MMS (Magnetospheric MultiScale) Constellation
MMS is a NASA solar-terrestrial probe constellation comprising four identically instrumented spacecraft that will use Earth's magnetosphere as a laboratory to study the microphysics of three fundamental plasma processes: magnetic reconnection, energetic particle acceleration, and turbulence. These processes occur in all astrophysical plasma systems but can be studied in situ only in our solar system and most efficiently only in Earth's magnetosphere, where they control the dynamics of the geospace environment and play an important role in the processes known as "space weather." 1) 2) 3) 4)
The fundamental plasma physics process of reconnection in the Earth's magnetosphere are to be monitored on temporal scales of milliseconds to seconds and on spacial scales of 10s to 100s of km. Three science objectives have been identified for the MMS mission. In priority order, these objectives are to:
• Determine the role played by electron inertial effects and turbulent dissipation in driving reconnection in the electron diffusion region
• Determine the rate of magnetic reconnection and the parameters that control it
• Determine the role played by ion inertial effects in the physics of reconnection.
The overall science objective is to investigate the physics of magnetic reconnection: 5)
- In the plasma universe magnetic fields connect and disconnect explosively transferring energy into the electrons and ions that make up the plasma.
- MMS will reveal for the first time the small-scale 3D structure and dynamics in the heart of reconnection regions occurring naturally in Earth's geospace plasma environment.
- Only by measuring the behavior of the particles and fields in three-dimensions in the diffusion region can we learn fully how reconnection proceeds.
- The four MMS spacecraft will be placed "surgically" in a tetrahedral formation into the diffusion region.
Understanding the physics of reconnection:
- The most important goal of MMS is to conduct a definitive experiment to determine what causes magnetic field lines to reconnect in a collisionless plasma.
- Magnetic Reconnection is evident throughout the plasma universe
- Yet it occurs in the near vicinity of Earth. It can be studied internally by man-made probes using the Earth's magnetosphere as a laboratory.
- The 4 spacecraft MMS flying in an adjustable pyramid-like formation through the core of these dynamic and impulsive energy conversion regions allows the 3D structure to be investigated.
- Through detailed measurements of the local plasmas and fields scientists will understand the physics that governs one of the most important drivers of space weather.
- ..... and learn how charged particles are energized on the sun and in astrophysical bodies throughout the universe.
Figure 1: Schematic view of the reconnection region (image credit: NASA)
The MMS mission employs four identically instrumented spinning spacecraft orbiting the Earth in a tetrahedral configuration to conduct definitive investigations of magnetic reconnection in key boundary regions of the Earth's magnetosphere. The process of magnetic reconnection, which controls the flow of energy, mass, and momentum within and across plasma boundaries, occurs throughout the universe and is fundamental to our understanding of astrophysical and solar system plasmas. It is only in the Earth's magnetosphere, however, that it is readily accessible for sustained study through the in situ measurement of plasma properties and of the electric and magnetic fields that govern the behavior of the plasmas.
Through high-resolution measurements made by each spacecraft, whose separations can be varied from tens of km to a few hundreds of kilometers, MMS will probe the crucial microscopic physics involved in these fundamental processes; determine the 3D geometry of the plasma, field, and current structures associated with them; and relate their micro-scale dimension to phenomena occurring on the mesoscale. By acquiring data simultaneously at multiple points in space, MMS will be able to differentiate between spatial variations and temporal evolution, thus removing the space-time ambiguity that has limited single-spacecraft studies of magnetospheric plasma processes.
The MMS mission is managed by the Heliophysics Division for NASA's Science Mission Directorate at NASA/HQ. NASA/GSFC will build the four spacecraft and the intersatellite ranging and communication system. Southwest Research Institute (SwRI) of San Antonio, TX leads the science investigation and development of the instrument suite together with numerous partners, including the University of New Hampshire, NASA/GSFC, Johns Hopkins University /Applied Physics Laboratory (JHU/APL), LASP (Laboratory for Atmospheric and Space Physics) at the University of Colorado, Boulder, and international partners in Austria (IWF), Sweden (KTH and IRF-U), France (CETP) and Japan.
The technology requirements of the MMS mission call for:
• On-board propulsion
• Intersatellite communication
• Autonomous operation.
Figure 2: Artist's rendition of the MMS mission in Earth's magnetosphere (image credit: NASA, SwRI)
Project development status:
• In August 2013, the MMS mission is in Phase-D, Integration and Test.
