DSCOVR (Deep Space Climate Observatory)
DSCOVR is the former renamed NASA/NOAA mission Triana, proposed in 1998 by then Vice President Al Gore. The goal of Triana was to observe Earth as a planet (i.e. continuous full disk observation of the sunlit Earth) from L1, the first Lagrangian Point in the Earth-Sun system. The mission was named Triana after Rodrigo de Triana, the lookout, who spotted the New World on Christopher Columbus's first expedition in 1492. 1)
Some background: The Triana mission development proceeded for 21 months, (a launch of the observatory was planned for early 2002), before it was de-manifested from the Space Shuttle in the spring of 2001 by the new Bush Administration in office. The official reasons stated were: "A constrained Shuttle flight rate of six per year, established in the 2001 NASA budget planning process, required that NASA give priority to the primary ISS (International Space Station) payloads, a Hubble Space Telescope (HST) reboost was needed, and microgravity experiments were planned to be launched." Consequently, it was not possible to identify a definitive launch date for Triana. 2) 3) 4) 5) 6) 7) 8)
Figure 1: Illustration of the Triana observatory configuration for Shuttle flight (two views of the undeployed S/C)
Legend to Figure 1: The Shuttle configuration contained also GUS (Gyroscopic Upper Stage) next to the Observatory. GUS was intended to boost the Observatory from LEO to the LOI (Lissajous Orbit Insertion) point.
The Triana observatory has been in a state of "Stable Suspension" since November 2001 (stored in a clean room at GSFC. After the mission was placed into suspension, it was renamed to DSCOVR (Deep Space Climate Observatory).
In 2008, the stored spacecraft was removed from storage. Power-on tests were performed to assess the current status of the observatory. This testing was successful, finding DSCOVR in nearly perfect condition after over 7 years of storage. In 2009, NASA provided funding to refurbish and recalibrate the two Earth science instruments, NISTAR and EPIC. In 2011, after a decade of stable suspension, NOAA, NASA, and the USAF are aligning again with plans to fully refurbish and launch DSCOVR possibly in 2014 (Ref. 1). Triana provided a solid foundation for DSCOVR, including advanced work on all observatory subsystems.
While maintaining its original instrument complement, DSCOVR will have a new primary function to monitor space weather. The satellite's original purpose was to provide a near-continuous view of the entire Earth and make a live image available via the Internet.
The majority of the structure was intact, however the flight transponder, star tracker, IMU (gyros), and a reaction wheel had been deintegrated from the flight observatory and moved to multiple locations at GSFC. Some of the GSE (Ground Support Equipment) was impounded and some of the GSE had been reabsorbed into the GSFC infrastructure.
To distinguish this report from previous versions it was called "The Serotine Report." Named after the serotinous or "late opening" pine cone which lies dormant for many years until activated by high heat: such as fire. The name of the report also has become attached to that team.
The DSCOVR mission, to be refurbished at NOAA expense and launched by the USAF, gives a unique opportunity to NASA to obtain unprecedented time resolution solar wind measurements for a minimal cost. The DSCOVR spacecraft is already built (Figure 2) and requires only an 18 month refurbishment to be ready for an operational space weather and scientific research mission. 9) 10)
The 1 AU, near-Earth solar wind has been observed by a number of NASA and international spacecraft over the past decades. However, particle instrumentation technology limitations did not allow the direct observation of the varying properties of the thermal solar wind particles in the kinetic regime, which requires measurements with better than 1 Hz cadence. Observations at this kinetic scale are essential to understand how the solar wind is continuously heated as it propagates away from the Sun, how small scale magnetic reconnection operates in the 1 AU solar wind, and how interplanetary shocks can accelerate particles to high energies.
Table 1: History and current status 11)
• Solar wind measurements at Earth-Sun Lagrange Point 1 (L1)
• Provision of 1-3 day early warnings of geomagnetic storm intensity. The consequences of solar storms are becoming more significant as society becomes increasingly dependent on technologies, from satellites to the electrical grid, that can be disrupted by a major storm. 15)
• DSCOVR will replace the NASA ACE (Advanced Composition Explorer) spacecraft in orbit since 1997
• Data downlink via the international RTSWnet (Real Time Solar Wind Network).
Mechanical structure: A full environmental test program, including vibration, strength, mass properties, acoustic, pyro shock, and alignment activities, is currently planned for DSCOVR. The Serotine team estimate includes funding to refurbish and/or rebuild portions of the mechanical GSE needed for this testing. During this study, the Serotine team rebuilt and certified the lifting GSE for the DSCOVR spacecraft and the EPIC instrument.
GN&C (Guidance Navigation and Control): The approach for DSCOVR includes using the original ACS dynamic simulator and not upgrading to a more costly advanced system, such as the one used for LRO (Lunar Reconnaissance Orbiter).
The DSCOVR star tracker was tested during this study using a star field simulator that attached to the tracker shade. While this is not a comprehensive performance test, the tracker did perform nominally. The current plan is to remove the star tracker and return it to Ball Aerospace for refurbishment. This will include some minor star tracker shade adjustments. Based on the testing performed by the Serotine team, no issues are expected during this refurbishment.
During the Serotine study, the DSCOVR reaction wheels were powered on and commanded to an assortment of speeds. All wheels performed nominally. The wheels were developed in-house at GSFC, and as part of DSCOVR's refurbishment effort, will be removed from the spacecraft and sent to the development lab for comprehensive performance testing and evaluation. These wheels can not be disassembled due to their pressed on assembly techniques. However, stored samples of the original grease will be tested to verify the integrity of the wheels' lubricant, which can not be directly sampled. This grease was also tested a few years ago and showed no degradation. The same grease was also used in an identical wheel that continues to perform flawlessly in its eighth year of powered life testing. Based on this and recent DSCOVR spacecraft testing, the Serotine team does not expect any issues with the DSCOVR wheels.
