ELSA-d (End-of-Life Service by Astroscale)-d (demonstration)
Astroscale UK Ltd is part of Astroscale Pte Ltd, a Singapore-based satellite services company that was founded in 2013 with the objective of developing innovative solutions against the growing number of space debris. The company's mission is to actively contribute to the sustainable use of the space environment by developing scalable and innovative on-orbit technologies, in order to safely remove the most threatening debris in orbit. 1)
In November 2017, Astroscale Pte Ltd (with HQs in Singapore) and SSTL (Surrey Satellite Technology Ltd. of Surrey, UK) signed a Memorandum of Understanding to pursue joint opportunities in areas of innovative on-orbit technologies and missions designed to safeguard the orbital environment for future generations. 2) 3) 4)
Astroscale and SSTL have agreed to long-term strategic cooperation that further positions the companies to compete globally in the growing small satellite and orbital debris removal markets. Together the companies will seek to identify ambitious debris removal projects and joint offerings for competitive small satellite missions in Japan. As a first step, Astroscale has contracted SSTL to supply a "Target" satellite and avionics for its inaugural End-of-Life Service by Astroscale-demonstration (ELSA-d) mission, which will simulate capture of orbital debris and is designed to validate key technologies for end-of-life spacecraft retrieval and disposal services.
Astroscale hopes to establish a long-term presence in the UK, starting with the establishment earlier this year of an office and mission control center in Harwell. This cooperation will lead to further investment in the UK, including potential establishment of a supply chain.
"We are very excited to welcome SSTL to the ELSA-d mission and to begin this ambitious strategic partnership," said Nobu Okada, Founder and CEO of Astroscale. "SSTL is synonymous with reliable and cost-effective small satellites and we are thrilled that they will provide a key component to ELSA-d. We are confident that this is only the beginning of mutually beneficial long-term relationship."
Sir Martin Sweeting, Executive Chairman of SSTL added, "We are extremely pleased to be working with Astroscale, a like-minded, innovation-driven company seeking to make space business viable for commercial operators. This practical solution to debris mitigation has the potential to provide a cost-effective approach for ensuring the long-term sustainability of the space environment for all."
The ELSA-d mission comprises of a "Chaser" satellite (Astroscale) and the Target satellite (SSTL), and will demonstrate key technologies necessary for orbital debris removal such as rendezvous & docking and proximity operations. Astroscale will design and manufacture the Chaser at its R&D office in Tokyo, using avionics from SSTL. It will be equipped with optical sensing instruments and a redundant capture mechanism.
The small satellite remote sensing and orbital debris removal markets are predicted to grow significantly in the coming years. By agreeing to long-term strategic cooperation, Astroscale and SSTL are now positioned to take advantage of these opportunities and positively impact future generations.
The Target and the Chaser will be attached for launch and deorbit, but while on-orbit, will be deployed in a series of three increasingly complex separation and capture maneuvers using rendezvous and docking algorithms. A docking plate with optical markers will be attached to the Target, allowing the Chaser to identify and estimate attitude during the docking.
SSTL's Target satellite incorporates S-band communications, GPS positioning, a 3-axis control system and laser retro-reflector. A variant of the SSTL-42 constellation platform family designed for operational missions in the 5-100 kg range, it will also fly an HD camera and lighting to record the capture sequences during eclipse.
Astroscale is designing and developing the highly maneuverable "Chaser" minisatellite of ~160 kg in its R&D office in Tokyo (Figure 1), using avionics from SSTL. In addition, SSTL is supplying the "Target" microsatellite of ~ 20 kg (Figure 2).
ELSA-d is a twin small satellite mission scheduled to launch in 2020, which will demonstrate key rendezvous and docking technologies, and proximity operational concepts in readiness for provision of a commercial deorbit service in 2020 as constellations are starting production and deployment. These technologies and demonstrations are also highly relevant for in-orbit assembly missions. 5)
Figure 1: Illustration of the ELSA-d mission: showing the Chaser with the attached Target satellite (image credit: Astroscale)
ELSA-d is an IOD (In-Orbit Demonstration) for key end-of-life technology and capabilities of future debris removal missions. In Astroscale, EOL (End-of-Life) and ADR (Active Debris Removal) have the following distinction: EOL is concerned with removal of future entities that are launched with a DP (Docking Plate) for semi-cooperative removal, while ADR is concerned with removal of existing entities in space that do not have a DP and are fully non-cooperative. 6)
The two ELSA-d spacecraft are launched stacked together. The chaser is equipped with proximity rendezvous technologies and a magnetic capture mechanism, whereas the target has a DP which enables it to be captured. With the chaser repeatedly releasing and capturing the target, a series of demonstrations can be undertaken including: target search, target inspection, target rendezvous, and both non-tumbling and tumbling capture. — ELSA-d will be operated from the UK at the National In-orbit Servicing Control Center Facility, developed by Astroscale as a key part of the ground segment.
