ADM-Aeolus (Atmospheric Dynamics Mission)
ADM-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 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 ADM-Aeolus spacecraft (image credit: ESA/ESTEC)
The S/C mass at launch is about 1300 kg of which 450 kg are allocated to the payload. Its size is 4.6 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: A launch of ADM-Aeolus is expected for mid-2014 (the previous launch date was 2011). The launch can be performed by any of the small launchers, like Vega, Rockot or Dnepr.
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. 23) 24) 25)
Orbit: Sun-synchronous orbit, altitude = 400 km (mean), inclination = 97.0º, local equator crossing time at 18:00 (on ascending node) and at 06:00 hours (dawn-dusk orbit), 7-day repeat cycle (109 orbits).
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 5: 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. 26)
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. 27)
Figure 6: 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 6: 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 7. 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 7: Nominal measurement geometry and coverage of ADM-Aeolus (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 track of 50 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 50 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 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 150 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. 28) 29) 30) 31) 32) 33) 34) 35) 36) 37) 38) 39) 40) 41) 42)
Figure 8: The ALADIN telescope fabricated in silicon carbide (image credit: EADS Astrium SAS)
Figure 9: 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 150 mJ pulses @355 nm
- Pulse repetition frequency of 100 Hz
- 12 s “bursts” every 28 s
• RLH (Reference Laser Head) 43)
- 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 10: 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 150 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 11: Photo of the RLH unit (image credit: Tesat-Spacecom, ESA)
Figure 12: 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 13: 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. 44)
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 14: Illustration of TRO layout (image credit: Kayser Threde GmbH)
Figure 15: ALADIN receiver optics with Rayleigh & Mie spectrometers (image credit: ESA)
Figure 16: 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 3: Observational requirements and performance of ALADIN
Table 4: 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 17: Detector unit with accumulation CCD (image credit: e2V)
Figure 18: Overview of the ALADIN instrument (image credit: EADS Astrium SAS)
Figure 19: Artist's conception of ADM-Aeolus observations (image credit: ESA/ESTEC)
Table 5: ADM-Aeolus observational requirements (goals are shown in brackets)
Figure 20: 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 21: 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. 42).
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.
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. Aeolus launch is now expected in mid 2014 (Ref. 42).
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. 45) 46)
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. 47)
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 3 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 6: Summary of ADM-Aeolus data products (Ref. 42)
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. 50) 51) 52) 53) 54) 55) 56) 57)
Table 7: 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. 58)
• 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. 59) 60)
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.
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 23 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 23: 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 24. The main instrument parameters for the satellite ALADIN and the A2D are summarized in Table 8.
Figure 24: Schematic optical layout of the A2D instrument (image credit: DLR)
Table 8: Specifications of the satellite ALADIN and measured performance of the A2D
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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