The DSCOVR propulsion subsystem is a monopropellant blowdown propulsion system. It consists of 1 monopropellant tank, 10 thrusters, 1 dual coil latching isolation valve, 2 pressure transducers. The DSCOVR propulsion control system is known as the EVD, part of the utility hub. Moog Inc. of Niagara Falls, N.Y. provided the MONARC-5 4.5 N monopropellant engines and thruster valves to NASA for the Triana mission in 1999. Moog provided critical technical support for recertification of the engines for the DSCOVR mission after more than 13 years in storage. The 5 N engines will be used for attitude control and minor adjustments to maintain the satellite's orbit at L1. DSCOVR has two key burns: Mid-Course Correction (MCC) and Lissajous Orbit Insertion (LOI). The thrusters will be used to perform the MCC for 50 minutes and LOI for 5.3 hours. After reaching L1, DSCOVR will perform station-keeping and momentum unloading with the thrusters. 16)
Figure 3: Earthward side of the deployed DSCOVR spacecraft and its component/sensor complement (image credit: NASA)
Figure 4: Sunward side of the deployed DSCOVR spacecraft (image credit: NASA)
Table 2: Mission roles of the joint NOAA, NASA and USAF program
Development status of DSCOVR project:
• Nov. 21, 2014: The DSCOVR spacecraft was transported by truck from NASA/GSFC in Maryland to an Astrotech satellite processing facility in Titusville, Fla., just outside the gates of Kennedy Space Center. 21)
• Status of DSCOVR project in May 2014:
- Spacecraft and Sensors are fully integrated
- Successfully completed Electromagnetic Interference & Compatibility (EMI/EMC), Mass Properties, Thermal Vacuum/Balance, Acoustics tests; Vibration testing is last major remaining environmental test.
- Observatory scheduled for October 2014 shipment to Launch site.
Figure 5: Photo of the DSCOVR spacecraft at NASA/GSFC prior to shipment to Cape Canaveral, FL (image credit: NASA)
Figure 6: Photo of the preparations of NOAA's DSCOVR spacecraft in the Astrotech payload processing facility of Titusville, FL (image credit: NASA)
Launch: The DSCOVR spacecraft of NOAA was launched on February, 11, 2015 (23:03.02 UTC) on a Falcon-9 v1.1 vehicle of SpaceX from SLC40 (Space Launch Complex 40) of Cape Canaveral Air Force Station, FL. 22) 23) 24) 25)
Orbit: Lissajous orbit about L1, the first Lagrangian Point (L1) in the Earth-Sun system (1.5 million km from Earth in the direction of the Sun). DSCOVR will be on-station in 110 days.
Figure 7: Artist's illustration of the DSCOVR spacecraft at L1 (image credit: NOAA) 28)
Second attempt of SpaceX to recover the first stage of the Falcon-9 vehicle:
As the second stage embarks on its journey into the Solar System, the first stage will begin its return to Earth, marking the second attempt undertaken by SpaceX to land a first stage booster on the newly commissioned (ASDS (Autonomous Spaceport Drone Ship) - a 91 m x 52 m floating platform equipped with GPS systems and powerful azimuth thrusters to maintain a precise spot in the Atlantic Ocean that is also programmed into the first stage of Falcon-9. 29)
The first attempt to return the first stage came on the Dragon SpX-5 mission ( launch on January 10, 2015) after several previous attempts to soft-land first stages in the Ocean that demonstrated the overall architecture of the delicate return sequence. The SpX-5 first stage successfully made it through its high velocity burns occurring outside the atmosphere and during re-entry, and began atmospheric guidance using four grid fins to find its way back to the landing platform.
Recovery attempt cancelled:
Out in the Atlantic Ocean, the ASDS (Autonomous Spaceport Drone Ship) was on station, beginning a holding pattern in a precise position using its GPS system and powerful azimuth thrusters for stabilization to ensure it would be in the correct spot that was also programmed into the computers of the first stage of Falcon-9. Over the course of the day on Feb. 11, it became clear that the ASDS could not hold its position due to rough seas and one of the four azimuth thrusters used for stationkeeping being out of operation. 30) 31)
As a result, SpaceX opted to move the ASDS out of the landing zone and have the first stage make its landing attempt, pretending the Ocean was firm ground to land on - delivering more data on atmospheric flight and the targeting of a precise spot without endangering the landing platform. The ASDS and its support ship were in a safe position, tasked with recording telemetry from the Falcon-9 and receiving video and data from the drone ship.
SpaceX had to revert from a planned landing on ASDS to a soft splashdown attempt due to rough seas in the Atlantic that prevented the drone ship from keeping its position.
But the first stage of the Falcon-9 vehicle went through all the preprogrammed reentry maneuvers after separation:
Two minutes and 44 seconds after liftoff, the first stage shut down its nine Merlin 1D engines after fulfilling its primary objective of delivering the second stage to a trajectory from where it could boost DSCOVR to its L1 Transfer Orbit. This marked the longest burn of an F-9 first stage in a mission with either soft splashdown or ASDS landing attempt, driven by the injection target of the spacecraft requiring more performance than a Dragon mission into Low Earth Orbit.
With the second stage on its way, the first stage embarked on its journey back to Earth. Classed as a secondary objective, the soft splashdown of the first stage nevertheless was right in the center of attention as part of SpaceX's quest toward re-usability that will involve flying booster stages back to landing sites for refurbishment and re-use for a drastic reduction in the overall cost of space flight.
Heading back in, the first stage kept a stable posture with its engines pointing forward to make a short 19-second reentry burn once reaching 70 km in altitude. This burn slowed the vehicle down from over 1,200 m/s and provided shielding of the engine compartment to create protection from the most strenuous environments occurring at reentry.
Flying through the dense atmosphere, the first stage was to use the grid fins to maintain stable yaw and roll rates and control its pitch-trim to control the downrange travel distance and begin targeting the precise landing spot. The grid fins of the Falcon-9 made their debut on the SpX-5 mission and provided excellent guidance throughout the entire velocity regime from supersonic entry velocity, through the transonic transition and into subsonic speeds.
To avoid the grid fins running out of hydraulic fluid prior to landing like they did on their first flight, the system flew with a larger margin of hydraulic fluid. The four fins can be individually controlled in a two-degree of freedom type design, rotating and tilting at the same time, allowing for complex guidance and control during atmospheric flight.
Beginning its 28-second landing burn using its Center Engine alone, the Falcon-9 was to make last fine corrections of its flight path, ideally coming to a nearly vertical descent. Ten seconds prior to touchdown, the four landing legs were to deploy using pressurized Helium to extend the legs to their deployed position. Landing was planned to occur at a speed of less than 6 m/s under the power of the center engine creating a thrust to weight ratio greater than one.
