Aeolus / formerly ADM (Atmospheric Dynamics Mission)
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).
• 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. 31)
- 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 6: 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 7: 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. 32)
- 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 8.
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 8: 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 9. 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 9: 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 10.
Figure 10: 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 33)
- 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 11: 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. 34)
Figure 12: 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 13: 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 14: 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. 35)
- 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 15: 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 16: 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 17: 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. 36)
- 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 18: 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. 37)
Figure 19: 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. 38)
- 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 20: 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 21: 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. 39)
- 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 22: 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. 40)
- 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 23: 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. 41)
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. 42)
Figure 24: 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 24: 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 25. 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 25: 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).
Sensor complement: (ALADIN)
ALADIN (Atmospheric Laser Doppler Instrument):
The instrument is being developed by Airbus DS (former EADS Astrium SAS), Toulouse, France as prime contractor of an industrial consortium. ALADIN is an incoherent direct detection lidar incorporating a fringe-imaging receiver (analyzing aerosol and cloud backscatter) and a double-edge receiver (analyzing molecular backscatter). The lidar emits laser pulses towards the atmosphere, then acquires, samples, and retrieves the frequency of the backscattered signal. The overall ALADIN instrument architecture is based on a 60 mJ diode-pumped frequency-tripled Nd:YAG laser operating in the ultraviolet (solid-state laser technology). The instrument consists of three major elements: a transmitter, a combined Mie and Rayleigh backscattering receiver assembly, and the opto-mechanical subsystem (a telescope with a 1.5 diameter). After integration, the telescope wavefront error has been measured within the specification (better than half a wavelength). This is a key parameter for minimizing the bias error on the wind speed. 43) 44) 45) 46) 47) 48) 49) 50) 51) 52) 53) 54) 55) 56) 57) 58)
Figure 26: The ALADIN telescope fabricated in silicon carbide (image credit: EADS Astrium SAS)
Figure 27: Functional architecture of ALADIN (image credit: EADS Astrium SAS)
TxA (Transmitter Laser Assembly). The transmitter laser is a diode pumped solid state laser (Nd:YAG). The TxA is composed of:
• PLH (Power Laser Head)
- Diode-pumped Nd-YAG laser
- Emits 60 mJ pulses @355 nm
- Pulse repetition frequency of 100 Hz
- 12 s "bursts" every 28 s
• RLH (Reference Laser Head) 59)
- Highly stable seed laser (a few MHz)
- Tunable over 7 GHz
• PLH and RLH are being conductively cooled
• TLE (Transmitter Laser Electronics)
- High current and voltage driver
- Transmitter control and synchronization.
Figure 28: Configuration of the TxA (image credit: EADS Astrium SAS)
The power laser is composed of a low power oscillator (10 mJ output energy) and two power amplifiers to generate light pulses with 150 mJ energy at the fundamental wavelength of Nd:YAG (1064 nm). This is converted to 60 mJ pulses in the UV (355 nm) by a frequency tripler. The oscillator is actively Q-switched by a Pockels cell. A seed laser is used as frequency reference. The injection seeding technique is used to achieve a single frequency mode with a low-power continuous wave (CW) single frequency laser. The power laser is conductively cooled via heat pipes. The transmitter assembly will be operated in burst mode with 100 Hz PRF during 7 seconds (plus a 5 second warm-up time), in intervals of 28 seconds. There are two fully redundant transmitters, each including two laser heads (Power Laser Head and Reference Laser Head), and a TLE (Transmitter Laser Electronics) module.
Figure 29: Photo of the RLH unit (image credit: Tesat-Spacecom, ESA)
Figure 30: The PLH mechanical structure of ALADIN (image credit: Galileo Avionica, ESA)
Receiver assembly: A combined Mie and Rayleigh backscattering receiver is implemented. The receiver assembly includes the transmit/receive switch (polarization-based), a set of relay optics and diplexers for beam transport and laser reference calibration, a blocking interference filter, the Mie and Rayleigh receivers (spectrometers), and two DFU (Detection Frontend Units).
Figure 31: Illustration of the Rayleigh spectrometer unit (image credit: EADS Astrium)
• The Mie receiver consists of a Fizeau spectrometer. The received backscatter signal produces a linear fringe whose position is directly linked to the wind velocity. The resolution of the Fizeau interferometer is 100 MHz (equivalent to 18 m/s). The wind value is determined by the fringe centroid position to better than a tenth of the resolution. The backscattered signals are detected by a thinned back-illuminated silicon CCD detector working in an accumulation mode which allows photon counting. In the Mie channel, the Doppler shift is estimated by measuring the displacement of straight fringes produced by either a Fizeau or a two-wave interferometer.
• The Raleigh receiver employs a dual-filter (also referred to as double-edge) Fabry-Perot interferometer (where the Doppler shift is estimated from the variation of the signal transmitted through two filters located on both sides of the broad Rayleigh spectrum) with a 2 GHz resolution and 5 GHz spacing. It analyzes the wings of the Rayleigh spectrum with a CCD. The etalon is split into two zones, which are imaged separately on the detector. The wind velocity is proportional to the relative difference between the intensities of the two etalons.