• On August 31, 2012, NASA's MMS mission passed the SIR (System Integration Review) which deems a mission ready to integrate instruments onto the spacecraft. 6)
• In May 2005, NASA selected the SwRI instrument suite team to work with GSFC in the MMS project for the mission formulation. In May 2009, the project passed the preliminary design review. In August 2010, the project completed its mission critical design review (CDR). The C/D Phase of the project started in 2011. 7) 8)
The four spin-stabilized spacecraft are being developed and integrated at GSFC. Each satellite has an octagonal shape that is approximately 3.5 m wide and 1.2 m high. The satellites spin at 3 rpm during science operations. There are 8 deployable booms per satellite: four 60 m wire booms in the spin plane for electric field sensors, two 12.5 m booms in the axial plane for electric field sensors, and two 5 m booms in the spin plane for the magnetometers. 9) 10) 11) 12)
The aluminum structure has a modular design to simplify I&T (Integration & Testing) consisting of a propulsion assembly, separation system/thrust tube, instrument deck, and spacecraft deck.
ACS (Attitude Control Subsystem): The ACS keeps the spacecraft to within ±0.5º of the desired orientation during science operations and implements on-board closed loop maneuver control. The ACS sensor complement consists of a star tracker (ST) with four separate heads and two redundant DSS (Digital Sun Sensors), one being a cold backup unit. The four ST heads output four independent quaternions at 4 Hz (that is, 16 independent attitude measurements/s). Once per spin period (~20 sec), the DSS will output a pulse indicating sun-crossing through the sensor FOV slit and a measurement of the sun elevation from the body X-Y-plane (the +Z-axis is the nominal spin axis). 13)
Star sensors and sun sensors provide attitude data, and accelerometers provide acceleration and ΔV data. Thrusters are used as actuators. A GPS receiver on board each spacecraft provides absolute position information. In addition, each spacecraft employs an IRAS (Inter-spacecraft Ranging and Alarm System) to determine its location relative to the other three spacecraft.
The propulsion subsystem is a mono-propellant blowdown system with 12 thrusters sized to achieve both small formation maintenance maneuvers and large apogee raise maneuvers. Approximately 360 kg of propellant will be contained in four titanium tanks per spacecraft. 14)
Figure 3: Side view of a MMS spacecraft (image credit: NASA/GSFC)
EPS (Electrical Power System): The EPS is a 'direct energy transfer system' employing a battery dominated bus. Power to the spacecraft is supplied from 8 identical body-mounted solar array panels that are electrostatically and magnetically clean. The battery is sized to provide power during the 4 hour eclipses. - The thermal design is passive using thermostatically controlled heaters.
RF communications: Ground communications occur over a single S-band frequency for uplink to all four spacecraft and a single S-band frequency for downlink from all four spacecraft. Real-time coverage of all critical commands, including post launch separation and all maneuvers, will be accomplished through TDRSS (Tracking and Data Relay Satellite System) of NASA.
The spacecraft bus avionics performs command and telemetry processing, timing distribution, solar array regulation, battery charge management, and thruster control. Orbit determination is performed on-board using weak signal GPS processing. Each satellite including instruments, fuel, and margin has a mass of ~ 1,250 kg. The power budget at end of life with instruments and margin is approximately 318 W. The mission design life is 2 years.
Each MMS spacecraft is being developed using the standard GSFC protoflight testing approach. The first unit is being tested to qualification levels and the remaining units to acceptance levels. Design heritage from previous GSFC in-house development efforts are being used where possible, although each unit having heritage will go through the full design and test process. Commercially available components are being purchased competitively. Planned procurements include accelerometers, star cameras, sun sensors, batteries, solar arrays, transponders, thrusters, propellant tanks, and separation systems.
Spacecraft to instrument suite integration and constellation level testing will occur at GSFC. Each instrument suite will be integrated with the spacecraft bus to form the MMS observatory. Performance and environmental testing will be performed on each observatory. Additional testing will be performed in the stacked launch configuration.