Elon Musk tweeted that the first stage landed within 10 m of its target based on navigation data received from the booster that also showed that the stage was upright and had reached a good velocity for touchdown. Had the Autonomous Spaceport Drone Ship been in position in better weather conditions, the landing would have succeeded (Ref. 30).
Status of mission:
• March 2, 2017: On February 26, 2017, the skies above Argentina dimmed and the landscape darkened as the Moon moved in front of the Sun, partially blocking its rays. The same thing happened that day in Chile and Angola, as a "ring of fire" (annular eclipse) appeared over the South Atlantic. 32)
- An annular eclipse occurs when the Moon passes in front of the Sun but is too far from Earth to completely obscure it. This geometry leaves the Sun's edges exposed in a red-orange ring. The NASA satellites DSCOVR and Aqua caught several earthly views of the event.
- The animation of Figure 9 shows a static view of Earth and the progression of the eclipse shadow. It is composed from 13 separate images acquired by the MODIS (Moderate Resolution Imaging Spectroradiometer ) instruments on NASA's Aqua and Terra satellites, as well as the VIIRS (Visible Infrared Imaging Radiometer Suite) sensor on the Suomi-NPP satellite. Each satellite follows the same orbital path but passes over points on Earth at slightly different times in a two-hour span.
- But the February event will soon be eclipsed, so to speak, by another one. On August 21, 2017, a total solar eclipse will cross the entire continental United States for the first time in nearly four decades. The total eclipse will take about 90 minutes to cross from Oregon to South Carolina. During that time, 11 NASA-funded science teams will gather observations of the Sun's tenuous atmosphere (corona)—which is too faint to see next to the bright solar disk — and of how the eclipse affects Earth and its atmosphere.
- "When the Moon blocks out the Sun during a total eclipse, those regions of Earth that are in the direct path of totality become dark as night for almost three minutes," said Steve Clarke, director of NASA's Heliophysics Division, in an earlier interview. "This will be one of the best-observed eclipses to date, and we plan to take advantage of this unique opportunity to learn as much as we can about the Sun and its effects on Earth."
Figure 8: This animation was assembled from three images acquired on February 26 by NASA's EPIC (Earth Polychromatic Imaging Camera), a four-megapixel CCD (Charge-Coupled Device) and Cassegrain telescope on the DSCOVR satellite. In that view, both the Earth and the lunar shadow move (image credit: NASA Earth Observatory, DSCOVR EPIC team)
Figure 9: This animation shows a static view of Earth and the progression of the eclipse shadow (image credit: NASA Earth Observatory, images by Joshua Stevens, using MODIS and VIIRS data)
• February 2, 2017: NASA has upgraded its website that provides daily views of the Earth from the Lagrangian Point L1 (1.5 million km away). NASA's EPIC (Earth Polychromatic Imaging Camera) camera imagery website was recently updated allowing the public to choose natural or enhanced color images of the Earth and even zoom into an area on the globe. 33)
- "The 'enhanced' color images make land features more visible," said Sasha Marshak, DSCOVR deputy project scientist at NASA/GSFC (Goddard Space Flight Center), Greenbelt, Maryland. "This is achieved by enhancing low intensity pixel values. The effect of atmospheric haze caused by air molecular scattering and attenuation of solar light by ozone has been also removed."
- EPIC is a 4 Mpixel CCD camera and telescope aboard NOAA's DSCOVR satellite that takes 10 narrow-band spectral images of the entire sunlit face of Earth from 317 - 780 nm. EPIC takes a new picture approximately every hour from mid-April to mid-October or every two hours for the rest of the year. EPIC images reveal how the planet would look to human eyes, capturing the ever-changing motion of clouds and weather systems and the fixed features of Earth such as deserts, forests, and the distinct blues of different seas.
- The EPIC website upgrade includes a new magnification feature where users get a zoomed-in look at an area under their cursor. Magnified areas appear in a circular box on screen.
Figure 10: An EPIC Natural Color image (left) and an Enhanced Color image (right) of the Earth, acquired on January 26, 2017 (image cedit: NASA/NOAA)
• On July 20, 2016, the DSCOVR project, together with the DSCOVR Earth Sensors science team and the Atmospheric Science Data Center (ASDC) at NASA/LaRC (Langley Research Center), released Level 1 data for EPIC (Earth Polychromatic Imaging Camera). Release of the National Institute of Standards and Technology (NIST) Advanced Radiometer (NISTAR) Level 1 data was expected shortly thereafter. The released datasets have initial versions of instrument calibration and geolocation applied. The EPIC data are not yet stray-light corrected but this is expected later in the year. The released data are available from June 2015 through the current day via the ASDC at https://eosweb.larc.nasa.gov. EPIC has obtained two spectacular sequences of lunar transit images since being in operation. The most recent transit images of the Earth and the moon are shown in Figure 11. 34) The full lunar transit gallery can be seen at: 35) 36)
Figure 11: On July 5, 2016, the moon passed between the DSCOVR spacecraft ) and Earth. Over a period of about four hours, the EPIC (Earth Polychromatic Imaging Camera) instrument snapped a series of images of the far side of the moon, which is never seen by observers on Earth's surface, passing by. Meanwhile, in the backdrop, Earth rotates. The background changes throughout the series, first showing Australia and the Pacific and gradually revealing Asia and Africa (image credit: NASA)
• March 10, 2016: DSCOVR was built to provide a distinct perspective on our planet. Yesterday, it added another first to its collection of unique snapshots. While residents of islands and nations in the Western Pacific looked up in the early morning hours to observe a total eclipse of the Sun, DSCOVR looked down from space and captured the shadow of the Moon marching across Earth's sunlit face. 37)
- "What is unique for us is that being near the Sun-Earth line, we follow the complete passage of the lunar shadow from one edge of the Earth to the other," said Adam Szabo, NASA's project scientist for DSCOVR. "A geosynchronous satellite would have to be lucky to have the middle of an eclipse at noon local time for it. I am not aware of anybody ever capturing the full eclipse in one set of images or video."
- In this, the only total solar eclipse of 2016, the shadow of the new Moon starts crossing the Indian Ocean and marches past Indonesia and Australia into the open waters and islands of Oceania (Melanesia, Micronesia, and Polynesia) and the Pacific Ocean. Note how the shadow moves in the same direction as Earth rotates. The bright spot in the center of each disk is sunglint—the reflection of sunlight directly back at the EPIC camera.