Figure 2: Illustration of the ELSA-d Target microsatellite (image credit: SSTL)
The mission will validate an innovative capture mechanism, as well as the CONOPS (Concept of Operations) for capturing and removing non-tumbling and tumbling semi-controlled targets from orbit. The Target and the Chaser will be attached for launch and deorbit, but will be deployed while on-orbit in a series of three increasingly complex separation and capture maneuvers using rendezvous and docking algorithms. A docking plate with optical markers will be attached to the Target, allowing the Chaser to identify and estimate attitude during the docking.
SSTL's Target satellite incorporates S-band communications, GPS positioning, a 3-axis control system and laser retro-reflector. A variant of the SSTL-42 constellation platform family designed for operational missions in the 5-100 kg range, it will also fly an HD camera and lighting to record the capture sequences during eclipse. These latter two technologies are potentially key technologies to support future in-orbit assembly operations.
Following successful demonstration, the "Chaser" spacecraft is intended to be mass produced to provide an on-demand service for constellation missions.
The ELSA-d mission is funded through private capital, and the project will address all the necessary regulatory aspects.
Figure 3: ELSA-d Chaser and Target: with Target at the front (detached) and Chaser in the background (image credit: Astroscale)
The key features of the mission are summarized in Table 1. The core constituents of the mission include a rendezvous (RDV) and docking suite and a magnetic capture system. Other elements include classical bus elements, such as power, propulsion, communications and processing.
Table 1: ELSA-d mission features
Key Commercial and Mission Factors
There were a range of key design factors for the ELSA-d mission. Being in an immature commercial and legal market, ELSA-d is a market leader in this domain. Astroscale attempts to engage in discussions with multiple parties to develop doctrine, standards, and regulation critical to active debris removal. 7)
• Astroscale is in discussions with the UK Space Agency (the mission licensing agency) to ensure a licensed chaser design.
• Astroscale is in preliminary discussions with UK insurance providers to understand future insurance standards in IOS (In-Orbit Servicing).
• Astroscale is part of various standardization and policy-development committees to ensure lessons learned are fed into future doctrine and that ELSA-d and future missions are in alignment with the future direction of policy.
• Astroscale interfaces with legal structures to ensure future design for legal compliance, including entities such as IADC (Inter-Agency Space Debris Coordination Committee) and UNCOPUOS (UN Committee on the Peaceful Uses of Outer Space).
A key design factor in ELSA-d is mission safety. These aspects encompass all areas across the mission development, including:
• Safety evacuations and passively safe trajectories (passive /active aborts, predefined evacuation point, protected safety ellipse insertion)
• Collision Avoidance Maneuvers (CAMs)
• Ground segment and operator oversight (including manual experimental abort)
• Protected critical mission functions (including reversion to higher levels of hardware and software authority)
• Safety critical computing [including multi-level FDIR (Failure Detection, Isolation and Recovery ), mostly fail-safe and some fail-operational]
• Architectural redundancy (some units are semi-hot redundant, some cold redundant)
• High-fidelity ground-based simulation (full on-ground simulation of all operational sequences before execution).
CONOPS (Concept of Operations)
The mission CONOPS activities are shown in Figure 4 and are divided into 7 phases as follows. Between demonstration phases, when the chaser and target are docked, they can enter a routine phase which is power and thermal safe. The phases are designed to generally increase in complexity ensuring less risky demonstrations are attempted first.
Phase 1 to 2: Launch and Commissioning
The chaser and target are launched together into the operational orbit of roughly 550 km. The chaser undergoes commissioning, testing interfaces with the ground segment, ensuring subsystems (where possible) are calibrated, and resulting in a system ready to start the demonstrations. The target is activated using the TAU (Target Activation Unit) and undergoes the majority of its commissioning prior to separation.
