Aeolus / formerly ADM (Atmospheric Dynamics Mission)Spacecraft Launch Mission Status Sensor Complement Ground Segment References
Aeolus is an ESA (European Space Agency) Earth Explorer Core Mission -a science-oriented mission within its Living Planet Program. The primary objective is to provide wind profile measurements for an improved analysis of the global three-dimensional wind field. The aim of the mission is to provide global observations of wind profiles with a vertical resolution that will satisfy the accuracy requirements of WMO (World Meteorological Organization). Such knowledge is crucial to the understanding of the atmospheric dynamics, including the global transport of energy, water, aerosols, chemicals and other airborne materials - to be able to deal with many aspects of climate research and climate and weather prediction. ADM-Aeolus represents a demonstration project for the Global Climate Observing System (GCOS). 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)
The measurement data will allow achievement of the primary goals of Aeolus:
- Provision of accurate wind profiles throughout the troposphere and lower stratosphere eliminating a major deficiency in the Global Observing System
- Direct contribution to the study of the Earth's global energy budget
- Provision of data for the study of the global atmospheric circulation and related features, such as precipitation systems, the El Niño and the Southern Oscillation phenomena and stratospheric/tropospheric exchange.
The secondary mission objectives are related to the provision of data sets for model variation and short-term "windclimatologies" allowing experts to:
- Validate climate models through the use of high quality wind profiles from a global measurement system
- Improve their understanding of atmospheric dynamics and the global atmospheric transport and cycling of energy, water, aerosols, chemicals and other airborne materials.
- Generate a number of derived products such as cloud top altitudes, aerosol properties and tropospheric height.
The ADM-Aeolus measurements will be assimilated in numerical forecasting models, in order to enhance the quality of operational short- and medium-range predictions. Expected improvements are mainly due to the excellent horizontal and vertical sampling capabilities of the instrument, combined with a continuous availability of its data products within 3 hours after sensing.
Note: In the works of the Greek poet Homer, Aeolus is the controller of the winds and ruler of the floating island of Aeolia. In the Odyssey, he gave Odysseus a favorable wind and a bag in which the unfavorable winds were confined. Odysseus' companions opened the bag; the winds escaped and drove them back to the island. Although he appears as a human in Homer, Aeolus later was described as a minor god.
The ADM-Aeolus mission makes use of a single observation instrument, namely ALADIN (Atmospheric Laser Doppler Instrument), employing the DWL (Doppler Wind Lidar) measurement technique. The retrieval of wind speed relies on direct measurement along the LOS (Line-of-Sight) by lidar using Doppler shift information from atmospheric molecules and particles advected by wind. The ALADIN observations will serve as input for NWP (Numerical Weather Prediction) models. An extensive pre-development evaluation and assessment program of ALADIN laser component technology was started in 2000.
ADM-Aeolus is seen as a pre-operational mission, demonstrating new laser technology and paving the way for future meteorological satellites to measure the Earth's wind.
Although the ADM-Aeolus satellite is a new design, the platform is based on a heritage from other ESA missions developed by Airbus DS (former EADS Astrium) including CryoSat, and Rosetta. The aim has been to build a spacecraft that is relatively simple to operate. This reduces the operating costs throughout its lifetime, and is also important for the future since similar Aeolus-type satellites are later envisaged for operational use.
The S/C structure, consisting of aluminum honeycomb elements, uses a conventional box-shaped spacecraft design (derived from Mars Express), upon which the observation instrument is mounted via three isostatic bipods. The electronic boxes of the bus and the associated satellite equipment are mounted on the side panels.
The spacecraft is three-axis stabilized with AOCS (Attitude and Orbit Control Subsystem), using thrusters, reaction wheels and magnetorquers as actuators, and magnetometers, coarse Earth sun sensors, inertial measurement units, rate measurement units, AST (Autonomous Star Tracker), and a GPS receiver as sensors. The orbit is maintained by 5 N thrusters. 14)
Table 1: AOCS elements of Aeolus
Magnetometer: The magnetometer (developed at LusoSpace, Portugal) of the ADM-Aeolus spacecraft employs the AMR (Anisotropic Magneto Resistive) technology. The rationale for using the AMR detector for the magnetometer development was due to several advantages over fluxgate technology: 15)
- Detector production repeatability
- Lower cost
- Easier integration in a PCB (Printed Circuit Board)
- Possibility to generate external magnetic field in the chip by mean of built in coils.
The magnetometer is a small (credit card surface dimension) and robust unit that can be used for several LEO missions. Two flight models of the magnetometer will be flown on ADM-Aeolus. In addition, a qualification model will fly on PROBA-2 as a passenger to provide more flight heritage and in orbit data.
Table 2: Performance parameters of the magnetometer
Figure 1: Photo of the AMR magnetometer (image credit: LusoSpace, ESA)
EPS (Electric Power Subsystem): Electric power is provided by two deployable solar wings of 14.5 m2 of total surface area. The triple-junction GaAs cells of the solar arrays provide over 2.4 kW of power (with 1.4 kW of average power required). The solar arrays are articulated toward the sun to optimize their power output. Use of SADM (Solar Array Drive Mechanism) for attitude regulation of the wings. The design includes a standard PCDU (Power Control and Distribution Unit) responsible for solar array power conditioning and distribution. A Li-ion battery of 64 Ah capacity is being used for eclipse phases and LEOP (Launch and Early Orbit Phase). 16)
On-board autonomy: The spacecraft is being designed to include a large amount of on-board autonomy in all mission phases such that ground contact is needed no more than once every 5 days even in the case of anomaly.
On-board data handling is performed by an ERC-32 radiation tolerant processor with 6 MByte system RAM. The subsystems are linked via MIL-STD-1533 data bus to the central processor. A solid-state memory provides a capacity of 8 Gbit on-board data storage.
Aeolus is conceived to allow simple in-flight operation. The satellite has a five-day autonomy in case of any single onboard failure, so that a single operator shift is sufficient to monitor the satellite. In addition, the orbit has a seven day repeat cycle, so that the complete operations timeline is repeated on a weekly cycle, thus minimizing the effort for mission planning.
At the heart of the avionics architecture are the CDMU (Command and Data Management Unit) manufactured by RUAG, Sweden and the PCDU (Power Conversion and Distribution Unit) manufactured by Patria, Finland. 17) 18)
The CDMU includes redundant processor modules interfaced by a MIL-STD-1553 bus protocol based ICB to IO boards providing input / output services including thruster drivers, mass memory units for measurement data storage and the TTR (Telemetry Telecommand and Reconfiguration) boards incorporating, telecommand packet decoders, telemetry encoders, RMs (Reconfiguration Modules) and SGM (Safe Guard Memory). The RMs monitor alarms generated autonomously within the PM (Processor Module) or from the APSW (Application Software) and perform reconfiguration and restart of the PMs accordingly.
The SGM is a permanently powered memory used to preserve data during PM reconfigurations and restarts. Each PM has two software images stored in non-volatile memory, a nominal mode image and a safe mode image. The RMs select which image to download into RAM and execute.
Except for the AST (Autonomous Star Tracker) subsystem, the CDMU is interfaced to all external units either via discrete lines provided by the IO boards or via an external MIL-STD-1553 bus. Each PM includes separate bus controllers allowing the active PM to control both the ICB (Internal Control Bus) and the external MIL-STD-1553 bus independently. The AST, manufactured by Terma in Denmark, is interfaced directly to each PM via an RS422 HSUART (High Speed Universal Asynchronous Receiver Transmitter).
The PCDU, which interfaces to the CDMU via the external Mil-STD-1553 bus, provides regulated and unregulated power outlets, shunt and battery charge control, solar array deployment thermal knife control and individually switched heater lines for thermal control. The power outlets supplying the TT&C receivers and the reconfiguration units are non-switchable and are protected by FCLs (Foldback Current Limiters). All other outlets are switched and protected by LCLs (Latching Current Limiters). The shunt regulation and battery charge control is fully implemented in the PCDU electronics and requires no involvement from ground or the on-board software under both nominal and failure conditions. Thermal knife drivers and deployment micro-switch status acquisition and conditioning are provided to support solar array deployment.
Figure 2: Overview of the avionics system (image credit: EADS Astrium Ltd.)
On-board autonomy architecture: One of the simplest methods to achieve on-board autonomy is to implement an on-board schedule that is loaded fully under ground responsibility. Such an autonomy approach is straight forward to test and validate since only basic functionalities such as command insertion, command deletion and command execution at scheduled time have to be tested. In particular there is no need to develop and test any logic relating one command to another and there is no need to develop and test any logic for selecting which commands to schedule. This was the approach adopted for ADM-Aeolus with two simple schedules being implemented, one based on time and the other based on orbit position.
Although this approach works well under nominal circumstances, it is not tolerant to failures that occur in the system such that, by the time the commands are due for execution, they are no longer valid or allowed. In particular such a system design approach is vulnerable to the following:
1) The scheduled commands address a physical unit that has failed and has been replaced by its redundant unit.
2) A scheduled command fails to execute successfully because a reconfiguration is occurring.
3) Commands to one unit are only allowable if another unit or subsystem is in a particular state and must not be executed if this condition is not met.
4) Scheduled commands are part of a functional sequence of commands and so are dependent on the successful execution of previous scheduled commands.
5) Complex critical operations, such as solar arrays deployment, require the execution of decision branches and must be executed even if the CDMU is reconfigured or restarted.
During the design stage the potential vulnerability of the AEOLUS scheduled operations to the above cases was assessed and the solutions taken to avoid them (Ref. 17).
FDIR (Failure Detection, Isolation and Recovery):
The overall FDIR concept adopted in Aeolus is driven by the objective to minimize ground intervention both during nominal operations and in failure scenarios.
The autonomous multi-layer FDIR architecture must include monitors to identify all failures that:
- Directly endanger the unit itself or risk propagation to other units as identified in the Satellite and lower level FMECAs (Failure Modes and Effects Criticality Analysis)
- Corrupt or significantly degrade functions necessary for the correct functioning of the spacecraft in the current spacecraft mode / configuration [these failures may be identified in the FMECAs and HSIAs (Hardware Software Interaction Analysis) or may be "feared events"]
- Corrupt or significantly degrade functions necessary for data dissemination to the ground.
A high speed FDIR MIL-STD-1553 bus was established to monitor bus protocol status messages to identify a loss of communication and allow start of recovery within 1 second. For each unit, feared events are identified based on the function of the unit in the overall design and also based on the satellite and unit FMECA and HSIA documents (Ref. 17).
The Aeolus FDIR concept is built around top-down onboard control architecture: (Ref. 12)
• At the highest level hot redundant TTR(TM, TC and Reconfiguration) boards within the CDMU contain Reconfiguration Modules which oversee the health and function of the CDMU and flight software by monitoring hardware alarm inputs and performing CDMU resets, reconfigurations and switches to Safe Mode as appropriate.
• At the next level the CDMU application software monitors and controls the spacecraft units by monitoring on board parameters and autonomously sending control commands in response to parameter out of range events.
• At the lowest level some units perform their own built-in health checks and report this through the TM to the CDMU software.
For the platform functions, the FDIR needs to ensure that the spacecraft can safely recover from single level failures either by resuming operations autonomously or by switching to predefined redundant configurations. For ALADIN, the FDIR needs to ensure instrument safety by both stopping scheduled operations and switching the instrument into a safe and stable configuration or by switching ALADIN into Survival mode.
