CICERO (Community Initiative for Continuing Earth Radio Occultation) pathfinder mission
GNSS RO profiles provide robust, fundamental measurements of key variables required in climate research and weather forecasting. These data are also of great value in the study of "space weather" – disruptive storms in the ionosphere. GNSS RO has been particularly useful for NWP (Numerical Weather Prediction), providing a significant improvement in global forecasting over the past seven years. The primary measurement made by GNSS RO is atmospheric refractivity, a reflection of temperature, density, pressure, and water vapor content. 1)
CICERO is a planned constellation of LEO (Low-Earth Orbiting) microsatellites for performing GPS and Galileo radio occultation (GNSS-RO) of Earth's atmosphere and surface remote sensing by GNSS reflection. The overall goal of the system is to deliver critical data on the state of the Earth to scientists and decision makers worldwide. Products will include high-accuracy profiles of atmospheric pressure, temperature, and moisture; 3D maps of the electron distribution in the ionosphere; and a variety of ocean and ice properties. Principal applications will be weather forecasting, climate research, and space weather monitoring. 2) 3) 4) 5) 6) 7) 8) 9)
The CICERO program is a community approach - a "people's campaign" for continuing GNSS-RO (Radio Occultation) observations. The overall goal of the CICERO mission is to provide an array of 24 microsatellites in LEO, designed to perform Global Navigation Satellite System (GNSS) radio occultation measurements.
The main objective of the CICERO pathfinder mission is to deploy two microsatellites to demonstrate the capabilities of the CICERO small satellite bus while enhancing weather and climate forecasting capabilities.
Orbit of the constellation: 500 km circular initial orbit, 750 km circular final orbit, inclination = 72º; later add 28º inclination. The altitudes and orbits are optimized to maximize global distribution of occultation events.
• Phase 1: 6 orbital planes, 2 SC each (1st launch)
• Phase 2: 12 orbital planes, 2 SC each (2nd launch)
• Phase n: Missions of opportunity and other orbits.
Figure 1: The planned CICERO constellation (image credit: GeoOptics)
Background: In 2007, GeoOptics LLC, a commercial company of Pasadena, CA (founded in 2006), announced the operation of the CICERO project, which will eventually consist of 100 microsatellites in LEO (Low-Earth Orbit) performing GNSS-RO (Global Positioning System and Galileo atmospheric - Radio Occultation). 10)
Note: GeoOptics, an international consortium, was founded by working scientists to establish a new model of community based space development for the public good that could change the way the world collects and disseminates Earth observational data. The CICERO Project will inaugurate this model with a potent new technique for weather and climate sensing known as Global Positioning System Radio Occultation, or GPS-RO.
Until the launch of FormoSat-3 / COSMIC in April 2006, there had been little GPS-RO data available for performing forecast impact studies. The major source had been the German CHAMP satellite (launch July 15, 2000), which has delivered 150-200 profiles per day globally. Nonetheless, some evaluations were attempted. A European team (Healy and Thépaut, 2006) added a mere 80 GPS profiles to a data set that included hundreds of thousands of standard satellite and ground measurements and 20,000 AIRS profiles, every 12 hours. The minuscule GPS data set boosted the total information content of the data by 4%, on top of a 10% increase provided by the AIRS data. On a per-profile basis, that's roughly 100 times the information content from GPS-RO than from any previous remote sensing measurement. 11) 12) 13)
GeoOptics partners in the CICERO consortium (Ref. 9):
• Broad Reach Engineering – Founding Partner, Instrument and Flight System Contractor, Mission Design, Marketing
• Moog Inc. (NY) – Propulsion systems, "green" propellant
• ESRI (Environmental Systems Research Institute, Redlands, CA) – Data systems and applications software
• CU / LASP (University of Colorado / Laboratory for Atmospheric and Space Physics, Boulder, CO) – Flight instruments, systems engineering, science research, constellation operations, education and outreach 14)
• University of North Dakota – Mirror operations & data center, science, engineering, education and public outreach
• MMA Design – Spacecraft mechanical systems development
• SRI – special ionospheric and space weather micro-sensors.