Phase 3: Capture without Tumbling
A TSM (Target Separation Mechanism) holds the target and chaser together during launch and phase 3 is the first time the target is separated; once separated, the magnetic capture system is used to repeatedly capture and release the target, so the TSM is no longer in use. The majority of the target commissioning has already been undertaken, so any remaining commissioning is performed. The chaser has the ability to position itself at set distances behind the target, which are defined as specific holding points (these include for example Point A and Point B, 10 m and 5 m behind the target, respectively). At Points A and B, the chaser performs a navigation check-out and calibration using its rendezvous sensors. This is the first time these sensors can be tested in space, since they can't be tested whilst the target is docked. Finally, the target is commanded to hold a set attitude and the chaser goes in for capture utilizing the docking plate on the target for guidance. There are several sub-phases of the final capture including target acquisition and tracking, and velocity, position and roll synchronization, but these are easier in the non-tumbling case than the tumbling phase 4 case.
Figure 4: This figure shows the mission CONOPS through 7 mission phases, progressing from launch and commissioning (phase 1 to 2), initial non-tumbling capture (phase 3), tumbling capture (phase 4), target search demonstration (phase 5), to final re-orbiting and passivation (phase 6 to 7), image credit: Astroscale
Phase 4: Capture with Tumbling
This phase is the more dynamically complex version of phase 3. The phase also contains two sub-demonstrations - INVD and Diagnosis. INVD (Inertial Navigation Validation Demonstration) tests the full rendezvous sensor suite. Diagnosis is a fly-around performed to visually inspect the target. Diagnosis simulates a full service, where images of the target are taken and downloaded to the ground for operator inspection before capture. After these two demonstrations, tumbling capture is performed. The target is commanded to follow a natural motion tumbling attitude profile. The chaser performs the sub-phases of final capture listed in phase 3. Part of the capture involves taking images of the tumbling target which are downloaded to ground and post-processed to extract target attitude. There, the FDS (flight dynamics system) in the ground segment calculates a trajectory to move and orient the chaser with the target such that the chaser is always facing the target DP. The trajectory is uploaded and executed to align the chaser and target, whereby settling is then used for final alignment before capture.
Phase 5: Relative Navigation Demonstration
This phase is a critical one in testing target search capabilities. The chaser separates and thrusts away from the target until its sensors lose the target at long range. The chaser moves into a safety ellipse, simulating first approach to an uncooperative target as in a full service mission. In a full mission, a combination of sensor data, including GPS and ground tracking, is used for the FDS to calculate a trajectory to insert the chaser on to a rendezvous trajectory with the target. In the ELSA-d mission, the FDS is still used but the demonstration is performed off-line. The chaser comes within a medium range of the target, eventually performing an absolute to relative navigation handover to transfer to relative navigation technologies and to make the final approach and non-tumbling capture.
Phase 6 to 7: Re-orbit and Passivation
In the final phase, the chaser performs a re-orbit maneuver to reduce the target altitude. This simulates the final de-orbit in a full mission. At a lower altitude, the craft is passivated. Both chaser and target proceed to an uncontrolled de-orbit burning up on re-entry. The mission at all times maintains 25 year debris mitigation compliance, as the initial demonstration altitude is only 550 km. The full duration of the mission is expected to last up to 6 months, including non-demonstration (routine) phase periods.
In phase 3, as part of a safety test, a manual safety abort can be performed by the operator prior to capture to test an active abort scenario, which the chaser will perform if any fault conditions are identified during final rendezvous stages.
Subject to fuel availability, entire phases can be repeated. For example, phase 3 could be attempted twice to develop greater experience.
The mission CONOPS is designed in a fluid manner that give operators the final decision in spacecraft operations, and making up-to-date decisions about undertaking demonstrations based on satellite health and performance.
Capabilities and Technologies
Overview of Key Innovations: The following are key innovative capabilities in the ELSA-d mission.
1) End-to-end rendezvous solution including far-range and short-range approaches : Rendezvous and docking in space is among the most complicated technical challenges. To date, only manual docking or some limited autonomous docking (with many constraints) has ever been attempted in space (e.g. ATV, Orbital Express, ETS-7, Dragon). ELSA-d utilizes an integrated suite of technology for rendezvous and capture including both hardware (processing, sensing and control) and software (guidance and navigation algorithms, control laws), enabling these complicated scenarios to be undertaken in space efficiently and safely.
