nSight-1 CubeSat mission in QB50 constellation
nSight-1 is the first privately funded, commercially developed, South African satellite. It was developed by SCS Space in Cape Town to demonstrate capability and to obtain flight heritage on subsystems that include a novel multispectral imager. The nSight-1 satellite is a 2U CubeSat that is part of the QB50 constellation that was launched and deployed from the ISS (International Space Station) in the second quarter of 2017. 1)
Background: QB50 is a constellation of 2U (10 cm x 10 cm x 20 cm) and 3U (10 cm x 10 cm x 30 cm) CubeSats that is coordinated and partially sponsored by the EU (European Union). Initially intended to consist of 50 satellites (hence the name), QB50 has finally resulted in 36 satellites being selected and delivered for launch. These satellites were built by as many institutions and groups across 21 countries. Twenty-eight of QB50's satellites were launched aboard an Atlas V rocket on the OA-7 "John Glenn" resupply mission to the ISS. OA-7 was successfully captured and docked to the ISS on 22 April 2017.
Deployment of the 28 ISS-bound QB50 satellites into orbit was performed by NanoRacks in several batches over a period spanning 16 to 25 May 2017.
The second part of the QB50 constellation consists of eight satellites that are scheduled for launch aboard a PSLV into a 500 km Sun synchronous polar orbit.
As a member of the QB50 constellation, nSight-1 carries a scientific payload for measuring the lower thermosphere – a region of the Earth's atmosphere that is currently not very well characterized.
nSight-1 makes use of the 2U CubeSat standard. Figure 1 shows a rendering of the satellites layout and main components. Many of the platform requirements are driven by the QB50 project. One such design constraint is the inclusion of the FIPEX (Flux-Φ-Probe Experiment) science payload that collects information about atmospheric oxygen in the lower thermosphere.
A custom-built break-out board was developed to take care of specific interfacing elements, where components do not all conform to "standard" CubeSat electrical interfaces. The Gecko Imager, VHF/UHF transceiver, OBC and ADCS module and Break-out Board were all designed and assembled locally in South Africa.
Radio Transceiver: The satellite communicates with the ground station using a VHF uplink and UHF downlink with 9600 bit/s maximum data rate. The constraint of a relatively low data rate drives the need for efficient bandwidth unitization for earth observation operations. This need is addressed by several on-board image processing options as described later in this paper. The satellite uses a deployable crossed dipole type antenna supplied by ISISpace B.V.
EPS (Electrical Power Subsystem): nSight-1 makes use of body-fixed as well as deployable solar panels covering a total area of 20 cm x 30 cm to maximize power generation in the nominal flying orientation. The power subsystem is comprised of a commercial EPS module that includes the power conditioning, charging electronics and batteries. The EPS faced scrutiny from reviewers because of the risk that malfunctioning batteries would pose to the ISS.
ADCS (Attitude Determination and Control Subsystem): The ADCS, designed and built by CubeSpace of Stellenbosch, South Africa, is similar to the ADCS that flew successfully on the QB50 precursor P1 and P2 satellites. A Y-axis momentum wheel was combined with magnetorquers in such a way that attitude is controlled while simultaneously managing wheel momentum and damping nutation.
This allows the imaging payload to remain pointing towards nadir and the FIPEX sensor pointing along the velocity vector to within 10 degrees to satisfy the QB50 requirement. This configuration will also result in minimal drag, thereby prolonging the satellite's life in orbit.
Attitude sensing is performed using a combination of a 3-axis magnetic field sensor, a fine sun sensor and nadir sensor. The fine sun and nadir sensors are also cameras with wide field of view that perform image processing to detect the sun and nadir vectors relative to the satellite body.
Figure 1: nSight-1's major components (South-African developed components shown in red), image credit: SCS Space
The ADCS unit contains a CubeComputer as the satellite's OBC, it is a single radiation tolerant microprocessor board that performs both attitude control and house-keeping tasks. The attitude control system contains various controllers, state estimators, orbit, magnetic field and sensor models to do satellite detumbling, Y-momentum 3-axis stabilization and nutation damping. External to the ADCS stack is a deployable 3-axis magnetometer to assist with magnetic detumbling and to be the only attitude sensor during eclipse.