Figure 4: Schematic view of the element arrangement within a MMS spacecraft (image credit: NASA)
Figure 5: Illustration of the MMS spacecraft (image credit: NASA)
Figure 6: Stacked launch configuration of the MMS spacecraft (image credit: NASA)
Figure 7: Photo of two MMS observatories (June 26, 2013) stacked up for shock testing to make sure they can withstand the launch environment (image credit: NASA)
Figure 8: MMS Four Separate – View of all four spacecraft in the MMS cleanroom getting prepared for stacking operations (image credit: NASA, Chris Gunn)
Launch: The four MMS satellites were launched on March 13, 2015 (02:44:00 UTC) on an Atlas-5-421 vehicle. The launch provider was ULA, the launch site was the Cape Canaveral Air Force Station, FL, USA. 17) 18) 19)
Orbits of the constellation:
MMS is designed to fly four identical spin-stabilized spacecraft in a tetrahedral formation in a set of highly elliptical orbits.
Because reconnection manifests itself in the Earth's magnetosphere at two locations with differing scale sizes and magnetic field orientations, a two-phase orbit strategy has been developed to test the universality of the mechanisms at work and to better understand how it controls planetary space weather. Phase 1 will probe reconnection sites at the mid-latitude dayside magnetopause, while Phase 2 focuses on reconnection sites that occur within the nightside magnetic neutral sheet. 20) 21)
Figure 10: Schematic view of Earth's magnetic field with the two regions of particular interest shown in red (image credit: SwRI)
The MMS mission aims to improve on the Cluster mission is several ways. Like Cluster, MMS will deploy a formation of four spacecraft moving in a close tetrahedral formation about a highly elliptical orbit with each of the spacecraft carrying a suite of instruments for in situ measurements of electric and magnetic fields and charged particle composition. The orbit selection and relative spacing are quite different with the mission intending to fly two distinct science phases each with multiple formation scale sizes.
• Phase 1: day side of magnetic field 1.2 RE x 12 RE. MMS will probe reconnection sites at the mid-latitude dayside magnetopause (red region on the left in Figure 10). Here the interplanetary magnetic field (IMF) merges with the geomagnetic field, transferring mass, momentum, and energy to the magnetopause. The solar wind flow transports the merged IMF/geomagnetic field lines toward the nightside, causing a build up of magnetic flux in the magnetotail.
In Phase 1, the formation flies with a relative spacing ranging from 10-160 km in a 1.2 x 12 RE orbit. Primary science is taken when the tetrahedron is at distances greater than 9 RE from the Earth and is within 30º of the Earth-Sun line, on the sunward side.
• Phase 2: night side of magnetic field 1.2 RE x 25 RE. The MSS constellation will investigate reconnection sites in the nightside magnetotail (red region on right side of Figure 10), where reconnection releases the magnetic energy stored in the tail in explosive events known as magnetospheric substorms and allows the magnetic flux stripped away from the dayside magnetopause by the solar wind/magnetosphere interaction to return to the dayside.
In Phase 2, the relative spacing is 30-400 km of the spacecraft occurs in a 1.2 x 25 RE orbit. Primary science is taken when the tetrahedron is at distances greater than 15 RE from the Earth and is within 30-40º of the Earth-Sun line on the night-ward side.
Orbits: HEO (Highly Elliptical Orbit) of 1.2 RE (perigee) x 12 RE (apogee), inclination = 28º. Note 1 RE = 6371 km. As the orbit evolves during Phase 1, the spacecraft will sample reconnection sites at different locations on the dayside magnetopause.
A transfer phase connects the two science phases by the execution of a series of maneuvers to raise each spacecraft's apogee while keeping the formation from drifting too far apart. The variable distances from Earth and inter-spacecraft spacing allows the MMS formation to act as a science instrument (Figure 11), with the inter-spacecraft distances being chosen to match the scale sizes of the physics occurring at different locations in the Earth's magnetosphere (Ref. 21).
Execution of the basic mission design requires that both deterministic and random maneuvers be performed in operations. The deterministic maneuvers are used to change the overall orbital characteristics of the formation and the random maneuvers arise from the need to maintain the formation against the relative drift that builds up over time. The number, size, and direction of these maintenance maneuvers are unknown a priori and are very dependent on the realization of a number of error sources (natural and man-made) in the system.
Earth's J2 term in the geopotential causes differential evolution of the orbit states of each spacecraft and is the largest natural perturbation. Lunar gravitational affects also play a significant role in Phase 2, because of the large apogee, causing both relative drift and, on occasion, a secular lowering of the perigee altitude below acceptable limits. Taken as a whole, these natural perturbations are well understood and predictable, depending deterministically on the initial orbital state and its subsequent evolution. The formation lifetime, defined as the time between maintenance maneuvers, in the presence of these natural perturbations, is on the order of 40-70 days.