The optomechanical subsystem of ALADIN uses a Cassegrain afocal telescope for both functions of laser emission and backscatter reception. The optomechanical architecture employs the monostatic observation concept: i.e., the transmit and receive beams propagate through the same telescope. This architecture allows to limit the instrument FOV: to ameliorate for instance the daytime performance, and to relax the telescope and optics stability requirements. TRO (Transmit-Receive Optics) is a major subsystem of ALADIN, directing the laser pulses towards the atmosphere, generating internal reference signals and feeding the atmospheric return signal into the subsequent optical analyzers. 60)
The telescope design employs isothermal and lightweight techniques based on SiC (Silicon Carbide) type ceramic mirrors and structures. This concept provides the needed optical quality and stability without a focusing or alignment mechanism. Star trackers for attitude sensing are mounted on the telescope structure to minimize the misalignment between the optical axis and the telescope's line-of-sight.
Figure 32: Illustration of TRO layout (image credit: Kayser Threde GmbH)
Figure 33: ALADIN receiver optics with Rayleigh & Mie spectrometers (image credit: ESA)
Figure 34: Illustration of some telescope elements (image credit: EADS Astrium SAS)
The instrument transmits raw source data consisting of the accumulated spectra from the Mie receiver and the flux intensities from the Rayleigh receiver. These data are provided for strips of 50 km length and a horizontal resolution down to 3.5 km. In the vertical direction, many layers or volume cells of the various altitude bins (nominally -1 km to 16.5 km height for the Mie channel, and 0.5 km to 26.5 km for the Rayleigh channel, but other scenarios can be uplinked in flight) are measured; the instrument looks into a fixed direction (quasi perpendicular to the flight path and 35º away from nadir) and provides a vertical wind profile along the line of sight. In addition to these source data, laser internal calibration and attitude data are transmitted, as well as the receiver response calibration data.
The instrument performance considers the SNR error for each channel at the indicated altitude range. In addition, systematic bias errors are taken into account. When no ground echo is retrieved, the measurement bias is not cancelled; the total measurement error is slightly deteriorated. - For the Mie channel, the LOS (Line-of-Sight) wind error is below the requirement of 0.6 m/s for altitudes from 0 to 2 km in height. For the Rayleigh channel, the LOS wind error is below the requirement (except a marginal performance around 16 km).
Table 4: Observational requirements and performance of ALADIN
Table 5: Major instrument parameters of ALADIN
The ALADIN instrument on ADM-Aeolus employs several novel technologies, like:
- Fizeau interferometer for aerosol return
- Sequential Fabry-Perot interferometer for molecular return
- Accumulation CCD as detector (also referred to as ACCD).
Figure 35: Detector unit with accumulation CCD (image credit: e2V)
Figure 36: Overview of the ALADIN instrument (image credit: EADS Astrium SAS)
Figure 37: Artist's conception of ADM-Aeolus observations (image credit: ESA/ESTEC)
Table 6: ADM-Aeolus observational requirements (goals are shown in brackets)
Figure 38: Aeolus overall HLOS wind measurement performance for nominal atmospheric conditions (image credit: ESA)
A programmable sequencer is implemented for the detector permitting configuration changes with regard to vertical altitude resolution and range coverage. The vertical resolution can be varied from 250 m to 2 km or more. However, the measurement accuracy is only obtained for the nominal vertical resolution of 1 km. The altitude range is limited to 30 km. The horizontal (along-track) onboard accumulation length can also be changed between a distance of 1.0 km and 3.5 km.
In addition to the horizontal line-of-sight (HLOS) velocity measurements, ALADIN is able to provide information on cloud characteristics over the depth of the atmosphere, as well as aerosol measurements in the troposphere. These include:
• Cloud top height (notably cirrus top and base)
• Cloud cover
• Cloud and aerosol extinction and optical thickness
• Identification of multi-layer clouds
• Lower troposphere aerosol stratification
• The height of the tropopause
• The height of the PBL (Planetary Boundary Layer).
Figure 39: The ADM-Aeolus measurement and sampling concept (image credit: ESA)
Change of operational principle (change from burst mode to continuous mode for ALADIN laser): The change of operational principle of the laser transmitter had minor impact on the other sub-systems of ALADIN and on the platform. Exchange of FPGA (Field Programmable Gate Arrays) in the TLE (Transmitter Laser Electronics), the DEU (Detection Electronics Units) and the ALADIN Control & Data Management unit (ACDM) as well as minor modifications of the operation software and the ground processing software are required (Ref. 57).
The laser transmitter is continuing to be the greatest development challenge. Delays in the transmitter program have resulted from two main problem areas, namely LIC (Laser-Induced Contamination) caused by the interaction of the high power UV beam with outgassing materials in the vicinity of optics, and LID (Laser-Induced Damage) due to the fact that some of the optics are near the "state of the art" in terms of surviving the high fluences of the laser, particularly in the UV section. 61)
One of the most extensive test programs for LID has been undertaken by DLR Stuttgart, on each of the coating lots of all flight optics, along with a number of endurance tests, in order to demonstrate sufficient LIDT margins for the duration of the mission.
The first flight model of ALADIN laser has been integrated and the second flight model integration is being prepared. Once the lasers are fully characterized and delivered, integration of ALADIN will resume.
Spacecraft operations are performed at ESOC (Darmstadt, Germany) using the Kiruna TT&C station. The instrument 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.