Figure 2: Illustration of the CubeSpace Y-momentum ADCS stack (image credit: SCS Space)
AIT (Assembly, Integration and Testing): Generally speaking, teams participating in the QB50 project were involved for all of the three years leading up to launch. The nSight-1 project was started in April 2016 and finally delivered for launch integration in October 2016. This very short 6-month timeframe necessitated several design choices, such as the inclusion of COTS (Commercial-Of-The-Shelf) components as opposed to new developments. Another hallmark of the project is the inclusion of largely South-African produced components, such as the ADCS module, the imaging payload and the VHF/UHF transceiver.
As with the design, the testing of the CubeSat was also greatly prescribed by the QB50 project coordinator, to make sure project and launcher requirements were adhered to. The satellite had to undergo vibration, thermal and vacuum testing. In addition to the environmental tests, it was also crucial to demonstrate the end-to-end functioning of the science and imager payloads. This includes over-the-air commanding, telemetry download and payload data downloading using the actual ground station equipment. The entire testing campaign was completed in one month.
Electronic components, including the Gecko imager's SU and CU, were tested to survive 30 krad of TID (Total Ionizing Dose).
Acceptance vibration testing was performed at a local South African test facility according to launch profiles supplied by the QB50 program requirements. Following a test-as-you-fly principle the satellite was integrated in a representative launch pod mechanical structure for the testing.
Thermal vacuum testing was performed at local Stellenbosch University facilities and created the necessary confidence that the satellite will survive and perform as expected in the space environment.
Figure 3: nSight-1 CubeSat undergoing vibration testing (image credit: SCS Space)
Launch: The Cygnus CRS (commercial Resupply Services) OA-7 capsule was launched on April 18, 2017 (15:11 UTC) by ULA (United Launch Alliance) on an Atlas-5 401 vehicle from the Air Force Station SLC-41, at Cape Canaveral,FL. — Prior to launch, the CRS OA-7 mission was given the name S.S. John Glenn, in honor of astronaut and senator of Ohio, John Glenn, the first US astronaut to orbit the Earth on Mercury 6 and the oldest to go to space on STS-95. John Glenn passed away in December 2016 at age 95. 2) 3)
Operational Requirements and Protocols: The NanoRacks-QB50 CubeSats were delivered to the ISS already integrated within a NanoRacks CubeSat Deployer (NRCSD). Onboard of ISS, a crew member will transfer each NRCSD from the launch vehicle to the JEM/Kibo. Visual inspection for damage to each NRCSD will be performed. When CubeSat deployment operations begin, the NRCSDs are unpacked, mounted on the JAXA MPEP (Multi-Purpose Experiment Platform) and placed on the JEM airlock slide table for transfer outside the ISS. A crew member operates the JRMS (JEM Remote Manipulating System) – to grapple and position for deployment. The CubeSats are deployed, when the JAXA ground controllers command a specific NRCSD.
Orbit: Near-circular orbit, altitude of ~ 400 km, inclination of 51.6º (β angle variation: 0-75º).
This file is seen as a supplementary to the file: "ISS: Cygnus CRS OA-7 mission of Orbital ATK" on the eoPortal — which contains all payloads of the flight.
List of QB50 x 28 :
Twentyeight CubeSats of the international QB50 constellation, a European FP7 (7th Framework Program) Project for Facilitating Access to Space and managed by the Von Karman Institute for Fluid Dynamics in Brussels, Belgium, were flown to the ISS for subsequent deployment (atmospheric research). The 28 CubeSats (all 2U except one with a 3U form factor) of the QB50 constellation were integrated into 11 NanoRacks 6U deployers. 4) 5) 6)
- Aalto-2 of Aalto University, Aalto, Finland is hosting the m-NLP (Multi-Needle Langmuir Probe) payload.
- Aoxiang-1 of NPU (Northwestern Polytechnical University), China/Belgium, carries a FIPEX (Flux-Phi Probe Experiment) payload.
- Atlantis of the University of Michigan,USA carries a FIPEX payload.
- BeEagleSat of Istanbul Technical University and Halvesan (a defence contractor in Turkey owned by the government) carries a m-NLP payload.
- Challenger of the University of Colorado, Boulder, CO, carries an INMS payload.
- Columbia, built by the Universidad del Turabo, Gurabo, Puerto Rico/USA, carries a FIPEX payload.
