STEREO (Solar-Terrestrial Relations Observatory)
The STEREO mission is a strategic element of NASA's Sun-Earth Connection program, a multi-institutional and international collaboration involving participants from France, Germany, the Netherlands, Belgium, Switzerland, the United States, and the United Kingdom. STEREO is the third mission of NASA's Solar Terrestrial Probes program. The overall objective is to increase the understanding of the origin and consequences of CMEs (Coronal Mass Ejections). 1) 2) 3) 4) 5) 6) 7)
Over a period of two to three decades, solar physicists have come to realize that the extremely energetic solar storms known as CMEs are the form of solar activity felt most forcefully at Earth. CMEs impacting Earth’s environment are the primary cause of major geomagnetic storms, and they are associated with nearly all of the largest solar proton events, which are streams of protons ejected from the Sun with so much energy that they can pose a significant radiation hazard to men and machines in space. The growth in society’s reliance on technology has led to an increased vulnerability to impacts from the space environment and, hence, to an importance in understanding the multifaceted influence of the Sun and CMEs on Earth. 8)
The mission consists of two nearly identical spacecraft in heliocentric elliptical orbit in the ecliptic plane at approximately 1 AU from the sun: one drifting ahead of the Earth and one behind. Simultaneous measurements are obtained by the satellites instruments at gradually increasing separations over the course of the two-year mission. The STEREO mission observation capabilities are:
• First stereo viewing of Sun from out-of-Earth-orbit vantage points
• First imaging and tracking of space disturbances from Sun to Earth
• First continuous determination of interplanetary shock positions by radio triangulation
• First simultaneous imaging of solar activity with in-situ measurement of energetic particles at 1 AU.
Figure 1: Artist's rendition of a STEREO spacecraft (image credit: NASA)
The STEREO S/C bus consists of a box-like structure of size 2.03 m x 1.22 m x 1.14 m (deployed length of 6.47 m). The S/C is built and integrated by JHU/APL with NASA/GSFC procuring the instruments and launch vehicle. The satellite is three-axis stabilized. Attitude sensing is provided by a star camera, two DSAD (Digital Solar Attitude Detector) systems, an IMU (Inertial Management Unit), and a pointing error signal generated by the instrument guide telescope. Actuation is provided by four reaction wheels and 12 thrusters. A S/C pointing accuracy of 7 arcsec (control) and 0.1 arcsec (knowledge) is provided. Each S/C utilizes four sets of solar arrays, each of which experiences a two stage deployment on orbit. The deployed solar arrays generate an average power of 475 W (EOL). A Super NiCd battery of 21 Ah is used for eclipse phase operations. 9) 10) 11) 12) 13)
The bus is designed around the IEM (Integrated Electronics Module) and the PSE (Power System Electronics) unit. The IEM contains the RF uplink and downlink cards, power converter cards, and an AIE (Attitude Interface Electronics) card. The cards within the IEM communicate over a PCI (Parallel Communication Interface). The Power System Electronics unit contains relays and control circuitry for the S/C power system. A MIL-STD-1553 bus architecture is used for on-board command and telemetry transmission between the IEM, the instruments, attitude sensors, and the PSE. Each spacecraft has a mass of about 620 kg (wet) and a design life of two years. 14)
Figure 2: Photo of the STEREO flight stack before spin balance (image credit: JHU/APL)
G&C (Guidance and Control) system: The Sun-pointed imaging instruments require very tight pointing of their boresights and small jitter with a significantly relaxed roll requirement. The two coronagraphs, COR1 and COR2, along with the EUVI and GT (Guide Telescope) are mounted on an optical bench in the middle of the spacecraft, and together they compose the SCIP (Sun-Centered Imaging Package). The three Sun-pointing instruments on SCIP (see also Figure 23) require very tight pointing of their boresights toward the center of the Sun to observe CME initiation and propagation and a very stable platform, i.e., minimal jitter, to prevent image smear and enable 3-D image reconstruction. 15)
Pointing requirements: The nominal STEREO mission attitude keeps the spacecraft +x axis centered on the Sun with the Earth in the x–z plane on the –z side of the spacecraft (Figure 3). This attitude is maintained at all times except during instrument- and antenna-calibration maneuvers and during propulsive maneuvers (orbit adjustments that occurred in the phasing orbits and the dumps of accumulated angular momentum that occur routinely on both spacecraft throughout the mission).
Figure 3: Nominal STEREO Sun-pointing orientation (image credit: JHU/APL)
The fine Sun-pointing requirements are driven by the SECCHI instruments, specifically those mounted on the SCIP, to provide clear images requirements refer to three terms, accuracy, jitter, and and to enable image-to-image correlation (Figure 4).
Figure 4: Imaging-time relationships driving STEREO pointing requirements (image credit: JHU/APL)
G&C system: Each of the nearly identical STEREO spacecraft is three-axis-stabilized with fixed solar arrays for power generation as well as a steerable HGA (High Gain Antenna) for heliocentric orbit communications. A complement of Sun sensors, a ST (Star Tracker), redundant IMUs (Inertial Measurement Units), the GT, reaction wheel assemblies, and a monopropellant hydrazine propulsion system are used by the G&C system to keep the two STEREO spacecraft three-axis-stabilized. The system is selectively redundant.
All sensor data and actuator commands are fed over a MIL-STD-1553B data bus to/from the onboard flight computers (Figure 5). The G&C algorithms run in the flight computer at 50 Hz and determine spacecraft state and compute errors, and issue commands to the actuators (wheels and thrusters) to maintain control.
The single, nonredundant ST on each spacecraft provides fine, three-axis inertial pointing knowledge during all phases of the mission at 10 Hz. After initial GT acquisition, the ST is primarily used to provide roll knowledge. A single IMU (the second is a cold spare) contains gyros and accelerometers that provide three-axis, incremental angle, and linear-velocity (only used when firing thrusters) measurements. IMU data (100 Hz) are provided to the flight algorithms, which use the last valid measurements in the control loop to propagate the inertial attitude knowledge between ST updates.
Figure 5: G&C component interconnections (image credit: JHU/APL)
The GT, part of the SCIP instrument suite, provides the fine-pointing error signal. Its axes are nominally aligned with the spacecraft axes (Figure 6). It reports to the G&C (at 250 Hz) the y and z components of the apparent unit vector to the Sun as measured in the GT coordinate system (the assumption being that the x-axis component is approximately one). It has an acquisition range of slightly >15 arcmin and a fine-pointing range of approximately 70 arcsec. The GT is able to report the off-Sun pointing error to an accuracy of 0.4 arcsec, 3σ, in each axis for a single measurement; averaging can further reduce the error. Five 250-Hz samples are passed to the G&C flight computer for processing every 20 ms (50 Hz).
