The European Space Agency (ESA)’s wind mission, Aeolus, was launched on 22 August 2018. It is the fifth of the ESA Earth Explorer family of missions and its main objective is to demonstrate the potential of Doppler Wind Lidar (DWL) in space for improving weather forecasts and the understanding of the role of atmospheric dynamics in climate variability. Aeolus carries a single instrument called the Atmospheric LAser Doppler INstrument (ALADIN): a high spectral resolution DWL which operates at 355nm which is the first of its kind to be flown in space.
Aeolus provides profiles of single horizontal line-of-sight winds (primary product) in near-real-time (NRT), and profiles of atmospheric backscatter and extinction. The instrument samples the atmosphere from about 30 km down to the Earth’s surface, or down to optically thick clouds. The required precision of the wind observations is 1-2.5 m/s in the troposphere and 3-5 m/s in the stratosphere while the systematic error requirement is less than 0.7 m/s. The mission spin-off products include information about aerosol and cloud layers. The satellite flies in a polar dusk/dawn orbit (6 am/pm local time ascending node), providing ~16 orbits per 24 hours with an orbit repeat cycle of 7 days. Global scientific payload data acquisition is guaranteed with the combined usage of Svalbard and Troll X-band receiving stations.
After more than 3 years in orbit and despite some performance issues related to its instrument ALADIN, Aeolus has achieved its objectives and designed-life time. Positive impact on the weather forecast has been demonstrated by multiple NWP centres world-wide. Four of the main European meteorological centres are now assimilating Aeolus winds operationally thanks to the excellent data timeliness and continuous improvement of the ground based processors.
The status of the Aeolus mission will be presented, including overall performance, planned operations and exploitation. Scope of the paper is also to inform about the programmatic highlights and future challenges until the end of its operational life time.

The European Space Agency’s (ESA) Aeolus satellite was launched on 22 August 2018 from Centre Spatial Guyanais in Kourou, French Guyana. The first atmospheric returns and first wind measurements were retrieved within the first two weeks of operations after launch. Aeolus has continually provided wind measurements since this time with only limited outages in availability. Aeolus successfully met it’s nominal mission lifetime at the end of November 2021, and it has been agreed to extend the mission operations until the end of 2022.

It was discovered that the atmospheric return signals from the ALADIN instrument were lower than expected before launch by a factor of x2 to x3. Extensive investigations were performed which led to the conclusion that the most likely cause for the loss in signal was an over-illumination of the 88µm diameter instrument field stop. In addition to this initial loss, it was found that the output energy of the first flight laser transmitter (FM-A) was decreasing. An investigation showed that this decrease was due to a continual misalignment of the laser master oscillator. The decrease in the FM-A UV energy and the resulting degradation of the wind measurement random error led to the decision to change to the second flight laser (FM-B) in June 2019. The FM-B laser has performed well in the subsequent 2.5yrs of operations retaining UV output energies above 60mJ and has recently provided UV output energies around 90mJ, corresponding to the levels that were reached in the on-ground thermal vacuum performance tests.

Despite the good performance of the FM-B transmitter, losses in the atmospheric return signal were evident. These losses were eventually traced to the emit path of the instrument (i.e. those optics after the laser transmitter up to the telescope output), largely due to independent measurements of the ALADIN emit energy by the Pierre Auger Observatory in Argentina, which indicated some 50% reduction in the emit energy between 2019 and 2022. This has led to recent interest to revert back to the FM-A transmitter (correcting for the misalignment) or to lowering the Aeolus altitude from the current 320km to 255km, in order to improve the signal returns and extend the mission duration.

Several issues with systematic bias in the wind measurements were also discovered. Pixels with slightly increased dark current levels in the memory zone of the Accumulation Charge-Coupled Devices (ACCDs), so called “hot pixels”, were found, with a new hot pixel arising approximately every 2-3 weeks. A novel pseudo-dark current correction method was employed in order to correct for these hot pixels. An important sub-orbital bias caused by differing background illuminations of the ALADIN telescope due to albedo variations was also discovered. This was mitigated by employing a correction which utilised the ALADIN telescope temperatures.

The Aeolus data has been extensively analysed by a number of meteorological centres and found to have a positive impact on NWP forecasts, particularly in the tropics and polar regions. These very positive results, along with the successful in-orbit demonstration of the measurement concept and associated technologies utilised on Aeolus, resulted in a statement of interest from EUMETSAT in a future, operational DWL mission in the 2030 to mid-2040’s timeframe.

This paper will summarise the performance of Aeolus’ ALADIN instrument in its 3+ years of operations, detailing the issues that have been described briefly above, as well as the mitigation actions taken, and drawing lessons learned for a future operational Doppler wind lidar mission.