- ExAlta-1 (Experimental Albertan-1), a 3U CubeSat of the University of Alberta, Canada, is equipped with m-NLP.
- DUTHSat was built by the University of Thrace, Greece. It features the M-NLP payload.
- HAVELSAT of Istanbul Technical University and Halvesan (a defence contractor in Turkey owned by the government); it carries the m-NLP payload.
- Hoopoe, of the Herzliya Science Center, Israel, is equipped with the m-NLP payload. The CubeSat, named for Israel's national bird, the Duchifat-2 (in English, Hoopoe-2), was built by Israeli high school students. More than 80 Israeli teenagers from around the country—in grades 9-12—came to Herzliya Science Center to help build the tiny 1.8 kg 2U CubeSat, a type of miniaturized satellite for space research. 7)
- INSPIRE-2 of the University of Sydney, Australia, features the m-NLP payload.
- LilacSat-1 of HIT (Harbin Institute of Technology), China/Belgium, carries the INMS (Ion-Neutral Mass Spectrometer) payload.
- LINK (Little Intelligent Nanosatellite of KAIST), Korea, is equipped with an INMS payload.
- NJUST-1 of Nanjing University of Science and Technology, China/Belgium, is equipped with an INMS payload.
- nSIGHT-1, developed by SCS-Space of Cape Town, South Africa. The CubeSat carries the FIPEX payload.
- PHOENIX of the National Cheng Kung University, Taiwan, carries an INMS payload.
- PolyITAN-2-SAU of the National Technical University, Ukraine, carries a FIPEX payload.
- qbee50-LTU-OC of the Lula University of Technology, Sweden, and partner Open Cosmos Ltd of England carries a FIPEX payload.
- QBITO of the Universidad Politécnica de Madrid, Spain, carries an INMS payload.
- SNUSat-1 and SNUSat-1B of the Seoul National University, Korea, both satellites carry the FIPEX payloads.
- SOMP-2 (Student's Oxygen Measurement Project 2) of the Technical University of Dresden, Germany, features the FIPEX payload.
- SpaceCube was built by Ecole des Mines Paristech of France; it carries the m-NLP payload.
- SUSat of the University of Adelaide, Australia, the CubeSat features an INMS payload.
- UNSW-ECO of the University of New South Wales, Australia. It is equipped with the INMS payload. - In addition, the UNSW-ECO features a total of four experiments including a GPS receiver, and two boards testing radiation-robust software and self-healing electronics. The fourth experiment is to test the satellite's chassis, built using a 3D-printed material never before flown in space. 8)
- UPSat was built by the University of Patras (Greece) and the Libre Space Foundation. UPSat is the first CubeSat to be based on open-sources software. DUTHSat it features the m-NLP (Multi-Needle Langmuir Probe) payload.
- X-CuveSat was built by the Ecole Polytechnique, the CubeSat carries the FIPEX payload.
- ZA-AeroSat, developed by Stellenbosch University, Stellenbosch, South Africa. It carries the FIPEX payload.
The NanoRacks-QB50 project uses the ISS to deploy a constellation of 28 CubeSats, from a total of 36, in order to study the upper reaches of the Earth's atmosphere over a period of 1 to 2 years. This constellation is the result of an international collaboration involving academia and research institutes from 23 different countries around the world. The project, coordinated by the QB50 Consortium, receives funding from the European Union's Seventh Framework Program for Research and Technological Development. The QB50 satellites conduct coordinated measurements on a poorly studied and previously inaccessible zone of the atmosphere referred to as the thermosphere. The project monitors different gaseous molecules and electrical properties of the thermosphere to better understand space weather and its long-term trends. 9)
- A specific nSight-1 mission objective is to allow for the testing of the newly developed "SCS Gecko Imager." Up until July 2017, due to our limited data downlink budget, the imager was operated in single-snapshot mode and a number of images were successfully processed and downloaded. Since August, the imaging testing has now entered the sequential-imagine phase which allows for partially overlapping images to be taken in quick succession during a single power-on cycle.