Figure 6: GT coordinate system and Sun vector measurement (image credit: JHU/APL)
Actuators: Four reaction wheel assemblies, arranged in a typical, pyramidal configuration, are the primary actuators for each STEREO spacecraft. The wheels, models RSI 12-75/607 manufactured by Rockwell-Collins, Deutschland GmbH (formerly Teldix), have a torque capability of 75 mNm and an angular momentum storage capacity of 12 Nms. All four wheels are operated simultaneously to maintain three-axis attitude control while providing an extra degree of freedom to redistribute angular momentum among the wheels. The wheels were carefully selected and placed on each spacecraft in an effort to minimize disturbances at the SCIP induced by static and dynamic wheel imbalances.
Dumping of accumulated angular momentum due to solar-pressure torques is accomplished with a monopropellant hydrazine blowdown propulsion system. Twelve thrusters, each nominally providing 4.4 N of force, are arranged in three sets of four with any set of four thrusters providing three-axis attitude control torque capability (via double-canting of each thruster) in addition to the nominal force in the particular direction. During a momentum dump, the thrusters are used to maintain attitude control while the wheels are torqued to their commanded momentum states.
RF communications: The onboard solid-state recorder has a capacity of 7.5 Gbit. RF communications are provided in X-band. The downlink data rate is variable up to about 720 kbit/s. The downlink data modulation is bi-phase shift keyed (BPSK). The uplink modulation is phase modulated (PM). Mission operations are conducted by JHU/APL at the MOC (Mission Operations Center) in conjunction with each POC (Payload Operations Center) for instrument operations (located at NRL, UMN, UCB, CalTech, and UNH).
In addition to normal data transmissions, the two STEREO spacecraft also broadcast continuously a low-rate (~600 bit/s) set of data consisting of typically 1 minute summaries (or 5 minute in the case of SECCHI) to be used for space weather forecasting, much like is currently done with the ACE and SOHO data. Several participating NOAA and international ground tracking stations will collect the data and send it electronically to the SSC (STEREO Science Center) at GSFC where it will be processed into useful physical quantities and be placed onto the STEREO website (http://stereo.gsfc.nasa.gov/). The goal is to have the processed data available within 5 minutes of receipt at the tracking stations. 16) 17)
Figure 7: Overview of a deployed STEREO spacecraft (image credit: NASA)
S/C operations scenario: The design enables nearly autonomous operations with occasional uplink support and to recover science data on a daily basis. The instrument/satellite operations are decoupled, operating independently from each other. In this setup, the instrument science teams are responsible for the following tasks: a) planning, scheduling and generating of commands; b) instrument health; c) instrument calibration; and d) synchronization of instrument operations with the S/C.
The current standard for sun observations is the SOHO (Solar and Heliospheric Observatory) mission with its instrument package. The science objectives of the STEREO project are obviously set to improve on the SOHO observations and to find answers to such questions as:
• Are CMEs driven primarily by magnetic or non-magnetic forces?
• What configuration of the corona leads to CMEs?
• What mechanism initiates a CME?
• What accelerates CMEs?
• How does a CME interact with the heliosphere?
• How do CMEs cause space weather disturbances?
Orbit: Heliocentric elliptical orbit in the ecliptic plane drifting at about 22º per year from the Sun-Earth line (at nearly 1 AU). The mission profile calls for solar imaging and heliospheric environment sampling by two identical spacecraft (with identical sensor complements) at gradually increasing angular separations from Earth (one drifting ahead of Earth and one behind). 18)
• STEREO A leads Earth by 22º after 1 year
• STEREO B lags Earth by about 22º after 1 year
Figure 8: Orbital configuration of the STEREO spacecraft after about a year
Initial orbit scenario: For the first 3 months after launch, the observatories will fly in highly elliptical orbits called “phasing orbits”. These are orbits from a point close to Earth to one that extends just beyond the moon's orbit. Mission operations personnel at JHU/APL will synchronize the orbits of the two spacecraft to encounter the moon about 2 months after launch. At that point, one spacecraft will use the moon's gravity to redirect it to an orbit lagging “behind” Earth (STEREO B). About 1 month later, the second observatory will encounter the moon again and be redirected to its orbit “ahead” of Earth (STEREO A). After both spacecraft have left the Earth-moon vicinity, they will be in heliocentric orbits at nearly 1 AU. The nominal mission starts when both spacecraft are in heliocentric orbits (about 90 days after launch).
Figure 9: Schematic of initial phasing orbits of the STEREO mission (image credit: NASA, JHU/APL)
Figure 10: Orbital progression of the STEREO spacecraft over the mission life (image credit: JHU/APL, NASA)
According to Ref. 18), all of the STEREO maneuvers were performed extremely accurately. This performance combined with STEREO’s accurate launch have left the spacecraft with a generous supply of fuel, with an ~60 m/s ΔV capacity that could be used for future spacecraft operations.
The additional maneuvers for Behind ended up targeting its drift rate to -22.000º/year, exactly the desired value. In December 2008, the mission design team suggested modifying the maneuvers to achieve a -22.5º/year drift rate, which would add up to 360º and an Earth return in 2023, 16 years after launch. But with the current drift rates, Ahead’s closest approach to Earth in 2023 will be 8.2 million km on 20 August, whereas Behind’s will be 10.0 million km on 14 July. Using approximately half of the remaining propellant could change the current drift rates to 22.5º/year.
Figure 11: Illustration of the twin STEREO satellites (image credit: JHU/APL)
The STEREO mission makes use of simultaneous interplanetary spacecraft operation and stereographic image reconstruction to develop a comprehensive 3-D description of the sun and the heliosphere. Payload operations will be conducted remotely from each of the instruments' facilities:
• SECCHI - Naval Research Laboratory
• IMPACT - University of California, Berkeley
• PLASTIC - University of New Hampshire
• SWAVES - Paris Observatory, Meudon/University of Minnesota
Launch: A launch of the twin STEREO satellites took place on Oct. 26, 2006 (UTC) in a stacked configuration using a Delta-II 7925-10L vehicle from Cape Canaveral Air Force Station, FLA.