Redundancy princple: In case of on-board failure detection during any of the mission phases, the on-board control system will attempt to recover operational status by switching to redundant units. In order to avoid the loss of platform functions mandatory for the mission, the redundancy concept has to be such that a single failure does not cause permanent loss of essential platform functions. All units have to therefore be independent of their redundant alternatives. This includes provisions to prevent malfunction or elimination of redundant units by a common cause.
Figure 3: Overview of the major ADM-Aeolus spacecraft elements (image credit: ESA)
Figure 4: Artist's rendition of the deployed ADM-Aeolus spacecraft (image credit: ESA/ESTEC)
The S/C mass at launch is about 1360 kg of which 266 kg are allocated to the payload. Its size is 1.74 m x 1.9 m x 2.0 m in launch configuration, limited by the payload envelop. The solar arrays of 13 m span have three panels on each side. The design life is 3 years. The prime spacecraft contractor is EADS Astrium Ltd., Stevenage, UK (contract award in Oct. 2003). Further Astrium sites in Germany and France are involved in the spacecraft development. 19) 20) 21) 22)
RF communications: TT&C communications are based on standard S-band links, the uplink data rate is 2 kbit/s the downlink data rate is up to 8 kbit/s. The measurement data are dumped via an X-band transmitter with 10 Mbit/s data rate. S/C operations are performed at ESOC (Darmstadt, Germany) using the Kiruna TT&C station. - The measurement data are received nominally by the ground station in Svalbard (Spitzbergen). Additional X-band receiving stations (antenna diameter as small as 2.4 m) can easily be added to provide a shorter data delivery time.
Launch: The Aeolus spacecraft was launched on 22 August 2018 (21:20 GMT) on a Vega vehicle, designated VV12, from Kourou,French Guiana. Some 55 minutes later, Vega's upper stage delivered Aeolus into orbit and contact was established through the Troll ground station in Antarctica. The satellite is being controlled from ESA's ESOC (European Space Operations Center) in Darmstadt, Germany. Controllers will spend the next few months carefully checking and calibrating the mission as part of its commissioning phase. 23) 24) 25)
Figure 5: Aeolus heads for orbit (image credit: ESA/CNES/Arianespace)
On Sept. 7, 2016, ESA and Arianespace signed a contract to secure the launch of the Aeolus satellite. With this milestone, a better understanding of Earth's winds is another step closer. With the main technical hurdles resolved and the launch contract now in place.
Baseline change in the autumn of 2010: Change from burst mode to "continuous mode" operation.
Stable and complete versions of the end-to-end simulator and ground payload data processing software are available, but they need to be upgraded to support the new continuous mode of the ALADIN instrument. These significant changes to the instrument design have delayed the planned launch date to mid-2013. 28) 29) 30)
Orbit: Sun-synchronous orbit, altitude = 320 km (mean), inclination = 96.97º, local equator crossing time at 18:00 (on ascending node) and at 06:00 hours (dawn-dusk orbit), 7-day repeat cycle (111 orbits).
• April 26, 2022: Launched back in 2018, Aeolus has outlived its 36-month in-orbit design life – but going above and beyond, it continues to deliver excellent data. This shows that there's life yet in the satellite, meaning ESA's wind mission is now expected to continue shining a light on the wind for another year. 31)
- Heralding the start of the Aeolus Third Anniversary Conference in Taormina, Sicily, which highlighted the continued importance of this pioneering wind mission, the Aeolus Mission Manager, Tommaso Parrinello, said, "I believe that the best is still to come, and I'm pleased to announce that with a switch of the laser we are extending the lifetime of this remarkable mission hopefully for another year."
- Named after Aeolus, who in Greek mythology was appointed ‘keeper of the winds' by the Gods, Aeolus is a one-of-a-kind satellite that measures wind from space. It is one of ESA's Earth Explorers missions, which use advanced space technologies to answer critical questions about Earth's natural processes and the impact that human activity is having.
- Pulses of ultraviolet light fired from Aeolus' ALADIN laser towards Earth are reflected from air molecules and particles in the atmosphere. Two optical analysers measure the Doppler shift of the molecular scattering, ‘Rayleigh', and scattering from aerosols and water droplets, ‘Mie'. By analysing these Doppler shifts, it is possible to estimate wind speed and direction at various altitudes worldwide, making Aeolus the first satellite mission to deliver profiles of Earth's wind on a global scale.
- The uses for Aeolus wind data are many, from predicting the weather and improving climate models, to tracking events in near-realtime, such as the recent Hunga Tonga volcanic eruption.
- Despite exceeding its initial lifetime, meteorology experts at the Taormina conference expressed the value Aeolus data continue to have.
- "Forecast Sensitivity Observation Impact shows that Aeolus is amongst the most important satellite missions, which is an impressive result for a demonstrator," said Mike Rennie of the European Centre for Medium Range Weather Forecasts (ECMWF).
- Nothing connects us quite like the weather. Whether it's to understand what coat to wear, or to determine climate expectations tomorrow, being able to predict it as accurately as possible is key.
- Although Mike showed that although the positive impact of the data obtained in 2019 was roughly twice as big as it is now, Aeolus is still proving useful for numerical weather prediction.
- "Although the Rayleigh impact is gradually declining as the instrument noise increases, the Forecast Sensitivity Observation Impact shows that Aeolus is still beneficial," added Mike.
Figure 6: Observing System Experiments show that ESA's Aeolus mission significantly improves short-range forecasts, particularly in the Tropics and at mid-latitudes. The graph shows the standard deviation of changes when assimilating Aeolus data at an atmospheric pressure of 200 hPa (around 12km altitude), image credit: ECMWF–M. Rennie/ESA
- Gemma Halloran of the UK Met Office, where an expanded Aeolus dataset will be operational in May, concurred, saying, "Almost all weather models improved with the assimilation of Aeolus data."
- Vivien Pourret of Météo France also presented data that put Aeolus amongst the best instruments for improving weather forecasts, third overall in terms of improvement per observation. He noted, "The goal is to operationally assimilate Aeolus data for as long as possible."
Aeolus helped track Hunga Tonga eruption
- Aeolus is also proving helpful for tracking events such as volcanic eruptions, thanks to near-realtime data reaching the user within three hours via the Aeolus Virtual Research Environment. Earlier this year, scientists working on the Aeolus Data Science Innovation Cluster used the online visualisation tool to track the Hunga Tonga volcanic eruption.
- On 15 January 2022, a huge blip, or drop, in the Aeolus signal over the region of the eruption suggested the plume of volcanic ash must have reached an altitude above the range of Aeolus, as shown in the image of Figure 7. The image of Figure 8 uses data from three days later, from 18 January, and shows how Aeolus could track the volcanic plume widening and spreading westwards over Australia.
Figure 7: Tonga volcanic ash plume leaves its mark in Aeolus data. Despite exceeding its design life in orbit, ESA's Aeolus mission continues to deliver excellent data. The uses for Aeolus wind data are many, from predicting the weather and improving climate models, to tracking events in near-realtime, such as the recent Hunga Tonga volcanic eruption. The image shows how the ash from the eruption left its mark in Aeolus' measurements on 15 January 2022. A huge blip, or drop, can be seen in the Aeolus signal over the Tonga region, suggesting the plume of volcanic ash must have reached an altitude above the range of Aeolus (image credit: ESA)
Figure 8: Spread of Tonga volcanic ash shown in Aeolus data on 18 January 2022 (image credit: ESA)
- After increasing the satellite's range of measurements, by the end of January the whole plume was clearly visible in the stratosphere.
- The usefulness of such analyses was made clear by Anna Kampouri of the National Observatory of Athens, who in Taormina also showed how Aeolus data improved models of Mount Etna's ash plume as it travelled across Greece in March 2021.
- The effect is important to warn the airline industry of potential hazards, as encounters with ash clouds in high concentrations can reduce visibility and damage aircraft engines.
Figure 9: The Aeolus scientific community gathered in Taormina, Sicily, which very aptly lies just south of the Aeolian Islands named after the very same ‘keeper of the winds', and under Mount Etna, a volcano whose plumes can be better modelled thanks to the assimilation of Aeolus data (video credit: ESA)
The future is bright for Doppler wind lidars in space
- While Aeolus is set for at least another year, discussions in Taormina inevitably led to potential follow-on missions. "The value of Aeolus is not only scientific, but also economic and societal," said ESA's Director of Earth Observation Programmes Simonetta Cheli in her opening address in Taormina. "Following the success of Aeolus and the operational assimilation of data into weather forecast models, it's clear there is growing support for a follow-on mission."
• December 14, 2021: It's hard to believe that ESA's Aeolus wind mission has now been orbiting Earth for three years and, remarkably, exceeded its design life milestone. Aeolus has gone way further than its original goal of demonstrating that ground-breaking laser technology can deliver global profiles of the wind; its data are being distributed to weather forecasting services across the world in less than three hours of measurements being made in space. Moreover, Aeolus has laid the foundation for future Doppler wind lidar satellite missions. 32)
- Being such a dynamic and relatively invisible aspect of Earth's environment, the wind is particularly challenging to measure from space. Nevertheless, the need for these measurements was identified many years ago by, for example, the World Meteorological Organization which is responsible for the World Integrated Global Observing System. This system, which comprises a vast number of meteorological and environmental observations taken from the ground, ships, upper atmosphere and space, is used by meteorological services all over the world.
- As part of ESA's FutureEO programme, Aeolus is an Earth Explorer research mission. But it was also designed to demonstrate how sophisticated Doppler wind lidar technology can address the need for more wind measurements to improve weather forecasts.
- Aeolus' single instrument is called ALADIN. Its laser transmits short fast pulses of ultraviolet light towards Earth. This light bounces off air molecules and particles such as dust in the atmosphere. The small fraction of light that scatters back towards the satellite is collected by a large telescope. All of this allows the horizontal speed of the world's winds to be measured in the lowermost 30 km of the atmosphere.
- Over the last three years, scientists have been using information from Aeolus to understand more about the systems that influence our weather and climate.
- However, its greatest achievement is the fact that the quality of Aeolus' data are so good that meteorological centres have been feeding the data into daily weather forecasting models since January 2020.
- This has been particularly relevant during the Covid pandemic, which, in the spring of 2020, led to a drop in the number of commercial flights that normally provide unique measurements of wind, temperature and pressure along their flight paths. With fewer measurements being made available from aircraft for weather forecasts, Aeolus has been an important contributor in helping to fill the gap.
Figure 10: Lidar concept. The state-of-the-art Aladin instrument incorporates two powerful lasers, a large telescope and very sensitive receivers. The laser generates ultraviolet light that is beamed towards Earth. This light bounces off air molecules and small particles such as dust, ice and droplets of water in the atmosphere. The fraction of light that is scattered back towards the satellite is collected by Aladin's telescope and measured (image credit: ESA)
- ESA's Aeolus mission scientist, Anne Grete Straume, said, "Aeolus has been a great boost to helping us understand the complexities of Earth's wind systems and how they influence the weather and the climate as described in a recent paper published in Geophysical Research Letters. 33)
- "The paper shows how Aeolus' observations in the tropical upper troposphere and lower stratosphere have helped correct weather models to better represent the atmospheric flow by capturing wind shear caused by Kelvin waves.