Contracts from NOAA for commercial data purchase:
• In September 2016, NOAA made its first two awards under the Commercial Weather Data Pilot program created by Congress last year. The winners are Spire Global of San Francisco with the Lemur constellation and GeoOptics of Pasadena, CA with the CICERO constellation, both of which will provide radio occultation data to NOAA for evaluation to determine whether commercial data can be incorporated into NOAA's numerical weather models. 15) 16)
- Congress provided $3 million to NOAA in the FY2016 Commerce, Justice, Science appropriations act (Division B of the FY2016 Consolidated Appropriations Act) for the pilot program. It required NOAA to enter into at least one pilot project through an open competitive process to purchase, evaluate and calibrate commercial weather data and to submit a report on how it would implement the project. NOAA publicly released that report in April.
- The idea originated in the House-passed Weather Research and Forecasting Innovation Act (H.R. 1561) sponsored by Rep. Frank Lucas (R-OK) and Rep. Jim Bridenstine (R-OK). Bridenstine chairs the Environment Subcommittee of the House Science, Space, and Technology (SS&T) Committee and also serves on the House Armed Services Committee. He led efforts to include a provision in the pending FY2017 National Defense Authorization Act for DOD to create a similar program.
- Under the contracts, the two companies will provide GNSS radio occultation data to NOAA by April 30, 2017 to demonstrate data quality and potential value to NOAA's weather forecasts and warnings. NOAA/NESDIS (National Environmental Satellite, Data, and Information Service) will assess the data through the end of FY2017 and issue a report in early FY2018. The contract award amounts were $370,000 for Spire and $695,000 for GeoOptics.
- NOAA already uses GPS radio occultation (GPO-RO) data in its forecasts. The data are acquired by the six-satellite Formosat/COSMIC constellation, a joint program with Taiwan. NOAA is requesting funds for a COSMIC-2 follow-on.
• Prior to the NOAA contract, Spire already built commercial demand for its services from other customers. In addition to other weather agencies, Peter Platzer, CEO of Spire Global, said there are multiple markets beyond just government that have an interest in limiting the economic impact of extreme weather events by using space-based observations. 17)
- "Our customers are organizations that are highly knowledgeable in the weather space that have the capability to run numerical weather prediction models, and hence have the skill to consume GPS-RO data. There are significantly more private organizations and nongovernmental organizations that actually have this capability than we were aware of," said Platzer.
- Spire presented the world's first commercially collected and processed GPS-RO profiles this year at the IROWG (International Radio Occultation Working Group) conference. Platzer said the company was able to demonstrate that a commercial company could collect GPS-RO profiles and perform the necessary processing to convert them to atmospheric profiles. While highlighting that this feat was something many doubted could be done commercially, Platzer quickly adds that Spire relied heavily on government-supported research from NOAA, NASA, the Alfred Wegener Institute, the DLR, and others.
• While GeoOptics and Spire Global were awarded this first contract by NOAA, another company, PlanetiQ of Boulder, CO, also has plans to launch weather satellites in early 2018.
CICERO mission design from a microsatellite to a 6U CubeSat form factor
Originally, GeoOptics Inc. of Pasadena, CA, developed CICERO 115 kg microsatellites together with the UC/LASP (University of Colorado /Laboratory for Atmospheric and Space Physics) using the TriG GNSS-RO payload developed to fly on the FORMOSAT 7 / COSMIC-2 mission. Later during development, the massive miniaturization of the GNSS-RO payload, called CION (CICERO Instrument for GPS-RO), allowed the satellite design to be changed to a much smaller 6U CubeSat based version built by Tyvak Nanosatellite Systems.
In 2016, Tyvak Nanosatellite Systems Inc. of Irvine, CA signed a contract with GeoOptics Inc. of Pasadena, CA, to build the first tech demo CICERO (Community Initiative for Continuous Earth Remote Observation) satellite. Spacecraft development is underway and will be ready to launch within a few months of the contract start date. 18)
Tyvak will utilize its core 6U Endeavor platform to provide the proper technical performance and mission assurance traits necessary for GeoOptics to carry forward its business plan of delivering data to scientists and decision-makers around the world from sensors deployed on a constellation of small satellites in low Earth orbit. GeoOptics' quest to revolutionize data availability for earth science will finally move forward with this partnership.