Figure 12: Artist's view of the twin STEREO spacecraft studying the sun (image credit: NASA, JHU/APL)
• January 2014: Both STEREO spacecraft are currently performing as expected and taking science data on a regular basis. 19)
Relevancy to the heliophysics research objectives: STEREO is very much a ‘Great Observatory’ mission, especially in the extended phase. Its relevance to other solar, as well as planetary and interplanetary, missions is unquestionably high. It spans from near Earth solar and interplanetary (i.e., SOHO, SDO, ACE, WIND, and IBEX) to planetary (i.e., Messenger, Venus Express, Mars Express, and MAVEN). As much as STEREO will benefit from SOHO, ACE and Wind, those missions will benefit from STEREO’s complement of instruments and side view perspective. The planetary missions will get three times the opportunity to have conjunctions and oppositions with well instrumented spacecraft than they do now, when only near Earth instrumentation is available. By utilization of new results from the Van Allen Probes, it is expected that there will be new advances in understanding of the coupling of solar generated variability in radiation, fields, and particles that will enable advanced predictive capability for space weather research. 20)
Figure 13: Orbital positions of the STEREO-A and -B observatories for 25 January 2014 (image credit: NASA/GSFC, SSC) 21)
• Nov. 26, 2013: The STEREO spacecraft is monitoring Comet ISON as it approaches the sun. 22)
• The two STEREO spacecraft and their payloads are operating nominally in February 2013. 23)
• On October 26, 2012, the twin STEREO spacecraft completed their 6th anniversary on orbit. 24)
Figure 14: Since its launch in 2006, the STEREO spacecraft have drifted further and further apart to gain different views of the sun (image credit: NASA/GSFC)
Legend to Figure 14: On Sept. 1, 2012, the two spacecraft and and the Solar Dynamics Observatory (at Earth) formed an equal-sided triangle, with each observatory providing overlapping views of the entire sun. By providing such unique viewpoints, STEREO has offered scientists the ability to see all sides of the sun simultaneously for the first time in history, augmented with a view from Earth's perspective by NASA's Solar Dynamics Observatory (SDO). In addition to giving researchers a view of active regions on the sun before they even come over the horizon, combining two views is crucial for three-dimensional observations of the giant filaments that dance off the sun's surface or the massive eruptions of solar material known as CMEs (Coronal Mass Ejections).
• Observation of a very large CME from the far side of the sun: On July 23, 2012, a massive cloud of solar material erupted off the sun's far side, zooming out into space, passing one of NASA's STEREO spacecraft along the way. Using the STEREO data, scientists at NASA's Goddard Space Flight Center in Greenbelt, MD, clocked this giant cloud, known as a CME (Coronal Mass Ejection), as traveling between 2900 and 3500 km/s as it left the sun. Measuring a CME at this speed, traveling in a direction safely away from Earth, represents a fantastic opportunity for researchers studying the sun's effects. 25)
On July 23, STEREO-A lay — from Earth's perspective — to the right side and a little behind the sun, the perfect place for observing this CME, which would otherwise have been hard to measure from Earth. The SOHO (Solar Heliospheric Observatory) spacecraft, an ESA and NASA mission, also observed the CME. It is the combination of observations from both missions that helps make scientists confident in the large velocities they measured for this event.
STEREO-A could observe the speed of the CME as it burst from the sun, and it provided even more data some 17 hours later as the CME physically swept by – having slowed down by then to about 1200 km/s. STEREO has instruments to measure the magnetic field strength, which in this case was four times as strong as the most common CMEs. When a CME with strong magnetic fields arrives near Earth, it can cause something called a geomagnetic storm that disrupts Earth's own magnetic environment and can potentially affect satellite operations or in worst-case scenarios induce electric currents in the ground that can affect power grids.
The event also pushed a burst of fast protons out from the sun. The number of charged particles near STEREO jumped 100,000 times within an hour of the CME's start. When such bursts of solar particles invade Earth's magnetic field they are referred to as a solar radiation storm, and they can block high frequency radio communications as used, for example, by airline pilots. Like the CME, this SEP (Solar Energetic Particle) storm event is also the most intense storm ever measured by STEREO. While the CME was not directed toward Earth, the SEP did – at a much lower intensity than at STEREO – affect Earth as well, offering scientists a chance to study how such events can widen so dramatically as they travel through space.
• The two STEREO spacecraft and their payloads are operating nominally in 2012 (Ref. 6).
• On Oct. 25, 2011, the two STEREO spacecraft celebrated their 5th on-orbit anniversary. Over the last five years, each STEREO spacecraft has moved to a position in its orbit where it can capture side-view images of anything the sun sends our way.
• The first complete image of the solar far side, the half of the sun invisible from Earth, was taken on June 1, 2011. The composite image was assembled from NASA's two STEREO probes. The STEREO-A data is shown on the left half of image and the STEREO-B data on the right of Figure 15. The image is aligned so that solar north is directly up. The seam between the two images is inclined because the plane of Earth’s — and STEREO's — orbit, known as the "ecliptic", is inclined with respect to the sun's axis of rotation. The data was collected by the SECCHI instrument suite. 29)
Figure 15: First complete image of the far side of the sun taken on June 1, 2011 (image credit: NASA)
• On Feb. 6, 2011, NASA's twin STEREO probes moved into position on opposite sides of the sun (180º apart, Figures 16 and 17) providing a view of the entire sun - front and back (this is the first time in history to see both sides of the sun simultaneously). This permits the project to view solar activity in its full 3-dimensional extent. 30)
Until now, the project had to wait about 2 weeks until the rear side active regions of the sun rotated into view on the front side. But no longer. On average the sun rotates in about 27 days – faster at the equator and slower at the poles. The new far side data will allow much faster detection of solar storms which in turn will enable faster predictions of space weather which potentially can severely impact sensitive technological infrastructure on Earth and throughout the solar system. 31)
Figure 16: Orbital location of the STEREO-A and -B constellation on February 6, 2011 (image credit: NASA, Universe Today)
Figure 17: On Feb. 6, 2011, the two STEREO spacecraft were precisely 180º apart in their orbit permitting a full view of the Sun (image credit: NASA)
• The twin STEREO spacecraft (called “Behind” and “Ahead” denoting their relative positions in space), now almost 120º apart, captured this large and dramatic prominence eruption over about a 30-hour period between Sept. 26-27, 2009. 32)
Figure 18: Image composition of an eruption on the Sun taken by the STEREO twin spacecraft constellation (image credit: NASA)
• In February 2009, a huge CME (Coronal Mass Ejection) occurred on the sun which was witnessed by the twin spacecraft. The angular position of STEREO-B was directly over the blast site while STEREO-A was stationed at right angles — a perfect geometry for cracking the phenomenon on the sun known as the "solar tsunami." The scale of a solar tsunami is so staggering that solar physicists doubted their senses in the past when observing such an event. It rose up higher than Earth itself and rippled out from a central point in a circular pattern millions of kilometers in circumference. Skeptical observers suggested it might be a shadow of some kind—a trick of the eye—but surely not a real wave. 33) 34) 35)
The twin STEREO spacecraft confirmed their reality in February 2009 when sunspot 11012 unexpectedly erupted. The blast hurled a billion-ton cloud of gas (a CME) into space and sent a tsunami racing along the sun's surface. STEREO recorded the wave from two positions separated by 90º, giving researchers an unprecedented view of the event which is termed as a fast-mode MHD (Magnetohydrodynamic) wave. The one STEREO saw reared up about 100,000 km high, and raced outward at 250 km/s packing as much energy as 2400 megatons of TNT (1022 joule).