In contrast, the man-made perturbations due to knowledge and execution errors in the maneuver process generically dominate the natural perturbations. Taken together, this combination of knowledge and execution error causes a post-maneuver evolution that is different from the one desired. The situation is further complicated by the fact that each spacecraft experiences its own realizations of these errors with no correlation between them. The man-made error sources associated with maneuver operations generally lower the formation lifetime to a range of 5-20 days.
The variable nature in the number of and type of maneuvers coupled with the „cooperative effect‟ of the errors makes pre-launch planning and operational support of the MMS formation flying problem very difficult. Questions associated with the stability of the design, such as how much fuel to budget, how maneuvers will be performed, and when to schedule ground assets to support the maneuvers, cannot be answered with simple analytical models. It was to address this need that the MMS flight dynamics group developed the End-to-End (ETE) code. The ETE code is designed to perform Monte Carlo simulations of the entire MMS operations phase from launch until the end of Phase 2 science. It combines high-fidelity orbit propagation, realistic models of the propulsion system, and models for navigation knowledge and maneuver execution errors into an event-driven framework that allows it to produce different maneuver scenarios in response to the different errors it encounters. Statistical reduction of the resulting data is then used to allocate resources for the actual launch campaign (Ref. 21).
Figure 12: Illustration of orbits in Phase 1 and Phase 2 (image credit: NASA, SwRI)
Figure 13: An artist's concept of the four MMS spacecraft flying in formation through the space around Earth (image credit: NASA)
MMS Navigator System:
Each spacecraft will fly the GSFC-developed IRAS (Inter-spacecraft Ranging and Alarm System), which consists of the Navigator GPS receiver integrated with a crosslink transceiver and a high quality frequency reference [i.e. an ultra-stable oscillator (USO)]. The tracking loops in the Navigator receiver are tuned to acquire low strength GPS signals to increase the number of GPS Space Vehicles (SVs) that can be acquired at high altitudes. This receiver has been demonstrated to reduce the acquisition threshold below 25 dB Hz as compared with a threshold of 35 dBHz that is typical for GPS receivers designed for LEO (Low Earth Orbiting) satellites. Each IRAS will also acquire and transmit one-way crosslink range measurements from the other formation members at intervals of 4 minutes. The GPS pseudorange (PR) and crosslink range measurements and associated state vectors for each of the formation members will be provided as data via the intersatellite link. 23) 24) 25) 26) 27) 28) 29) 30) 31)
To perform on-board orbit determination, the IRAS hosts the GEONS (GPS Enhanced Onboard Navigation System) flight software. GEONS is a flight software package developed by NASA to provide onboard orbit determination for a wide range of orbit types. GEONS is capable of using GPS measurements and intersatellite crosslink measurements to simultaneously estimate absolute and relative orbital states. GEONS employs an EKF (Extended Kalman Filter) augmented with physically representative models for gravity, atmospheric drag, solar radiation pressure, clock bias and drift to provide accurate state estimation and a realistic state error covariance.
The Navigator GPS receiver employs a weak signal tracking technology that significantly improves reception of GPS signals when the spacecraft is above 10 Re. Thrust acceleration measurements from an accelerometer within the ACS (Attitude Control System) are recorded during maneuvers and incorporated into the dynamics modeling in the EKF. The estimated states are periodically downlinked and used in the MMS FDOA (Flight Dynamics Operations Area) to generate definitive and predictive products for support of mission and science operations. "Definitive" refers to the portion of the ephemeris that is produced by EKF estimates using measurements. "Predicted" ephemerides are propagated from the last point of the definitive ephemeris.
Table 1: GEONS configuration on MMS
Figure 14 provides an overview of the MMS navigation operations concept. Ground operations support for all MMS spacecraft is provided from a single MOC (Mission Operations Center ), which includes a FDOA (Flight Dynamics Operations Area) that supports MMS navigation operations, as well as maneuver planning, conjunction assessment, and attitude ground operations.