2) Search for targets and approach with absolute to relative navigation hand-over : Searching for and discovering an object in space is a complex technical challenge. ELSA-d's search is performed by using absolute navigation (ground-based radar or optical methodologies plus the chaser's GPS system) to get within a knowledge boundary. On first acquisition of the target, relative navigation is switched to in an absolute to relative navigation handover phase. Final approach is achieved using relative navigation.
3) Fly-around inspections of target with operator assessment : A fly-around (diagnosis) stage enables an operator to visually examine the chaser before final approach. This may be useful if communications with the target have been lost.
4) Docking plate to enable semi-cooperative removal : The DP is a core part of ELSA-d's rendezvous suite, providing a point of contact on the target for a magnetic capture system, and also provides an optically controlled surface for GNC. The DP turns the capture into a semi-cooperative case, compared to the more complicated uncooperative case.
5) Magnetic capture of non-tumbling and tumbling targets : AS has developed an innovative magnetic capture technology for use in capture. The technology improves on the shortcomings of both tethered systems (tether dynamic issues, complexity / jamming of a reeling mechanism, difficulty in controlling target attitude) and robotic systems (degree of complexity, cost).
6) Re-orbit, de-orbit and passivation capabilities : ELSA-d uses chemical propulsion to provide both re-orbiting and de-orbiting capability. A re-orbit to a lower altitude simulates immediate evacuation from the operating altitude, which is needed in future missions to quickly take a satellite out of harms way from other satellites in that orbit.
7) Mission designed with safety evacuations and passively safe trajectories in mind : Mission safety is of paramount importance to ELSA-d to ensure there is no further debris generation in space. Safety is also a large part of having a licensed mission design. The mission's range of safety features includes (but is not limited to): collision avoidance maneuvers (passive and active aborts), ability to move to an evacuation point, ability to enter a protected safety ellipse, and ground segment oversight during critical phases.
8) Ground segment designed specifically for in-orbit servicing : Unlike a conventional ground segment, ELSA-d's ground segment is specifically designed with in-orbit servicing in mind. Features include the ability to chain and align ground station passes to service longer demonstration scenarios while providing operator-in-the-loop safety.
Magnetic Capture System:
ELSA-d's capture system enables magnetic capture of tumbling objects using a specialized capture mechanism. The system has a set of small concentric permanent magnets which are extended and retracted using a mechanism to allow connection with the docking plate on the target. Once it attaches to the docking plate, the capture system can also release when desired using an internal mechanism that slowly pushes the docking plate away. This enables repeated docking and undocking cycles.
The ELSA-d grappling interface is designed to be mounted on a target satellite and consists of a flat, disc-shaped DP (Docking Plate) on top of a supporting stand-off structure. It provides distinctive functions that make a defunct satellite easier to identify, assess, approach, capture, and de-orbit, thus minimizing future costs of removal. Specific characteristics of the Astroscale DP, shown in Figure 5, which facilitate navigation and capture include: optical markers for guidance and navigation in proximity operations, a flat reflective plane for precise distance and attitude measurement, and ferromagnetic material suitable for magnetic grappling concepts.
Figure 5: ELSA-d: A prototype of the DP (optical markers not visible), image credit: Astroscale
• In December 2018, Astroscale UK signed a cooperation agreement with ESA (European Space Agency) to exchange data and expertise related to space debris collision avoidance, environmental monitoring of debris and the development of monitoring techniques. 8)
The Astroscale/ESA technical agreement relates to Astroscale's ‘End-of-Life Service by Astroscale-demonstration' (ELSA-d) mission. ELSA-d will consist of two spacecraft, a Chaser and a Target, stacked together for launch and initial deployment into orbit. Then, the Chaser will repeatedly release and capture the Target in a series of live demonstrations to prove the technology necessary for debris removal, including rendezvous and docking and proximity operations.
Under this no-funds-exchanged agreement, ESA will provide Astroscale with collision assessments for the ELSA-d Chaser satellite, including collision avoidance maneuver recommendations and screening. This is similar to the services provided for ESA's own and other European missions by ESA's Space Debris Office, located in Darmstadt, Germany.