Figure 19: A solar tsunami seen by the STEREO spacecraft from orthogonal points of view (image credit: NASA)
Legend to Figure 19: The gray part of the animation has been contrast-enhanced by subtracting successive pairs of images, resulting in a "difference movie."
Solar tsunamis were discovered in 1997 by the SOHO (Solar and Heliospheric Observatory) of ESA. In May 1997, a CME came blasting up from an active region on the sun's surface, and SOHO recorded a tsunami rippling away from the blast site.
• The continuous progression of the two STEREO spacecraft orbits in the ecliptic plane is widening the observation capability of the sun. As of January 2009, more than 270º of solar longitude can be observed (Figure 10 and 20). The sun spins on its axis once every 25 days, so over the course of a month the whole sun does turn to face Earth, but a month is not nearly fast enough to keep track of events. 36) 37)
Since the sun spins counterclockwise in its orbit, STEREO-B gets a sneak preview of sunspots and coronal holes before they turn to face Earth-a boon for forecasters. In early 2009, STEREO-B enjoys a 3-day look-ahead advantage over Earth-based observatories. This has allowed researchers to predict geomagnetic storms as much as 72 hours earlier than ever before. On several occasions in late 2008, STEREO-B spotted a coronal hole spewing solar wind before any other spacecraft did.
• On April 23, 2007, the twin spacecraft have made the first three-dimensional images of the Sun. The new view will greatly aid scientists' ability to understand solar physics and thereby improve space weather forecasting. 39)
Figure 21: The combined images from all the STEREO HI instruments covering a wide swath of the sky—approximately 150º wide observed on Feb. 18, 2007 (image credit: NASA)
Legend to Figure 21: Literally half of the sky in a single view can be made by combining all of the coronagraph and HI images from the two STEREO spacecraft. The view covers the angular range from the Sun in the center out to ~90º in both directions along the ecliptic. Several planets as well as the Earth and Moon can be seen as can Comet McNaught. The Andromeda galaxy (M-31) can be seen at the top left, and the Milky Way is at the right (Ref. 8).
• With the second lunar swingby by the Behind spacecraft (STEREO-B) on January 21 2007, the STEREO mission entered into the main science phase of the mission. The two spacecraft have now escaped from Earth orbit and are in orbit around the Sun (to take on their mission orbits).
• The first sun observation images taken by the STEREO-A spacecraft were received on Dec. 4, 2006. The SECCHI instrument captured a very active region on the sun known as AR903 that produced a series of intense flares. A few days later during an unusually active solar period, the ”A” observatory captured images of a coronal mass ejection with one of SECCHI's two white-light coronagraphs.
• On Dec. 15, 2006, mission operations personnel at JHU/APL used lunar gravitational swingbys to alter the spacecraft orbits, redirecting the ”A” observatory to its orbit ”ahead” of Earth. STEREO-A flew past the moon at a distance of approximately 7,340 km. The STEREO-B spacecraft passed approximately 11,776 km above the lunar surface where gravity is slightly weaker. Although the ”B” observatory's orbit was slightly boosted, the spacecraft didn't undergo its full lunar gravitational assist until Jan. 21, 2007 when it re-encountered the moon. The spacecraft then came within approximately 8,818 km of the surface, swinging past the moon in the opposite direction of the ”A” spacecraft and into an orbit ”behind” Earth.
Sensor complement: (SECCHI, IMPACT, PLASTIC, SWAVES)
The sensor complement consists of four scientific instruments (two instruments and two instrument suites, with a total of 13 instruments per observatory). All STEREO instruments involve participation and collaboration of teams from a number of institutions. This applies to instrument design and development as well as to instrument operations, data sharing and analysis. 40)
The overall measurement strategy calls for:
• Imaging of the solar atmosphere and heliosphere from two perspectives simultaneously
• Tracking of disturbances in 3-D from their onset at the sun to beyond Earth's orbit
• Measurement of energetic particles generated by the disturbances
• Sampling of fields and particles in the disturbances as they pass Earth's orbit.
Table 2: Overview of STEREO measurement objectives
SECCHI (Sun-Earth Connection Coronal and Heliospheric Investigation)
SECCHI is an instrument suite, PI: Russell A. Howard of NRL. The instrument is named after one of the first solar physicists, namely A. Pietro Secchi (1818-1878), who used the new medium of photography to record solar eclipses. The overall objective is to study the 3-D evolution of CMEs from birth at the sun's surface through the corona and interplanetary medium to its eventual impact at Earth.
SECCHI consists of a suite of remote sensing instruments: an EUVI (Extreme Ultraviolet Imager), two white-light coronagraphs (COR1 and COR2), and an HI (Heliospheric Imager) package. The EUVI+COR1+COR2 devices are collectively referred to as SCIP (Sun Centered Imaging Package). The SEB (SECCHI Electronics Box) supports all of the SECCHI instruments. The total mass of the SECCHI instrument is 69.3 kg (SCIP=44.6 kg, HI= 11.3 kg, SEB= 9.2 kg), total power = 50 W. 41) 42) 43) 44)
Table 3: SECCHI instrument performance overview
Figure 22: Photo of the SECCHI optical bench with its thermal tent removed (image credit: NASA)
Legend of Figure 22: From left to right are the guide telescope, the COR1, the EUVI, and COR2 telescopes. HI1 and HI2 are mounted to the exterior of the observatory separately.
Figure 23: Top view of SECCHI-SCIP assembly with thermal tent (image credit: NRL)
The SCIP (Solar Centered Imaging Package) contains three compact telescopes to view the solar disk and solar corona. The SCIP structure provides the required alignments, stiffness, and modularity to the three instruments. SCIP also contains the GT (Guide Telescope) that provides fine-pointing information to the S/C attitude control system and to FPS (Fine Pointing System) of the EUVI. The pointing strategy is based on pointing the S/C to minimize the scattered light in COR1 and relying upon the SCIP structure to assure that COR2 and EUVI are within acceptable pointing tolerances. The SCIP and the HI assemblies have their own camera electronics inside their enclosures. These units are known as CEB (Camera Electronics Box). The SCIP camera electronics are composed of three circuit cards with a common backplane interface. The CCD (Charge Coupled Device) camera electronics are designed for independent control of all instrument (COR1, COR2, HI, and EUVI) CCDs, a camera readout rate of 1 Mpixel/s through either of two CCD output ports, and 14-bit data quantization.