Figure 11: Aeolus tightens up wind models. The paper in the Geophysical Research Letters describes how Aeolus' observations in the tropical upper troposphere and lower stratosphere have helped correct the ECMWF weather model to better represent the atmospheric flow by capturing wind shear caused by Kelvin waves. Kelvin waves triggered, for example, by strong convective towers pushing airmasses upwards, are the main drivers of tropical weather patterns. Phenomenon such as the Asian monsoon and the Intertropical Convergence Zone (ITCZ), which are key to tropical rainfall, are found in these regions. The figure shows the standard deviation of the difference in zonal winds for ECMWF model analysis over a period of six months, at 200 hPa level, with and without Aeolus winds. The larger the difference, the more Aeolus winds change the model wind fields. Aeolus winds also change the model in the South Pacific and South Atlantic convergence zones (SPCZ and SACZ), which is home to the southern hemispheric storm region and the ‘roaring forties'. In these areas, the use of Aeolus wind observations ties the model closer also to other sparse but very accurate weather observations, for example by routine weather balloon soundings (image credit: ECMWF–M. Rennie)
- "Thanks to the quality and uniqueness of the data, four European weather centres have been using Aeolus' data for their daily forecasts since 2020, and India's National Centre for Medium Range Weather Forecasting centre also started benefiting from Aeolus this year. This demonstrates that Aeolus has clearly achieved a key objective of being used for daily forecasts, but also demonstrates how the technology can be used for follow-on missions."
- The Aeolus mission was under development for several years before it was finally launched in 2018. The lidar technology was completely new and challenging to realise.
- ESA's Aeolus Payload Manager, Denny Wernham, noted, "Aeolus was extremely challenging to develop. It was designed as a demonstrator mission and astonishingly we still have it in good health and delivering valuable data for science and weather forecasting three years after going live in orbit. Thanks to Aeolus, we have gained valuable experience and knowledge for the development of possible future Doppler wind lidar satellites in space."
- While the mission has certainly demonstrated that this laser technology works in space, an observatory in Argentina that searches for cosmic rays has also discovered that spaceborne lidars could help cross-calibrate the energy scales of different cosmic-ray observatories.
- Scientists from the Institute for Astroparticle Physics of the Karlsruhe Institute of Technology in Germany and the National Institute for Nuclear Physics in Italy who study cosmic rays from outer space using information from the Pierre Auger Observatory in Argentina, noticed an unexpected reoccurring signal in their data. Together with scientists from the Institute of Atmospheric Physics of the German Aerospace Center, they figured out that the observatory was detecting a signal emitted by Aeolus.
- The observatory is being used to study the origin of ultrahigh-energy cosmic rays. Wide-field optical telescopes detect fluorescence radiation emitted from nitrogen molecules excited as cosmic-ray-induced particle cascades. The strongest fluorescence lines are in the ultraviolet, close to the 355 nm frequency of the Aeolus laser. Aeolus' laser signal sweeps across the observatory's view every week.
- Michael Unger, from the Karlsruhe Institute of Technology, explained, "We plan to use this laser beam from space for systematic studies of the density of aerosols above the observatory and for the calibration of our telescopes. Future satellite-based lidar missions could be designed to aid the cross-calibration of the energy scales of different cosmic-ray observatories."
- The observatory is also helping ESA to understand more about the complexities of spaceborne lasers.
- Toni Tolker-Nielsen, Acting Director for ESA's Earth Observation Programmes, added, "The Pierre Auger Observatory's work has also been extremely important in providing new insights and independent evidence that will help us in our further understand the technical complexities of using lasers in space. These results confirm that cosmic ray observatories can offer an independent and powerful method to measure the performance of Earth observation satellite lasers, paving the way to future collaboration with other missions."
Figure 12: Pierre Auger Observatory Fluorescence Detector. Scientists from the Institute for Astroparticle Physics of the Karlsruhe Institute of Technology in Germany and the National Institute for Nuclear Physics in Italy who study cosmic rays from outer space using information from the Pierre Auger Observatory in Argentina, noticed an unexpected reoccurring signal in their data. Together with scientists from the Institute of Atmospheric Physics of the German Aerospace Center, they figured out that the observatory was detecting a signal emitted by ESA's Aeolus wind mission (image credit: S. Saffi)
• September 20, 2021: For a team of scientists and technicians from Europe and the US, the fact of ‘going back to the office' this September has meant heading off to the Cabo Verde islands in the Atlantic – not to extend their summer holidays, but for a complex international experiment campaign that will scrutinize the data being delivered by one of today's most innovative Earth observation satellites: ESA's Aeolus wind mission. 34)
- Since it was launched three years ago, Aeolus has far exceeded expectations and frequently hailed a remarkable success. It was developed as a research mission and to demonstrate how novel laser technology could deliver vertical profiles of Earth's wind. These measurements were much needed, for example by the World Meteorological Organization's Global Observing System, which is a coordinated system of methods and facilities for making meteorological and environmental observations on a global scale.
- Despite Aeolus being built as a research and demonstrator mission, it has proven to be so good that, for more than a year now, its data have been distributed publicly to forecasting services and scientific users in less than three hours of measurements being made from space (see Figure 18).
- Playing such an important part in forecasting, and with a potential follow-on satellite mission on the table, it is critical to ensure that its data are accurate, particularly for forecasts in the Tropics where large weather systems develop and where Aeolus is said to be making a real difference.
- Hence, scientists from ESA, NASA, the German Aerospace Center (DLR), the French National Centre for Scientific Research, the CNES French space agency, Météo-France, Atmospheres Spatial Observations Laboratory, the National Observatory of Athens, the Leibniz Institute for Tropospheric Research, the University of Nova Gorica, the Ocean Science Centre Mindelo, and from many other institutes are all joining forces in Cabo Verde and also in the Virgin Islands for the Aeolus tropical Atlantic campaign.
- The Cabo Verde islands lie about 600 km off the coast west Africa. This tropical location is not only relevant for Aeolus, but it is also where strong winds often carry desert dust and aerosols from the African continent across the islands, making it an ideal place for investigating cloud–aerosol interaction and atmospheric dynamics.
- Throughout the month, this intrepid team are taking measurements of the wind, aerosols and clouds with a range of instrumentation on different aircraft flying at different altitudes.
Figure 13: Desert dust blows from Africa. Captured by the Copernicus Sentinel-3 mission on 24 June 2021, this image shows desert dust blowing from the continent of Africa out across the Atlantic Ocean. Several of the small islands that make up the archipelago of Cabo Verde can be seen peeking out from beneath the clouds. These volcanic islands lie in the Atlantic Ocean about 570 km off the west coast of Senegal and Mauritania, which frame the image on the right. The sand comes mainly from the Sahara and Sahel region. Owing to Cabo Verde's position and the trade winds, these storms are not uncommon. - During September 2021, Cabo Verde is the site of an extensive field campaign to validate data from ESA's Aeolus wind mission (image credit: ESA, the image contains modified Copernicus Sentinel-3 data (2021), processed by ESA, CC BY-SA 3.0 IGO)
- Many flights are even coinciding with Aeolus as it orbits above. Measurements are also being taken by lasers and radars on the ground. This is all providing a wealth of data to compare with that from Aeolus and to support the science to tropical weather.
- Thorsten Fehr, head of the atmospheric section at ESA, said, "We had hoped to run the field campaign last year, but of course the COVID pandemic thwarted our plans. It is an extremely complicated campaign and has been a mammoth task for us and our teams to arrange.
Figure 14: For a team of scientists and technicians from Europe and the US, the fact of ‘going back to the office' this September has meant heading off to the Cabo Verde islands in the Atlantic – not to extend their summer holidays, but for a complex international experiment campaign that will scrutinize the data being delivered by one of today's most innovative Earth observation satellites: ESA's Aeolus wind mission. - This intrepid team is taking measurements of the wind, aerosols and clouds with a range of instrumentation on different aircraft flying at different altitudes. As the photo shows, the aircraft are packed with instruments. Many flights are even planned to coincide with Aeolus as it orbits above. Measurements are also being taken by lasers and radars on the ground. This is all providing a wealth of data to compare with that from Aeolus and to support the science to tropical weather (image credit: ESA)
- "This is truly an international effort and we are all thrilled to have the campaign now well underway, especially given COVID. I can't thank everyone enough for all the work they've done to make it a reality.
- "This extraordinary experiment campaign brings huge benefits, not only to our Aeolus mission, but also to our upcoming EarthCARE mission that is set to advance our understanding of the role that clouds and aerosols play in reflecting incident solar radiation back out to space and how the trap infrared radiation emitted from Earth's surface.
- "In addition, the data we collect will help in development of an Earth Explorer mission concept called Wivern, which aims to measure wind in clouds.
- "You could never achieve an experiment of this scale without working together. International collaboration is key to so much of what we do, and we naturally build strong bonds our colleagues. So, we were deeply saddened by the sudden loss of a dear NASA colleague last week, which understandably led to NASA having to suspend their operations. Our thoughts and sincere condolences go to Gail Skofronick-Jackson's family, friends and colleagues."
- ESA's Aeolus mission manager, Tommaso Parrinello, said, "We are all extremely shocked by the tragic loss of Gail. NASA had been supporting our campaign in the Virgin Islands well before the fleet of European aircraft arrived in Cabo Verde and they had planned to join the team here for their second part of the campaign.
- "We now hope that we will be able to resume this part of the campaign next year."
Figure 15: Two aircraft in Cabo Verde for Aeolus. For a team of scientists and technicians from Europe and the US, the fact of ‘going back to the office' this September has meant heading off to the Cabo Verde islands in the Atlantic – not to extend their summer holidays, but for a complex international experiment campaign that will scrutinize the data being delivered by one of today's most innovative Earth observation satellites: ESA's Aeolus wind mission. - This intrepid team are taking measurements of the wind, aerosols and clouds with a range of instrumentation on different aircraft flying at different altitudes. Many flights are even planned to coincide with Aeolus as it orbits above. Measurements are also being taken by lasers and radars on the ground. This is all providing a wealth of data to compare with that from Aeolus and to support the science to tropical weather (image credit: ESA)
• February 4, 2021: As this winter's polar vortex currently sends extreme icy blasts of Arctic weather to some parts of the northern hemisphere such as the northeast of the US, scientists are using wind information from ESA's Aeolus satellite to shed more light on this complex phenomenon. 35)
Figure 16: The animation uses data from ESA's Aeolus wind satellite and shows how the polar vortex in the lower stratosphere changed between 1 December 2020 and 1 February 2021. The first few plots at the beginning of December show the vortex in a comparatively normal state, but in mid-December patches of blue wind appear, and the wind is going backwards relative to normal conditions. Scientists are using wind information from Aeolus to shed more light on this complex phenomenon that can disrupt the weather at lower latitudes (animation image credit: University of Bath/C. Wright)
- The polar vortex is a huge mass of frigid air high above the North Pole in the polar stratosphere. It is surrounded by a strong jet of air swirling counter-clockwise along the vortex's boundary. The vortex tends to be much stronger in the winter, keeping bitter cold air locked in around the Arctic.
- However, sometimes the vortex can weaken, become distorted or even split into two and meander further south, affecting the weather and jetstream further down in the troposphere, potentially bringing unusually cold weather and snow to lower latitudes.
- One meteorological event that can disturb the polar vortex is known as a ‘sudden stratospheric warming', which is what has been happening over the last couple of months. Sudden stratospheric warmings happen to some extent every year, but the current event has been categorized as major, and is less common.