The two primary components of the Ground Segment are the FOS (Flight Operations Segment) and the PDS (Payload Data Segment). The Aeolus ground segment at ESOC is scheduled to use the latest version of the SCOS-2000 mission control system (version 5).
For the complete mission duration (launch up to the end of mission, when ground contact to the spacecraft/payload is terminated), facilities and services will be provided to the PDS (Payload Data Segment) located at ESA/ESRIN (Frascati, Italy) for planning of scientific data acquisition. This will include the uplink of instrument operation timelines as well as the provision of scientific data downlink schedules based on the predicted spacecraft orbit. The PDS will be responsible for measurement data acquisition via the X-band station network, the preprocessing of scientific data, and the scientific data archiving and distribution to the Meteorological Centers and general scientific community. 62) 63)
The FOCC (Flight Operations Control Center) will operate from a dedicated control room at ESOC. Data processing will be done at ESA/ESRIN, while wind profile retrieval will be done by the ECMWF (European Centre for Medium-Range Weather Forecasts), UK. Data ground processing to be completed within five minutes after reception. 64)
The key operational requirements for Aeolus driving the overall mission operations concepts are:
• Aeolus should return near global measurements of wind speed
• Data measurements should be collected with an availability of 0.95
• The collected data should be delivered to the Data Center in less than three hours from the time of measurement
• Full 5 day autonomy including a sustainable safe mode
• Orbit related driven specifications for repeat periods, orbit period, pattern of ground station passes, and frequency of orbit control maneuvers.
Operational automation in the ground segment: ADM-Aeolus opted to be one of the first missions to utilize the mission automation systems developed as part of ESOC infrastructure, namely MATIS (Mission Automation System) and SMF (Services Management Framework). An initial simple automation approach has been taken to allow Aeolus to set automation targets which would have no impact on FOCC readiness for flight. Two simple initial targets have been set:
- Automation of Control Center to TT&C station link configuration pre-pass and post-pass
- Automation of playback of the X-band HK data dumps.
A further automation phase will cover TM monitoring, analysis and reporting.
The PDS will be in charge of the science data reception via X-band and of various processing, archiving and product dissemination tasks. It will include the X-band acquisition station located in Svalbard (Norway), the APF (Aeolus Processing Facility ) located in Tromsø (Norway) for the processing and dissemination of the Level 1B and Level 2A products, and the Level 2 Processing Facility (L2/Met PF) hosted by the ECMWF (European Centre for Medium Range Weather Forecast) in Reading (UK).
The primary data product of the mission will be the Level 1B data set, comprising calibrated wind velocity observations for both Mie and Rayleigh channels, with various additional annotation parameters. With the continuous mode laser operation, each observation profile will be constructed by the averaging of N on-board accumulated measurements of P consecutive pulses. Typical figures for N and P are, respectively, 30 and 20, leading to an observation horizontal integration length of 90 km, with less than 1% data gap between successive observations (instead of 50 km in burst mode with 150 km data gap between observations). The different values provided in Table 4 correspond to the horizontal integration length that needs to be considered in order to meet the wind velocity random error requirement.
The Level 1B products will be globally delivered to a number of meteorological service centers within 3 hours after sensing (NRT service) and for selected regions within 30 minutes after sensing (QRT service, e.g. within 30 minutes).
Higher level products will include information on clouds and aerosols optical properties (Level 2A), as well as consolidated horizontal line-of-sight wind observations (Level 2B), after temperature/pressure corrections and scene classification of the measurements within one observation. The assimilation of Level 2B data in the ECMWF operational forecast model will provide the so-called Aeolus assisted wind products (Level 2C).
Table 7: Summary of ADM-Aeolus data products (Ref. 57)
Preparatory campaigns for the verification of the measurement principle
An A2D (Aladin Airborne Demonstrator) instrument was developed by EADS Astrium SAS to demonstrate and validate the capability of ALADIN. Installation and testing of the A2D on ground was performed with first atmospheric signal in October 2005. The two functional test-flights (Oct. 18 and 20, 2005) were performed with signal from clear atmosphere, clouds and ground. The measurements demonstrated that the aircraft integration and testing was successful. These were probably the first flights of an airborne, direct-detection Doppler wind lidar worldwide. 67) 68) 69) 70) 71) 72) 73) 74)
Table 8: Overview of the A2D validation campaigns on the Falcon aircraft
• In August 2009, DLR performed a campaign on Germany's highest mountain, the Zugspitze. The clean mountain air was needed to provide the right conditions to investigate what effects the atmosphere would have on the return signal of the satellite's core instrument. The objective was to accurately measure the spectrum of the backscattered laser light from a lidar to further improve the measurements of wind speed. The experiments were carried out by DLR at the Environmental Research Station Schneefernerhaus observatory, which is located 2650 m above sea level. The science measurements were done with the A2D. 75)
• In March 2010, a DLR team conducted a flight campaign of 2 weeks in Iceland, performing a total of six flights over Iceland, over the ocean between Iceland and Greenland and over the Greenland glacier plateau. The aim of this DLR-led campaign with A2D was to investigate details of the instrument operations strategy and to refine the ADM-Aeolus data processors that will provide the mission's wind products. 76) 77)
Two different wind lidar instruments – the A2D (ALADIN Airborne Demonstrator), and a reference wind lidar operating at an infrared wavelength of two microns – were operated onboard DLR's Falcon 20E aircraft, and both performed well throughout the campaign.