"After exhaustive market research, Tyvak stood out as the company with the best technology and mentality to partner with. We couldn't find anyone else with a comparable integrated approach well catered toward high performance and high reliability missions," Thomas Yunck, Founder and CTO of GeoOptics stated.
The overall mission architecture, spacecraft design and development, payload integration, operations and data communication will be Tyvak's responsibility. 19)
Nanosatellite evolution: Nanosatellite have evolved to support powerful instruments for advanced operational capabilities and high mission utility.
- Operationally, they can fulfil critical government or commercial missions within a substantial mission life of >3 years.
- High utility: high level of performance, reliability and confidence in mission success.
• Tyvak is developing the GeoOptics CICERO constellation of satellites
- Completed the first three satellites in the constellation
- Intended to gather vital data on the Earth's weather, climate and environment
• The program utilizes Tyvak's Endeavour small satellite platform
- 6U CubeSat, high-speed communications, 3-axis high performance attitude control
- The CICERO constellation will test and demonstrate the newly developed instrument and the integrated satellites
• Data gathered will provide:
- Weather monitoring and forecasting information to NOAA (National Oceanic and Atmospheric Administration)
- Science and weather data to other government and commercial customers.
Figure 2: The GPS Radio Occultation instrument development emphasized an building an instrument of equal quality to previous instruments (image credit: GeoOptics, Tyvak)
CICERO Radio Occultation Constellation
- Demonstration and operational constellation of radio occultation observation satellites for a commercial customer
- Data products to be delivered to customer as part of a commercial data buy for near real-time weather data.
• Technologies developed and demonstrated
- Scientific RO (Radio Occultation) instrumentation made in collaboration with JPL
- Very low EMI (Electromagnetic Interference) spacecraft, extensive RF shielding
• Program challenges & experiences
- Low EMI requirement
- GPS antenna and RO payload development.
Figure 3: Illustration of the 6U Cubesat CICERO (image credit: Tyvak)
Figure 4: Artist's rendition of the deployed CICERO 6U CubeSat (image credit: Tyvak)
The CICERO-6 integrated 6U CubeSat has a mass of ~ 10kg, a solar array providing ~ 21 W of power.
Table 1: Key parameters of the 6U CICERO-6 CubeSat
Designs goals achieved and general capabilities:
• Near 100% duty cycle
- Fixed deployable solar arrays to allow nearly 100% duty cycle operations of the CION (CICERO Instrument for GPS-RO) instrument
• Data capacity
- 2 Mbit/s X-band transmitter can downlink all RO data collected in 1 orbit along with associated back-orbit telemetry
- Compatibility with KSAT (Kongsberg Satellite Services AS) Network for fast data delivery
• Algorithms to target RO opportunities
- Calculated on the ground and then event scripts for vehicle slews for optimal RO opportunities are executed
- Will be automated on the vehicle using onboard propagator in future batches
• GPS RO instrument improvements
- Updated software to support GLONASS
- Software upload and re-flash capabilities.
Launch: The CICERO-6 nanosatellite was launched on June 23, 2017 (03:59 UTC) as a secondary payload (co-passenger) on the PSLV-C38 vehicle in XL configuration of ISRO from SDSC (Satish Dhawan Space Center), India. The primary payload on this flight was CartoSat-2E (~712 kg), the sixth satellite in the Cartosat-2 series (total launch mass of ~955 kg). 20) 21)
Orbit: Sun-synchronous near-circular orbit, altitude of 505 km, inclination = 97.44º, LTDN (Local Time on Descending Node) at 9:30 hours.
Secondary payloads: 30 satellites (co-passengers) with a total mass of 243 kg):
The 29 international customer nanosatellites were launched as part of the commercial arrangements between Antrix (Antrix Corporation Limited), a Government of India company under DOS (Department of Space) and the commercial arm of ISRO and the International customers.
The Dutch company ISISpace (Innovative Solutions In Space) of Delft accommodated most of the secondary payloads aboard the multi-satellite launch (responsible for manifesting a total of 23 satellites on this particular launch). Engineers stowed the CubeSats in QuadPacks before shipping them to the Indian launch site. 22)
Among these 23 satellites, there are 8 CubeSats that will complement and complete the QB50 constellation for upper atmospheric research (www.qb50.eu). This project, sponsored by the European Commision's FP7 is managed by the Von Karman Institute from Belgium and ISISpace has been one of the consortium partners of QB50 since the start. The CubeSats launched by ISISpace into polar orbit will work together with the CubeSats deployed from the International Space Station in May.