ESA will also present a request for tracking support for the ELSA-d Target and Chaser satellites to the ILRS (International Laser Ranging Service), which will include both the Target and Chaser on the ILRS' list of objects to be followed.
Astroscale will install satellite laser ranging reflectors on the Chaser and Target satellites to enable high-precision orbit determination and provide GPS measurements at least twice a day.
Director of Astroscale UK Ltd, John Auburn, commented, "Astroscale is launching its pioneering ELSA-d mission in early 2020 and is delighted to work with ESA in several crucial technical areas. ELSA-d is scheduled to be operational for around six months, so it is great to utilize its time in orbit to collect useful scientific data in addition to testing our de-orbiting technologies."
The agreement also provides ESA with access to camera time over three observation periods. The camera will be used to test the principle of detecting small pieces of space debris crossing the field of view, which will generate important data to be used by ESA in developing a future, larger optical instrument. ESA and Astroscale will further work together on the feasibility of the observation approach and identify a workable solution.
In return, ESA will provide support and advice on its flight-proven ground control software products that will be used by ELSA-d, primarily through the review of mission control design documentation and advice on configuration and system validation and testing.
Figure 6: Astroscale's Space Debris Removal Mission, ELSA-d (image credit: Astroscale)
Launch: A launch of the ELSA-d mission (Chaser and Target as secondary payloads) is planned for early 2020 on a Soyuz-2-1b Fregat-M vehicle from the Baikonur Cosmodrome, Kazakhstan.
Ground Segment and Operations
ELSA-d utilizes the National In-orbit Servicing Ground Segment Facility hosted at the Satellite Applications Catapult (UK) and developed by AS (prime) with Catapult, RHEA, GMV, and SciSys subcontracts. The facility has been developed as a multi-mission facility with a long-term view to provide capability for a variety of IOS missions.
The control center has, at its core, a Mission Control System for the chaser spacecraft and one for the target spacecraft. The center interfaces to a number of external entities including Astroscale's own ground station in Totsuka (Japan), external ground stations for contact with the chaser and target, and a ground support center in Tokyo. It is built in the virtualized environment of a CEMS cloud infrastructure. Satellite communications are based on CCSDS standards and a core suite of ESA software tools are part of the system. The main components are as follows:
1) MCS (Mission Control System): The MCS is responsible for controlling and monitoring the spacecraft. It is based on ESA MICONYS SCOS-2000 framework. The Mission Database (MIB) is considered a sub-component of the MCS. The MCS relies on the File Based Operations (FBO) approach which guarantees improved reliability on command sequences, bandwidth optimization, file compression, and use of standard file formats.
2) FDS (Flight Dynamics System): The FDS determines both the position and the orientation of satellites, and enables the planning and execution of required maneuvers. It is responsible for LEOP calibrations, planning docking and orbit maintenance, conjunction avoidance maneuvers with other resident space objects, de-orbiting and re-entry planning, and station-keeping operations. Classical functionalities, such as orbit and attitude determination, use advanced filtering techniques to fuse multiple measurements sources and absolute/relative navigation features. In addition, the FDS provides IOS specific functionalities such as diagnosis trajectory optimization, active abort maneuver reconstruction, safety orbit planning and capture trajectory optimization. A 2D/3D visualization tool is built-in to the FDS.
3) IPS (Image Processing System): The IPS is in charge of the estimation of the docking plate attitude information from a live stream of images taken by the chaser spacecraft. It includes advanced feature detection capabilities that allow the chaser to estimate the docking plate attitude information even when it is not directly visible.
4) MPS (Mission Planning System): The MPS is used to plan activities and use of resources (e.g. data budget). The MPS receives data from the FDS, the MIB, and the Mission Operations Preparation Tool (MOIS Preparation) to construct a coherent schedule, which can then be uplinked to the satellite, executed and verified by telemetry. The MPS is able to automatically negotiate passes with the ground stations providers based on the user needs. In addition to classical MPS functionalities, it automatically manages passes over ground stations in order to have (when possible) an uninterrupted stream of telemetry when switching from one ground station to the other.