• In May 2015, DLR is using its Falcon research aircraft to test an aircraft-based version of the wind measurement laser technology. From their temporary base in Iceland, the researchers are flying over the ice sheets of southern Greenland. As they do so, another proven wind lidar that was used over Iceland to take volcanic ash measurements during the eruption of Eyjafjallajökull in 2010 is being used as a reference and comparison instrument on board the Falcon. The United States aerospace agency, NASA, is also in Iceland, supporting the campaign with its own research aircraft and measurement equipment. 78)
- Windiest place on Earth: Europe's weather systems are formed in the arctic polar region around Iceland and Greenland. Small anomalies that occur where cold air masses from the polar regions meet warmer air masses can lead to the development of weather systems. The Icelandic depressions here are well known. In addition, the polar region of Greenland is of particular interest in climate research because of the rising temperatures in the Arctic and the associated retreat of polar ice sheets. "In the current research flight campaign, we are calibrating the new wind lidar above the extensive ice fields of Greenland – testing our algorithms in the process – to make sure that, later on, everything runs smoothly in space," says Oliver Reitebuch from the DLR Institute of Atmospheric Physics. In particular, the southern tip of Greenland – the windiest place in the world – is the perfect testing ground for the new wind measurement technology, as it is especially challenging, with pronounced tip jets and strong jet streams.
- NASA and DLR joint flights: "We are conducting several flights per day above the permanent ice of Greenland and, in doing so, are acquiring comparative data from a summit station operated by our US research colleagues at an altitude of 3200 m," says DLR test pilot Philipp Weber. After taking off from Iceland, the Falcon crew makes a refuelling stop at Kangerlussuaq in Greenland and then spends two hours crisscrossing Greenland. In total, some 10 test flights above Greenland are planned, which will mostly take place in coordination with the NASA DC-8 research aircraft. The data from the NASA DC-8 and the DLR Falcon will then be compared. Two lidar instruments are being used on board the NASA DC-8, in addition to measurement probes that are ejected from the aircraft via a chute.
- Scattered light makes wind fields visible: At present, the major wind fields over the oceans are still detected optically by weather satellites tracking cloud movements, or measured indirectly using radar signals reflected by the wave motion on the surface of oceans. "The wind lidar measurements will enable the project team to directly measure wind speeds from ground level up to an altitude of 20 km with significantly greater accuracy. Depending on the altitude, the project can achieve a resolution of between 500 to 1000 m while doing so," explains Reitebuch. "With the Doppler lidar –a laser pulse sequence is emitted into a wind field at a precisely defined wavelength. Depending on the movement of the wind field, the light is reflected back with a very small change in wavelength. From this, the team can determine the wind speed," continues Reitebuch. Using this technology, the DLR researchers will be capable of accurately determining changes as small as one ten billionths of a wavelength.
- Small anomalies with large effects on the weather: In addition to testing the wind lidar above Greenland, the DLR atmospheric researchers are acquiring data on the formation and development of Icelandic depressions. The researchers hope to better understand how low-pressure systems arise from small anomalies over Iceland, Greenland and the North Atlantic in a short time. "From Iceland, measurements can be performed in the strong jet streams over the North Atlantic. Detailed knowledge of the wind distribution is particularly important because a lack of wind data very quickly leads to errors in weather forecasting models," says Reitebuch. "These errors affect the accurate forecasting of the development of low-pressure systems, which often move towards Europe and, due to their high winds and heavy rainfall, have a significant effect on our daily lives."
- The DLR ADM (Atmospheric Dynamics Mission) research flight campaign over Iceland and Greenland is a DLR contribution to the ESA ADM-Aeolus mission. Involved in this mission are the DLR Institute of Atmospheric Physics, DLR Flight Experiments, ESA and the University of Leeds, which has a wind lidar installed at the summit station in Greenland for the mission to perform comparison measurements from the ground. This mission is being carried out in cooperation with NASA. For the first time in the world, four wind lidar instruments on two aircraft are being used at the same time.
Figure 41: Photo of the DLR Falcon (foreground) and the NASA DC-8 aircraft prior to the joint research flight campaign from Iceland (image credit: DLR)
Design and setup of the ALADIN airborne demonstrator:
The core of the A2D is based on the ALADIN receiver and transmitter from the pre-development program of ESA and is therefore representative of the actual satellite instrument. The optical receiver of the A2D was space qualified with respect to its thermal vacuum and vibration environment during the pre-development phase.
The A2D is a nonscanning lidar as the satellite instrument. Thus, only one LOS component of the three-dimensional wind vector is measured in contrast to most other direct-detection wind lidars, which are equipped with a scanning device. The LOS wind is measured perpendicular to the aircraft roll axis, with an off-nadir angle of 208. The A2D is designed to be operated on the DLR Falcon 20 aircraft, a twin-engine jet with a pressurized cabin allowing a maximum payload of 1.1 ton, a flight altitude of up to 12 km, and range of up to 3700 km.
