ISS Utilization: SAGE-III
ISS Utilization: SAGE-III (Stratospheric Aerosol and Gas Experiment-III)
SAGE-III, a NASA instrument built as part of the EOS (Earth Observing System) program, is an existing grating spectrometer instrument that measures near-UV/visible/near-IR energy through the Earth's limb during solar and lunar occultations and, during the daytime side of the orbit, limb scattering.
The President's FY 2011 budget provides an opportunity to refurbish and recalibrate SAGE-III for a possible flight on the ISS by 2014. Observing from the ISS, SAGE-III will provide near-global, long-term measurements of key components of the Earth's atmosphere important to climate and chemical processes. The most important of these key measurements are the vertical distribution of aerosols from the upper troposphere through the stratosphere, and ozone from the upper troposphere through the mesosphere. In addition, SAGE-III will provide high vertical resolution profile measurements of trace gases such as water vapor and nitrogen dioxide.
Three copies of SAGE-III were produced. One instrument was mounted on the Meteor-3M spacecraft that launched in 2001 and a second was carefully packed away awaiting a flight of opportunity. The SAGE- III - ISS and the other two SAGE-III instruments were developed and managed by NASA/LaRC (Langley Research Center) and built by Ball Aerospace in Boulder, CO.
The SAGE family of instruments was pivotal in making accurate measurements of the amount of ozone loss in Earth's atmosphere. SAGE has also played a key role in measuring the onset of ozone recovery resulting from the internationally mandated policy changes that regulated chlorine-containing chemicals, the Montreal Protocol, which was passed in 1987. 1)
• SAGE-III/ISS was delivered to NASA/LaRC and is in storage since 2004
• Instrument assessment, including powered radiometric testing, was performed in 2009 and concluded that the instrument is still an excellent flight candidate
• An ESA-provided HPS (Hexapod Pointing Platform) will be used to compensate for major variations in ISS orientation
• A detailed accommodation study with the ISS program is being performed to determine the best location to mount SAGE-III. Mounting location selection criteria include:
- Clear measurement FOV (Field of View)
- ISS environment including contamination and disturbances.
The objective of SAGE-III is to provide global, long-term measurements of key components of the Earth's atmosphere. The most important of these are the vertical distribution of aerosols and ozone from the upper troposphere through the stratosphere. In addition, SAGE-III also provides unique measurements of temperature in the stratosphere and mesosphere and profiles of trace gases such as water vapor and nitrogen dioxide that play significant roles in atmospheric radiative and chemical processes.
Table 1: The SAGE-III/ISS legacy
For nearly thirty years, NASA's SAGE (Stratospheric Aerosol and Gas Experiment) family of remote-sensing- satellite instruments continuously measured stratospheric ozone (O3) concentrations, aerosols, water vapor, and other trace gases. However, there has been a nearly decade-long gap in SAGE measurements since the SAGE-III Meteor-3M (launched in 2001) ended on March 6, 2006.
The first SAGE mission (SAGE-I) launched February 18, 1979, on the ARM-B (Applications Explorer Mission-B) satellite of NASA. The mission collected valuable data for nearly three years until the satellite's power system failed. SAGE-II launched onboard the ERBS (Earth Radiation Budget Satellite) in October 1984, and observed stratospheric O3 from 1984 until 2005. Data from SAGE II were integral in confirming human-driven changes to O3 concentrations in the stratosphere, and thus influenced the decisions to negotiate the Montreal Protocol in 1987. Later, observations from SAGE-II showed that O3 in the stratosphere stopped decreasing in response to the actions agreed to in the treaty. 5)
• January 17, 2017: The SAGE III instrument was delivered from NASA's Langley Research Center to NASA Kennedy Space Center (KSC) in November 2015 and is now completely ready for launch. "Final powered testing was completed at the KSC Space Station Processing Facility during the past year and in December 2016 the instrument payload and the Nadir Viewing Platform were handed over to SpaceX for installation into the Dragon Trunk of the Falcon 9 rocket. The integrated trunk is now awaiting launch," Marilee M. Roell, SAGE III Science Manager at NASA/LaRC, told SpaceFlight Insider. 6)
• In November 2015, SAGE-III was shipped to KSC (Kennedy Space Center) in Florida. 7)
Figure 1: Photo of the SAGE-III project team at NASA/LaRC when the SAGE-III payload left for KSC (image credit: NASA/LaRC)
• In February 2015, the HPS (Hexapod Pointing System) of ESA (European Space Agency) was delivered to NASA/LaRC. HPS is a key component to the SAGE-III instrument to automatically adjust the pointing of SAGE-III to compensate for variations in the orientation between the space station and Earth during its orbit, maintaining the instrument perpendicular to the Earth's surface. - HPS was several years in storage prior to NASA/LaRC delivery. 8)
Figure 2: The SAGE-III instrument on top is shown with the Hexapod Pointing System in a clean room of NASA/LaRC (image credit: NASA, David C. Bowman)
Launch: SAGE-III was launched on the commercial SpaceX Falcon 9 v1.1 FT vehicle and Dragon spacecraft CRS-10 (Cargo Resupply-10) on February 19, 2017. The launch site was NASA's historic Launch Complex 39 Pad A of the Kennedy Space Center, Cape Canaveral SLC-39A, FL. 9) 10) 11)
Table 2: CRS-10 mission overview 12)
Orbit of the ISS: Near-circular orbit with an altitude of ~400 km, inclination = 51.6º.