- From its position of about 1.5 million km from Earth and toward the Sun, DSCOVR maintains a constant view of the sunlit face of the planet. EPIC acquires images using ten different spectral filters—from ultraviolet to near infrared—to produce a variety of science products. Natural-color images are generated by combining three separate monochrome exposures (red, green, and blue channels) taken in quick succession.
- According to Szabo, the satellite normally collects images at all ten wavelengths about once every 108 minutes (with just one image at full resolution). For this eclipse, the EPIC team collected full-resolution images every 20 minutes on just the red, green, and blue channels. This allowed the satellite to collect 13 images spanning the entire four hours and twenty minutes of the eclipse.
- In addition to the EPIC camera, DSCOVR carries NISTAR (National Institute of Standards and Technology Advanced Radiometer), an instrument that measures how much solar energy is being radiated back into space from Earth. In coming weeks, scientists will be analyzing NISTAR data to quantify how the eclipse changed the incoming and outgoing radiation for those few hours.
- Situated in a stable orbit between the Sun and Earth, DSCOVR's primary mission is to monitor the solar wind for space weather forecasters at NOAA (National Oceanic and Atmospheric Administration). Its secondary mission is to provide daily color views of our planet as it rotates through the day. The satellite was built and launched through a partnership between NASA, NOAA, and the U.S. Air Force.
Figure 12: The moon's shadow on Earth as seen from the EPIC instrument on the DSCOVR satellite at L1 during the total solar eclipse on March 9, 2016 (image credit: NASA Earth Observatory)
• On February 11, 2016, the DSCOVR spacecraft was launched one year ago. The deep-space mission DSCOVR will soon replace NASA's 17-year old ACE research satellite as America's primary warning system for geomagnetic storms and solar wind data. NASA will be responsible for coordinating ACE's continued role in space weather research. 38)
• On October 28, 2015, NOAA officially took command of its DSCOVR satellite. NASA, in charge of both the launch and activation of the satellite, has officially handed over satellite operations to NOAA's DSCOVR team. Next, the team will optimize the final space weather instrument settings and the satellite will soon begin normal operation. 39)
- DSCOVR will give NOAA's SWPC (Space Weather Prediction Center) forecasters higher-quality measurements of solar wind conditions, improving their ability to monitor and warn of severe and potentially dangerous space weather events. DSCOVR will be able to provide warnings 15 to 60 minutes before solar storms reach Earth.
• On Oct. 19, 2015, NASA launched a new website so the world can see images of the full, sunlit side of the Earth every day. Once a day NASA will post at least a dozen new color images of Earth acquired from 12 to 36 hours earlier by NASA's EPIC (Earth Polychromatic Imaging Camera). Each daily sequence of images will show the Earth as it rotates, thus revealing the whole globe over the course of a day. The new website also features an archive of EPIC images searchable by date and continent. 40)
- The primary objective of NOAA's DSCOVR mission is to maintain the nation's real-time solar wind monitoring capabilities, which are critical to the accuracy and lead time of space weather alerts and forecasts from NOAA. NASA has two Earth-observing instruments on the spacecraft. EPIC's images of Earth allow scientists to study daily variations over the entire globe in such features as vegetation, ozone, aerosols, and cloud height and reflectivity.
Figure 13: An Earth image series of the EPIC instrument can be recovered daily at http://epic.gsfc.nasa.gov/
• August 6, 2015: A NASA camera aboard the DSCOVR (Deep Space Climate Observatory) spacecraft has captured a unique view of the Moon as it passed between the spacecraft and Earth. The 4 Mpixel CCD camera (EPIC) and telescope on the DSCOVR satellite maintains a constant view of the fully illuminated Earth as it rotates, providing daily scientific observations of ozone, vegetation, cloud height, and airborne aerosols. About twice a year the camera will capture images of the Moon and Earth together as the orbit of DSCOVR crosses the orbital plane of the Moon. 41) 42)
Figure 14: On July 16, 2015, NASA's EPIC (Earth Polychromatic Imaging Camera) instrument on DSCOVR captured a unique view of the Moon as it passed between the spacecraft and Earth. The image shows the fully illuminated "dark side" of the Moon that is not visible from Earth. The DSCOVR spacecraft is located at the Lagrangian Point L1, located about 1.5 million km from Earth in the direction of the sun (image credit: NASA)
Figure 15: The lunar far side lacks the large, dark, basaltic plains, or maria, that are so prominent on the Earth-facing side. The largest far side features are Mare Moscoviense (Sea of Moscow) in the upper left and Tsiolkovsky crater in the lower left (image credit: NASA, DSCOVR EPIC Team)
• July 20, 2015: The EPIC (Earth Polychromatic Imaging Camera) instrument aboard the DSCOVR spacecraft has returned its first view of the entire sunlit side of Earth from L1, 1.5 million km away in the direction of the sun. The color images of Earth from NASA's EPIC are generated by combining three separate images to create a photographic-quality image. The camera takes a series of 10 images using different narrowband filters — from ultraviolet to near infrared — to produce a variety of science products. The red, green and blue channel images are used in these Earth images. 43) 44)
- "This first DSCOVR image of our planet demonstrates the unique and important benefits of Earth observation from space," said NASA Administrator Charlie Bolden. "As a former astronaut who's been privileged to view the Earth from orbit, I want everyone to be able to see and appreciate our planet as an integrated, interacting system. DSCOVR's observations of Earth, as well as its measurements and early warnings of space weather events caused by the sun, will help every person to monitor the ever-changing Earth, and to understand how our planet fits into its neighborhood in the solar system."
- These initial Earth images show the effects of sunlight scattered by air molecules, giving the images a characteristic bluish tint. The EPIC team now is working on a rendering of these images that emphasizes land features and removes this atmospheric effect. Once the instrument begins regular data acquisition, new images will be available every day, 12 to 36 hours after they are acquired by EPIC. These images will be posted to a dedicated web page by September.
- "The high quality of the EPIC images exceeded all of our expectations in resolution," said Adam Szabo, DSCOVR project scientist at NASA/GSFC (Goddard Space Flight Center) in Greenbelt, Maryland. "The images clearly show desert sand structures, river systems and complex cloud patterns. There will be a huge wealth of new data for scientists to explore."
- The primary objective of DSCOVR is to maintain the nation's realtime solar wind monitoring capabilities, which are critical to the accuracy and lead time of space weather alerts and forecasts from NOAA.