The installation of the A2D inside the Falcon aircraft is shown in Figure 42 with the telescope, the mechanical aircraft frame, and the thermal hood of the receiver system. The mechanical frame holding the telescope, receiver, and laser is mounted via vibration-damping shock mounts to the seat rails of the aircraft. The mechanical frame of the 10.6 µm heterodyne wind infrared Doppler lidar, which has proven its aircraft vibration-damping behavior needed for coherent detection, was adapted to hold the A2D laser, optical receiver, and telescope.
The laser beam is directed toward the atmosphere via a window in the bottom fuselage of the aircraft cabin. The electronic units operating the A2D are installed in 19 inch aircraft racks and are controlled by two operators. The total volume of the system is 3 m3, the mass is 550 kg, and the mean power consumption is 2.5 kW. Finite element simulations were performed to minimize the overall weight, providing high stiffness for the transmit and receive optical path, and to prove airworthiness.
Figure 42: The A2D instrument installed in the Falcon 20 aircraft of DLR during initial tests in Oct. 2005 (image credit: DLR)
Optical design overview: The narrowband single-frequency laser pulses at 354.89 nm vacuum wavelength are generated by an Nd:YAG laser. The circularly polarized laser pulses are transmitted via three reflecting mirrors through the aircraft window (or one reflecting mirror in case of ground operation) toward the atmosphere. The last reflecting mirror is placed on the telescope optical axis and thus a coaxial transmit–receive system is obtained.
The backscattered photons from the atmosphere are collected by a 20 cm aperture Cassegrain telescope and directed to the optical receiver via an optical relay with two lenses and two mirrors. After passing the front optic with field and aperture stop, the light is directed toward the two spectrometers. The Rayleigh spectrometer uses the double-edge technique with a sequential Fabry–Perot interferometer, whereas the Mie spectrometer is based on a Fizeau interferometer. For both the Rayleigh and the Mie spectrometer, an ACCD (Accumulation CCD) detector is used, and the electronic signal is digitized after preamplification. The sequential implementation of the Fabry–Perot interferometer and the ACCD are patented by Astrium.
The optical beam path with about 60 optical elements and the alignment sensitivities were studied in detail with an optical ray-tracing model. The principle layout of the A2D optical design is shown in Figure 43. The main instrument parameters for the satellite ALADIN and the A2D are summarized in Table 9.
Figure 43: Schematic optical layout of the A2D instrument (image credit: DLR)
Table 9: Specifications of the satellite ALADIN and measured performance of the A2D
Development status of the spacecraft and ALADIN
• August 16, 2018: Measuring 4.5 m across, this relatively small antenna in Australia, dubbed NNO-2, will be the first to hear from the soon-to-be-launched Aeolus satellite, the first ever to measure winds on Earth from Space. 79)
- Aeolus' first steps after separation will include the automatic unfolding of its solar ‘wings' and turning its antenna to face Earth to start sending signals. Only then will teams on the ground be able to get any sign from the satellite that all is well.
- Since 2015, NNO-2 has been pointing to space, listening for signals from rockets and newly launched satellites and transmitting instructions and commands to them from engineers on Earth.
• August 9, 2018: As preparations for the launch of ESA's latest Earth Explorer continue on track, the team at Europe's Spaceport in French Guiana has bid farewell to the Aeolus satellite as it was sealed from view in its Vega rocket fairing. Liftoff is set for 21 August at 21:20 GMT (23:20 CEST). 80)
- Since its arrival at the launch site in early July, Aeolus has been thoroughly tested and fuelled with hydrazine.
- Like all of ESA's Earth Explorer missions, Aeolus will fill a gap in our knowledge of how our planet works and show how novel technology can be used to observe Earth from space.
Figure 44: Encapsulation - Aeolus carries one of the most sophisticated instruments ever to be put into orbit. The first of its kind, the Aladin instrument includes revolutionary laser technology to generate pulses of ultraviolet light that are beamed down into the atmosphere to profile the world's winds – a completely new approach to measuring the wind from space (image credit: ESA/CNES/Arianespace)
• August 2, 2018: With liftoff less than three weeks away, ESA's Aeolus satellite has been fuelled and is almost ready to be sealed within its Vega rocket fairing. 81)
Figure 45: With liftoff less than three weeks away, ESA's Aeolus satellite has been fuelled and is almost ready to be sealed within its Vega rocket fairing. Getting a satellite ready to be launched involves a long list of jobs, some of which are trickier than others. Since hydrazine is extremely toxic, only specialists dressed in bulky astronaut-like suits remained in the cleanroom for the duration of the activity (image credit: ESA/CNES/Arianespace)
• July 24, 2018: The launch of Aeolus — ESA's mission to map Earth's wind in realtime — is getting close, with the satellite due for lift-off on 21 August from Europe's Spaceport in Kourou, French Guiana. With the wind in their sails, mission teams are busily preparing this unique satellite for its upcoming journey. 82)
- Aeolus will carry a sophisticated atmospheric laser Doppler instrument, dubbed ALADIN. Combining two powerful lasers, a large telescope and extremely sensitive receivers, it is one of the most advanced instruments ever put into orbit.
- Currently one of the biggest challenges in making accurate weather predictions is gathering enough information about Earth's wind. Aeolus will be the first-ever satellite to directly measure winds from space, at all altitudes, from Earth's surface through the troposphere and up 30 km to the stratosphere — providing information that will significantly improve the quality of weather forecasts.