SAGE-III will be robotically mounted onto the ELC, a NVP (Nadir Viewing Platform) on the International Space Station.
SAGE-III will be transferred from the Dragon Trunk attached to a FRAM (Passive Flight Releasable Attachment Mechanism) Adapter Plate 3 (PFAP 3), and installed to the ELC-4 (ExPRESS Logistics Carrier -4) site via the SSMS (Space Station Remote Manipulator System). The SAGE-III investigation is then mounted on a preinstalled NVP (Nadir Viewing Platform) that provides the required interface and structure. This move requires the use of the SPDM (Special Purpose Dexterous Manipulator) and temporary stowage on the SPDM's EOTP (Enhanced ORU Temporary Platform) FRAM Site.
The SAGE-III instrumentation has a total mass of 527 kg and a data rate of 2150 MB/day.
Figure 3: Prior to launch, the SAGE-III instrumentation will be mounted inside the Dragon trunk of SpaceX (image credit: NASA)
Figure 4: Planned NASA Earth Science Instruments for the International Space Station(image credit: NASA) 13)
Figure 5: This illustration shows the Instrument Payload attached to the Nadir Viewing Platform on the ELC-4 (image credit: NASA)
• March 8, 2017: Just a little more than two weeks after its Feb. 19 launch on a SpaceX Falcon-9/Dragon spacecraft, SAGE-III (Stratospheric Aerosol and Gas Experiment III) is now safely installed on the outside of the International Space Station, where it will monitor ozone and aerosols in Earth's upper atmosphere. — In a highly choreographed sequence of events, the station's robotic Canadarm2 removed the instrument payload and its NVP (Nadir Viewing Platform) from the Dragon trunk and installed them on the ExPRESS Logistics Carrier platform. The entire sequence took about four days. The NVP allows SAGE-III to face nadir, or down, toward Earth. 14)
- Final installation of the instrument payload, which includes SAGE-III and its hexapod pointing system, took place March 7.
- "With a flawless launch and the on-orbit assembly sequence behind us, the SAGE-III and the space station teams have completed a major milestone," said SAGE-III Project Manager Mike Cisewski. "Our team is ready to proceed with payload commissioning and our long-term work of extending the SAGE data record from the station."
- Activation and calibration of SAGE-III will take approximately 90 days. Brooke Thornton, SAGE-III mission operations manager, and the mission operations team will closely monitor those activities from the Flight Mission Support Center at NASA/LaRC (Langley Research Center) in Hampton, Virginia.
- "The operations team has been working so hard to get fully prepared for this and now that it is here it's almost a relief," said Thornton. "This team has operated and tested this payload for nearly five years now, so I know we have the best team to get us through activation and all the extensive calibration activities. We're ready to get into routine operations."