- Such dramatic events cause the strong wind around the edge of the polar vortex to weaken or reverse, leading the temperature of the polar stratosphere to rise rapidly by tens of degrees Celsius.
- Since these events can trigger extreme weather in Europe and North America, they are of scientific and practical interest. However, the processes involved are not fully understood, and until recently there have been major technical challenges in measuring wind from space, which is needed to measure and monitor such a large-scale event.
Figure 17: Based on data from ESA's Aeolus wind mission, the image shows how the polar vortex in the lower stratosphere changed between 1 December 2020 and 1 February 2021. The first few plots at the beginning of December show the vortex in a comparatively normal state, but in mid-December patches of blue wind appear, and the wind is going backwards relative to normal conditions (image credit: University of Bath/C. Wright)
- Fortunately, scientists now have ESA's Aeolus satellite at hand to help understand more about why and how the polar vortex is pushed off balance.
- Aeolus is the first satellite in orbit to profile directly Earth's winds from space.
- It works by emitting short, powerful pulses of ultraviolet light from a laser and measures the Doppler shift from the very small amount of light that is scattered back to the instrument from molecules and particles to deliver profiles of the horizontal speed of the world's winds mostly in the east-west direction in the lowermost 26 km of the atmosphere.
- Although Aeolus only measures wind in the lower part of the atmosphere, the lower part of the current stratospheric polar vortex jet leaves a signature in the satellite's data.
- Corwin Wright, Royal Society research fellow at the University of Bath in the UK, said, "Changes in the wind structure in a sudden stratospheric warming event have never been observed directly at a global scale before. So far, our understanding of these changes has been developed using point measurements, measurements along localized aircraft flight tracks, through the use of temperature observations, and, primarily, computer models and assimilative analyses.
- Anne Grete Straume, ESA's Aeolus mission scientist, commented, "We are currently observing a polar vortex event where we see it split into two, with one spinning mass of air over the North Atlantic and one over the North Pacific.
- "The split leads to changes in the tropospheric circulation allowing cold air masses from the poles to more easily escape down to lower latitudes. At the moment, parts of North America seem to be experiencing colder weather than Europe, although we have seen events of cold air reaching quite far south in Europe over the past few weeks causing, for example, heavy snowfall in Spain.
- "What scientists would also like to understand is whether sudden stratospheric warming events might become more frequent owing to climate change. Also for this, Aeolus wind data will be very important to better understand the mechanisms triggering these weather events.
- "It is early days yet to draw any scientific conclusions from our Aeolus data, but work is certainly underway to shed new light on why this seasonal phenomenon can sometimes be extreme – watch this space."
Figure 18: Profiling the world's winds. The Aeolus mission was not only built to advance our understanding of atmospheric dynamics, but also to provide much-needed information to improve weather forecasts. The satellite carries the first wind lidar in space, which can probe the lowermost 30 km of the atmosphere to provide profiles of wind, aerosols and clouds along the satellite's orbital path. The laser system emits short powerful pulses of ultraviolet light down into the atmosphere. The telescope collects the light that is backscattered from air molecules, particles of dust and droplets of water. The receiver analyses the Doppler shift of the backscattered signal to determine the speed and direction of the wind at various altitudes below the satellite. These near-realtime observations will improve the accuracy of numerical weather and climate prediction and advance our understanding of atmospheric dynamics and processes relevant to climate variability (image credit: ESA/ATG medialab)
Figure 19: This image of snow in the Great Lakes region in the US was captured by the Copernicus Sentinel-3 mission's ocean and land color instrument on 3 February 2021. While there are reports of record-low ice cover on the lakes this year, there has, nevertheless, been heavy snowfall across the Midwest and Great Lakes over the last few days. Snow has also hit the northeast US. It is thought that this winter's polar vortex is currently sending extreme icy blasts of Arctic weather to some parts of the northern hemisphere. Scientists are using wind information from ESA's Aeolus satellite to shed more light on the complex polar vortex phenomenon (image credit: ESA, the image contains modified Copernicus Sentinel data (2021), processed by ESA, CC BY-SA 3.0 IGO)
• May 12, 2020: To increase the data downlink possibilities from ESA's Aeolus wind mission, a new receiving antenna in Antarctica has been built. This supplements the ground station in Svalbard and also helps guarantee data delivery in near realtime for weather forecasting. 36)
Figure 20: New Antarctic ground station for Aeolus increases data flow (image credit: Kongsberg Satellite Services)
• May 12, 2020: Delivering new information about Earth's winds, ESA's Aeolus mission has already been hailed a success. Today, this remarkable satellite mission has yet again achieved new heights: its data are now being distributed publicly to forecasting services and scientific users in less than three hours of measurements being made from space. 37)
- Aeolus is one of ESA's Earth Explorer missions, which all set out to demonstrate how new ways of observing Earth can advance our understanding of how the planet works as a system.
- Carrying one of the most sophisticated instruments ever to be put into orbit, Aeolus is the first satellite mission to directly profile Earth's winds from space.
- It works by emitting short, powerful pulses of ultraviolet light from a laser and measures the Doppler shift from the very small amount of light that is scattered back to the instrument from molecules and particles to deliver vertical profiles that show the horizontal speed of the world's winds in the lowermost 26 km of the atmosphere.
- ESA's Director of Earth Observation Programs, Josef Aschbacher, said, "Aeolus was never going to be an easy satellite mission to develop, and, indeed, it took some years to get it right before it could be launched. The wait was certainly worth it though, and in the 20 months that it has been in orbit, it has gone from strength to strength that will lead to benefits for science and society alike.
- "And, thanks to all the teams involved and in agreement with EUMETSAT, we are very proud to announce that as of today, Aeolus' data are being distributed in near-real time for numerical weather prediction beyond the Aeolus core user community."
- ESA's Peggy Fischer said, "A huge amount of work has gone into perfecting Aeolus' data before today's public release. This satellite technology is completely new so we have had to understand and correct certain biases in the data that were not known before launch.
- "To do this, key Aeolus experts from different organizations worked together in the Data Innovation and Science Cluster team – the Aeolus DISC, to validate and optimize the data processing and bias correction methods."
- ESA's Jonas von Bismarck, added "As the last and particularly tricky bit of the puzzle, a bias related to temperature variations across the instrument's telescope was corrected, making the data ready to be used in numerical weather prediction without the forecast centers having to carry out further complex corrections."
- The ECMWF (European Centre for Medium-Range Weather Forecasts) in the UK has already been including Aeolus data in their forecasts since January, relying on their own bias correction scheme.
Figure 21: Wind profile from Aeolus 6 May 2020. Carrying breakthrough laser technology, the Aeolus satellite – an ESA Earth Explorer mission – was launched in August 2018. It is the first satellite mission to profile Earth's winds directly from space. Its data are not only being used to understand how wind, pressure, temperature and humidity are interlinked to contribute to climate research, but are also now being used in near-realtime for weather forecasting. This image is an example of Level-2B Rayleigh wind velocity in m/s second over Europe on 6 May 2020 at 06:00 UTC (image credit: ESA/VirES)
Figure 22: Aeolus data flow. ESA's European Space Operations Centre, ESOC, in Germany operates the Aeolus satellite via the ground station in Kiruna, Sweden. The scientific data, however, are downlinked from the satellite to the ground station in Svalbard, Norway. The satellite completes an orbit around Earth every 90 minutes. Thanks to the ground station's northerly position, the satellite's polar orbit takes it within view of the ground station in the vast majority of its orbits so that data can be downlinked directly. Once the data have been received in Svalbard they are sent to Tromsø for processing. From Tromsø, the data are sent for further processing to the European Centre for Medium-Range Weather Forecasts in Reading, UK, and to ESA's center of Earth observation, ESRIN, in Frascati, Italy. ESRIN is responsible for making the data available to users (video credit: ESA/ATG medialab)
• April 21, 2020: We are all too aware that COVID-19 is a serious threat to health, is putting huge pressure on healthcare systems and it could leave the global economy struggling for years to come. With lockdown measures in force across the globe, the pandemic is also affecting aspects of everyday life that may not be so obvious. The drop in commercial flights, for example, has led to fewer measurements for weather forecasts, but fortunately, ESA's Aeolus satellite mission is helping to fill the gap. 38)
Figure 23: With lockdown measures in force across the globe, the COVID-19 pandemic is also affecting aspects of everyday life that may not be so obvious. The drop in commercial flights, for example, has led to fewer measurements for weather forecasts, but fortunately, ESA's Aeolus satellite mission is helping. The ECMWF (European Centre for Medium-Range Weather Forecasts) uses wind information from ESA's Aeolus satellite, and this is partly filling the gap caused by having fewer measurements from aircraft (image credit: Stocksnap/Pixabay)
- The COVID-19 pandemic is affecting countless industries across the globe. The travel industry is one of the hardest hit with an unprecedented decline in air traffic. Under normal circumstances, commercial aircraft equipped with sensors supply measurements of temperature, wind speed and wind direction in the atmosphere below 13 km. Without these measurements, the weather forecasts we take for granted everyday would be much less accurate.
- Florian Pappenberger from the European Centre for Medium-Range Weather Forecasts (ECMWF) in the UK, said, "Measurements from aircraft across Europe have dropped by 90%. We are still able to forecast the weather reliably several days ahead, but due to COVID-19 we may have temporarily lost as much skill as we gained in several years of scientific development."
- The weather is a product of chaotic processes and even very small changes in the atmosphere can lead to completely different weather conditions in the long term. This is why it is important to have the best understanding possible of the current state of the atmosphere before starting to calculate what the weather will be like days and weeks ahead.
Figure 24: Impact of aircraft and Aeolus data in ECMWF forecasts before and after the COVID-19-related reduction in air traffic. Forecast Sensitivity to Observation Influence (FSOI) measures how various observing systems influence the ECMWF numerical weather forecast quality. The figure shows the total impact [Mega J/kg] of aircraft data and Aeolus data for two weeks before and after the reduction in aircraft data owing to COVID-19. The impact of Aeolus has increased by 23% (image credit: ECMWF)
- ESA's Aeolus mission was built to demonstrate how new spaceborne technology could profile Earth's winds to understand how wind, pressure, temperature and humidity are interlinked – contributing to climate research and to forecasting the weather.
- It works by emitting short, powerful pulses of ultraviolet light from a laser and measures the Doppler shift from the very small amount of light that is scattered back to the instrument from these molecules and particles to deliver vertical profiles that show the horizontal speed of the world's winds in the lowermost 30 km of the atmosphere.
- Aeolus has not only proved successful as a technology demonstrator and of value to science, but has surpassed expectations – and now meteorologists are already using its data operationally to improve weather forecasts.
- Lars Isaksen from ECMWF, said, "Satellite data provide a lot of information on temperature and humidity fields, but less on wind fields. In January 2020, ECMWF started using wind information from the Aeolus satellite and we can now use these data to partly fill the gap caused by having fewer measurements from aircraft."
Figure 25: The data counts of aircraft weather data received at ECMWF from 3 March to 14 April 2020. The dramatic reduction relates to reduced air traffic due to COVID-19 (image credit: ECMWF)
- ESA's Jonas von Bismarck added, "The technology that Aeolus carries is exceptional and certainly proving its worth. We were all thrilled when ECMWF started using its data for weather forecasting, but we never expected a situation that's been brought about by COVID-19 – and we now see the mission playing an important role during this awful crisis."