- Paolo Ferri, Head of Mission Operations at ESA adds, "The Aeolus mission will be a wonderful addition to our fleet of satellites that continually observe Earth bringing us incredible insights into our planet, in particular into the complex world of atmospheric dynamics and climate processes — systems that not only affect our everyday lives but also have huge consequences for our future."
Figure 46: Earth's wind patterns: The movement of air constitutes the general circulation of the atmosphere, transporting heat away from equatorial regions towards the poles, and returning cooler air to the tropics. Atmospheric circulation in each hemisphere consists of three cells - the Hadley, Ferrel and polar cells. High-speed wind fields, known as ‘jets', are associated with large temperature differences (image credit: ESA/AOES Medialab)
• July 10, 2018: With the campaign to launch ESA's Aeolus wind satellite on 21 August well underway, the satellite's telescope has been opened and inspected to make sure it is perfectly clean and shiny. 83)
- While Aeolus' novel laser technology is arguably the sexy part of the instrument, its telescope, which measures around 1.5 m across, is pretty dominant and equally important. It is used to collect backscattered light from the atmosphere and direct it to the receiver. In short, the laser system generates a series of short pulses of ultraviolet light which are beamed down into the atmosphere. The telescope collects the light backscattered from particles of gas and dust in the atmosphere. The time between sending the light pulse and receiving the signal back determines the distance to the ‘scatterers' and therefore the altitude above Earth. As the scattering particles are moving in the wind, the wavelength of the scattered light is shifted by a small amount as a function of speed. The Doppler wind lidar measures this change so that the velocity of the wind can be determined.
- It is clearly important to make sure that the instrument is absolutely spotless, so engineers at the launch site in Kourou have first turned to the telescope and given it a close inspection.
Figure 47: Aeolus shiny telescope (image credit: ESA)
• July 6, 2018: Having set sail from France on 15 June - Global Wind Day, ESA's Aeolus wind satellite has arrived safe and sound at the launch site in French Guiana. - While almost all satellites travel by aircraft, Aeolus' journey was rather different – it travelled all the way across the Atlantic from Saint Nazare, western France to the Port of Cayenne, French Guiana by ship. 84)
- Aeolus carries one of the most sophisticated instruments ever to be put into orbit. A 12-day journey was undertaken to avoid potential damage caused by air re-pressurization during descent had the satellite travelled by air – a quicker but decidedly riskier option.
- Upon its long-awaited arrival, the team unloaded Aeolus and its support equipment. The containers were then carefully positioned on a truck to be transported to the launch site about 60 km away, where the satellite container was moved into the airlock, to stabilize after its long journey.
- The satellite was then removed from its container, placed on its integration trolley for testing and connected to its electrical support equipment. Initial checks indicate that Aeolus has withstood its journey from France in good condition.
- ESA's Aeolus project manager, Anders Elfving, said, "We are obviously all extremely pleased that Aeolus has now arrived at the launch site. An awful lot of work and planning went into making sure it arrived safe and sound – now it's full steam ahead for preparing the satellite for liftoff on 21 August."
- A range of checks will be carried out on the satellite in the cleanroom before the scheduled liftoff on a Vega rocket on 21 August at 21:20 GMT (23:20 CEST) from Europe's spaceport near Kourou.
Figure 48: ESA's Aeolus wind satellite on the integration trolley in Kourou, French Guiana (image credit: ESA)
• June 15, 2018: Today is Global Wind Day, which couldn't be more apt for ESA's Aeolus wind satellite to begin its voyage to the launch site in French Guiana. And, while almost all satellites journey by aircraft, Aeolus is different, it's going by ship. 85)
- Since the ALADIN instrument is sensitive to pressure change, ESA and Airbus Defence and Space engineers decided that the safest way for it to journey from France, where it has been going through testing, to French Guiana would be by ship.
- Denny Wernham, ESA's Aeolus instrument expert, explains, "Going by ship may seem a little strange, after all it will take around 12 days to get there instead of a matter of hours, but if, for whatever reason, the aircraft had to descend rapidly and there was a sudden increase in air pressure, Aeolus' instrument could be damaged.
- "It was designed, of course, to allow for the pressure drop during launch ascent so that it could be taken into orbit, but not for a fast descent. So basically, once it's up, it's up.
- "So, today we see our beloved satellite and all of its support equipment being loaded onto a ship in Saint Nazaire in western France and set forth across the Atlantic. And, indeed, it is kind of ironic: our high-tech wind satellite is travelling by a means that many years ago relied on the wind."
- Aeolus has, without doubt, been a challenging satellite mission to develop. Nevertheless, this long-awaited mission is now set to not only improve our understanding of how the atmosphere works and contribute to climate change research, but will also help to predict extreme events such as hurricanes. It will also help to better understand and model large-scale wind patterns driving weather such as El Niño.