EUVI (Extreme Ultraviolet Imager). EUVI observes the chromosphere and innermost corona underlying the same portions of the corona and the heliosphere observed by COR1, COR2, and HI. Four EUV emission lines between 17.1 and 30.4 nm are observed. The EUVI design uses a small Ritchey-Chretien telescope of EIT (EUV Imaging Telescope) heritage on SOHO and of TRACE (Transition Region and Coronal Explorer) mission heritage. The improved mirror coatings, detector array, and telemetry allocation provide a higher sensitivity and image cadence of the EUVI instrument than previously possible. The optical system of EUVI, consisting of entrance filter, shutter, filter wheel, primary mirror and active secondary mirror, and a CCD array, provides pixel-limited imaging throughout the FOV in a compact envelope. The EUVI optics are fully baffled. The CCD detector assembly uses a backside-illuminated, backside-thinned and pacified array of size 2k x 2k with an EUV quantum efficiency >70%. The detector is passively cooled by a radiator. EUVI has its own ISS to satisfy its jitter requirements. The ISS includes the EUVI secondary mirror as the actuator and GT as the high-accuracy angular motion sensor. The EUVI secondary mirror is actuated to compensate for the S/C jitter. The GT is used to provide fine-pointing angular measurements to orient the boresights of the SECCHI telescopes to point toward the geometric center of the sun. The circular full sun FOV is ±1.7 Rsun. 45)
COR1 (Coronagraph1 Imager). The objective is to explore the inner corona in white light and pB down to 1.25 Rsun. COR1 observes the inner (1.4 - 4 Rsun) corona with a high frequency and polarization precision. The COR1 optical design is an all-refractive 7-element system. The instrument FOV is is 1.25-4 Rsun for a full field width of 2.13º and an aperture diameter of 36 mm. The spectral range is 650-750 nm. Images are collected at three different linear polarization angles. The occulter is attached to the second optical element in COR1, a doublet field lens. The final stage consists of two doublets, along with the filter and polarization optics. The stage takes the chromatically aberrated image of the corona, near the occulter, and relays it onto the CCD detector. The 2k x 2k detector data is summed on-board to 1k x 1k images to produce 7.5 arcsec pixels.
Figure 24: Schematic view of SECCHI-COR1 flight unit (image credit: NRL, NASA)
Figure 25: Photo of the SECCHI-COR1 flight unit (image credit: NRL, NASA)
COR2 (Coronagraph2 Imager). The objective is to explore the outer corona (2-15 Rsun) in white light and pB with high spatial and temporal resolutions. The COR2 design is of LASCO heritage on SOHO. The optical design of the instrument uses a classical externally occulted Lyot design with an entrance aperture diameter of 30.5 mm and an overall length of 1348 mm. The spectral range and polarization technique is the same as COR1 to provide a seamless transition between the two regions.
Figure 26: Mechanical layout of the SECCHI-COR2 instrument (image credit: NRL)
Figure 27: Illustration of EUVI (image credit: NRL)
HI (Heliospheric Imager). The instrument assembly is being developed by a UK-led team involving the University of Birmingham and the Rutherford Appleton Laboratory (RAL), in collaboration with Centre Spatial de Liege (CSL), Belgium, and the Naval Research Laboratory (NRL), USA. The PI is Richard Harrison. The objective is to extend the concept of traditional externally occulted coronagraphs to a new regime, by observing the heliosphere from the sun to the Earth (12-215 Rsun). 46) 47) 48)
The HI is a wide-angle visible-light imaging system for the detection of coronal mass ejection (CME) events in interplanetary space and, in particular, of events directed towards the Earth.
The HI assembly consists of two instruments, HI-1 and HI-2, the exterior baffle systems, the hinged baffle cover, the focal plane packages, the HI camera electronics, and the radiators for the CCDs. - HI-1 looks at the inner heliosphere to within 3.28º of the sun with an opening angle of 20º. The HI-1 camera uses a five-vane forward linear baffle system and a matching linear internal occulter, a 20º full field angle lens, and a 2048 x 2048 pixel format CCD.
The HI-2 camera looks further out, from an elongation of 18.36º, with an opening angle of 70º. The HI-2 objective is set deep within the forward baffle system shadow at a diffraction angle of 16.5º. The HI-2 camera consists of a wide-angle fisheye lens and a CCD detector array.
Table 4: Characteristics of the HI instrument
Figure 28: SECCHI Heliospheric Imager (HI) concept (image credit: NRL)
Figure 29: SECCHI-HI instrument design (image credit: CSL, RAL)
The SEB (SECCHI Electronics Box) provides all command and data handling functions and is the only electronic interface with the S/C. SEB consists of a control computer, interfaces for the camera and CCD, fine-pointing and jitter control electronics, a housekeeping and data acquisition system, and power conversion. A RISC-based single board computer RAD6000 (32 MHz) is used. The image processing and compression software employs such compression routines as Rice (lossless) and a lossy wavelet compression (H-compress) technique.
Figure 30: Accommodation of SECCHI components on the STEREO spacecraft (image credit: NRL, NASA)
IMPACT (In-situ Measurements of Particles and CME Transients)
IMPACT (In-situ Measurements of Particles and CME Transients), PI: Janet G. Luhmann of the Space Sciences Laboratory of UCB (University of California at Berkeley). The IMPACT investigation science focuses on the magnetic connections to the sun and topology of interplanetary CME-related interplanetary disturbances, and the energetic particles that precede and accompany these disturbances as they move toward Earth. IMPACT addresses these with a combination of solar wind electron and solar energetic particle instrumentation that, with the PLASTIC solar wind ion measurements provide the comprehensive, identical in-situ measurements at the two STEREO spacecraft locations necessary to relate what is seen in the SECCHI images to the space weather environment at 1 AU.
IMPACT's solar wind electron instruments (SWEA, STE) and magnetometer (MAG) are mounted on the STEREO boom, while the SEP (Solar Energetic Particles) instruments are on the spacecraft body. All of these instruments use the IDPU (IMPACT Instrument Data Processing Unit), which serves as a single interface to the spacecraft. The PLASTIC solar wind ion instrument also uses the IMPACT IDPU. IMPACT's boom instruments consist of the electron instruments SWEA (Solar Wind Electron Analyzer) and STE (Suprathermal Electron Telescope) and the magnetometer. The SEP package is also a multi-instrument (4) system, but it is more physically integrated, and so included under one heading here.