- In addition to space weather instruments, DSCOVR carries a second NASA sensor — the NISTAR (National Institute of Science and Technology Advanced Radiometer ). Data from the NASA science instruments will be processed at the agency's DSCOVR Science Operations Center in Greenbelt, Maryland. This data will be archived and distributed by the Atmospheric Science Data Center at NASA/LaRC (Langley Research Center) in Hampton, Virginia.
Figure 16: Earth as seen on July 6, 2015 from a distance of one million miles by NASA's EPIC instrument aboard NOAA's DSCOVR (Deep Space Climate Observatory) spacecraft (image credit: NASA)
• June 8, 2015: More than 100 days after it launched, NOAA's DSCOVR (Deep Space Climate Observatory) satellite has reached its orbit position, about one 1.5 million km from Earth. Once final instrument checks are completed, DSCOVR, which will provide improved measurements of solar wind conditions to enhance NOAA's ability to warn of potentially harmful solar activity, will be the nation's first operational space weather satellite in deep space. Its orbit between Earth and the sun is at a location called the Lagrange point 1, or L1, which gives DSCOVR a unique vantage point to see the Earth and sun. The satellite is expected to begin operations this summer. 45)
- Data from DSCOVR, coupled with a new forecast model set to come online in 2016, will enable NOAA's space weather forecasters to predict geomagnetic storm magnitude on a regional basis. Geomagnetic storms occur when plasma and magnetic fields streaming from the sun impact Earth's magnetic field. Large magnetic eruptions from the sun have the potential to bring major disruptions to power grids, aviation, telecommunications, and GPS systems.
- DSCOVR will eventually replace NASA's ACE (Advanced Composition Explorer) research satellite as America's primary warning system for solar magnetic storms headed towards Earth. ACE will continue to provide valuable research data to the science community.
- In addition to space weather-monitoring instruments, DSCOVR is carrying two NASA Earth-observing instruments that will gather a range of measurements from ozone and aerosol amounts, to changes in Earth's radiation budget—the balance between incoming radiation (largely from the sun) and that which is reflected from Earth. This balance affects our climate.
• February 24, 2015: In just 12 days after it launched, NOAA's DSCOVR satellite has reached the halfway mark to the L1 position. In 12 days, DSCOVR has traveled approximately 0.8 million km. However, just as a ball loses speed at the top of its arc, DSCOVR is losing speed. As DSCOVR moves further away from the earth, the sun's gravity comes into play and bends the trajectory. This results in curved trajectory versus a "straight-line" approach. It will take DSCOVR another 100 days to travel the remaining distance to L1 (at 1.5 million km from Earth). This will put the expected arrival time of DSCOVR at L1 around the beginning of June. 46)
Sensor complement: (EPIC, NISTAR, PlasMag instrument suite, PHA)
The primary objective of the sensor complement is to measure the solar wind (np, vp, tp) and the interplanetary magnetic field at 240 RE forward of the Earth.
The DSCOVR solar wind/IMF data will be downlinked to the Real Time Solar Wind Network (RTSWnet). 47)
Secondary mission objectives: Earth observations. The Earth viewing instruments on DSCOVR have a continuous view of the entire sunlit face of the Earth.
Table 3: Level 1 science requirements (Ref. 20)
EPIC (Earth Polychromatic Imaging Camera)
The EPIC instrument was managed by SIO (Scripps Institution of Oceanography) at USCD (University of California at San Diego) and built by LMATC (Lockheed Martin's Advanced Technology Center) in Palo Alto, CA. The objective is to measure ozone amounts, aerosol amounts, cloud height and phase, hotspot land properties (a view of the land from angles where shadows are a minimum), and UV radiation estimates at the Earth's surface. EPIC is able to view the entire sunlit Earth from sunrise to sunset at an almost constant scattering angle between 165‐178º. The EPIC channels were selected to match closely with TOMS in the UV region and with MODIS in the visible range; hence, the data products will be very similar and can be directly compared. These comparisons will validate both the calibration and data reduction algorithms.
The instrument system consists of the EPIC instrument, MEB (Mechanisms Electronics Box) and EC (EPIC Computer) which controls the instrument and interfaces to the spacecraft avionics. EPIC's telescope, built by SSG Inc., is a reflecting Ritchey-Chrétien design with an aperture diameter of 30.5 cm, f 9.38, a FOV (Field of View) of 0.61º, and an angular sampling resolution of 1.07 arcsec. Once at L1, Earth (plus 100 km) varies from 0.45º to 0.53º full width. 48) 49)
Figure 17: Photo of the EPIC instrument system (image credit: NASA)
Table 4: EPIC performance parameters
Figure 18: Schematic view of the Cassegrain telescope (image credit: NASA, Ref. 11)
Table 5: Expected EPIC data products
• Ozone: total column
• Aerosol properties: aerosol index, aerosol optical thickness, aerosol height
• Cloud & surface properties: cloud fraction, cloud height, surface albedo
• Vegetation properties: vegetation index and LAI (Leaf Area Index)
• RGB: colored image of the Earth's sunlit face.
EPIC challenges (Ref. 11):
• Geolocation: The spacecraft jitter is expected to be on the order of 1 pixel. This increases EPIC's effective field of view. The ‘edge' of the Earth and the outline of the continents will have to be used to exactly geo-locate the images. This is especially important for the algorithms based on ratios of channels.
• Stray light: EPIC's (spatial) stray light is significant and must be corrected. A very complex stray light correction algorithm is being developed. It is based on laboratory measured point spread functions and calculations of an optical model.
• Instrument stability: The radiometric stability of EPIC will be tracked using the measured reflectivity over ice-covered surfaces and by periodic images of the moon's sunlit face at a nearly constant phase angle when it is furthest from the Earth as seen from L1.
Figure 19: EPIC UV cloud reflectivity will determine the cloud amounts and changes between midday and sunrise-sunset conditions (image credit: NASA)
NISTAR (National Institute of Standards and Technology Advanced Radiometer)
The NISTAR cavity radiometer was designed and developed by NIST (National Institute of Standards and Technologies) of Gaithersburg, MD and Ball Aerospace of Boulder, CO. The objective was to measure the radiance output from the sunlit Earth over a broad portion of the spectrum (UV and VIS as reflected radiation, IR as emitted radiation) in order to detect changes in the Earth's energy balance (climate studies).