- Dr Isaksen added, "While Aeolus is certainly helping to fill the gap, we are also releasing more radiosondes to help maintain the reliability of the weather forecasts during the crisis."
- Dr Isaksen added, "While Aeolus is certainly helping to fill the gap, we are also releasing more radiosondes to help maintain the reliability of the weather forecasts during the crisis."
• January 10, 2020: ESA's Aeolus satellite has been returning profiles of Earth's winds since 3 September 2018, just after it was launched – and after months of careful testing these measurements are considered so good that the ECMWF (European Centre for Medium-Range Weather Forecasts) is now using them in their forecasts. 39)
- The decision to include new measurements in weather forecasts is never taken lightly; it takes a lot of work to understand the data properly and ensure that they are of good quality.
- It is extremely unusual for a completely new type of satellite data to be ready for practical use in forecasts so soon after launch. Nevertheless, this extraordinary satellite has surpassed expectations and, as of today, Aeolus will be improving our forecasts, from one-day forecasts to those forecasting the weather more than a week ahead.
- Boasting a number of ‘firsts', Aeolus is the first satellite mission to provide profiles of Earth's wind in cloud-free air globally, carries the first instrument of its kind, and uses a novel approach to measuring the wind from space.
- Its novel Doppler wind lidar instrument, which comprises a powerful laser, a large telescope and a very sensitive receiver, emits short, powerful pulses of ultraviolet light down into the atmosphere and measures the shifts in wavelength of the laser light scattering off molecules and particles moving in the wind.
- Aeolus was designed to fill the lack of wind-profile measurements in the weather observation network and, therefore, to play a key role in increasing our understanding of the workings of the atmosphere, contribute to climate research and also improve weather forecasting.
- Before forecasters could assimilate Aeolus' data into weather forecasts, some serious testing and quality checks had to be done.
Figure 26: This extraordinary satellite has surpassed expectations and, as of 9 January 2020, Aeolus will be improving our forecasts, from one-day forecasts to those forecasting the weather more than a week ahead. These plots show how the assimilation of (bias corrected) Aeolus data reduces wind forecast errors (blue shading), particularly in the southern hemisphere and in the tropics, several days ahead. The 10 hPa pressure level corresponds to about 30 km altitude. Cross-hatching indicates statistical significance at the 95% level. The experiment covers the period from 2 August to 28 December 2019 (image credit: ECMWF). 40)
- ESA's Aeolus mission manager, Tommaso Parrinello, said, "During the first year of Aeolus' life in orbit, ESA and the Aeolus Data Innovation Science Cluster team worked hard to characterize and calibrate this ground-breaking satellite instrument and understand exactly how it was working in space.
- "They were helped by scientists across the world who compared wind measurements taken from the ground and from aircraft with those from Aeolus.
- "While we did find that we had to switch to the instrument's second laser transmitter to boost power, the mission is proving to be an excellent way of measuring the wind – so much so that we now see data being assimilated into forecasts, which we are absolutely thrilled about."
- Michael Rennie from the ECMWF explains, "We had to assess the impact that Aeolus would have on the weather forecasts before deciding to ingest them operationally – and this involved checking the data quality with the forecast and other observations, and running a host of experiments to see if Aeolus consistently improves the forecasts, and by how much.
- "Our experiments showed that, indeed, Aeolus had a positive impact, and this makes a big difference, particularly over parts of the world where there is a lack of other wind observations.
- "The biggest improvement is in tropical regions and in the southern hemisphere. We also see that measurements from Aeolus are among the most important instruments in space for forecast quality, which is hugely impressive considering that Aeolus actually gives us less than 1% of the measurements we use in daily forecasts."
- With the operational assimilation of Aeolus data at ECMWF, a major milestone for this novel mission has been reached. Other operational weather centers across the world are also seeing positive impact of Aeolus observations and plan to start assimilating data during the course of this year.
- This mission milestone also paves the way for a possible future fleet of operational Doppler wind lidar satellites in space.
Figure 27: Weather room at ECMWF. As of 9 January 2020, Aeolus will be improving our forecasts, from one-day forecasts to those forecasting the weather more than a week ahead (image credit: ECMWF)
• November 12, 2019: Tests carried out show that new wind profile observations from ESA's Aeolus satellite significantly improve weather forecasts – particularly in the southern hemisphere and the tropics. 41)
- Carrying breakthrough laser technology, the Aeolus satellite – an ESA Earth Explorer mission – was launched in August 2018. It is the first satellite mission to provide profiles of Earth's winds globally.
- Unexpectedly, Aeolus observations turn out to have small ‘biases' in their data. As is normal for any satellite mission, successfully correcting these biases is an important part of optimizing the use of the satellite's observations.
- Over the past year, scientists at the European Centre for Medium-Range Weather Forecasts (ECMWF), in close collaboration with ESA, the German Aerospace Center (DLR), the software company DoRIT, the Royal Netherlands Meteorological Institute (KNMI) and Météo-France have been making big strides in understanding these inconsistencies.
- Tests carried out at ECMWF show that when Aeolus data are combined with short-range forecast information in a process called data assimilation, the short-range forecasts used are improved.
Figure 28: Aeolus wind data for Hurricane Dorian. Aeolus horizontal-line-of-sight wind observations measured in the direction of the laser beam and projected onto the horizontal plane, on 1 September 2019 between about 6ºN and 42ºN. Aeolus was launched in 2018 to test the usefulness of direct wind profile observations from space for numerical weather prediction. It works by measuring the backscatter of laser light from air molecules (‘Rayleigh-clear' data) and from clouds and aerosols (‘Mie-cloudy' data), image credit: ESA/ECMWF
- The data have been found to be significantly closer to other wind, temperature and humidity observations than when Aeolus data are not assimilated – especially in the southern hemisphere and the tropics which are less covered by conventional observations in the northern hemisphere.
- Tommaso Parrinello, Aeolus Mission Manager at ESA, comments, "I am impressed with the achievements of the ESA-funded Aeolus team of engineers. With Aeolus's first functioning Doppler wind lidar in space, complex biases can appear but I am extremely pleased that the team has found a physically based correction to solve them.
- "As early as 15 months after launch, ECMWF and several other numerical weather prediction centers have shown large improvements in weather forecasts when Aeolus data is assimilated in test experiments. This is a success story thanks to the close collaboration between ESA, ECMWF, other weather prediction centers and all scientists involved."
- ECMWF, in collaboration with other scientists, has shown that Aeolus biases are closely correlated with small variations in the temperature distribution across the large mirror used in the Aeolus instrument's telescope.
- ECMWF's Mike Rennie adds, "We have been able to identify and correct some of these biases successfully. This finding will enable us to refine our bias correction, since those temperatures are measured in space and available in real time."
- "Aeolus engineers and scientists are now investigating why such temperature differences cause wind biases and if the mirror temperatures can be controlled better."
- ECMWF will continue to work closely with ESA on ways to minimize such biases in Aeolus data, which can be applied to future follow-on missions.
Figure 29: These plots show how the assimilation of Aeolus data reduces wind forecast errors (blue shading) in large parts of the southern hemisphere and the tropics throughout the troposphere and beyond (10 hPa corresponds to about 30 km altitude). In the northern hemisphere, forecasts improve mainly in the polar region. Cross-hatching indicates statistical significance at the 95% level. The experiment covers the period from 2 August to 18 October 2019 (image credit: ESA/ECMWF)
• October 2019: Aeolus hosts ALADIN, the first spaceborne DWL (Doppler Wind Lidar) world-wide. The satellite is providing consistent and positive results and it is expected that first public data will be released in Q1 2020. 42)
- The Aeolus primary mission objective is to demonstrate the DWL technique for measuring wind profiles from space, intended for operational assimilation in Numerical Weather Prediction (NWP) models. The wind observations will also be used to advance atmospheric dynamics research, process studies and for evaluation of climate models.
- The wind observations will also be used to advance atmospheric dynamics research, process studies and for evaluation of climate models. Mission spin-off products are profiles of cloud and aerosol optical properties. The Aeolus mission selection was motivated by the need for more abundant direct wind profile measurements in the World Meteorological Organization (WMO) Global Observing System (GOS). Aeolus winds will hence contribute to mitigate the current wind observation deficit. Meteorological Centers world-wide are currently preparing to ingest Aeolus winds near-real-time in their operational weather models, as soon as the data is of sufficiently good quality. This is expected towards the end of 2019.
- The main product from Aeolus is the HLOS (Horizontally projected Line-Of-Sight) wind profile observations (approximately zonally oriented) from the surface up to 25-30 km altitude. The atmospheric backscattered signal for the individual laser pulses are averaged on-board to yield ~3 km measurements along-track. These measurements are further averaged on-ground to observations, representing horizontal scales up to ~88 km. The vertical resolution of the winds varies from 0.25 to 2 km, and is optimized along the orbit according to the climatological region. The HLOS wind observation random error (precision) requirement is 1 m/s in the PBL (Planetary Boundary Layer), 2.5 m/s in the free troposphere and 3-5 m/s in the stratosphere. The bias (systematic error) requirement is 0.7 m/s.
- The Aeolus Level 2A product contains profiles of particle and molecular parallel-polarized backscatter and extinction coefficients, scattering ratios and backscatter-to-extinction ratios. From these parameters it is possible to derive particle layer height, multi-layer cloud/aerosol stratification, cloud/aerosol optical depths and some information on cloud/aerosol type. Other products will be developed during the mission operational phase.
- The Aeolus data has been available to its CAL/VAL teams (including NWP centers) world-wide since December 2018, and will be publicly released from the ESA Aeolus Data Dissemination Facility and via EUMETCAST as soon as the initial product CAL/VAL has been concluded. The first pubic data release (wind product) is expected in Q1 2020.
Aeolus satellite in orbit experience/status
- After initial acquisition of the correct orbit, the In-Situ Cleaning System (ICS) which provides a low pressure of oxygen for the high power laser emission path of the instrument was initiated. The oxygen provided by the ICS is needed in order to prevent laser-induced contamination from occurring on the laser optics.
- After this was successfully achieved, the laser was switched on in discrete, increasing energy steps, with the LBM (Laser Beam Monitoring) mode of the instrument applied in order to ensure that the laser fluence was within the margins necessary to avoid laser-induced damage to the instrument. The laser was set to its full energy setting on the 3rd of September. The initial UV energy was 65mJ (lower than the 80mJ for the same set-point achieved in ground tests).
- The next stages were to perform the adjustment of the ALADIN telescope focus on the reception path of the instrument and then to calibrate the ALADIN spectrometers. ALADIN has two sequential spectrometers which are designed to measure the Doppler shift from the backscattered signal return due to the wind. The Mie spectrometer, used to measure the backscatter returns from particles and aerosols, is based upon a Fizeau spectrometer, which images a fringe whose position on the CCD is dependent on the frequency of the returned signal. The Rayleigh spectrometer, is based upon two Fabry-Perot etalons with slightly different path lengths which act as two filters slightly displaced in frequency space. The difference in the signals transmitted by the two filters gives the frequency shift of the backscattered signal returns, broadened by Brownian motion, from the molecules in the atmosphere. These adjustments and calibrations were all successfully executed and placed the ALADIN instrument in a position to deliver the first wind measurements through the Earth's atmosphere from space. The laser UV energy for the first year of the mission is shown in Figure 30.