SWEA (Solar Wind Electron Analyzer). The objective is designed to measure the distribution function of the solar wind core and halo electrons from about 1 eV to several keV, with high spectral and angular resolution over practically the full spherical range. This capability allows the distinction between these components in detail during both undisturbed periods and the passage of CME generated disturbances, when the interplanetary field rotates far out of the ecliptic plane.
Figure 31: Illustration of SWEA (image credit: UCB/SSL)
STE (Suprathermal Electron Telescope). STE is a new instrument that covers electrons in the energy range of about 2-20 keV with approximately 50 times the sensitivity of previous instruments. These higher energy electrons are frequently accelerated in flares or impulsive flare-like bursts, and they produce solar type III radio bursts as they escape the sun. They provide a unique tracer of the footpoints of CME and normal interplanetary magnetic fields, and allow the measurement of the length of those field lines. In addition, an electron superhalo is continuously present in the interplanetary medium at those energies. STE utilizes passively cooled SSDs (Silicon Semiconductor Devices) that measure all energies simultaneously. The STE consists of two detector arrays of four SSDs in a row, each about 0.1 cm2 in area and about 500 μm thick. Each array looks through a rectangular opening that provides a FOV of about 20º x 80º for each SSD with the 80º direction perpendicular to the ecliptic. Adjacent FOVs are offset for a total FOV of about 80º x 80º. The two arrays are mounted back-to-back, looking in opposite directions, centered about 25º from the average Parker Spiral field direction. STE is located just inboard of SWEA on the STEREO boom to clear its FOV and remain in shadow.
Figure 32: The STE instrument (image credit: UCB/SSL)
MAG (Magnetometer). The MAG system is a simplified version of the magnetometers flown on Mars Global Surveyor and Lunar Prospector. It is a triaxial fluxgate design, mounted on the STEREO boom (about 4m) just inboard of the SWEA and STE instruments. The fluxgate sensors use a ring core geometry, with magnetic cores consisting of permalloy. The units are compact, low power, and ultra-stable. To optimize sensitivity at the low field values to be found in interplanetary space, the magnetometer dynamic range is divided into 8 ranges that are automatically switched whenever the field being measured exceeds or falls below predetermined levels. The maximum range is sufficiently large to allow IMPACT MAG magnetic field measurements in the Earth's magnetic field during the commissioning phase orbits.
Figure 33: The MAG instrument (image credit: GSFC)
SEP (Solar Energetic Particles) package. SEP consists of 4 separate instruments that cover the full solar energetic particle energy, flux and composition ranges needed to meet the STEREO science goals. The SEP instruments include the SEPT (Solar Electron Proton Telescope), SIT (Suprathermal Ion Telescope), LET (Low Energy Telescope), and the HET (High Energy Telescope). Together, the SEP instruments measure electrons, protons and heavier ions from several tens of keV to approximately 100 MeV (or MeV/nucleon).
Figure 34: The SEP package instruments on STEREO (image credit: UCB/SSL)
• SEPT (Solar Electron Proton Telescope). SEPT consists of two dual, double-ended magnet/foil solid state detector particle telescopes that cleanly separate and measure electrons in the energy range 20-400 keV and protons from 20-7000 keV, while providing anisotropy information through the use of several FOVs. Each SSD detector in SEPT is 300 μm thick and 0.53 cm2 in area. A rare-earth permanent magnet is used to sweep away electrons for ion detection, while a parylene foil transmits electrons but stops protons. SEPT is divided into two pieces for FOV reasons. The SEPT-E telescope is housed with the rest of the SEP package, located on the body of the S/C. It looks in the ecliptic plane along the Parker Spiral magnetic field direction, both forward and backward. SEPT-N/S is housed separately at a different spacecraft location and looks out of the ecliptic plane perpendicular to the nominal magnetic field, both north and south. The viewing cones for the SEPT telescopes are each about 60º.
Figure 35: Illustration of the SEPT instrument (image credit: NASA)
• SIT (Suprathermal Ion Telescope) of GSFC, MPI and University of Maryland. SIT is a time-of-flight ion mass spectrometer that measures elemental composition of He-Fe ions over the energy range of about 30 keV/nucleon to 2 MeV/nucleon. The FOV angles are 17º x 44º, with the 44º angle in the ecliptic plane, centered approximately 60º from the spacecraft-sun line to avoid sunlight while still intercepting significant numbers of Parker Spiral field controlled energetic ion fluxes. The telescope analyzes ions that enter through thin entrance foils and stop in a solid state detector. A time-of-flight approach for determining the composition utilizes start and stop times obtained from secondary electrons entering a MCP (MicroChannel Plate) detector system. The MCP and SSD areas are each 6.0 cm2. The SIT geometric factor allows study of even small SEP events.
Figure 36: The SIT instrument (image credit: UCB/SSL)
• LET (Low Energy Telescope) of Caltech, GSFC and JPL. LET is a special double-fan arrangement of 14 solid-state detectors designed to measure protons and helium ions from about 1.5 to 13 MeV/nucleon, and heavier ions from about 2 to 30 MeV/nucleon. LET uses a standard dE/dx vs. E-technique, identifying particles that stop at depths of about 20-70 μm and about 70-2000 μm corresponding to two general energy ranges. The large FOV spans from 20º above to 20º below the ecliptic plane and extends 65º to either side of the forward and backward Parker Spiral field directions in the ecliptic plane. Like SIT, LET's large geometric factor also ensures the detection of even small SEP events.
Figure 37: The LET instrument (image credit: UCB/SSL)
• HET (High Energy Telescope). HET also uses the solid-state detector, dE/dx vs. E-approach, but in a six-detector, more traditional linear arrangement designed to measure protons and helium ions to 100 MeV/nucleon and energetic electrons to 5 MeV. HET identifies particles that stop at depths of 1 to 8 μm in the detectors. Some information will also be obtained on heavier nuclei up through Fe using the dE/dx vs. E signatures and ranges together, and penetrating particles will be analyzed. The FOV covers a 47.5º cone around the Parker Spiral field direction. SIT, LET and HET are all packaged together with the SEPT-E subsystem and a SEP CPU (Central Processing Unit) in the main SEP package on the spacecraft body, but they use the IMPACT IDPU to interface with the spacecraft together with the rest of IMPACT.