The NISTAR instrument package includes four detectors: three active-cavity electrical substitution radiometers and one silicon photodiode channel to measure the "total Earth reflected and emitted radiant power" received in the direction of the spacecraft. 50) 51)
NISTAR measures the absolute irradiance integrated over the entire sunlit face of the Earth in 4 broadband channels minute-by-minute. The measurements performed by NISTAR will be of high accuracy, on the order of 0.1%. NISTAR provides 3 broadband channels (A, B, C) plus a photodiode channel.
A) A UV to far infrared (0.2 µm to 100 µm) channel to measure the total radiant power UV, VIS and IR wavelengths emerging from the Earth.
B) A solar (0.2 µm to 4 µm) channel to measure reflected solar radiance in UV, VIS and NIR wavelengths.
C) A near infrared (0.7 µm to 1.1 µm) channel to measure the reflected IR solar radiation
D) Photodiode channel (0.2 µm to 1.1 µm) for monitoring of radiometer filter elements (channel for calibration reference).
Figure 20: NISTAR radiance-wavelength graph of the channel measurements (image credit: NASA, Ref. 20)
The goal of NISTAR is to measure the Earth's energy balance (solar input and Earth reflection and radiation to space) with sufficient accuracy (0.1%) to improve our understanding of the effects of changes caused by human activities and natural phenomena.
In 2010, the NISTAR instrument was re-calibrated against a portable version of the NIST SIRCUS (Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources) facility. The calibration was performed with the NISTAR space-flight instrument in a thermal vacuum chamber in a clean-room environment at NIST. This calibration included system-level measurements of the relative spectral response of the NISTAR bands using a wavelength-tunable laser, and absolute responsivity measurements of each of the four NISTAR detectors at 532 nm. The standard uncertainty of the absolute responsivity calibration obtained using this technique was 0.12 % (k=1). 52) 53)
Figure 21: Photo of the NISTAR instrument under test (image credit: NASA)
NISTAR has a FOV of 1º, sufficient to see and image the full Earth disk whose FOV is approximately 0.5º as seen from L1. The photodiode channel has been included in order to obtain a faster time series (<1 s) than what can be obtained by the cavity radiometers. The channel is used to provide for the tracking stability of the filters, to verify co‐alignment of NISTAR and EPIC, and to continuously observe the solar reflected broadband radiation from Earth with high temporal resolution.
Figure 22: Photo of the NISTAR instrument (image credit: NASA)
A PTC (Positive Temperature Coefficient) thermistor and a wire‐wound low‐temperature coefficient heater are bonded to the outside of the 30º conical receiver cavity. The absolute cavity radiometers are designed for optimum power measurements (in the tens of μW range). The optical signal incident on the receiver is only 1 μWcm‐2, however, the emission from the receiver cavity to space is estimated to be 30 μWcm‐2 when the shutter is open. There are four digital control loops, three receiver cavity control loops and one for the heat sink. The PTC temperature sensor resistance measurements are performed with AC‐bridge circuits operating between 35 and 155 Hz. The NISTAR absolute cavity radiometers are designed for a noise floor of less than 10 nW (defined as the level at which the SNR is equal to one for a single one second measurement). The NISTAR electronics have a measurement resolution of 10 mΩ and internal equivalent noise of less than that. The NISTAR total instrument mass is 25.5 kg.
DSCOVR's location at the L1 observing position, rather than in Earth orbit, will permit long integration times, since no scanning will be required. A radiometric accuracy of 0.1-0.2% is expected, a 10-fold improvement in accuracy over current Earth-orbiting satellite data. These will be the only measurements of the entire Earth's reflected and emitted radiation at the retro reflection angle. As such, NISTAR will provide important missing data not obtainable by any Earth-orbiting satellite.
Figure 23: Photo of the NISTAR instrument (image credit: SIO)
PlasMag instrument suite:
The PlasMag (Plasma Magnetometer) instrument suite of NASA/GSFC and MIT (Massachusetts Institute of Technology) is a comprehensive science and space-weather package that includes a fluxgate vector magnetometer, not present on SOHO, a Faraday Cup solar wind positive ion detector and a top-hat electron electrostatic analyzer. This instrument cluster provides high time resolution measurements in real time and represents the next generation of upstream solar wind monitors intended to provide continuity of measurements started by IMP-8, WIND, SOHO and ACE. 54)
Faraday Cup: The PlasMag Faraday Cup will provide very high time resolution (0.5 second) solar wind bulk properties in three dimensions, which coupled with magnetic field data (20 vectors/second), will allow the investigation of solar wind waves and turbulence at unprecedented time resolution. This, in turn, will allow new insights into the basic plasma properties: the process of turbulent cascade and the rate of reconnection. Both topics are critical in understanding the nature of coronal heating.
The electron electrostatic analyzer will allow the continual observation of the 3D electron distribution function for various solar wind conditions. Special attention will be given to the supra-thermal component or "strahl" that follows the interplanetary field lines very closely and provides the closest link to the formation of the solar wind in the upper corona. It provides a way of identifying large magnetic loops that are still connected at both ends to the solar corona.
It is important that DSCOVR be at L1 before the ACE mission ends, to allow for cross calibration of the solar instruments, to augment solar wind early warning, and to eventually replace ACE. The DSCOVR PlasMag fluxgate magnetometer will provide crucial continuity of observation of this important interplanetary solar wind parameter. The magnetic field measurements will allow, among other things, the connection of the photospheric magnetic sector structure to 1 AU heliospheric current sheet observations.
Figure 24: Photo of the PlasMag electron electrostatic analyzer (left) and the PlasMag Faraday Cup (right), image credit: NASA
Electron spectrometer: The objective of the top-hat electron spectrometer is to provide high time resolution (<1 s) solar wind electron, full 3D distribution function observations. While electron measurements have an inherently higher uncertainty, the electron spectrometer may extend space weather monitoring to unusually high speed events. The electron spectrometer sits at the tip of mag boom to gain a nearly 4π FOV (Ref. 6).
Figure 25: Illustration of the boom-mounted electron spectrometer (image credit: NASA)
PHA (Pulse Height Analyzer)
The objective of PHA is to monitor the effect of high energy particles on spacecraft electronics. PHA is an instrument designed and built by GSFC. It was previously flown of STS-95 (Oct. 1998).