As can be observed from the figure, the first laser transmitter (FM-A) operated for a duration of around 9 months, accumulating just over 1 billion shots. There was a monotonic decrease in the laser energy which resulted in several energy adjustments being made. Investigations showed that the energy decrease was due to a misalignment of the master oscillator leading to a decrease in the energy supplied to the amplifiers. Furthermore, at the beginning of this year, there was an reboot anomaly on the GPS unit on the satellite which led to the ALADIN instrument being switched off for around 1 month.
Figure 30: Laser transmitter UV energy over the first year of the Aeolus mission (image credit: ESA)
- In June of this year, it was decided to switch to the second flight laser (FM-B) which is not showing the same energy decrease as the first, and is currently stabilizing to a level around 60mJ which is adjudged sufficient by the science teams.
- Quite early in the mission, it was noticed that there was a significant bias introduced into the final wind product data which was related to specific layers within the 24 layers that Aeolus measures through the atmosphere. It was also noticed that the number of layers that were impacted was growing. Investigations led to the discovery that there were "hot" pixels i.e. pixels with an elevated signal level, appearing on the accumulation CCDs for both the Mie and Rayleigh spectrometers. Although the investigations are continuing in to the root cause of these, an in-orbit fix was found whereby pseudo dark current measurements are made regularly by setting the altitude bins below ground level.
- The impact of performing this pseudo dark current calibration can be clearly seen in Figure 31. The bias introduced by the "hot" pixels can be seen as streaks in discrete altitude bins on the left hand side of the figure. The correction was introduced in mid June (shown by the green line in the figure). The streaks have been completely eliminated by the introduction of the in-orbit correction.
Figure 31: Wind data from Aeolus showing the impacts of the "hot" pixels before and after the introduction of a pseudo dark current calibration (image credit: ESA)
- In general terms, apart from the reboot anomaly with the GPS unit, which has also occurred on other satellites that use the similar units, the spacecraft has performed very well. There have been a small number of reconfigurations of the star-tracker and on 2 September, there was an avoidance maneuver which was successfully undertaken by Aeolus in order to avoid the Starlink 44 satellite. Apart from these, there are no major issues to report with any of the platform subsystems and units to date and there is sufficient fuel and oxygen to complete the mission lifetime of three years.
- Aeolus first results: The initial assessment of the Aeolus primary product, the L2B wind profile observations, has been done by the partners of the Aeolus Data Innovation and Science Cluster (DISC). ECMWF, KNMI and MétéoFrance have developed the Aeolus L2B processor and processing facility, which includes product quality monitoring using the ECMWF weather model. ECMWF is running the operational L2B product facility as part of the Aeolus ground segment. The Aeolus processing facility worked extremely well from the start of the mission, allowing for good quality L2B winds being available from the Payload Data Ground Segment already 2 days after the laser switch-on. The Aeolus Rayleigh and Mie wind observations for 12 September 2019 are shown in Figure 32.
Figure 32: Aeolus molecular (Rayleigh, upper panel) and particle (Mie, lower panel) backscatter winds above the Earth geoid (vertical axis) along the orbit from Antarctica (left) to the North Atlantic (right) on 12 September 2019. Aeolus measures in clear air, in and below optically thin clouds, and down to and on top of optically thick clouds. Areas below thick clouds are shown in white. Blue colors indicate Westerly winds and red colors Easterly winds in m/s. The lower part of the stratospheric jet around Antarctica can be seen, and is connected with the tropospheric polar jet and the subtropical jet in the Southern Hemisphere. On the Northern hemisphere, the subtropical jet and polar jet stream can be seen in the troposphere. The tropical Easterly winds are also well visible (image credit: ESA)
- Results from the first data quality assessment done by ECMWF as part of the DISC team and Aeolus CAL/VAL teams, comparing Aeolus winds with NWP models, ground-based and airborne observations world-wide, are very consistent and positive. They show that the Aeolus wind random errors are compliant to the mission requirements in the free troposphere for the Mie channel for laser output energies above about 65 mJ, and slightly above for the Rayleigh channel. However, positive NWP impact has shown to be larger for the Rayleigh winds in most cases due to the uniqueness of the data and their large vertical coverage. The assessment has also shown that the bias requirements can be expected to be met after further optimization of the instrument calibration and data processing.
- First NWP impact experiments by leading meteorological centers world-wide show positive impact of the Aeolus observations particularly in the tropical troposphere and southern hemisphere where direct wind observations are sparse. The impact is comparable to the impact from other satellite-based observations which have been assimilated for many years and have much larger data volumes. This is very impressive, considering that the Aeolus observations contribute with less than 1% of the total number of observations used by forecast models. This demonstrates the great potential of the Doppler Wind Lidar technology for operational meteorological missions. Further results from the Aeolus CAL/VAL teams and NWP centers assimilating Aeolus data will be shown at the next Aeolus workshop in March 2020.
Aeolus lessons learned
- As part of the exercise for preparing for future lidar missions, an extensive lessons learned activity has been conducted on the Aeolus satellite and the ALADIN instrument in particular. The main lessons learned are summarized below:
a) The conductance of thermal interfaces of highly dissipative units can change in-orbit. Any mechanical distortion arising from this change should be decoupled from alignment sensitive items.
b) Low pressure reduces the laser damage threshold of optics. Use non-porous optical coatings. Ensure that you have a margin of at least x2 for the laser induced damage threshold.
c) Laser induced contamination results when there is a non-oxidizing environment. Low pressures of oxygen are successful in avoiding these highly absorbing deposits.
d) Be careful when performing tests in sub pupil on large telescopes on instruments with a restricted field of view as this can add uncertainties in the radiometric budgets of the instrument.
e) For future missions, improve the accuracy and acquisition rates for telemetries which are key to understanding and controlling the performance of high power lasers.
Table 3: The safety situation in space with an ever increasing LEO environment 43)
- Data is constantly being issued by the 18th Space Control Squadron of the US Air Force, who monitor objects orbiting in Earth's skies, providing information to operators about any potential close approach.
- With this data, ESA and others are able to calculate the probability of collision between their spacecraft and all other artificial objects in orbit.
- About a week ago, the US data suggested a potential ‘conjunction' at 11:02 UTC on Monday, 2 September, between ESA's Aeolus satellite and Starlink44 – one of the first 60 satellites recently launched in SpaceX's mega constellation, planned to be a 12 000 strong fleet by mid-2020.
- Experts in ESA's Space Debris Office worked to calculate the collision probability, combining information on the expected miss distance, conjunction geometry and uncertainties in orbit information.
Figure 33: Predicted conjunction between Aeolus and Starlink 44 (image credit: ESA)
- As days passed, the probability of collision continued to increase, and by Wednesday 28 August the team decided to reach out to Starlink to discuss their options. Within a day, the Starlink team informed ESA that they had no plan to take action at this point.
- ESA's threshold for conducting an avoidance maneuver is a collision probability of more than 1 in 10 000, which was reached for the first time on Thursday evening (29 August).
- An avoidance maneuver was prepared which would increase Aeolus' altitude by 350 m, ensuring it would comfortably pass over the other satellite, and the team continued to monitor the situation.
- On Sunday (1 September), as the probability continued to increase, the final decision was made to implement the maneuver, and the commands were sent to the spacecraft from ESA's mission control center in Darmstadt, Germany.
- At this moment, chances of collision were around 1 in 1000, 10 times higher than the threshold.
- On Monday morning (2 September), the commands triggered a series of thruster burns at 10:14, 10:17 and 10:18 UTC, half an orbit before the potential collision.
- About half an hour after the conjunction was predicted, Aeolus contacted home as expected. This was the first reassurance that the maneuver was correctly executed and the satellite was OK.
- Since then, teams on the ground have continued to receive scientific data from the spacecraft, meaning operations are back to normal science-gathering mode.
- Contact with Starlink early in the process allowed ESA to take conflict-free action later, knowing the second spacecraft would remain where models expected it to be.
- Since the first satellite launch in 1957, more than 5500 launches have lifted over 9000 satellites into space. Of these, only about 2000 are currently functioning, which explains why 90% of ESA's avoidance maneuvers are the result of derelict and uncontrollable ‘space debris'.
- In the years to come, constellations of thousands of satellites are set to change the space environment, vastly increasing the number of active, operational spacecraft in orbit.
- This new technology brings enormous benefits to people on Earth, including global internet access and precise location services, but constellations also bring with them challenges in creating a safe and sustainable space environment.
- "No one was at fault here, but this example does show the urgent need for proper space traffic management, with clear communication protocols and more automation," explains Holger.
- "This is how air traffic control has worked for many decades, and now space operators need to get together to define automated maneuver coordination."
- As the number of satellites in orbit rapidly increases, today's 'manual' collision avoidance process will become impossible, and automated systems are becoming necessary to protect our space infrastructure.
- Collision avoidance maneuvers take a lot of time to prepare – from determining the future orbital positions of functioning spacecraft, to calculating the risk of collision and the many possible outcomes of different actions.
- ESA is preparing to automate this process using artificial intelligence, speeding up the processes of data crunching and risk analysis, from the initial warning of a potential conjunction to the satellite finally moving out of the way.
- Such use of space-based communication links can save precious time when sending maneuver commands at the last minute.
- Under its Space Safety activities, ESA plans to invest in technologies required to automatically process collision warnings, coordinate maneuvers with other operators and send the commands to spacecraft entirely automatically, ensuring the benefits of space can continue to be enjoyed for generations to come.
• July 23, 2019: ESA's Aeolus satellite, which carries the world's first space Doppler wind lidar, has been delivering high-quality global measurements of Earth's wind since it was launched almost a year ago. However, part of the instrument, the laser transmitter, has been slowly losing energy. As a result, ESA decided to switch over to the instrument's second laser – and the mission is now back on top form. 44)
Figure 34: Shortly after switching over from the first to the second laser, Aeolus is delivering high-quality measurements of Earth's wind. Currently, instrument and data processing refinements are ongoing, which will enhance the data product quality even more in the coming weeks. The figure shows measurements by Aeolus while crossing the African continent between Turkey (on the right) and the Southern Ocean (left). Aeolus measures winds from the surface up to about 25 km altitude. Strong easterly winds are visible around the tropopause at 15 km altitude over north Africa (green, yellow and orange), and the strong westerly winds (blue and purple colors) in the upper troposphere and lower stratosphere as the satellite moves into the area of the ‘roaring forties' over the Southern Ocean. Thick clouds block the laser signal and hence prevent measurements to be taken within or below the clouds (white areas between 0 and 10 km altitude), image credit: ESA
- Developing novel space technology is always a challenge, and despite the multitude of tests that are done in the development and build phases, engineers can never be absolutely certain that it will work in the environment of space.
- Aeolus is, without doubt, a pioneering satellite mission – it carries the first instrument of its kind and uses a completely new approach to measuring wind from space.
- The instrument, called Aladin, not only comprises the laser transmitters, but also one of the largest telescopes ESA has put into orbit and very sensitive receivers that measure the minute shifts in wavelength of light generated by the movement of molecules and particles in the atmosphere caused by the wind.