Figure 6: The SAGE-III is visible here on its new home on the International Space Station's ExPRESS Logistics Carrier platform (image credit: NASA)
The SAGE-III investigation observes the vertical profile of Earth's atmosphere by measuring the extent of our protective ozone layer, nitrogen dioxide (NO2) levels, water vapor and aerosols in the atmosphere. Atmospheric occultation measurements (i.e., measuring light transmitted through the atmosphere) are performed as the sun or moon is rising or setting. By measuring the composition of the middle and lower atmosphere from the unique vantage point of the ISS, researchers can monitor and help to better understand and quantify the long-term changes and our impact on Earth. 15) 16) 17) 18)
The science goals of the mission are to:
• Assess the state of recovery in the distribution of O3
• Re-establish the aerosol measurementsneeded by both climate and O3 models; and
• Gain further insight into key processes contributing to O3 and aerosol variability.
Figure 7: By using the sun and moon as light sources, SAGE can detect O3, aerosols and other trace gases in the atmosphere (image credit: NASA)
SAGE -III is the first instrument to measure the composition of the middle and lower atmosphere from the ISS. The orbit of the ISS provides a perfect vantage point from which to acquire measurements of this region of the atmosphere. The station's orbital path will allow SAGE-III to observe O3 during all seasons and over a large portion of the globe. SAGE-III on ISS will also measure O3 concentrations deeper into the atmosphere than previous SAGE measurements, reaching down into the troposphere. Another benefit of flying onboard the ISS is that scientists and engineers will also have near-continuous communications with the payload.
SAGE-III relies upon the flight-proven designs used in the SAM-I (Stratospheric Aerosol Measurement-I) and SAGE -I and -II instruments. SAGE-III, like its predecessors, is a grating spectrometer that measures ultraviolet/visible energy. However, SAGE-III has a few upgrades. The new design incorporates CCD (Charge Coupled Device) array detectors and a 16 bit A/D converter. The new CCD array detector enhances the measurement capability and may allow for new experimental data products like methane (CH4), bromine monoxide (BrO), and iodine monoxide (IO).Combined, these devices allow for wavelength calibration, a self-consistent determination of the viewing geometry, lunar occultation measurements and expanded wavelength coverage.
SAGE- III on ISS consists of two separate payloads—the Instrument Payload and the Nadir Viewing Platform. Combined, the SAGE-III payloads have a mass of 527 kg and a data rate of 2150 MB/day.
The Instrument Payload includes a SA (Sensor Assembly), IAM (Interface Adapter Module), a DMP (Disturbance Monitoring Package), the Hexapod Pointing System[HEU (Hexapod Electronics Unit and) HMA ( Hexapod Mechanical Assembly)], two CMP (Contamination Monitoring Packages), and the ICE (Instrument Control Electronics) box (Figure 8).
The SAGE-III SA (Sensor Assembly) — a grating spectrometer that measures ultraviolet (UV) and visible light and has a two-axis pointing system—consists of three subsystems: the scan head, imaging optics, and the spectrometer detector (Figure 9). These subsystems are employed to acquire light from either the sun or moon by vertically scanning across them. Once the instrument is powered on, light that is brought into the spectrometer by the telescope is broken up into UV, visible, and infrared wavelengths from 280 to 1040 nm by the grating spectrometer and sent to the CCD array. The measurements are made using a ratio: the amount of light passing through the atmosphere compared to the amount of light coming directly from the sun out-side the atmosphere. By measuring the amount of absorption of radiation at various heights throughout the atmosphere at different wavelengths, SAGE-III can infer the vertical profiles of O3, aerosols, water vapor, and NO2. Additional aerosol information is provided by a discrete photodiode at 1550 nm.
Figure 9: Illustration of the three subsystems that make up the SAGE-III SA(Sensor Assembly), image credit: NASA
Several busy operations onboard the ISS can interfere with science observations, e.g., visiting instrument traffic and thruster operations. To avoid contamination from such operations, the Instrument Payload includes two CMP (Contamination Monitoring Packages) to monitor the environment surrounding the instrument. If the space station environment contains elevated contamination levels, a transparent contamination door will close to protect the instrument's sensors while allowing measurements to continue.
The IAM (Interface Adapter Module) acts as the "brain" of the instrument payload, providing power and computing to the payload and acting as the interface between the instrument and the space station. The DMP (Disturbance Monitoring Package) is a miniature inertial measurement unit that will measure all small motions from space station operations. These measurements will be used to help identify and reduce noise in the instrument signal caused by the space station's vibrations. The HPS (Hexapod Pointing System) supports the payload and keeps the instrument level with respect to Earth while in orbit (see next chapter on HPS).