• PHA is a small, low power, HiLRS (High Linear Energy Transfer Radiation Spectrometer) to evaluate SEUs (Single Event Effects) on microelectronics in the space environment
• PHA provides in-flight measurement of a spectrum of ionizing particle energy, charge, and mass.
Figure 26: Photo of the PHA instrument (image credit: NASA)
The NOAA-OSD (Office of Systems Development) Ground Systems Division (GSD) provides technical and engineering consultation for satellite ground systems and supporting facilities. The support GSD provides encompasses the entire end-to-end satellite data flow from spacecraft and instrument commanding, to capture of raw satellite data telemetry streams, and finally to processing and distribution of derived imagery and data products for consumption by worldwide users. 55) 56)
• Provide expertise in satellite ground systems design with emphasis in telemetry and command, instrument data processing, image display and analysis, product generation, data archiving, communications, and associated ground equipment, infrastructure and facilities
• Plan ground system activities leading to new satellite systems, including the introduction of significant new services and products from Research-to-Operations and International satellite missions
• Evaluate new technologies for satisfying current and future ground systems requirements, from raw data capture through distribution of products to users
• Lead ground systems development efforts including specifying, procuring, installing, and testing NESDIS hardware and software systems resulting from requirements generated during system planning activities.
• Work directly with NOAA-OSPO (Office of Satellite and Product Operations), NOAA-OSO (Office of Systems Operations), NASA and other agencies to ensure compatibility of future satellites with existing and future ground systems
• Provide support to the OSPO and the Center for SaTellite Applications and Research (STAR) for Information Technology refresh and enhancement projects.
Figure 27: Space/Ground communications normal operations (image credit: NASA, NOAA)
Figure 28: DSCOVR realtime data and product flow (image credit: NOAA)
1) Joe Burt, Bob Smith, "The DSCOVR Mission," Proceedings of the 2012 IEEE Aerospace Conference, Big Sky, Montana, USA, March 3-10, 2012
2) Francisco P. J. Valero, Jay Herman, Patrick Minnis, William D. Collins, Robert Sadourny, Warren Wiscombe, Ddan Lubin, Keith Ogilie, "Triana ‐ a Deep Space Earth and Solar Observatory," Report prepared for the National Academy of Sciences by the Triana Science Team (SIO, NASA/GSFC, NIST, LaRC, ARC, NCAR, LMD, LM, LANL, VT)
3) J. G. Watzin, "The Triana Mission ‐ A Pathfinder Mission to Explore the Utility of using Deep Space in Conducting Earth Observation," Proceedings of the 51st IAF Congress, Rio de Janeiro, Brazil, Oct. 2‐6, 2000
4) S. A. W. Gerstl, F. P. J. Valero, "The Triana Satellite Mission from L1 for Global Vegetation Monitoring," Proceedings of IEEE/IGARSS'99, Vol. I, Hamburg, June 28‐July 2, 1999
6) Quang-Viet Nguyen, "Deep Space Climate Observatory (DSCOVR) Mission Briefing," Heliophysics Subcommittee Meeting, NASA HQ, Washington, DC, Feb. 27-28, 2012, URL: http://science.nasa.gov/media/medialibrary/2012/03/01/Nguyen-DSCOVR-Mission-Briefing-HPS-v01b.pdf
7) "Deep Space Climate Observatory," Wikipedia, URL: http://en.wikipedia.org/wiki/Deep_Space_Climate_Observatory
8) Jeff Foust, "People re-DSCOVR an existing program," April 12, 2013, URL: http://www.spacepolitics.com/2013/04/12/people-re-dscovr-an-existing-program/
9) A. Szabo, K. W. Ogilvie, A. F. Viñas, E. J. Summerlin, "Solar Wind Kinetic Physics - High Time Resolution Solar Wind Measurements from the DSCOVR Mission," DSCOVR White Paper, URL: http://www8.nationalacademies.org/SSBSurvey/DetailFileDisplay.aspx?id=750&parm_type=HDS
10) D. J. Knipp, D. A. Biesecker, "Changing of the Guard: Satellite Will Warn Earth of Solar Storms," Eos, Vol. 96, doi:2015EO026579, published on 24 March 2015, URL: https://eos.org/project-updates/changing-of-the-guard-satellite-will-warn-earth-of-solar-storms
11) Alexander Cede, Jay Herman, Alexander Marshak, "Overview of the Earth Science Instruments on the NASA DSCOVR mission," ICAP (International Cooperative for Aerosol Prediction), ESA/ESRIN, Frascati, Italy, May 14 – 17, 2012, URL: http://bobcat.aero.und.edu/jzhang/ICAP/AERP/MeetingPDFs-RemoteSensing/Cede-DSCOVR%20%28NXPowerLite%29.pdf
12) Francisco P. J. Valero, Brett C. Bush, Shelly K. Pope, V. Ramanathan, Jay Herman, Keith Ogilvie, Warren J. Wiscombe, Steven Lorentz, Joseph Rice, Patrick Minnis, Peter Pilewskie, William D. Collins, Claude Basdevant, Bernard Legras, Hector Teitelbaum, G. Louis Smith, Alan Strahler, Claudio Tomasi, Ankie J. M. Piters, John Burrows, Irina Melnikova, "NRC Earth Science Decadal Survey-Mission Concept Earth Sciences from the Astronomer's Perspective, a Deep Space Climate Observatory (DSCOVR) ,"NRC (National Research Council), URL: http://geo-cape.larc.nasa.gov/docs/DSCOVR_RFI_030.pdf
17) "NOAA Satellite and Information Service -Deep Space Climate Observatory (DSCOVR)," URL: http://www.nesdis.noaa.gov/DSCOVR/pdf/DSCOVR%20-%20Program%20Overview%20Info%20Sheet.pdf
18) "New NOAA spacecraft readies for launch next month," NOAA, Dec. 18, 2014, URL: http://www.noaanews.noaa.gov/stories2014/20141218_DSCOVR.html