5) Automation System (MOIS): The automation system is responsible for the automatic execution of schedules produced by the MPS and preparation system. It also provides validation and test harness components which can be used for testing before going operational, and a validator component which can be used alongside the MCS to validate that correct procedures are being executed.
6) GSCG (Ground Station Control System): The GSCG is based on the ESA SCOS2000 NIS component. It is used to interface the MCS and ground stations conforming to the CCSDS SLE standard.
7) SIM (Simulator): The simulator is implemented using the ESA SIMULUS Simulation framework. It provides an end-to-end simulation of both the chaser and target spacecraft. The simulator includes a highly realistic chaser model that emulates the OBCs. This feature not only allows AS to run faster than realtime simulations but also to run on-board software in a plug-and-play fashion. The purpose of the simulator is testing and validation of operational procedures and databases, support the training of operators, and execution of simulation campaigns. Moreover, the simulator is used for testing and validating the on-board software in an operational environment.
The mission will be split into four main phases with respect to operations: LEOP, Commissioning, Critical Phases and non-Critical Phases. Docking and rendezvous will be performed during Critical Phases, and the mission will be continuously operated. Astroscale will be fully responsible for conducting all the operations for the chaser and target at the control center in Harwell.
The control room has several operator desks; each desk includes a thin client for remotely connecting to the data center. As all the MCC systems are running in a virtualized environment inside of the data center, the positions and roles of the desks are flexible. The data center will be replicated in two different locations in order to ensure the reliability of the MCC. In addition, every system will be composed of a primary and backup server, where data is replicated in near real-time across servers and data center locations. The control center and associated communication channels are designed with data encryption in mind.
In summary, the ELSA-d mission is an important step towards fully operational EOL (End of Life) and ADR (Active Debris Removal) missions by maturing technologies and capabilities necessary for future services. In particular, the ELSA-d mission will not just space-prove future payload technologies but will also go through almost the full series of CONOPS expected in a full servicing mission with a demonstration target.
Next Steps: ELSA-d is Astroscale's first IOD mission that is part of a roadmap of other IODs and capability developments for future EOL and ADR services. Presently, Astroscale is working with future customers and is in the early stages of developing a supply chain capable of enabling high volume production of chasers.
2) "SSTL and ASTROSCALE team up for Orbital Debris Removal missions," SSTL Press Release, 21 November 2017, URL: https://www.sstl.co.uk/media-hub/latest-news/2017/sstl
4) Jeff Foust, "Astroscale to partner with SSTL on orbital debris removal mission," Space News, 21 November 2018, URL. https://spacenews.com/astroscale-to-partner-
5) S. Eckersley, C. Saunders, D. Gooding, M. Sweeting, C. Whiting, M. Ferris, J. Friend, L. Forward, G. Aglietti, A. Nanjangud, P. Blacker, C. Underwood, C. Bridges, P. Bianco, "In-Orbit Assembly of Large Spacecraft Using Small Spacecraft and Innovative Technologies," Proceedings of the 69th IAC (International Astronautical Congress) Bremen, Germany, 1-5 October 2018, paper: IAC-18.B4.6A.1, p. 10, URL: https://iafastro.directory/iac/proceedings/IAC-18/IAC-18/B4/6A/manuscripts/IAC-18,B4,6A,1,x43225.pdf
6) Chris Blackerby, Akira Okamoto, Kohei Fujimoto, Nobu Okada, Jason L. Forshaw, John Auburn, "ELSA-d: An in-orbit End-of-Life Demonstration Mission," Proceedings of the 69th IAC (International Astronautical Congress) Bremen, Germany, 1-5 October 2018, paper: IAC-18.A6.5, URL: http://astroscale.com/wp-content/uploads/2018/
7) Chris Blackerby, Nobu Okada, Akira Okamoto, Eriko Yamamoto, Charity Weeden, Jason Forshaw, John Auburn, Karen Rogers, "Perspectives from a Venture Space Company on Regulatory Frameworks for Addressing Space Debris," Proceedings of the 69th IAC (International Astronautical Congress) Bremen, Germany, 1-5 October 2018, paper: IAC-18.A6.8.6, URL: https://iafastro.directory/iac/proceedings/IAC-18
8) "Astroscale enters technical cooperation with European Space Agency," Astroscale Press Release, 12 December 2018, URL: https://astroscale.com/astroscale-enters-technical-
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 (email@example.com).