Figure 4: The SCS Gecko imager has successfully produced sequential partly overlapping images taken of Vredefort Dome, an impact crater listed by UNESCO as a World Heritage Site in South Africa. The crater rim with the Vaal River passing through are visible in the central part of the image (image credit: SCS Space)
- Another significant milestone achievement relates to the second main payload incorporated as part of the QB50 project of the European Commission, the FIPEX atmospheric science payload. This instrument is currently operating daily (twice the contracted frequency) collecting data for roughly 10 minutes at 11:00 UTC. The data is enhanced with satellite altitude metadata before it is uploaded to the prime investigator's (The von Karman Institute for Fluid Dynamics) database.
- SCS Space CEO, Hendrik Burger, confirms that "nSight-1 is currently the only QB50 satellite delivering FIPEX science data to the QB50 project as other satellites in orbit are still in their commissioning phase. We are very proud that, as part of an international satellite project, our satellite is in its intended orbit and successfully responding to commands from our ground station."
• Commissioning and early operations of nSight-1: After release from the ISS, the satellite was spinning mainly around its lowest moment of inertia X-axis at approximately 5º/s. Due to a configuration mistake in the magnetometer mounting the initial magnetic detumbling controller actually increased the X-axis spin rate to around 60º/s. Fortunately, the error was quickly corrected and a fast Bdot detumbling controller dumped the angular body rates within an orbit. Figure 5 shows the Y-axis angular rate as measured by a solid-state MEMS rate sensor as the body spin is damped within 60 minutes (Ref. 1).
Figure 5: Detumbling result as measured by the Y-axis rate sensor (image credit: SCS Aerospace Group)
Once the body was successfully detumbled a Y-Thomson magnetic controller was enabled to bring the satellite to a controlled Y-spin rate normal to the orbit plane (the body pitch rotates with small roll and yaw angles within the orbit plane). Figure 6 shows the magnetometer measurement and Figure 11 the Y-rate measured during Y-Thomson control to a -2.5º/s reference body rate.
Figure 6: The magnetometer measured body vector towards an accurate Y-Thomson spin (image credit: SCS Aerospace Group)
The next step was to calibrate the magnetometer in-orbit for orthogonality, gain and bias. This was done by measuring the magnetic field vector for an orbit during a slow body spin. The whole orbit data (WOD) of these measurements was then downloaded and the magnetometer vector magnitude compared with an IGRF model's magnitude at the same orbit location. An attitude independent calibration procedure10 was used to obtain the magnetometer calibration sensitivity matrix (gain and orthogonality) and offset (bias) vector.
The final step was to stabilize the satellite 3-axis at zero roll, pitch and yaw angles (nadir and FIPEX pointing toward the velocity vector).
• Deployment of the 28 ISS-bound QB50 satellites into orbit was performed by NanoRacks. On May 26, 2017, NanoRacks successfully deployed the company's 171st CubeSat via the NRCSD (NanoRacks CubeSat Deployer) on the ISS (International Space Station), and the company's 182nd space station CubeSat deployed overall. This cycle completes the NRCSD-11 and NRCSD-12 missions. 12)
Table 1: List of the 17 QB50 CubeSats which were deployed in the NRCSD-12 cycle of NanoRacks, completed on May 26, 2017
Sensor complement of nSight-1: (FIPEX, Gecko imager)
FIPEX (Flux-Φ Probe EXperiment)
FIPEX was developed by the Technical University of Dresden, Germany and supplied as part of the QB50 program agreement. Its goal is to measure the time resolved behaviour of atomic oxygen flux in the upper atmosphere and lower thermosphere. This flux is of general importance in spaceflight as it interacts with spacecraft surface materials, e.g. causing erosion. 13)
The FIPEX instrument is equipped with solid oxide electrolyte micro-sensors that are able to distinguish atomic and molecular oxygen at very low ambient pressures down to 10-10 mbar. The sensor utilizes the amperometric three electrode principle where electrical current is measured a noble metal ceramic compound heated to approximately 660°C.
The measurement is based on solid oxide electrolyte micro-sensors. For oxygen conducting solid state electrolytes, e.g. yttrium-doped zirconia, the conductivity starts at high temperatures and so the sensor operates at an elevated temperature of 600-700ºC, heated by an electrical resistance. Oxygen is "pumped" from one electrode to the other by an applied direct voltage at tree electrode polarization control and in accordance with Faraday's law; the measured current in is proportional to the mass flux by electrolysis. To distinguish between atomic oxygen (AO) and molecular oxygen (O2) sensor elements with different cathode materials being used.