After more than three years of operations of ESA’s Wind mission, launched on 22 August 2018, the Aeolus Payload Data Ground Segment continues to ensure successfully global X-band data acquisition with the combined usage of the Svalbard and Troll X-band ground stations, seamless mission planning operations, uninterrupted systematic science data production in Near Real Time (within 3 hours from sensing) and easy data access and discovery.

The Payload Data Ground Segment is based on a distributed architecture that foresees the first level of processing up to the preliminary wind observations and scientific aerosol/cloud profile products combined with the telemetry acquisition service, hosted and operated by KSAT in Norway. The second level of processing including the scientific wind observations is performed by the European Centre for Medium-Range Weather Forecasts (ECMWF) based in England for further assimilation and usage by multiple Numerical Weather Prediction (NWP) centres world-wide.
The Aeolus data is made available to meteo and expert users via the ESA Earth Observation Gateway through a dedicated dissemination system and archived for data preservation. Data quality. monitoring calibration and scientific processors evolution is performed by an international expert consortium, the Aeolus DISC (Data, Innovation, and Science Cluster).
The PDGS team at the ESA/ESRIN centre in Frascati has been responsible of the coordination and execution of the Payload Data Ground Segment operations, including the deployment in operations of various processing baselines and the public dissemination to users of the Aeolus products once the relevant quality standards were met. In addition, the PDGS team is responsible for the challenging mission planning of the instruments operations and has supported the execution of three reprocessing campaigns allowing mission data to be made available to users in the latest processing baselines, ensuring the highest possible data quality standards.
Along the years efforts have been put also in providing the Aeolus data user community with a modern concept of extended access to Earth Observation (EO) data. The VirES for Aeolus system provides a highly interactive data manipulation and retrieval web interface for the official Aeolus data products. The VirES Service will be extended with a Virtual Research Environment (VRE) that will become operational at the beginning of 2022 in order to provide data manipulation capabilities to users.
The architecture, current status and performance of the Aeolus PDGS will be presented, with focus on the main activities performed in more than three years of operations of the ground segment of this extremely challenging mission.

Assuring and improving Aeolus' data quality is a collaborative effort of the ESA Aeolus team, the Aeolus DISC (Data Innovation and Science Cluster) as well as more than 30 individual Cal/Val teams. The Aeolus satellite has brought cutting edge UV DWL (Doppler Wind Lidar) technology to space for the first time. This required to be prepared and committed to support and maximise the performance from the ground, being ready for unknown biases and error product-sources. ESA, DISC, Cal/Val as well as the ASAG (Aeolus Science and Data Quality Advisory Group) have worked together closely during mission in order to react to findings from one of the performance monitoring tools, such as the monitoring of Aeolus winds against winds from Numerical Weather Predication centers (NWP monitoring), and the DISC has half yearly released new processing baselines, each of them improving the data quality and/or compensating for newly discovered anomalies and biases, but also reacting to performance decreases of the Aladin instrument over time. The presentation will introduce the many groups involved in maximizing Aeolus' data quality via on-ground activities, and their tasks, along with results, highlight and achieved milestones.

The Aeolus DISC (Data, Innovation, and Science Cluster) is a core element in ESA's data quality framework for the Aeolus mission, comprised of an international expert consortium to study and improve the data quality of Aeolus products. The tasks of the Aeolus DISC are various and include among others the instrument and data quality monitoring, the calibration and characterization of the instrument, and the refinement of the retrieval algorithms and the processor evolution. Additionally, the Aeolus DISC supports ESA with data reprocessing campaigns, provides support to data users and performs impact assessment studies.

The Aeolus DISC's mission experts and expert centres have long track records in supporting and contributing to Aeolus, covering instrumental aspects, laser-atmosphere interaction, calibration and validation (e.g. aircraft campaigns), wind product and processor development, as well as the development of optical, aerosol and cloud processors and products.

In this presentation, we will summarize the achievements of the Aeolus DISC for the data quality of the various Aeolus products. We will especially focus on how the constant instrument and data monitoring supported the evolution of the Aeolus processing chain. Past, present and future processor changes will be described and their impact on Aeolus NRT and reprocessed data quality will be illustrated.

Various processor improvements, developed by the Aeolus DISC after launch, lead to a drastic reduction of the systematic bias of the Aeolus wind products down to below 1 m/s on a global scale. Examples of such processor improvements are the hot pixel correction for enhanced dark current rates observed on the Aeolus detectors (Weiler et al., 2021a) and a correction for thermal changes of the instrument’s large telescope along the orbit (Weiler et al., 2021b).