PLASTIC (PLAsma and SupraThermal Ion and Composition)
PLASTIC instrument, PI: Antoinette Galvin of UNH (University of New Hampshire), collaboration of UNH, University of Bern, Switzerland, MPE Garching and University of Kiel, Germany, and NASA/GSFC. The objective of PLASTIC is to study coronal SW (Solar Wind) and solar wind-heliospheric processes. The science objective is to measure ions in the energy-per-charge range of 0.3 to 100 keV/e. The instrument performs three functions in one package: 49) 50)
• Measurement of the distribution functions of solar wind protons and alpha particles (providing density, velocity, kinetic temperature and its anisotropy), with a time resolution of about one minute.
• The SW sector also provides, on at least five minute resolution, the elemental composition, charge state distribution, kinetic temperature, and velocity of the more abundant solar wind heavy ions (e.g., C, O, Ne, Mg, Si, and Fe).
• The PLASTIC Wide Angle Partition measures the distribution functions of suprathermal ions H through Fe, with a comparatively large geometrical factor that allows the study of suprathermal particles, including shock-accelerated particles and pick up ions. Together with IMPACT measurements, PLASTIC completes the required STEREO mission in-situ observations.
The PLASTIC instrument design is based on the CIS/CODIF (Cluster Ion Spectrometer/Composition and Distribution Functions) sensor onboard the Cluster mission (with a much improved mass resolution to resolve individual ionic charge states of heavy ions in the solar wind); PLASTIC includes an electrostatic deflection analyzer, a time-of-flight section, and solid-state detectors (SSD's) for energy measurement. The instrument provides a large IFOV (resolved in-ecliptic and in polar angles) with measurements taken at high time resolution (1-5 minutes) spanning an ion energy range of 0.3-100 keV/e. To handle a large range of particle fluxes, the entrance system of PLASTIC employs collection apertures with different geometric factors for the bulk solar wind (H ~96%, He ~4%) and for the heavier, less-abundant ions (<1%) and suprathermals.
Figure 38: Illustration of the PLASTIC instrument (image credit: UNH, NASA)
SWAVES is an interplanetary radio burst tracker device. PI: Jean-Louis H. Bougeret of CNRS (Centre National de la Recherche Scientifique) Observatory of Paris, with instrument management by the University of Minnesota. The goal of the SWAVES radio and plasma waves investigation is to obtain unique and critical observations for all primary science objectives of the STEREO mission, the generation of CMEs, their evolution and their interaction with Earth's magnetosphere. SWAVES can probe a CME from lift-off to Earth by detecting the coronal and interplanetary (IP) shock of the most powerful CMEs, providing a radial profile through spectral imaging, determining the radial velocity from about 2 Rsun (from center of sun) to Earth, measuring the density of the volume of the heliosphere between the sun and Earth, and measuring important in situ properties of the IP shock, magnetic cloud, and density compression in the fast solar wind stream that follows. 51) 52)
SWAVES achieves these goals by measuring IP (Interplanetary) type II and type III radio bursts, both remotely and in-situ (the type II radio bursts are signatures of the propagation of CMEs, while type III radio bursts represent the propagation of energetic electrons associated with flares).
• SWAVES antenna system (see Figure 11 and 39). SWAVES uses three mutually orthogonal monopole stacer antenna elements as its prime sensors (built by the University of California, Berkeley), each 6 meters in length. The three monopoles are deployed away from the sun so that they remain out of the fields of view of sunward looking instruments. The antenna design optimizes the radio burst tracking in the 16MHz - 30kHz range and maintains a high SNR for expected solar type II, type III and other solar and IP radio emissions.
• Preamplifiers. A high input impedance preamplifier is connected to each of the three electric monopoles.
• Radio receivers. There are five radio receivers in the SWAVES instrument to cover the following frequency ranges: 1) LFR Lo: 10-40 kHz; 2) LFR Hi: 40-160 kHz; 3) HFR: 0.125-16 MHz; 4) FFR1: 32 MHz fixed frequency; 5) TDS: 250,000 samples/s time series snapshots.
• Housekeeping and low-rate science. This system provides digitized and selectable housekeeping data to monitor the status and health of the various parts of the instrument.
• Calibration. The instrument contains its own internal calibration circuitry. An internal calibration sequence will be invoked about once per day.
The focal point of SWAVES instrument commanding and operations will be performed at the UMN (University of Minnesota).
Figure 39: The SWAVES antenna system (image credit: NASA)
Space weather data. “Real-time” in situ space weather data products from IMPACT and PLASTIC, consisting of approximately 1 minute cadence information on solar wind electrons and ions, solar energetic particles and magnetic fields are packaged in the data stream from the IMPACT IDPU with the regular coded science and housekeeping telemetry from the instruments. The bulk of the space weather data is expected to come from SECCHI, producing about 7 images/hour (256 x 256 pixels). In addition, SWAVES is producing a 1 minute averaged dynamic spectrum.
STEREO mission technology summary
• Stereoscopic image processing
• Light-weight coronagraphs
• High sensitivity, low-scatter, wide angle photometry
• CME radio tracker
• Autonomous operation - beacon mode.
The STEREO ground system was developed with the intention that the two spacecraft could be operated simultaneously and independently of each other. To achieve this goal, the ground system architecture is such that for most MOC (Mission Operations Center) functions, there are separate hardware/software components for each spacecraft. This approach minimizes the risk of confusion among operators and other personnel, and it allows for the simultaneous operation of both spacecraft. 53)
Figure 40: Overview of the STEREO ground system architecture (image credit: JHU/APL)
The STEREO mission operations were designed around the data return requirement of an average of 5 Gbit/day of data, and although loss of some data was deemed acceptable, maximizing data return was kept a priority. After 6 months in heliocentric orbit, STEREO returned a daily average of 7.0 Gbit of data from the Ahead spacecraft and 7.7 Gbit of data from the Behind spacecraft.
STEREO employs decoupled spacecraft bus/instrument operations modeled after the TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics) program. Operating only the spacecraft bus and engaging a highly automated system, STEREO mission operations are able to use a small team to safely operate two spacecraft simultaneously. The STEREO ground system was developed to support this concept of highly automated operations from the APL MOC (Mission Operations Center) and multiple POCs (Payload Operations Centers). In addition, the STEREO ground system includes a SDS (STEREO Data Server) for the distribution of all of the data and products required for operation of the STEREO observatories, as well as two HIL (Hardware-In-the-Loop) simulators for verification of commanding and analysis of anomalies.
The STEREO ground system is segmented into several distinct network “zones.” This design ensures the integrity of the STEREO spacecraft as well as STEREO and APL network resources. All real-time command and control of the STEREO spacecraft originates in the STEREO ground system Restricted IONet (Internet Protocol Operational Network). The Restricted IONet is a secure NASA network that places secure controls on the users and facilities. Each spacecraft has a primary and a backup command workstation in the Restricted IONet. Dedicated Memory Allocation Examiner (or “MAX”) workstations for each spacecraft are also located on this network. Additionally, each spacecraft has a third command workstation, one that is remotely located in a separate building on the APL campus. These workstations would be used to ensure spacecraft health and safety in the event that the entire STEREO MOC becomes inaccessible.