The ISILaunch19 manifest also includes several payloads that were initially scheduled for a launch on a Falcon 9 mission through Spaceflight's Sherpa mission, a launch that was significantly delayed due to various factors. ISL and Spaceflight have jointly worked on re-manifesting these payloads to this PSLV launch to make sure our customer's satellites were launched earlier. Such a re-manifesting of payloads clearly shows the added value of the ISILaunch Services of ISISpace to serve the CubeSat community, offering access-to-space flexibility towards customers of launch services by operating across multiple launch vehicles and missions in parallel.
Figure 5: Overview of timeline and satellites handled by ISISpace in their ISILaunch 19 campaign (image credit: ISISpace)
• NIUSAT (Noorul Islam University Satellite), located in Kumarakovil, Thuckalay, Kanyakumari District Tamil Nadu, India. NIUSAT is an Earth observation nanosatellite (15 kg). NIUSAT features a RGP camera with a ground resolution of 25 m and a frame size of 50 km x 50 km.
• CE-SAT-1 (Canon Electric Satellite-1), a microsatellite (50 kg) of Canon Electronics Space Technology Laboratory, Japan. The microsatellite features an optical imaging system based on a 40 cm diameter Cassegrain telescope.
• Max Valier, a nanosatellite (16 kg) of the "Max Valier" school Bolzano and the "Oskar von Miller" school Merano, in South Tyrol, Italy.
• D-SAT (Deorbit Satellite), a 3U CubeSat mission (3.5 kg) by the Italian company D-Orbit to demonstrate active end-of-life reentry.
• 3 Diamond nanosatellites (Blue, Green, Red) of Sky and Space Global, UK, developed by GomSpace ApS of Denmark. The three 3U CubeSats (each 6 kg) are pathfinders for Sky and Space Global's 200 Satellite LEO constellation.
• Pegasus, a nanosatellite (2U CubeSat) of FH Wiener Neustadt, Austria (thermosphere research). Pegasus is a member of the QB50 constellation with the m-NLP payload.
• InflateSail, a 3U CubeSat of SSC (Surrey Space Centre) at the University of Surrey, UK (technology demonstration nanosatellite). Part of the QB50 constellation. Inflatesail is designed to test a deployable sail as a means to deorbit the satellite. Inflatesail will use a 3.3 m sail at the end of a 1m boom deployed from the body of the satellite.
• UCLSat (University College London Satellite), a 2U CubeSat of UCL with the INMS (Ion and Neutral Mass Spectrometer), ionosphere research. UCLSat is part of the QB50 constellation.
• NUDTSat (National University of Defense Technology Satellite), Belgium, a 2U CubeSat of NUDT for ionosphere research. NUDTSat features a FIPEX instrument of the QB50 constellation.
• COMPASS-2 (DragSail CubeSat), a 3U CubeSat of FH Aachen, Germany (technology demonstration nanosatellite). COMPASS-2 is part of the QB50 constellation.
• LituanicaSAT-2, a 3U CubeSat of Vilnius University, Lithuania. The CubeSat is part of the QB50 constellation with a FIPEX payload.
• URSA MAIOR (University of Rome la SApienza MicroAttitude In ORbit testing), a 3U CubeSat to study the lower thermosphere. USRA MAIOR is a member of the QB50 constellation with the m-NLP payload.
• VZLUSat-1, a 2U CubeSat Czech technology nanosatellite of VTLU, developed in cooperation with Czech companies (RITE, HVP Plasma, 5M, TTS, IST) and universities (CVUT, University of West Bohemia). The nanosatellite carries on board the following experiments: a miniaturized X-ray telescope, composite material for radiation shielding, FIPEX, a QB50 instrument, to measure the concentration of oxygen in the thermosphere.
• SUCHAI-1 (Satellite of the University of Chile for Aerospace Investigation), a 1U CubeSat (1 kg).
• Venta-1, a nanosatellite (7.5 kg) of Ventspils University, Latvia, developed by Ventspils University College in cooperation with Ventspils High Technology Park, Bremen University of Applied Sciences and OHB Systems. Venta-1 carries an AIS (Automated Identification System ) receiver, which will pick up identification signals from ships at sea.