Figure 49: The ship, painted with ‘Airbus onboard', waiting for ESA's Aeolus satellite to arrive. The vessel will carry Aeolus from Saint Nazaire in western France to the launch site in French Guiana. Liftoff is scheduled for 21 August 2018 (image credit: ESA–G. Labruyere) 86)
• June 5, 2018: Like all of the Earth Explorers, Aeolus was built to show how cutting-edge space technology can shed new light on the intricate workings of our planet. This pioneering satellite uses powerful laser technology that probes the lowermost 30 km of our atmosphere to yield vertical profiles of the wind as well as information on aerosols and clouds. This will not only improve our understanding of how the atmosphere works and contribute to climate change research, but will also help to predict extreme events such as hurricanes and El Niño. 87)
Figure 50: Before ESA's Aeolus satellite is packed up and shipped to French Guiana for liftoff in August, media representatives had the chance to see this wind measuring Earth Explorer satellite standing proud in the Airbus Defence and Space cleanroom in Toulouse, France (image credit: ESA, M. Pedoussaut)
• February 7, 2018: ESA's Aeolus satellite has been particularly tricky to build. One of the main stumbling blocks has been getting its lasers to work in a vacuum, but recent tests on the satellite show that the vacuum or temperature of space won't get in the way of Aeolus measuring Earth's winds. 88)
- The ALADIN instrument shoots pulses of ultraviolet light down into the atmosphere and measures the backscattered signals from molecules and aerosols to profile the world's winds.
- "This will be the first time that we will be able to directly measure profiles of the global wind field from space in cloud-free conditions. It has been a major challenge for us all – our ESA engineers, industry, our Member States – to overcome many technical and programmatic challenges. I am grateful to everyone for having gone through this and for having trust in ESA to finally make it happen. We are now very close to seeing the fruits of a long endeavor," said Josef Aschbacher, ESA's Director of Earth Observation Programs.
- These vertical slices through the atmosphere, along with information on aerosols and clouds, will advance our knowledge of atmospheric dynamics and contribute to climate research.
- Since Aeolus will deliver measurements almost in realtime, it is also set to provide much-needed information to improve daily weather forecasts.
- The satellite's novel technology was under development for some years, but issues with the laser component of the instrument and with the optics, which have to survive exposure to the high-intensity laser pulses, were eventually resolved, and in 2016 the instrument was finally ready.
Figure 51: Laser reading: The image indicates that the laser carried on ESA's Aeolus satellite works well in a vacuum. ESA's Aeolus satellite spent nearly two months in a thermal–vacuum chamber to make sure that its novel instrument will work as it should in space. Aeolus carries one of the most sophisticated instruments ever to be put into orbit: Aladin, with two powerful lasers, a large telescope and very sensitive receivers. It will be the first such satellite mission to measure Earth's winds from space. It actually carries two laser transmitters just in case one fails (image credit: ESA)
- ALADIN was then added to the satellite in the UK, after which the assembly was moved to France where it was shaken to simulate the rigors of liftoff.
- The last round of tests was carried out in CSL (Centre Spatial de Liège), Belgium, and involved putting the satellite in a thermal–vacuum chamber for almost two months (Figure 52).
- Once the satellite was safely inside, the air was pumped out and the chamber cooled by liquid nitrogen to simulate the environment of space – and then Aeolus was put through its paces.
- ESA's Aeolus project manager, Anders Elfving, said, "The test was exceptionally complex, not only because it was a tight fit with the satellite filling up most of the space in the chamber, but also because we had to make sure that the whole instrument's performance is tip-top. It was an extremely technical and delicate undertaking that included firing ALADIN's lasers at full power. The satellite as a whole came through with flying colors, and we are particularly pleased that the two laser transmitters performed brilliantly."
- With this milestone behind it, Aeolus has now been returned to France where it will have a few final tests before being shipped across the Atlantic to Europe's Spaceport in French Guiana for launch on a Vega rocket in the autumn.
- ALADIN was built by Airbus SAS in Toulouse, France, the satellite by Airbus Ltd. in Stevenage, UK, and the laser transmitters by Leonardo SpA in Florence and Pomezia, Italy.
Figure 52: Opening the thermal vacuum chamber after testing the spacecraft for nearly two months (image credit: Centre Spatial de Liège)
• November 2, 2017: With liftoff on the horizon, ESA's Aeolus satellite is going through its last round of tests to make sure that this complex mission will work in orbit. Over the next month, it is sitting in a large chamber that has had all the air sucked out to simulate the vacuum of space. 89)
- Aeolus carries one of the most sophisticated instruments ever to be put into orbit: Aladin, which includes two powerful lasers, a large telescope and very sensitive receivers. The laser generates ultraviolet light that is beamed down into the atmosphere to profile the world's winds – a completely new approach to measuring the wind from space.
- These vertical slices through the atmosphere, along with information it gathers on aerosols and clouds, will improve our understanding of atmospheric dynamics and contribute to climate research. As well as advancing science, Aeolus will play an important role in improving weather forecasts.
- With these difficulties in the past, the satellite is now undergoing final testing in Belgium before it is shipped to French Guiana for liftoff, which is scheduled for the middle of next year.
- After having spent this spring at Airbus Defence and Space in Toulouse, France, where it was checked that it could withstand the vibration and noise liftoff and its ride into space, Aeolus has been at the Centre Spatial de Liège since May. - Here, it has just been enclosed in the thermal–vacuum chamber for the next 30 days or so.
- With the satellite safely inside, the chamber door was closed a few days ago and the air was pumped out to create a vacuum.