Nadir Viewing Platform: To orient SAGE-III facing nadir, or toward Earth, a special L-shaped mounting bracket called the Nadir Viewing Platform (Figure 10) was designed, built, and tested at LaRC. The Nadir Viewing Platform will attach to ELC-4 (ExPRESS Logistics Carrier-4)) onboard the station, perpendicular to the plane of the ELC, providing the nadir-orientation needed by the Instrument Payload. It replicates the standard ELC exposed-payload attachment. The ELC-4 will provide electrical power and command and data-handling services, and the Nadir Viewing Platform provides electrical power and data services to the SAGE-III instrument. Other space station-based payloads have already used the Nadir Viewing Platform's design, turning a traditional ELC site into a nadir-oriented site.
Figure 10: Illustration of the SAGE-III nadir viewing platform (image credit: NASA/LaRC)
Figure 11: NASA/LaRC scientists are checking SAGE-III in preparation for its trip to the International Space Station (image credit: NASA/LaRC)
The SAGE-III event locations are modulated by the number of sunrise/sunset events experienced by the ISS in its inclined orbit plus some loss due to obscuration of the sun by the ISS platform and limitations to operations due to the presence of visiting spacecraft. Overall, the project expects to acquire 75% of the total events possible (~8000 solar events per year).
Figure 12: Nominal SAGE-III/ISS measurement locations (image credit: NASA/LaRC)
HPS (Hexapod Pointing System)
The ESA (European Space Agency) provided instrument HPS (Hexapod Pointing System) is intended to provide a stable nadir pointed platform to the NASA instrument SAGE-III. The Hexapod is one of the elements developed by ESA for NASA in return for ISS utilization rights prior to the launch of the ESA Columbus Laboratory.
Table 3: Some background on the development of Hexapod 20)
Already in 1999, lifetime and lubrication tests had been used as guidelines for flight unit screws selection of Hexapod. The rationale for this test campaign was the lack of information on roller screw mechanism performances and lubrication behavior for vacuum exposed long mission, where no maintenance is foreseen. 21)
The HPS will be carried on one ExPA (Express Pallet Adapter) nadir oriented payload accommodation facility, located on the Integrated Truss Assembly of the ISS. The core of the HPS positioning/pointing mechanism is made by six electromechanical linear actuators disposed as three trapezoids between two reference planes, indicated as the lower and the upper platforms. The lower platform is fixed to the ExPA and the position and the attitude of the upper one is defined by the lengths of the six linear actuators.
Figure 13: Schematic view of the Hexapod - SAGE-III layout (image credit: Alenia Aerospazio)
The HPS includes an external off-loading device designed to partially support the launch loads, thus optimizing the actuators performances for on-orbit environment. The control and pointing function are powered and commanded by an electronic subsystem, the Hexapod Control Unit. The HPS is a project developed under an ESA contract, with the involvement of Alenia Aerospazio (as prime contractor) and Carlo Gavazzi Space, with the participation of ADS International and AEA/ESTL to these lifetime tests activity.
Figure 14: Photo of the Hexapod development model (image credit: Alenia Aerospazio)
On November 25, 2005, the ESA Hexapod reached the end of its development phase with the completion of the Flight Acceptance Review at ESA/ESTEC. TAS-I (Thales Alenia Space-Italy), former Alenia Aerospazio, was responsible for the development, design, integration, and test of the Hexapod Pointing System. 22)
The Hexapod is a high accuracy pointing system, developed to support ISS external payloads. The flight unit of the Hexapod has a mass of 116 kg and consists of the HEU (Hexapod Electronic Unit) and the HMA (Hexapod Mechanical Assembly). The HEU handles power distribution, telemetry and telecommand management, data processing, and command and control. The HMA includes six linear actuators arranged as three trapezoids and connected to a bottom flange and an upper platform. Varying the lengths of the actuators provides the possibility to control the attitude and position of payloads attached to the upper platform in six degrees of freedom.
The HPS includes an external off-loading device designed to partially support the launch loads, thus optimizing the actuators performances for on-orbit environment. The control and pointing function are powered and commanded by an electronic subsystem, the Hexapod Control Unit.