20) Adam Szabo, "Deep Space Climate Observatory (DSCOVR)," 6th NASA Space Weather & Robotic Mission Operations Workshop, NASA/GSFC, Greenbelt, MD, September 17-18, 2014, URL: http://swrc.gsfc.nasa.gov/main//sites/swrc/presentations/2014_NASA_SWx-Workshop/Szabo_DSCOVR%20Overview%20Space%20Weather%20Workshop.pdf
21) "Deep Space Climate Observatory arrives at Florida launch site," Spaceflight Now, Nov. 21, 2014, URL: http://spaceflightnow.com/2014/11/21/deep-space-climate-observatory-arrives-at-florida-launch-site/
22) "NOAA's new deep space solar monitoring satellite launches," NOAA, Feb. 11, 2015, URL: http://www.noaanews.noaa.gov/stories2015/20150211-NOAA-new-deep-space-solar-monitoring-satellite-launches.html
24) Tim Reyers, "Politics Kept a Mission in Cold Storage and Delayed for 12 Years, Launching Feb. 9th," Universe Today, Jan. 19, 2015, URL: http://www.universetoday.com/118310/politics-kept-a-mission-in-cold-storage-for-10-years/
26) "SpaceX Awarded Two EELV Class Missions From The USAF," Space Travel, Dec. 07, 2012, URL: http://www.space-travel.com/reports/SPACEX_Awarded_Two_EELV_Class_Missions_From-The_USAF_999.html
27) John Leslie, "DSCOVR Mission Moves Forward to 2015 Launch," NASA. Sept. 10, 2013, URL: http://www.nasa.gov/content/goddard/dscovr-mission-moves-forward-to-2015-launch/#.UvSOp_vnNeM
28) William F. Denig, P Mulligan, "NOAA Operational Space Environment Monitoring," 94th American Meteorological Society Annual Meeting, Atlanta, GA, USA, Feb. 2-6, 2014, URL: https://ams.confex.com/ams/94Annual/webprogram/Handout-Paper238184/denig-ams2013_J1-2.pdf
29) Patrick Blau, "SpaceX Falcon 9 ready for DSCOVR Launch & 1st Stage Landing Attempt," Spaceflight 101, Feb. 7, 2015, URL: http://www.spaceflight101.com/dscovr-mission-updates.html
30) Patrick Blau, "DSCOVR embarks on long-awaited Mission after flawless Falcon 9 Launch — 1st Stage makes Soft Splashdown after scrubbed Landing Attempt due to rough Seas," Spaceflight 101, Feb. 11, 2015, URL: http://www.spaceflight101.com/dscovr-mission-updates.html
31) "SpaceX launches DSCOVR satellite to deep space orbit," SpaceX, Feb. 11, 2015, URL: http://www.spacex.com/news/2015/02/11/spacex-launches-dscovr-satellite-deep-space-orbit
32) "The Solar Ring of Fire," NASA Earth Observatory, March 2, 2017, URL: http://earthobservatory.nasa.gov/IOTD/view.php?id=89748
33) Rob Gutro, "NASA Makes an EPIC Update to Website for Daily Earth Pics," NASA, Feb. 2, 2017, URL: https://www.nasa.gov/feature/goddard/2017/nasa-makes-an-epic-update-to-website-for-daily-earth-pics
34) "The Earth Observer," NASA, July - August 2016. Volume 28, Issue 4, pp: 2-3, URL: http://eospso.nasa.gov/sites/default/files/eo_pdfs/July_August_2016_col_508_0.pdf
37) "An EPIC Eclipse," NASA Earth Observatory, March 10, 2016, URL: http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=87675
38) "Celebrating the first anniversary," NOAA, Feb. 11, 2016, URL: http://www.nesdis.noaa.gov/news_archives/dscovr_one_year_anniversary.html
41) "The Dark Side and the Bright Side," NASA, Earth Observatory Series, August 6, 2015, URL: http://earthobservatory.nasa.gov/IOTD/view.php?id=86353
42) Rob Gutro, "From a Million Miles Away, NASA Camera Shows Moon Crossing Face of Earth," NASA, Aug. 5, 2015, URL: http://www.nasa.gov/feature/goddard/from-a-million-miles-away-nasa-camera-shows-moon-crossing-face-of-earth
43) Steve Cole, Rob Gutro, "NASA Satellite Camera Provides "EPIC" View of Earth," NASA Release 15-151, July 20, 2015, URL: http://www.nasa.gov/press-release/nasa-satellite-camera-provides-epic-view-of-earth
44) Jason Major, "This is Our Planet From a Million Miles Away," Universe Today, July 20, 2015, URL: http://www.universetoday.com/121480/this-is-our-planet-from-a-million-miles-away/
45) "Nation's first operational satellite in deep space reaches final orbit," NOAA/NESDIS News Archive, June 8, 2015, URL: http://www.nesdis.noaa.gov/news_archives/DSCOVR_L1_orbit.html
47) Karen C. Fox, "NOAA's DSCOVR: Offering A New View of the Solar Wind," Feb. 6, 2015, URL: http://www.nasa.gov/content/goddard/dscovr-offering-a-new-view-of-the-solar-wind/#.VNcpjS7-Y_c
48) Enhanced Polychromatic Imafing Camera," NOAA, URL: http://www.nesdis.noaa.gov/DSCOVR/pdf/DSCOVR%20-%20EPIC%20Instrument%20Info%20Sheet.pdf
49) Rob Gutro, "NOAA's DSCOVR to provide 'EPIC' views of Earth," Phys.org, Jan. 7, 2015, URL: http://phys.org/news/2015-01-noaa-dscovr-epic-views-earth.html
50) J. P. Rice, S. R. Lorentz, K. Lykke, R. C. Smith,F. P. Valero, " NISTAR: The NIST Advanced Radiometer," American Geophysical Union (AGU), Fall Meeting 2011, Dec. 5-9, 2011, San Francisco, CA
51) "National Institute of Standards & Technology Advanced Radiometer (NISTAR)," URL: http://www.nesdis.noaa.gov/DSCOVR/pdf/DSCOVR%20-%20NISTAR%20Instrument%20Info%20Sheet.pdf
52) Joseph Rice, "Taking the Wraps off NISTAR," NIST, Dec. 2, 2014, URL: http://www.nist.gov/pml/div685/grp04/nistar-radiometer-scheduled-to-fly.cfm
54) "PLASMA-Magnetometer (PlasMag)," NOAA, URL: http://www.nesdis.noaa.gov/DSCOVR/pdf/DSCOVR%20-%20PlasMag%20Instrument%20Info%20Sheet.pdf
56) Mike Simpson, Doug Biesecker, "DSCOVR Overview," COPC, November 4, 2014, URL: http://www.ofcm.gov/copc/meetings/2014-02/pdf/09-COPC%20Presentation_DSCOVRchartinputs20141030v2.pdf
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).