Additionally, for aerosol and cloud retrievals, current processors were optimized and new processing routines were developed (Flament et al., 2021, and Ehlers et al., 2021). In particular, this includes a new feature mask and an optimal estimation retrieval based on EarthCare algorithms. Both products will be fully functional for the spring 2022 baseline release.




Weiler, F., Kanitz, T., Wernham, D., Rennie, M., Huber, D., Schillinger, M., Saint-Pe, O., Bell, R., Parrinello, T., and Reitebuch, O.: Characterization of dark current signal measurements of the ACCDs used on board the Aeolus satellite, Atmos. Meas. Tech., 14, 5153–5177, https://doi.org/10.5194/amt-14-5153-2021, 2021a.

Weiler, F., Rennie, M., Kanitz, T., Isaksen, L., Checa, E., de Kloe, J., Okunde, N., and Reitebuch, O.: Correction of wind bias for the lidar on board Aeolus using telescope temperatures, Atmos. Meas. Tech., 14, 7167–7185, https://doi.org/10.5194/amt-14-7167-2021, 2021b.

Flament, T., Trapon, D., Lacour, A., Dabas, A., Ehlers, F., and Huber, D.: Aeolus L2A Aerosol Optical Properties Product: Standard Correct Algorithm and Mie Correct Algorithm, Atmos. Meas. Tech. Discuss. [preprint], https://doi.org/10.5194/amt-2021-181, in review, 2021.

Ehlers, F., Flament, T., Dabas, A., Trapon, D., Lacour, A., Baars, H., and Straume-Lindner, A. G.: Optimization of Aeolus Optical Properties Products by Maximum-Likelihood Estimation, Atmos. Meas. Tech. Discuss. [preprint], https://doi.org/10.5194/amt-2021-212, in review, 2021.

Based on the experience gained during the pre-launch activities in the Aeolus processor development and validation campaign programmes, DLR has conducted an intensive performance monitoring for the mission since its launch in 2018.

Already in the decade bevor, various ground and airborne pre-launch validation campaigns had been performed in support of the Aeolus operations and processor development. An integral part of the early success of the mission was the deployment of the ALADIN Airborne Demonstrator (A2D), a prototype of the ALADIN Doppler Wind Lidar (DWL) instrument on-board Aeolus. With its high technological commonality, it allowed to study Aeolus-specific lidar topics under various atmospheric conditions already before launch. In addition, the scanning, coherent 2-µm DWL acted as a high-accuracy reference by providing wind vector profiles for the A2D and Aeolus validation. Airborne measurements were performed with the two instruments operated in parallel on the DLR Falcon research aircraft in a downward looking configuration. The comprehensive operational, technological and algorithm knowledge established at DLR during these pre-launch activities laid the basis for a twofold Aeolus performance monitoring, implemented to support the mission.

Firstly, performance-relevant instrument parameters of the laser and receiver optics as well as the detector signals evolution for the internal reference and atmospheric path were monitored and reported on a regular basis. Additionally, special operations for tests of the Aeolus lasers and instrument alignment were supported, both by applying the tools derived during the pre-launch period and with newly developed performance indicators. The second monitoring activity was covered by four post-launch airborne validation campaigns with a focus on the Aeolus wind product. Deploying the two DWLs onboard the Falcon, coordinated flights along a total distance of 26,000 km under the Aeolus track were performed in different geographical regions and during the main operational phases of the mission until September 2021. The collocated A2D and 2-µm DWL wind observations not only reflect the evolution of the error characteristics during each mission state, but also provide valuable recommendations for the optimization of the Aeolus wind retrieval and related quality control algorithms.

This contribution gives an overview of the Aeolus in-orbit instrument performance evolution and related results of the four airborne validation campaigns.

We highlight some of the scientific benefits of the Aeolus Doppler Wind Lidar mission since its launch in August 2018. Its scientific objectives are to improve weather forecasts and to advance the understanding of atmospheric dynamics and its interaction with the atmospheric energy and water cycle. A number of meteorological and science institutes across the world have demonstrated that the Aeolus mission objectives are being met, despite the measurements being noisier than expected. Its wind product is being operationally assimilated by five Numerical Weather Prediction (NWP) centres, thanks to demonstrated useful positive impact on NWP analyses and forecasts. Applications of its atmospheric optical properties product have been found, e.g., in the detection and tracking of smoke from the extreme Australian wildfires of 2020 and in atmospheric composition data assimilation. The winds are finding novel applications in atmospheric dynamics research, such as for tropical phenomena (Quasi-Biennial Oscillation disruption events), Sudden Stratospheric Warming events, detection of atmospheric gravity waves, and in the smoke generated vortex associated with the Australian wildfires. It has been applied in the assessment of other types of satellite derived wind information such as atmospheric motions vectors. The successes of Aeolus will hopefully lead to the approval of an operational follow-on mission.