ESA's development of the Hexapod marks the upgrading of hexapod-based positioning/pointing systems for space applications. The Hexapod is designed for five years of in-orbit operation without maintenance, though users will be able to up-link Hexapod flight-software updates.
Figure 15: Photo of the Hexapod assembly below the SAGE-III instrument (image credit: ESA)
Ground System and Data
The FMSC (Flight Mission Support Center) at NASA/LaRC, VA (Figure 16), together with the ISS Command and Control group at the MCC-H (Mission Control Center-Houston at NASA/JSC (Johnson Space Center) and the POIC (Payload Operations Integration Center) at NASA/MSFC (Marshall Space Flight Center) in Huntsville, AL, will command and operate the instrument, as well as manage and distribute the data. This includes oversight of the installation and instrument checkout, managing routine planning and command loads, data analysis, and resolution of any issues that may arise.
Figure 16: This graphic shows how data travel from SAGE-III on ISS to the SCF and get released through the ASDC (image credit: NASA)
To communicate with SAGE-III on ISS, the SPOC (SAGE-III Payload Operations Center) at the FMSC will send software commands to NASA's TDRSS (Tracking and Data Relay Satellite System). Once data are transmitted and collected, they will be processed at the SCF (Science Computing Facility) and released to the public directly through the ASDC (Atmospheric Science Data Center).
Measurements from SAGE-III on ISS will be validated as they have been during previous SAGE missions: Data from a number of independent instruments using a variety of techniques will be used to assess sensor biases and precision. Validation plans for SAGE-III on ISS include working with ongoing ground-based operations, including the Network for the Detection of Atmospheric Composition Change, spaceborne sensors, and balloon-based O3 measurements, including those from the Southern Hemisphere Additional Ozonesondes (SHADOZ) network.
Table 4: SAGE-III/ISS production products
Legend to Table 4: *or cloud-top altitude, **defined at 600 nm, TP = altitude of the tropopause.
In addition to the production products, the project plans to explore some promising research products including for both solar events (BrO, CH4, and IO) and lunar (NO2, NO3, OClO) event types.
In summary, data from SAGE- III on ISS, coupled with model results, will allow scientists to monitor the health of the ozone layer and track the recovery of stratospheric O3 since ratification of the Montreal Protocol. By the 2020s, expectations are that O3 will recover to about half of the amount lost from pre-1980 levels. SAGE-III on ISS will also be valuable in assessing the performance of OMPS flying on the Suomi-NPP (National Polar-orbiting Partnership) satellite and planned for additional flights on the JPSS (Joint Polar Satellite System). In addition, data from SAGE-III on ISS will help to re-institute aerosol measurements crucial for more-accurate, long-term climate and O3 concentration and distribution models. Finally, if there are any new threats to the ozone layer, SAGE-III on ISS data will help the scientific community identify the cause and assess the threat.
After SAGE-III on ISS, a new generation of instruments will be needed to continue the long-term record of stratospheric O3 and aerosol concentrations.
2) David Flittner, J. Zawodny, M. Cisewski, D. MacDonnell, R. Moore, L. Thomason, "Overview of Stratospheric Aerosol and Gas Experiment (SAGE III) on the International Space Station," AIAA Space 2010 Conference & Exposition: 'Future Earth Science Missions and Enabling Activities,' Aug. 30 to Sept. 2, 2010, Anahein CA, USA, URL: http://esto.nasa.gov/conferences/space2010/presentations/02_Flittner_SAGE_III.pdf
3) Tony Phillips, "Don't let this happen to your planet," NASA Science News, March 29, 2013, URL: http://science.nasa.gov/science-news/science-at-nasa/2013/29mar_sage3/