The sensor needs to be in free flow and to determine the actual flux the attitude of the satellite with respect to its direction of motion needs to be known.
Figure 7: FIPEX on QB50_precursor Engineering model during development tests (image credit: TU Dresden)
FIPEX in general is able to distinguish and to measure atomic and molecular oxygen at very low ambient pressures (down to 10-10 mbar partial pressure). The objective of the FIPEX on CubeSats is to measure the time resolved behavior of atomic oxygen flux in-situ in the upper atmosphere and lower thermosphere based on solid oxide electrolyte micro-sensors. Especially the flux of atomic oxygen is of general importance as it shows different interactions with spacecraft surfaces, e.g. erosion of the surface materials. Furthermore, using the atmosphere models, the prediction of total density and the partial pressures in higher latitudes are insufficient. It is well documented in the literature that the main models of the upper thermosphere (e.g. NRLMSISE, DTM, METM) show significant deviations in the prediction of the residual species over time, altitude and longitude of up to 470 percent.
The sensor is based on the amperometric three electrode principle where the electrical current is measured along the electrochemical polarization control on a noble metal ceramic compound heated to approximately 660°C. According to Faraday's law, this current is proportional to the mass flux by electrolysis. Thus, oxygen is non-dissociative adsorbed and transformed to oxygen ions under a potentiometric-Nernst-principle polarization control. These ions are conducted through the solid electrolyte towards the anode, where they recombine to oxygen molecules. Additionally, a diffusion barrier limits the oxygen flux to the cathode.
Figure 8: Amperometric principle for the AO sensor (image credit: TU Dresden)
If this flux limitation is high enough, the oxygen partial pressure almost vanishes at the cathode. In this particular case, the measured current is limited directly by the diffusion of the oxygen to the cathode and therefore a linear dependence on the oxygen partial pressure is achieved due to the diffusion law. Under low-pressure conditions, e.g. in LEO (Low Earth Orbit) the oxygen molecule flux is naturally limited by effusion laws. In order to distinguish the atomic oxygen (AO) from the molecular oxygen (O2) different cathode materials are used.
The core design features of this imager include its modular design, integrated high-speed flash memory and FPGA processor. The imager's data rate and storage capabilities can be scaled to accommodate an extensive array of imaging requirements, optics and focal plane assemblies.
Figure 9: Illustration of the Gecko imager (image credit: SCS Space)
The 2 Mpixel RGB SU and optics combine to give nSight-1 a 32 m GSD (Ground Sampling Distance) and 64 km swath from the ISS deployment altitude of 400 km. The sensor features a Bayer-pattern filter, allowing photorealistic color images to be captured.
Figure 10: Sensor unit for the Gecko imager (image credit: SCS Space)
The CU (Control Unit) on nSight-1 was configured to include 128 GB of high speed flash memory. Together with its integrated FPGA processor this enables the CU to capture images at up to 300 frames per second. Its modularity and processing power allows the CU to be adaptable to larger satellite payloads that will in future support a wider range of spectral bands and more powerful optics. The CU's flash memory controller includes automatic wear levelling and error correction functionality.
Figure 11: Illustration of the CU for the Gecko imager (image credit: SCS Space)
On-board image processing: Raw images are captured from the matrix sensor at a fidelity of 12 bits in snapshot (global shutter) mode, resulting in raw data files of approximately 3 MB per image frame. Raw image data is written directly to the integrated mass storage.
Although it is possible to download uncompressed full frames to the ground, this may not be desirable. Assuming an average overpass duration of 5 minutes, it will take at least 9 overpasses to download one complete uncompressed image frame using the 9600 bit/s UHF downlink. This does not yet take into account the protocol overhead, or the fact that both housekeeping and science data should be downloaded using the same link. Optionally-activated processing options were therefore implemented on the FPGA and OBC (On-Board Computer), including i) JPEG image compression and ii) thumbnail generation.
Future versions of the CU will include both lossless and lossy JPEG2000 compression implemented natively on the FPGA. On nSight-1 we instead implemented the standard discrete cosine transform 8-bit lossy RGB JPEG compression with adjustable quality factor.