Figure 35: The state-of-the-art Aladin instrument incorporates two powerful lasers, a large telescope and very sensitive receivers. The laser generates ultraviolet light that is beamed towards Earth. This light bounces off air molecules and small particles such as dust, ice and droplets of water in the atmosphere. The fraction of light that is scattered back towards the satellite is collected by Aladin's telescope and measured (image credit: ESA)
- Aladin, works by emitting short, powerful pulses of ultraviolet light from a laser and measures the Doppler shift from the very small amount of light that is scattered back to the instrument from these molecules and particles to deliver vertical profiles that show the speed of the world's winds in the lowermost 30 km of the atmosphere.
- While scientists and meteorology centers have been thrilled with the data produced by Aeolus, the first laser's energy was becoming a concern – and in June, energy levels dipped to the point that the quality of the wind data was set to be compromised.
- Tommaso Parrinello, ESA's Aeolus mission manager, said, "With the power from the first laser declining, we decided to turn it off and activate the second laser, which the instrument was equipped with to ensure we could address an issue such as this.
- "Switching to the second laser appears to have done the trick so we're back in business. And, we are confident that the instrument will remain in good shape for years to come."
Figure 36: This photo, which was taken in the cleanroom when Aeolus was being built, shows the instrument's two lasers. They are the two large square plate-like items in the middle. Aeolus carries the world's first space Doppler wind lidar. It works by emitting short, powerful pulses of ultraviolet light from a laser and measures the Doppler shift from the very small amount of light that is scattered back to the instrument from molecules and particles in the atmosphere to deliver vertical profiles that show the speed of the world's winds in the lowermost 30 km of the atmosphere (image credit: Airbus Defence and Space)
- Denny Wernham, ESA's Aeolus instrument manager, added, "The great news is that the second laser's energy is, so far, very stable, which is what we expected since this laser is actually better than the first. This is because we have more scope to adjust it in orbit to retain the performance needed.
- "I would like to stress that despite the first laser's drop in energy, it worked for nearly a year and provided a vital dataset for our stakeholders. It accumulated nearly one billion shots, which is a record for a high-power ultraviolet laser in space, and we can always go back to it if we need to later in the mission."
- The ECMWF (European Center for Medium Range Weather Forecasting) is also enthusiastic about the data now being delivered.
- Michael Rennie at ECMWF, said, "We were very happy to see the wind data after the switch, and given the fact that when Aeolus was using its first laser we could see that it can improve our weather forecasts off-line, we are expecting even better results with the new setup.
- "Towards the end of the year, we hope that we will be feeding data from Aeolus into our forecasts in real time."
- Anne Grete Straume, ESA's Aeolus mission scientist, added, "It is extremely good news for the mission and forecasters alike.
- "We are very much looking forward to seeing several weather-forecast impact assessments by European, American and Asian meteorological centers at a meeting with our community in September 2019.
- "These assessments compare the impact of Aeolus with the impact of measurements by other weather satellites and observations in the World Meteorological Organization Global Observing System.
- "Towards the end of 2019, further scientific studies will also start using Aeolus wind observations to learn more about the role of winds in the atmosphere–land–ocean system and how small and large-scale winds will alter as our climate changes."
• April 5, 2019: Assessing the accuracy of data being returned by completely new technology in space is a challenging task. But this is exactly what engineers and scientists have been dedicating their time to over the last months so that measurements of the world's winds being gathered by Aeolus can be fed confidently into weather forecast models. 45)
- Carrying breakthrough laser technology, the Aeolus satellite – an ESA Earth Explorer mission – was launched in August 2018. Its novel Aladin instrument, which comprises a powerful laser, a large telescope and a very sensitive receiver, measures the wind by emitting short, powerful pulses of ultraviolet light down into the atmosphere.
- It is the first satellite mission to provide profiles of Earth's wind globally. Its near-realtime observations will soon be made available to weather forecasters around the world. These observations are set to improve the accuracy of weather forecasts as well as advance our understanding of atmospheric dynamics and processes linked to climate variability.
- Before ESA can declare that the data good enough to be included in forecasts, the data have to be carefully calibrated and validated. Part of this process has involved gathering measurements of wind, aerosols and clouds from the ground, aircraft and from other satellites to compare them with measurements being delivered by Aeolus.
- Also, in preparation for ingesting the data into their forecasts, a number of weather forecasting centers around the world have started to compare the Aeolus winds with their models.
- So, after several months of calibration and validation exercises, around 100 scientists and engineers from universities, research institutes and weather centers in Europe, the US, Canada, Japan and China gathered recently at ESA's center of Earth observation in Frascati, Italy to review the latest results from the Aeolus data investigations.
Figure 37: The image shows winds measured by Aeolus over western Europe on 10 March 2019. Red indicates wind blowing from east to west (easterlies) and blue indicates wind blowing from west to east (westerlies). The strong westerly wind in the jet stream, with speeds of more than 200 km/hr, is clearly visible at the altitude of around 10 km. On this day, very strong winds extended from the jet stream all the way down to the surface and caused problems for traffic and construction, for example. Black areas indicate where the satellite could not measure winds owing to thick cloud layers (image credit: ECMWF–M. Rennie)
The European Center for Medium-range Weather Forecast (ECMWF) and the German Weather Service (DWD) preliminary analyses showed that Aeolus winds are improving forecasts, particularly in the troposphere, which is the part of the atmosphere between the ground and about 16 km high.
Lars Isaksen, principal scientist at ECMWF, said, "Aeolus' Aladin is the only instrument that provides wind profiles from space. Wind profiles, especially over remote areas, are very important for numerical weather prediction. ECMWF is heavily involved in processing, calibrating and validating the Aeolus wind data, and in just seven months after the satellite was launched, we and other weather centers have carried out numerous impact studies. These results are very promising and indicate that Aeolus winds will improve weather forecasts and help us better understand global wind circulation."
Examples of results presented at the workshop included the storm that hit the UK and parts of Europe on 10 March and Cyclone Idai that devastated Mozambique, Malawi and Zimbabwe.
Figure 38: Wind measured by the Aeolus satellite while crossing the Cyclone Idai west of Madagascar on 11 March 2019. Red indicates wind blowing from east to west (easterlies) and blue indicates wind blowing from west to east (westerlies). Since Aeolus measures wind in the cloud-free atmosphere, and within thin clouds and on top of thick clouds, the measurements here are those surrounding Idai. The black patch is the part of the cyclone, which was covered by a thick cover of spiral-shaped clouds. The image shows strong easterly winds north of the hurricane (in red on the left of the image), with wind speeds up to 150 km/hr (above 40 m/s). In the upper right corner (altitude of 22–25 km), the tropical stratospheric easterly jet can be seen in red, and lower down on the right (altitude of 10–16 km) the sub-tropical westerly jet in the southern hemisphere is visible in blue (image credit: ECMWF–M. Rennie)
Figure 39: Cyclone Idai west of Madagascar. Captured by the Copernicus Sentinel-3 mission, this image shows Cyclone Idai on 13 March 2019 west of Madagascar and heading for Mozambique. Here, the width of the storm is around 800–1000 km, but does not include the whole extent of Idai. The storm went on to cause widespread destruction in Mozambique, Malawi and Zimbabwe. With thousands of people losing their lives, and houses, roads and croplands submerged, the International Charter Space and Major Disasters and the Copernicus Emergency Mapping Service were triggered to supply maps of flooded areas based on satellite data to help emergency response efforts (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
- The value of having different satellite instruments observing the same weather event is important for gathering as much information as possible to improve the accuracy of weather forecasts and so that people affected by severe weather can take necessary action.
- Tommaso Parrinello, ESA's Aeolus Mission Manager, said, "We are really happy with the data Aeolus is returning. We also see how the mission can add complementary information to satellites carrying optical instruments such as the Copernicus Sentinel-3 and the satellites carrying radar such as the Copernicus Sentinel-1. While comparisons with ground-based instrumentation and weather models are currently ongoing to refine the calibration and data processing, we expect that the quality of the Aeolus data will be high enough around the end of this year – after which the data will be ready for scientific research and for weather forecasting."
• February 11, 2019: Since launch, engineers and scientists have been carefully checking the information that this pioneering mission is delivering on the world's winds – and now it's time for the next phase. Although our daily weather forecasts are pretty reliable, they still need to be improved further and to do this meteorologists urgently need direct measurements of the wind. 46)
- However, this is no easy task as extraordinary technology is needed to measure the wind from space.
- Nevertheless, ESA's Aeolus satellite has been designed to do just this. It carries the first instrument of its kind and uses a completely new approach to measuring wind.
- Since this is such novel and challenging technology, scientists and engineers have had their work cut out assessing how the satellite is functioning in orbit and checking the quality of the data it is returning.
- For example, they have been comparing this new data with modelled data at the ECMWF (European Center for Medium-Range Weather Forecasts) and have already established improvements to the forecast model thanks to the additional data from Aeolus.
- This will have a positive impact on weather forecast accuracy in general.
- ESA's Aeolus project manager, Anders Elfving, said, "This satellite mission is certainly a challenging one, but I'm very happy to say that we are now formally out of the commissioning phase, which encompasses the first four months of a mission's life in orbit when we do all the checks and tweaks."
- "We still have some work to do to make sure Aeolus delivers on its promise as we have to improve on the way the data is processed taking into account the peculiarities of its instrument. And, we must remember that this is a completely new type of mission, so we are learning all the time. We also have field campaigns going on all over the world to help with the process of calibration and validation. This means measurements of the wind are being taken from the ground, from balloons and from aircraft to compare with measurements we are getting from space.- At this stage, the results are expected to be announced in March."
- One recent field campaign has been carried out in Germany by DLR (German Aerospace Center). This involved flying an aircraft directly under Aeolus' orbital path and taking more or less simultaneous measurements with an airborne version of the satellite instrument.
Figure 40: Comparing wind measurements: As part of the working being done to calibrate and validate measurements from ESA's Aeolus wind satellite, scientists have been taking similar measurements from an aircraft carrying an airborne version of the satellite instrument. instrument. The pilot flies the plane under the satellite as it orbits above so that measurements of wind can be compared (image credit: ESA/DLR)
• February 7, 2019: Following the launch of Aeolus on 22 August 2018, scientists have been busy fine-tuning and calibrating this latest Earth Explorer satellite. Aeolus carries a revolutionary instrument, which comprises a powerful laser, a large telescope and a very sensitive receiver. It works by emitting short, powerful pulses –50 pulses per second –of ultraviolet light from a laser down into the atmosphere. The instrument then measures the backscattered signals from air molecules, dust particles and water droplets to provide vertical profiles that show the speed of the world's winds in the lowermost 30 km of the atmosphere. These measurements are needed to improve weather forecasts. As part of the working being done to calibrate this novel mission, scientists have been taking similar measurements from an aircraft carrying an airborne version of Aeolus' instrument. The pilot flies the plane under the satellite as it orbits above so that measurements of wind can be compared. 47)
Figure 41: Flying under Aeolus (video credit: ESA)
• September 12, 2018: Just one week after ESA's Aeolus satellite shone a light on our atmosphere and returned a taster of what's in store, this ground-breaking mission has again exceeded all expectations by delivering its first data on wind – a truly remarkable feat so early in its life in space. 48)
- Florence Rabier, Director General of the ECMWF (European Centre for Medium-Range Weather Forecasts), said, "We always knew that Aeolus would be an exceptional mission, but these first results have really impressed us. The satellite hasn't even been in orbit a month yet, but the results so far look extremely promising, far better than anyone expected at this early stage. We are very proud to be part of the mission. Aeolus looks set to provide some of the most substantial improvements to our weather forecasts that we've seen over the past decade."