The STEREO firewall isolates the Restricted IONet from the remaining components of the STEREO MOC ground system, which reside in the operations “demilitarized zone” (Ops DMZ) network (which has fewer restrictions and is a less secure network). There are 6 workstations assigned in the Ops DMZ, 3 per spacecraft, telemetry archive servers with 10 TB RAID (Redundant Array of Independent Disk) storage arrays (one system for each spacecraft), a second-level archive server, an SDS, and a database server. Additionally, each spacecraft has a dedicated HIL simulator with its own command and telemetry workstation.
An APL firewall isolates the Ops DMZ from the APL intranet. The STEREO development community functions in this network zone. Software development workstations and testbeds/mini-MOCs are connected to the APL intranet. This configuration allows the developers and testers to access their systems from other laboratory resources, such as employee desktop computers and laptops.
A separate dedicated network, known as the Science DMZ, supports instrument POCs located at APL. In addition to the APL-located instrument POCs, each instrument has primary POCs located at their home institutions, which also have access to the STEREO instrument command and telemetry servers, as well as the SDS, via an Internet connection. The SSC (STEREO Science Center) and the FDF (Flight Dynamics Facility), both located at GSFC (Goddard Space Flight Center) in Greenbelt, Maryland, also communicate with the STEREO MOC via the Internet by using FTP (File Transfer Protocol).
SDS (STEREO Data Server): The STEREO ground system consists of the APL STEREO MOC, the DSMS (Deep Space Mission System) ground stations, the instrument POCs, the FDF, and the SSC. To coordinate mission operations between these teams and return the science data to the mission scientists, a routine flow of data products is necessary. Most of these data products are available to the STEREO community through the SDS. The SDS consists of a webpage in the APL MOC where data products can be transferred to the community via FTP.
STEREO mission operations:
The APL STEREO MOT (Mission Operations Team) works in the MOC and has the primary responsibility of management of the spacecraft bus, including the development of command messages and the uplink to the spacecraft by way of the DSMS. Recovery of spacecraft bus engineering (state-of-health) telemetry and the performance analysis based on this telemetry are also performed at the MOC. The MOC receives instrument command messages from the POCs and, after verification that the command APIDs (Application Identifiers) are appropriate for the POC they came from, queues these for uplink to the spacecraft on the basis of start and expiration times appended to the command messages by the POC. The MOC does not directly verify any instrument commands and does not decommutate or analyze any instrument telemetry aside from currents and temperatures observed from the spacecraft side of the instruments. Each POC is individually responsible for the health and safety of its instrument. In addition to the POCs, the other external teams that support STEREO mission operations are the DSN (Deep Space Network); the FDF, which performs navigation for the STEREO mission; and the SSC, which is the primary archive of STEREO data and the focal point for education and public outreach.
External operations facilities:
• POCs: The POCs are the instrument operations centers that generate the commands for each of the four STEREO instrument suites (SECCHI, IMPACT, SWAVES, and PLASTIC) and monitor instrument health and safety. These centers are located at the Navel Research Laboratory in Washington, DC (SECCHI); the University of California, Berkeley (IMPACT), the University of Minnesota, Minneapolis (SWAVES); and the University of New Hampshire, Durham (PLASTIC). These are the home bases for the instrument POCs, but they are able to operate remotely when necessary, and they also maintain a presence at the MOC.
• DSN: The DSN is being used to provide communications to both spacecraft from launch to end of life. The use of all three DSN antenna facilities—Goldstone, Madrid, and Canberra—is required to determine the elevation component for the navigation of each spacecraft. Nominally, a 3.5-5 hour track, depending on spacecraft range, centered every 24 h per spacecraft will be conducted by using the 34 m beam wave guide subnet.
The MOC is connected to the DSN via Restricted IONet links. Commands will be flowed to the DSN by using the standard Space Link Extension Service over the Restricted IONet, and real-time telemetry will be flowed from the DSN to the MOC over the Restricted IONet by using legacy the UDP (User Datagram Protocol) service. Playback data received at the DSN station will be flowed to the Central Data Recorder at the JPL (Jet Propulsion Laboratory), where they will in turn be flowed in half-hour increments to the MOC via FTP as intermediate data recorder (IDR) files. Orbit data for each spacecraft will be provided to the DSN from the FDF for acquisition, and ranging data will be distributed from the DSN to the FDF for orbit determination purposes over the Restricted IONet.
• FDF (Flight Dynamics Facility): The FDF at the NASA GSFC determines the orbits of the observatory from tracking data provided by the DSN ground stations, and it generates predicted DSN station contact periods and predicted and definitive orbit data products. The FDF also generates orbital ephemeris data in support of orbit maneuvers that satisfy science and mission requirements, and it transfers this information to the STEREO MOC via the FDF Products Center.
• SSC (STEREO Science Center): The SSC is located at the NASA GSFC and serves four main functions for the STEREO mission. First, it is the prime archive of STEREO telemetry and data, and it serves that data to the international science community, and to the general public, through its own website. It is also the collection site, processing center, and distribution point for STEREO space weather beacon data. Science coordination between the STEREO instruments, and between STEREO and other observatories, is performed through the SSC. Finally, the SSC is the focal point for education and public outreach activities.
The STEREO observatories continually point at the Sun as they move away from the Earth at a rate of 22º/year while remaining approximately the same distance from the Sun throughout the mission. In this orbit, the solar array input power is sufficient to cover any instrument/spacecraft mode, and the observatory remains thermally stable. Each instrument has its own partitions to use on the SSR, which are typically set by the MOT to an overwrite mode. Thus, the instruments are left with no power, thermal, or SSR constraints other than managing the amount of data being placed in their SSR partitions that is ideal for the decoupled spacecraft/instrument operations concept employed by STEREO. This operations concept greatly simplifies mission operations and allows for a smaller operations team size and automated, unattended real-time operations for most of the spacecraft contacts.
Summary: The STEREO ground system is uniquely divided between the two spacecraft, allowing simultaneous communications with the two spacecraft while minimizing the risk of confusion. The mission operations tools used for planning, real-time operations, and assessment of spacecraft performance allow for the safe operation of two spacecraft simultaneously. Specifically, separation of command and memory management between the two spacecraft is rigorously maintained. STEREO mission operations are being accomplished with a minimal number of staff while the science data return is kept well above the minimum requirement for mission success.
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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.