- Denny Wernham, ESA's Aladin instrument manager, said, "It takes some time for the air and outgassing from the satellite to be pumped out of the chamber, but Aeolus finally faced ‘hard vacuum' on 31 October.
- "Tests are scheduled to run continuously over the next 33 days. We are particularly keen to see how well the laser transmits its pulses of ultraviolet light and the alignment of the instrument in this environment.
- Once these tests are done, the satellite will be transported back to Toulouse for final checks before being shipped across the Atlantic to Europe's Spaceport in French Guiana for launch on a Vega rocket.
• January 30, 2017: The road to realizing ESA's Aeolus mission may have been long and bumpy, but developing novel space technology is, by its very nature, challenging. With the satellite now equipped with its revolutionary instrument, the path ahead is much smoother as it heads to France to begin the last round of tests before being shipped to the launch site at the end of the year. 90)
- Aeolus carries one of the most sophisticated instruments ever to be put into orbit: ALADIN, with two powerful lasers, a large telescope and very sensitive receivers. It shoots pulses of ultraviolet light down into the atmosphere to profile the world's winds. This is a completely new approach to measuring the wind from space, which usually involves tracking cloud movement, measuring the roughness of the sea surface or inferring wind from temperature readings.
Figure 53: Now that Aeolus is equipped with its ALADIN instrument, it is ready to be moved from Airbus Defence and Space in the UK to their facilities in Toulouse, France. There it will start the last round of tests before being shipped to the launch site (image credit: Airbus DS)
Figure 54: Profiling the world's winds (image credit: ESA/ATG medialab)
Legend to Figure 54: The ADM-Aeolus mission will not only advance our understanding of atmospheric dynamics, but will also 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.
- Aeolus has been built mainly to advance our understanding of Earth. These vertical slices through the atmosphere, along with information on aerosols and clouds, will advance our knowledge of atmospheric dynamics and contribute to climate research. - However, Aeolus also has a very important practical role to play because its measurements will be delivered rapidly, improving weather forecasts. After its long development, ALADIN was finally ready to join the satellite at Airbus Defence and Space in Stevenage in the UK in August last year.
Figure 55: Standing proud: ESA's Aeolus satellite in the cleanroom at Airbus Defence and Space in Stevenage, UK. During the last half of 2016 the UK team with support of their colleagues from Toulouse in France worked tirelessly to integrate the ALADIN instrument into the satellite, to check that all is aligned and that the complete satellite is working flawlessly. As the sole measuring instrument on the Aeolus satellite, ALADIN comprises two powerful lasers, a large telescope and very sensitive receivers. It is designed to probe the lowermost 30 km of the atmosphere to provide profiles of wind, aerosols and clouds along the satellite's orbital path (image credit: Airbus DS)
- With the satellite now complete, it is time move it to Toulouse where it will be tested to make sure that it can withstand the vibration and noise of liftoff. — After this, ADM-Aeolus will go to Liege in Belgium to be checked in a thermal–vacuum chamber.
• August 2, 2016: After many years in development, ALADIN – the Doppler wind lidar to be carried on the Aeolus satellite – is ready to be shipped from Toulouse, France, to the UK to be installed on the satellite in preparation for liftoff by the end of 2017. Aeolus will be the first satellite mission to probe the wind globally. These vertical slices through the atmosphere, along with information on aerosols and clouds, will advance our knowledge of atmospheric dynamics and contribute to climate research. 91)
- Its 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.
Figure 56: Photo of the ALADIN telescope and instrumentation at Airbus DS in Toulouse to be shipped to Airbus DS UK for installation into the ADM/Aeolus spacecraft (image credit: Airbus DS)
• 2016: The ALADIN instrument is fully integrated and both laser transmitters are aligned for optimal performance. The In-situ Cleaning Subsystem was tested together with the latest satellite flight software. The satellite platform was finalized and checked out in preparation for mating with the ALADIN PLM. The Payload Data Ground Segment facilities are being prepared and undergoing tests. The Flight Operations Segment facilities are also being readied. 92)
• April 22. 2015: A lot of time has gone into developing the technology involved and testing both lasers. Despite numerous setbacks, in particular issues associated with them working properly in a vacuum, engineers at Selex-ES in Italy persevered. Thanks to their dedication and ingenuity, a major milestone for the mission has been achieved. Both lasers have now been delivered to Airbus Defence and Space in Toulouse, France, ready to be integrated into the rest of ALADIN. 93)
- Despite a number of setbacks, this cutting-edge piece of technology is now ready to be integrated into the rest of the satellite's instrument – a Doppler wind lidar called ALADIN. — ADM-Aeolus will provide profiles of the world's winds as well as information on aerosols and clouds. These profiles will not only advance our understanding of atmospheric dynamics, but will also offer much-needed information to improve weather forecasts.
- Thanks to these collective efforts, the project can now focus on the instrument and satellite integration and testing. This means a launch of ADM-Aeolus spacecraft can be done in 2016.
Figure 57: Photo of the ADM-Aeolus second ALADIN laser prior to closure showing the complexity of the 80 optical components contained within a relatively small space of 45 x 34 x 20 cm and a mass of ~30 kg (image credit: Selex-ES)
Figure 58: The completed ADM-Aeolus laser under testing at Selex-ES (image credit: Selex-ES)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org)