Bayer pattern demosaicing, RGB color space conversion, chroma subsampling and thumbnail downsampling take place in real time on the on-board CU FPGA processor. The pre-processed image data is then downloaded to the OBC, where the JPEG compression process takes place.
Due to OBC and satellite power constraints the JPEG engine was highly optimized for this platform, and supports both 4:4:4 and 4:2:2 color subsampling for enhanced compression ratios. Using 4:2:2 sub-sampled JPEG, image frames can easily be compressed to less than 15% of their equivalent 8-bit raw file sizes, depending on image content.
Image thumbnails are downsampled by a factor 8 and then aggressively compressed as 8-bit color JPEGs, resulting in thumbnail files that are typically between 4 kB and 7 kB each. This allows multiple thumbnails to be downloaded in a single overpass. From these, frames can be selected for high-resolution download on subsequent overpasses, as the large amount of on-board flash storage makes long-term retention a practical option. This prevents the unnecessary download of images obscured by clouds or images captured with inappropriate imaging settings (e.g. incorrect gain or exposure).
Figure 12: JPEG compression imaging pipeline (image credit: SCS Space)
A dedicated ground station was set up on South African soil to service nSight-1's daily data downlink needs. The location was chosen for its radio silence and very favourable horizon elevation profile.
The ground station equipment consists of Yagi antennas on tracking mechanisms and an IC-9100 radio. The MCS (Mission Control System) of a South African based company, Spaceteq, is utilized in conjunction with the CubeSupport software that serves as supporting ground software for CubeSpace products. The ground station has been in contact with the satellite since the first beacon after release into orbit.
Figure 13: Photo of the SCS Space Ground Station located near Grabouw, South Africa (image credit: SCS Space)
1) Daniël Francois Malan, Kannas Wiid, Hendrik Burger, Lourens Visagie, Willem Herman Steyn, "The Development of "nSight-1" - Earth Observation and Science in 2U," Proceedings of the 31st Annual AIAA/USU Conference on Small Satellites, Logan UT, USA, Aug. 5-10, 2017, paper: SSC17-X-10, URL: http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=3675&context=smallsat
2) "NASA Space Station Cargo Launches aboard Orbital ATK Resupply Mission," NASA, Release 17-029, April 18, 2017, URL: https://www.nasa.gov/press-release/nasa-space-station-cargo-launches-aboard-orbital-atk-resupply-mission
3) "Mission Page: OA-7 Space Station Cargo Resupply," Orbital ATK, April 18, 2017, URL: https://www.orbitalatk.com/news-room/feature-stories/OA7-Mission-Page/default.aspx?prid=180
4) Davide Masutti, "QB50-ISS CubeSats ready to be launched," Dec. 9, 2016, URL: https://www.qb50.eu/index.php/news/78-qb50-iss-ready-to-be-launched
5) US Commercial ELV Launch Manifest, March 5, 2017, URL: http://www.sworld.com.au/steven/space/uscom-man.txt
8) Andrew Dempster, "Australia's back in the satellite business with a new launch," Space Daily, April 20, 2017, URL: http://www.spacedaily.com/reports/Australias_back_in_the-satellite_business_with-a_new-launch_999.html
9) "NanoRacks-QB50," NASA, March 15, 2017, URL: https://www.nasa.gov/mission_pages/station/research/experiments/2539.html
10) "nSight-1 is Out of Sight in Space for 120 Days and That's a Good Thing!," Satnews Daily, 28 Sept. 2017, URL: http://www.satnews.com/story.php?number=1900013935
11) "nSight-1 120 days in orbit," SpaceRef, Sept. 28, 2017, URL: http://www.spaceref.com/news/viewpr.html?pid=51592
12) "NanoRacks Completes Largest ISS CubeSat Deployment Cycle To Date," NanoRacks, May 26, 2017, URL: http://nanoracks.com/wp-content/uploads/NanoRacks-Release-65-Largest-ISS-CubeSat-Deployment-Cycle-To-Date.pdf
13) "Manual for the FIPEX on QB50 Science Unit, PART: Flight Model," TU Dresden, December 1, 2014, Issue: 2.0.1, URL: https://www.qb50.eu/index.php/tech-docs/category/18-old-docs?download=84:fipex-user-manual
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).