- ESA's Aeolus mission scientist, Anne Grete Straume, explained, "These first wind data shown in the plot made by ECMWF are from one orbit. In the profile we can see large-scale easterly and westerly winds between Earth's surface and the lower stratosphere, including jet streams. In particular, you can see strong winds, called the Stratospheric Polar Vortex, around the South Pole. These winds play an important role in the depletion of the ozone layer over the South Pole at this time of the year."
- Named after Aeolus, who in Greek mythology was appointed ‘keeper of the winds' by the Gods, this novel mission is the fifth in the family of ESA's Earth Explorers, which address the most urgent Earth-science questions of our time.
- It carries the first instrument of its kind and uses a completely new approach to measuring the wind from space.
- ESA's Earth Explorer Program manager, Danilo Muzi, said, "Aeolus carries revolutionary laser technology to address one of the major deficits in the Global Observing System: the lack of direct global wind measurements. The essence of an Earth Explorer mission is to deliver data that advances our understanding of our home planet and that demonstrates cutting-edge space technology. With the first light measurements and now these amazing wind data, Aeolus has wowed us on both fronts."
Figure 42: First wind data from ESA's Aeolus satellite. These data are from three quarters of one orbit around Earth. The image shows large-scale easterly and westerly winds between Earth's surface and the lower stratosphere, including jet streams. As the satellite orbits from the Arctic towards the Antarctic, it senses, for example, strong westerly winds streams, called tropospheric vortices (shown in blue) each side of the equator at mid latitudes. Orbiting further towards the Antarctic, Aeolus senses the strong westerly winds (shown in blue left of Antarctica and in red right of Antarctica) circling the Antarctic continent in the troposphere and stratosphere (Stratospheric Polar Vortex). The overall direction of the wind is the same along the polar vortex, but because the Aeolus wind product is related to the viewing direction of the satellite, the color changes from blue to red as the satellite passes the Antarctic continent (image credit: ESA/ECMWF)
Figure 43: Ozone hole over Antarctica on 4 September 2018. Strong winds, called the Stratospheric Polar Vortex, around the South Pole play an important role in the depletion the ozone at this time of the year. Low ozone is shown in blue and high in pink (image credit: KNMI–Temis, released on 12 September 2018)
• September 5, 2018: The ALADIN instrument on Aeolus has been turned on and is now emitting pulses of ultraviolet light from its laser, which is fundamental to measuring Earth's wind. And, this remarkable mission has also already returned a tantalizing glimpse of the data it will provide. 49)
- Aeolus carries a revolutionary instrument, which comprises a powerful laser, a large telescope and a very sensitive receiver. It works by emitting short, powerful pulses – 50 pulses per second – of ultraviolet light from a laser down into the atmosphere. The instrument then measures the backscattered signals from air molecules, dust particles and water droplets to provide vertical profiles that show the speed of the world's winds in the lowermost 30 km of the atmosphere.
- The mission is now being commissioned for service – a phase that lasts about three months. One of the first things on the ‘to do' list was arguably the one of the most important: turn on the instrument and check that the laser works.
- ESA's Director of Earth Observation Programs, Josef Aschbacher, explained, "Aeolus is a world premiere. After the launch two weeks ago the whole community has been anxiously awaiting the switch-on of the ultra-violet laser, which is a real technological marvel. This has been successful. We have pioneered new technology for one of the largest data gaps in meteorology – global wind profiles in cloud-free atmosphere. I am grateful to all who have made this success possible."
- ESA's Aeolus project manager, Anders Elfving, added, "Aeolus has been one of the most challenging missions on ESA's books. And, unsurprisingly, we have had to overcome a number of technical challenges. After many years in development, we had absolute confidence that it would work in space, but it was still somewhat nerve-racking when we turned on the instrument a few days ago. But the years of work certainly appear to have paid off. After turning it on, we started slowly and steadily increasing the power. It is now emitting at high power – and we couldn't be happier."
- Richard Wimmer from Airbus Defence and Space noted, "It is a very exciting time to have Aeolus safely in orbit and doing what we and our industrial teams spent years building it to do."
- Michael Rennie from the ECMWF (European Centre for Medium-Range Weather Forecasts), added, "At this very early stage in the mission – just three days after the instrument was switched on – Aeolus has already exceeded expectations by delivering data that show clear features of the wind."
- With Aeolus instrument healthy and performing well, engineers will continue ticking off other items on the ‘commissioning to do list' so that in a few months Aeolus will be ready to deliver essential information to improve our knowledge of atmospheric dynamics, further climate research and improve weather forecasts.
Figure 44: First light from Aeolus. Following the launch of Aeolus on 22 August, this extraordinary satellite is not only emitting pulses of ultraviolet light from its laser, but has also measured light backscattered from air molecules and cloud tops. The measurements show a full orbit around Earth, from the Arctic to the Antarctic, and back. For calibration purposes the signal backscattered from Earth's surface is used, which is also seen in these results (image credit: ESA) .
• August 24, 2018: Having worked around the clock since the launch of Aeolus on 22 August, teams at ESA's control center in Germany have declared today that the critical first phase for Europe's wind mission is complete. 50)
- Once in orbit, Aeolus separated from the Vega launcher and began its free-flying journey, unfolding its solar arrays, turning its radio antenna toward Earth and sending signals to ground stations in Australia and Antarctica to signify that all is well.
- An initial radio signal from Aeolus was picked up at 00:15 CEST on 23 August by a special launcher tracking dish, dubbed NNO-2, at ESA's New Norcia station in Australia — the newest in the Agency's network of communication antennas.
- This first, simple, ‘hello' was followed just 15 minutes later by the official data link that was established at the Norwegian Troll Satellite Station in Antarctica. With this full data link, mission teams at ESOC became able to send commands to the satellite and receive the data it will go on to collect.
- Flight control teams guided the satellite through this tense period, working to ensure Aeolus was safely configured and ready for its next milestone: in-orbit commissioning.
- During the commissioning phase of a satellite, controllers nudge it slightly to optimize its position in orbit, and perform tests to ensure the health of its instruments. This step is unique for every satellite, and for Aeolus it is expected to last for several months.
- The main commissioning objective of Aeolus is to fully check out, calibrate and understand the behavior of all systems onboard the spacecraft, now that has taken up its new residence in space. The absolute centerpiece of this, ESA's newest satellite, will be the switch-on and first light of the hypermodern Aladin lidar instrument.
- Once this is done, the real challenge will be to fully calibrate, characterize and tune the instrument, finally making it able to get to work measuring Earth's winds.
The DWL (Doppler Wind Lidar) operation principle of ALADIN
DWL is an active observation technique; the instrument fires laser pulses towards the atmosphere and measures the resulting Doppler shift of the return signal, backscattered at different levels in the atmosphere. The frequency shift results from the relative motion of the scatter elements along the sensor line of sight. This motion relates to the mean wind in the observed volume (cell). The measurement volume is determined by the ground integration length of 50 km (sample size), the required height resolution and the width of the laser footprint. The measurements are repeated at intervals of 200 km.
Figure 45: Schematic illustration of the lidar backscatter technique (image credit: ESA)
Light is scattered either by interaction with aerosol or cloud particles (Mie scattering) or by interaction with air molecules (Rayleigh scattering). The two scattering mechanisms exhibit different spectral properties and different wavelength dependencies such that instruments evaluating only one signal type or both in separate processing chains can be constructed.
To improve the detection of the Rayleigh signal, the laser emits light pulses in the UV spectral region (355 nm). Detection of the backscatter light and analysis of the Doppler shift is done with high-resolution spectrometers (about 5 x 108 resolving power).
For lidar techniques where the shape of the backscattered light cannot be directly measured in detail, it is important to know what shape is expected in order to calculate the speed, abundance, temperature or chemical composition of molecules in the atmosphere. - The shape of the backscattered light is described by ‘Rayleigh-Brillouin scattering theory', where the Rayleigh scattering is related to the temperature and Brillouin scattering is related to pressure fluctuations in the atmosphere. The shape of the Rayleigh-Brillouin backscattered light is described by the ‘Tenti' model, which was created in the early 1970s. This model is used worldwide to interpret atmospheric lidar measurements. 51)
Although early ESA studies showed this model to be suitable for interpreting data from the Agency's satellites carrying lidars, it was decided to launch a new laboratory experiment, through ESA's General Studies Program, to see if there was still room for improvement. An advanced model would lead to even better accuracy in lidar measurements.
The study was led by Wim Ubachs of the Laser Centre at the VU University Amsterdam in the Netherlands. The team included participants from the VU University Amsterdam, the University of Nijmegen, Eindhoven University of Technology, the KNMI (Royal Netherlands Meteorological Institute) and the German Aerospace Center, DLR. 52)
Figure 46: Example of Rayleigh-Brillouin scattering of light emitted at a wavelength (green line) as a function of its intensity (I), at a pressure of one atmosphere (image credit: ESA)
Legend to Figure 46: The red line shows the emitted light after Rayleigh scattering by molecules. The blue line shows the light after both Rayleigh and Brillouin scattering.
Measurements of Rayleigh-Brillouin scattering were taken for a range of pressures and gases, representative of Earth's atmosphere. The measurements were compared to the Tenti model, and as a result the model could be improved. The experiment concluded that the updated Tenti model now describes the shape of the backscattered light from nitrogen and oxygen to within an accuracy of 98%. It was also confirmed that atmospheric water vapor does not affect the Rayleigh-Brillouin line shape. In addition, the scattering profiles from nitrogen, oxygen and air were shown to be the most accurate ever measured worldwide and will now form the basis for further scientific research into Rayleigh-Brillouin scattering.
The study has delivered a wide variety of profiles that are important, not only to ESA's lidar missions, but also to other scientists working with lidar instruments. Some important issues dealing with the understanding of the profiles related to wavelength, scattering angle and temperature dependencies and polarization effects are still open and will be further studied in a follow-on activity with ESA.
The satellite is flown with the ALADIN instrument pointing toward Earth in a plane quasi-perpendicular to the flight path and 35º offset from nadir in the anti-sun direction. The measurement geometry is depicted in Figure 47. The LOS is oriented such that the relative velocity at the intersection with the Earth is zero (yaw steering). All measurements are taken along the LOS. The Doppler shift of the backscatter signal reflects the relative wind speed along the LOS and has to be processed to a horizontal wind speed component, HLOS (Horizontal Line-of-Sight), referenced to the ground.
Figure 47: Nominal measurement geometry and coverage of the Aeolus mission (image credit: ESA/ESTEC)
The measurement volume of the return signal from a single shot is defined by the lateral extension of the transmitted beam (a few meters in diameter) and the time gating of the receiver, which is adapted to the desired vertical resolution (250 m to 2 km or more). Due to the fact that the signal from a single shot is too weak for the evaluation, 700 shots along a ground measurement track of 87 km have to be accumulated and integrated.
Measurement profile: The onboard instrument is operated at a duty cycle of 25% to obtain wind profile separation. An active operation cycle lasts 7 seconds (equivalent to about 87 km ground track), followed by a gap in observations of 21 seconds (equivalent of nearly 150 km ground track). Winds can be measured in clear air (i.e., above or in the absence of thick clouds), and within and through thin clouds (e.g., cirrus).