4) "Space Station Bound SAGE III is Full Steam Ahead," NASA, Feb. 22, 2013, URL: http://www.nasa.gov/mission_pages/station/research/news/sage_III.html
5) Heather Hansen, Kristyn Damadeo, "SAGE-III on ISS: Continuing the Data Record," The Earth Observer, November - December 2015. Volume 27, Issue 6, pp: 4-11, URL: http://eospso.nasa.gov/sites/default/files/eo_pdfs/Nov%20Dec%202015_508_col.pdf
6) Tomasz Nowakowski, "SAGE III to Provide Highly Accurate Measurements of Atmospheric Gases," Space Daily, Jan. 17, 2017, URL: http://www.spacedaily.com/reports/SAGE_III
7) Sam McDonald, "SAGE III Leaves Langley for Journey to the International Space Station," NASA/LaRC, Nov. 20, 2015, URL: http://fpd.larc.nasa.gov/sage-iii-leaves-langley-for-journey-to-iss.html
8) Michael Finneran, "European Space Agency Delivers Earth-observing Component to NASA," NASA, Feb. 25, 2015, URL: http://www.nasa.gov/larc/european-space-agency-delivers-earth-observing-component-to-nasa
9) Tabatha Thompson, Dan Huot, Stephanie Martin, "NASA Cargo Headed to Space Station Includes Important Experiments, Equipment," NASA, Feb. 19, 2017, Release 17-021 , URL: https://www.nasa.gov/press-release/nasa-cargo-headed-to-space-station-includes-important-experiments-equipment
10) "SpaceX CRS-10 Mission Overview," NASA, URL: https://www.nasa.gov/sites/default/files/atoms/files/spacex_crs-10_mission_overview.pdf
11) Ken Kremer, "NASA's Historic Pad 39A Back in Business with Maiden SpaceX Falcon 9 Blastoff to ISS and Booster Landing," Universe Today, Feb. 19, 2017, URL: http://www.universetoday.com/133508/nasas-historic-pad-39a-back-business-maiden-spacex-falcon-9-blastoff-iss-booster-landing/
12) "SpaceX CRS-10 Mission Overview," NASA, URL: https://www.nasa.gov/sites/default/files/atoms/files/spacex_crs-10_mission_overview.pdf
13) Steven Volz, "NASA Earth Science Flight Program Overview," Proceedings of JACIE 2014 (Joint Agency Commercial Imagery Evaluation) Workshop, Louisville, Kentucky, March 26-28, 2014, URL: https://calval.cr.usgs.gov/wordpress/wp-content/uploads/Volz_JACIE-Presentation.pdf
14) Joe Atkinson, "SAGE III Installed on Its New Home on the International Space Station," NASA, March 8, 2017, URL: https://www.nasa.gov/feature/langley/sage-iii-installed-on-its-new-home-on-the-international-space-station
15) Joseph M. Zawodny, "Stratospheric Aerosol and Gas Experiment III-ISS (SAGE III-ISS)," NASA Fact Sheet, April 3, 2013, URL: http://www.nasa.gov/mission_pages/station/research/experiments/1004.html#description
17) Kristyn Damadeo, "The SAGE Legacy's Next Chapter: SAGE III on the International Space Station," The Earth Observer, NASA, Sept.-Oct. 2013, Vol. 25, Issue 5, pp: 4-8, URL: http://eospso.gsfc.nasa.gov/sites/default/files/eo_pdfs/Sept_Oct_2013_508_color.pdf
18) Gloria Hernandez, Joseph M. Zawodny, Michael S. Cisewski, Brooke Thornton, Andrew Panetta, Marilee M. Roell, Marilee M. Roell, "On the Stratospheric Aerosol and Gas Experiment III on the International Space Station," 2014, URL: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140003897.pdf
19) "Staring at the Sun with SAGE-III," NASA/LaRC, March 8, 2013, URL: http://www.nasa.gov/multimedia/imagegallery/image_feature_2465.html
20) R. Stalio, P. Trampus, P. C. Galeone, R. Trucco, E. Anderson, K. Berens, "ISS-VIEW: A Software Tool for External Science Payloads Attached to the International Space Station," URL: http://www.esa.int/esapub/bulletin/bullet91/b91stal.htm
21) Piero Pochettino, Marino Ballesio, Daniele Gallieni, Steve Gill, "Hexapod / SAGE-III roller screws lifetime and lubrication tests," ESMATS (European Space mechanisms and Tribology Symposium)1999, Toulouse, France, URL: http://www.esmats.eu/esmatspapers/pastpapers/pdfs/1999/pochettini.pdf
22) Aldo Petrivelli, Dieter Isakeit, "Development of Hexapod reaches completion," ESA, Dec. 5, 2005, URL: http://www.esa.int/About_Us/Business_with_ESA/Development_of_Hexapod_reaches_completion
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).