horizontal coverage of Mie winds across the SAL, whereas A2D Rayleigh winds measured in this region, which are affected by Mie contamination through crosstalk and signal extinction, are effectively filtered out. Preliminary studies suggest that a refinement of the Aeolus wind retrieval may improve the Mie wind data coverage in aerosol regions.</p> <p>In addition, we studied the influence of different QC schemes on the validation results and developed a two-step QC approach that ensures effective outlier removal and compliance with the Aeolus mission requirements document. The QC scheme also improves the comparability of different validation studies and thus helps to facilitate the consolidation of the Aeolus error evaluation from different Cal/Val teams.</p> <p>The contribution presents comparative wind observations of Aeolus and the two DLR airborne wind lidar instruments from the JATAC with a focus on the error assessment and a potential improvement of the Aeolus wind data product.</p>

The Aeolus mission by the European Space Agency was launched in August 2018 and stopped operations in April 2023. Aeolus carried the direct-detection Atmospheric LAser Doppler INstrument (ALADIN). To support the preparation of Aeolus, the ALADIN Airborne Demonstrator (A2D) instrument was developed and applied in several field campaigns. Both ALADIN and A2D consist of so-called Rayleigh and Mie channels used to measure wind from both molecular and particulate backscatter signals. The Mie channel is based on the fringe-imaging technique, which relies on determining the spatial location of a linear interference pattern (fringe) that originated from multiple interference in a Fizeau spectrometer. The accuracy of the retrieved winds is among others depending on the analytic algorithm used for determining the fringe location on the detector. In this paper, the performance of two algorithms using Lorentzian and Voigt fit functions is investigated by applying them to A2D data that were acquired during the AVATAR-I airborne campaign. For performance validation, the data of a highly accurate heterodyne detection wind lidar (2-µm DWL) that was flown in parallel are used as a reference. In addition, a fast and non-fit-based algorithm based on a four-pixel intensity ratio approach (R 4) is developed. It is revealed that the Voigt-fit-based algorithm provides 50% more data points than the Lorentzian-based algorithm while applying a quality control that yields a similar random error of about 1.5 m/s. The R 4 algorithm is shown to deliver a similar accuracy as the Voigt-fit-based algorithms, with the advantage of a one to two orders of magnitude faster computation time. Principally, the R 4 algorithm can be adapted to other spectroscopic applications where sub-pixel knowledge of the location of measured peak profiles is needed.

In cooperation with the European Space Agency (ESA) and Airbus Defense and Space (DS), the German Aerospace Center (DLR) developed the ALADIN Airborne Demonstrator (A2D).Also operating at 355 nm wavelength, it is the prototype of the first direct-detection Doppler wind lidar instrument in space -ALADIN (Atmospheric Laser Doppler Instrument)the single payload of ESA's Aeolus mission.The A2D was deployed for extensive ground-based measurements and airborne campaigns in the years of mission preparation, with the goal to test operational procedures and refine the algorithms for the processing chain.Experience gained with the A2D supported the ground tests conducted with ALADIN before the launch of Aeolus in 2018.It also laid the foundation for the continuous performance monitoring of the mission, performed within the Data Innovation and Science Cluster (DISC), led by DLR.After launch, DLR executed four airborne campaigns with a focus on validating the Aeolus wind products during different phases of the mission and in diverse geographical and meteorological conditions.During these campaigns, the DLR Falcon aircraft was equipped with the A2D and a high-accuracy scanning heterodyne-detection 2-μm Doppler wind lidar (DWL), used as a reference.Complementary and synergistic results from both DWLs do not only allow for the characterization of the Aeolus and A2D wind errors, but also provide recommendations for the optimization of the Aeolus wind retrieval.Fielding the A2D as a test-bed and validation tool for the Aeolus mission has been a milestone for space lidars and vital for the success of Aeolus, as it has been providing efficient access to instrument specifics in the technological, analytical and scientific domains.The paper covers selected aspects of the airborne technology demonstration program that has supported Aeolus in becoming a successful explorer mission, able to provide high impact data for numerical weather prediction institutions around the world on an operational basis.