• Aalto-1, a Finnish student nanosatellite (3U CubeSat) of Aalto University, Aalto, Finland.
• ROBUSTA-1B (Radiation on Bipolar Test for University Satellite Application), a nanosatellite with a scientific experiment developed by the University of Montpellier students (France), a successor to the ROBUSTA satellite, which was launched in February 2012.
• skCube, a 1U CubeSat for educational and popularization outreach developed by SOSA (Slovak Organization for Space Activities ) at the University of Zilina. It is Slovakia's first satellite.
• CICERO-6 (Community Initiative for Cellular Earth Remote Observation-6), a 6U CubeSat of GeoOptics Inc. (~10 kg), Pasadena, CA, built by Tyvak Nanosatellite Systems. The objective is to demonstrate radio occultation observations for a commercial customer. CICERO-6 features CION (CICERO Instrument for GPS-RO) with a mass of 1.2 kg.
• Tyvak-53b, a technology 3U demonstrator by Tyvak Nanosatellite Systems, Inc. (Irvine, CA) to validate technology aimed at helping to deorbit small satellites.
• Lemur-2 x 8. Lemur-2 is commercial satellite constellation of Spire Global Inc., San Francisco, CA, The objective of the Lemur-2 constellation is ship tracking via AIS (Automatic Identification System) with SENSE. The STRATOS instrument makes use of GPS occultation measurements to determine temperature, pressure and humidity profiles of Earth's atmosphere for application in operational meteorology.
Sensor complement: (CION)
CION (CICERO Instrument for GPS-RO)
• Key challenges & experiences (Ref. 19).
- Miniaturize the instrument from a power and mass perspective but ensure end data product quality and support essential JPL heritage software compatibility
- The quality and effectiveness of collaboration is far superior when the collaborators are within driving distance. JPL has been on site for system test verifications and design reviews.
• CION instrument features
- RF inputs: 3 antenna inputs with 4 down converters each
- Processor: 1.2 GHz dual core Arm processor
- RAM: 1 GB flash: uo to 256 GB
- DSP: Programmable FPGA for digital signal processing
- Sub-channels: 16 GPS (8 dual frequency satellites)
- External clock: High performance oscillator (10 MHz, ~ 5 x 1012)
- External PPS (Precise Positioning Service) output and external event input
• Accommodations of CION
- Volume: 3U available
- 30 cm x 10 x x 6 cm utilized by final payload configuration
- Mass: 1.2 kg
- Power: 8 W at 12 VDC
- Communication interfaces available: Two RS 422, USB and Ethernet
• Antenna incorporates combiner into the phased array
- Eliminated need for external RF combiner
- Eliminates need for complex coax harnessing and associated mass
- Bus design has a RF gasket enclosure to prevent EMI
- Payload resides in its own separate volume within the 6U Cubesat
• External inputs
- Temperature controlled highly stable crystal oscillator for precise data clocking and timing
- External GPS for position, orbit determination
• Collection rate
- 1 kHz, 100 Hz, or 50 Hz native data collection rate from all channels.