During the first 3 years of the European Space Agency's Aeolus mission, the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt, DLR) performed four airborne campaigns deploying two different Doppler wind lidars (DWL) on board the DLR Falcon aircraft, aiming to validate the quality of the recent Aeolus Level 2B (L2B) wind data product (processor baseline 11 and 12). The first two campaigns, WindVal III (November–December 2018) and AVATAR-E (Aeolus Validation Through Airborne Lidars in Europe, May and June 2019), were conducted in Europe and provided first insights into the data quality at the beginning of the mission phase. The two later campaigns, AVATAR-I (Aeolus Validation Through Airborne Lidars in Iceland) and AVATAR-T (Aeolus Validation Through Airborne Lidars in the Tropics), were performed in regions of particular interest for the Aeolus validation: AVATAR-I was conducted from Keflavik, Iceland, between 9 September and 1 October 2019 to sample the high wind speeds in the vicinity of the polar jet stream; AVATAR-T was carried out from Sal, Cape Verde, between 6 and 28 September 2021 to measure winds in the Saharan dust-laden African easterly jet. Altogether, 10 Aeolus underflights were performed during AVATAR-I and 11 underflights during AVATAR-T, covering about 8000 and 11 000 km along the Aeolus measurement track, respectively. Based on these collocated measurements, statistical comparisons of Aeolus data with the reference lidar (2 µm DWL) as well as with in situ measurements by the Falcon were performed to determine the systematic and random errors of Rayleigh-clear and Mie-cloudy winds that are contained in the Aeolus L2B product. It is demonstrated that the systematic error almost fulfills the mission requirement of being below 0.7 m s−1 for both Rayleigh-clear and Mie-cloudy winds. The random error is shown to vary between 5.5 and 7.1 m s−1 for Rayleigh-clear winds and is thus larger than specified (2.5 m s−1), whereas it is close to the specifications for Mie-cloudy winds (2.7 to 2.9 m s−1). In addition, the dependency of the systematic and random errors on the actual wind speed, the geolocation, the scattering ratio, and the time difference between 2 µm DWL observation and satellite overflight is investigated and discussed. Thus, this work contributes to the characterization of the Aeolus data quality in different meteorological situations and allows one to investigate wind retrieval algorithm improvements for reprocessed Aeolus data sets.

Since the start of the European Space Agency's Aeolus mission in 2018, various studies were dedicated to the evaluation of its wind data quality and particularly to the determination of the systematic and random errors in the Rayleigh-clear and Mie-cloudy wind results provided in the Aeolus Level-2B (L2B) product. The quality control (QC) schemes applied in the analyses mostly rely on the estimated error (EE), reported in the L2B data, using different and often subjectively chosen thresholds for rejecting data outliers, thus hampering the comparability of different validation studies. This work gives insight into the calculation of the EE for the two receiver channels and reveals its limitations as a measure of the actual wind error due to its spatial and temporal variability. It is demonstrated that a precise error assessment of the Aeolus winds necessitates a careful statistical analysis, including a rigorous screening for gross errors to be compliant with the error definitions formulated in the Aeolus mission requirements. To this end, the modified Z score and normal quantile plots are shown to be useful statistical tools for effectively eliminating gross errors and for evaluating the normality of the wind error distribution in dependence on the applied QC scheme, respectively. The influence of different QC approaches and thresholds on key statistical parameters is discussed in the context of the Joint Aeolus Tropical Atlantic Campaign (JATAC), which was conducted in Cabo Verde in September 2021. Aeolus winds are compared against model background data from the European Centre for Medium-Range Weather Forecasts (ECMWF) before the assimilation of Aeolus winds and against wind data measured with the 2 µm heterodyne detection Doppler wind lidar (DWL) aboard the Falcon aircraft. The two studies make evident that the error distribution of the Mie-cloudy winds is strongly skewed with a preponderance of positively biased wind results distorting the statistics if not filtered out properly. Effective outlier removal is accomplished by applying a two-step QC based on the EE and the modified Z score, thereby ensuring an error distribution with a high degree of normality while retaining a large portion of wind results from the original dataset. After the utilization of the described QC approach, the systematic errors in the L2B Rayleigh-clear and Mie-cloudy winds are determined to be below 0.3 m s−1 with respect to both the ECMWF model background and the 2 µm DWL. Differences in the random errors relative to the two reference datasets (Mie vs. model is 5.3 m s−1, Mie vs. DWL is 4.1 m s−1, Rayleigh vs. model is 7.8 m s−1, and Rayleigh vs. DWL is 8.2 m s−1) are elaborated in the text.

In August 2018, the first-ever spaceborne wind lidar – Aeolus – was launched and has since then been providing global data of the horizontal wind field from the ground up to 30 km to improve numerical weather prediction. Aeolus is based on a single instrument called ALADIN (Atmospheric Laser Doppler Instrument), which comprises a single-frequency, ultraviolet solid-state laser in a master oscillator power amplifier configuration that emits nanosecond pulses into the atmosphere. High output energy and excellent frequency stability ensure a sufficient signal-to-noise ratio of the backscatter return needed for an accurate determination of the wind-induced Doppler frequency shift. To demonstrate the Aeolus measurement principle and to validate the corresponding wind data quality, the German Aerospace Center (DLR) started with airborne pre-launch validation activities already in 2007. After launch, these activities were extended by four campaigns over Europe, the North Atlantic region, and the tropics. In this talk, the challenges of the development and the operation of ALADIN are addressed and the successful accomplishment of the Aeolus mission is demonstrated using the results obtained from the post-launch validation campaigns.

The realization of the European Space Agency's Aeolus mission was supported by the long-standing development and field deployment of the Atmospheric LAser Doppler INstrument (ALADIN) Airborne Demonstrator (A2D) which, since the launch of the Aeolus satellite in 2018, has been serving as a key instrument for the validation of ALADIN, the first-ever Doppler wind lidar (DWL) in space. However, the validation capabilities of the A2D are compromised by deficiencies of the dual-channel receiver which, like its spaceborne counterpart, consists of a Rayleigh and a complementary Mie spectrometer for sensing the wind speed from both molecular and particulate backscatter signals, respectively. Whereas the accuracy and precision of the Rayleigh channel is limited by the spectrometer's high alignment sensitivity, especially in the near field of the instrument, large systematic Mie wind errors are caused by aberrations of the interferometer in combination with the temporal overlap of adjacent range gates during signal readout. The two error sources are mitigated by modifications of the A2D wind retrieval algorithm. A novel quality control scheme was implemented, which ensures that only backscatter return signals within a small angular range are further processed. Moreover, Mie wind results with large bias of opposing sign in adjacent range bins are vertically averaged. The resulting improvement of the A2D performance was evaluated in the context of two Aeolus airborne validation campaigns that were conducted between May and September 2019. Comparison of the A2D wind data against a high-accuracy, coherent DWL that was deployed in parallel on board the same aircraft shows that the retrieval refinements considerably decrease the random errors of the A2D line-of-sight (LOS) Rayleigh and Mie winds from about 2.0 to about 1.5 m s−1, demonstrating the capability of such a direct detection DWL. Furthermore, the measurement range of the Rayleigh channel could be largely extended by up to 2 km in the instrument's near field close to the aircraft. The Rayleigh and Mie systematic errors are below 0.5 m s−1 (LOS), hence allowing for an accurate assessment of the Aeolus wind errors during the September campaign. The latter revealed different biases of the Level 2B (L2B) Rayleigh-clear and Mie-cloudy horizontal LOS (HLOS) winds for ascending and descending orbits, as well as random errors of about 3 m s−1 (HLOS) for the Mie and close to 6 m s−1 (HLOS) for the Rayleigh winds, respectively. In addition to the Aeolus error evaluation, the present study discusses the applicability of the developed A2D algorithm modifications to the Aeolus processor, thereby offering prospects for improving the Aeolus wind data quality.

In August 2018, the European Space Agency (ESA) launched the first Doppler wind lidar into space, which has since then been providing continuous profiles of the horizontal line-of-sight wind component at a global scale. Aeolus data have been successfully assimilated into several numerical weather prediction (NWP) models and demonstrated a positive impact on the quality of the weather forecasts. To provide valuable input data for NWP models, a detailed characterization of the Aeolus instrumental performance as well as the realization and minimization of systematic error sources is crucial. In this paper, Aeolus interferometer spectral drifts and their potential as systematic error sources for the aerosol and wind products are investigated by means of instrument spectral registration (ISR) measurements that are performed on a weekly basis. During these measurements, the laser frequency is scanned over a range of 11 GHz in steps of 25 MHz and thus spectrally resolves the transmission curves of the Fizeau interferometer and the Fabry–Pérot interferometers (FPIs) used in Aeolus. Mathematical model functions are derived to analyze the measured transmission curves by means of non-linear fit procedures. The obtained fit parameters are used to draw conclusions about the Aeolus instrumental alignment and potentially ongoing drifts. The introduced instrumental functions and analysis tools may also be applied for upcoming missions using similar spectrometers as for instance EarthCARE (ESA), which is based on the Aeolus FPI design.

In single crystals of orthorhombic α‐AlOOH, known also as mineral diaspore, χ(3)‐nonlinear lasing by stimulated Raman scattering (SRS) and Raman‐induced four‐wave mixing (RFWM) is investigated. Picosecond pumping at 1.064 µm wavelength produces a broadband Stokes and anti‐Stokes frequency comb with up to 25 SRS‐ and RFWM‐generated emission lines. All observed Stokes and anti‐Stokes lasing components in the visible and near‐IR are identified and attributed to a single SRS‐promoting vibration mode with ωSRS ≈ 445 cm−1. The first Stokes steady‐state Raman gain coefficient in the visible spectral range is estimated to a value not less than 0.36 cm GW−1.

The Data Innovation and Science Cluster (DISC) is a core element of ESA's data quality strategy for the Aeolus mission, which was launched in August 2018. Aeolus provides for the first-time global observations of vertical profiles of horizontal wind information by using the first Doppler wind lidar in space. The Aeolus DISC is responsible for monitoring and improving the quality of the Aeolus aerosol and wind products, for the upgrade of the operational processors as well as for impact studies and support of data usage. It has been responsible for multiple significant processor upgrades which reduced the systematic error of the Aeolus observations drastically. Only due to the efforts of the Aeolus DISC team members prior to and after launch, the systematic error of the Aeolus wind products could be reduced to a global average below 1 m/s which was an important pre-requisite for making the data available to the public in May 2020 and for its use in operational weather prediction. In 2020, the reprocessing of earlier acquired Aeolus data, another important task of the Aeolus DISC, also started. In this way, also observations from June to December 2019 with significantly better quality could be made available to the public, and more data will follow this and next year. Without the thorough preparations and close collaboration between ESA and the Aeolus DISC over the past decade, many of these achievements would not have been possible.