Figure 6: Photo of CION (image credit: CICERO collaboration)
Figure 7: Evolution of a payload — CION end result (image credit: CICERO collaboration)
Figure 8: Final configuration of CICERO (image credit: CICERO collaboration)
1) Lee Jasper, Danielle Nuding, Elliot Barlow, Erik Hogan, Steven O'Keefe, Pete Withnell, Thomas Yunck, "CICERO - A Distributed Small Satellite Radio Occultation Pathfinder Mission," Proceedings of the 27th AIAA/USU Conference, Small Satellite Constellations, Logan, Utah, USA, Aug. 10-15, 2013, paper: SSC13-IV-5, URL: http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2936&context=smallsat
2) Thomas P. Yunck, "A Brisk Tour of GPS Radio Occultation - Past, Present and Future," May 17, 2007, URL: http://www.jcsda.noaa.gov/documents/seminardocs/JCSDASeminarYunck051707.pdf
4) C. McCormick, C. Lenz, D. Smith, T. Yunck, "Community Initiative for Continuing Earth Radio Occultation CICERO," Proceedings of the 21st Annual AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 13-16, 2007, SSC07-I-3
5) Thomas P. Yunck, Chris C. McCormick, "CICERO: A Community Initiative for Continuing Earth Radio Occultation," http://www.cosmic.ucar.edu/oct2007workshop/abstracts/Yunck_COS_WS_Abs.pdf
6) Christian Lenz, "The CICERO Constellation for Continuing & Sustainable RO Observations," Oct. 13, 2008, URL: http://www.cosmic.ucar.edu/gnssro/presentations/25_LenzCICERO_Christian_Lenz.ppt
7) Turner Brinton, "NOAA, Taiwan Developing Plan for Weather Satellite Program," Space News, April 12, 2010, p. 30, URL: http://www.spacenews.com/civil/100409-noaa-taiwan-plan-weather-satellite-program.html
9) Thomas P. Yunck, Chris C. McCormick, Christian Lenz, "CICERO - Community Initiative for Continuing Earth Radio Occultation, A Community Cooperative for Earth Science," International Workshop on GNSS Remote Sensing, Shanghai, China, Aug. 7-9, 2011, URL: http://18.104.22.168/gnss/PPT_GNSSRS2011.pdf
11) Thomas P. Yunck, "A Brisk Tour of GPS Radio Occultation - Past, Present and Future," May 17, 2007, URL: http://www.jcsda.noaa.gov/documents/seminardocs/JCSDASeminarYunck051707.pdf
12) Shuanggen Jin, G. P. Feng , S. Gleason, "Remote sensing using GNSS signals: Current status and future directions," Advances in Space Research, Vol. 47, 2011, pp. 1645-1653, URL: http://center.shao.ac.cn/geodesy/publications/2011ASR_Jin.pdf
13) T. P. Yunck, "CICERO: Community Initiative for Continuing Earth Radio Occultation," IROWG (International Radio Occultation Working Group) Annual Meeting, Estes Park, Colorado, USA, March 28-April 3, 2012, URL: http://www.irowg.org/docs/Presentation/yunck.pdf
14) "GeoOptics LLC and CU-Boulder Create a Partnership Around the CICERO System of Earth Satellites," Dec. 4, 2009, URL: http://www.prweb.com/releases/2009/12/prweb3295414.htm
15) Marcia S. Smith,"Spire, GeoOptics Win First NOAA Commercial Weather Data Contracts," SpacePolicyOnline.com, Sept. 16, 2016, URL: http://www.spacepolicyonline.com/news/spire-geooptics-win-first-noaa-commercial-weather-data-contracts
16) Jason Samenow, "NOAA awards first-ever satellite data contracts to private industry," Washington Post, Sept. 16, 2016, URL: https://www.washingtonpost.com/news/capital-weather-gang/wp/2016/09/16/noaa-awards-first-ever-satellite-data-contracts-to-private-industry/?utm_term=.f20614578f48
17) Caleb Henry, "Spire CEO: We are Launching Satellites Every Month," Satellite Today, October 26, 2016, URL: http://www.satellitetoday.com/nextspace/2016/10/26/spire-ceo-launching-satellites-every-month/
18) "TYVAK a Terran Orbital Company Signs Contract to Build First Tech Demo CICERO Satellite," Tyvak 2016, URL: http://www.tyvak.com/tyvak-a-terran-orbital-company-signs-contract-to-build-first-tech-demo-cicero-satellite/
19) Dave Williamson, "Small Satellites: The Execution and Launch of a GPS Radio Occultation Instrument in a 6U Nanosatellite," 33rd Space Symposium, Colorado Springs, CO, USA, April 3-6, 2017, URL of presentation: https://www.spacesymposium.org/sites/default/files/downloads/Williamson_Dave-GPS_Radio_Occultation_Talk-v1.pdf
20) "PSLV-C38 / Cartosat-2 Series Satellite," ISRO, June 23, 2017, URL: http://www.isro.gov.in/launcher/pslv-c38-cartosat-2-series-satellite
21) "Indian Launch Manifest of April 15, 2017," URL: http://www.sworld.com.au/steven/space/india-man.txt
22) Andra, "Successful ISILaunch19 campaign," ISILaunch19, June 23, 2017, URL: http://blog.isilaunch.com/successful-isilaunch19-campaign/
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).