Figure 14: Overview of MMS Navigation Operations Concept (image credit: NASA/GSFC)
Several challenging navigation requirements result from the fact that the majority of the MMS mission occurs above the GPS constellation, where GPS signal acquisition is sparse. Critical navigation requirements include:
- A maximum mean semi-major axis (SMA) error of 100 m above 3 Re within the definitive ephemeris; necessary to meet formation maintenance (FM) maneuver planning requirements
- A maximum relative definitive position error of 1%of the separation distance between spacecraft or 100 m, whichever is larger; necessary to meet science requirements
- A maximum relative predicted position error growth of 200 m/day; necessary to meet collision avoidance requirements
- A maximum USO clock bias error of 325 microseconds; necessary to meet a 1 millisecond maximum relative clock bias error science requirement.
GEONS Performance Assessment Process (Ref. 30):
During the initial nine weeks of the mission when the GSFC FDF (Flight Dynamics Facility) was the primary source of definitive MMS orbit solutions, an extensive analysis was performed to certify the GEONS solutions for MMS navigation. The certification process consist of evaluating GEONS inflight performance with respect to the associated onboard definitive navigation requirements and related ground predictive requirements. GEONS definitive performance is evaluated primarily by comparing GEONS solutions extracted from telemetry data downlinked from the MMS spacecraft versus FDF solutions, assumed as the reference for definitive orbit solutions. Predictive performance is evaluated by comparing the predictive solutions with both the definitive FDF and GEONS solutions. The predictive solutions are generated by propagating GEONS solutions in the Planning Products segment of the MMS FDOA ground system, which is based on FreeFlyer 6.9.1. The prediction process is configured to use a variable step Runge Kutta 8(9+) integrator with a maximum stepsize of 30 sec, a 21 x 21 EGM96 Earth gravity model, solar and lunar point mass gravity using DE 405, solar radiation pressure, and the Jacchia-Roberts atmospheric density model. The predictive solutions are used to support maneuver planning, conjunction assessment, and SN (Space Network) and DSN (Deep Space Network) contact scheduling and acquisition.
The comparisons are performed using the GGSS (GEONS Ground Support System), which is a component of the FDOA ground system. GGSS is an extensible ground software tool, developed to support space missions that use GEONS as the onboard navigation system. GGSS uses the NetBeans Platform module system as a base, which permits developers to leverage an existing collection of NetBeans plugins. GGSS performs a significant amount of data analysis and product generation using a custom MATLAB toolbox developed for the MMS mission. Because the MMS orbits are highly elliptical, time series trending is not adequate for performance measures that vary significantly over each orbit. Therefore, GGSS also performs period-folded trending in mean anomaly bins for many of the performance measures. Under the assumption that performance measures are approximately stationary within small mean anomaly bins across multiple orbits, period folding permits the computation of statistics such as means and 99% confidence intervals for each bin.
FDF provided orbit determination support for the first nine weeks of the MMS mission. During the remainder of the mission, FDF will maintain a cold backup orbit determination support capability. From the time of spacecraft separation through the first four weeks of the mission, the FDF definitive solutions were computed using the batch-least-squares estimator in the GTDS (Goddard Trajectory Determination System). Starting at April 7, 2015, the FDF definitive solutions were computed using the filter/backward smoother capability in the ODTK (Orbit Determination Tool Kit); these solutions additionally include realistic definitive covariance data. The FDF solutions were generated by processing range-rate measurements from DSN contacts and range and Doppler measurements from SN contacts. Later in the commissioning phase, the USN (Universal Space Network) station at Kiruna, Sweden provided additional Doppler tracking near perigee when SN passes were cancelled or not available. Kiruna Doppler tracking exhibited a relatively large bias throughout the commissioning in both the GTDS and ODTK solutions, which were applied in all GTDS solutions and estimated in ODTK solutions. Throughout the commissioning phase, the MMS satellite spin rate induced a large Doppler noise envelope of varying magnitude depending on the spin rate, which ranged from 3 rpm to 7 rpm.
The primary GEONS navigation calibration time span was 2015-133-17:00 UTC through 2015-136-17:00 UTC; this was a "quiescent" period in which no orbit or attitude maneuvers occurred. During this 3-day navigation calibration period, the FDF also delivered ODTK solutions in which the definitive attitude profile was used to model the effects of the MMS spacecraft spin on the tracking measurements. The Doppler residuals were significantly smaller when the spin effects were modeled. These solutions, referred to as "despun" solutions, are used to generate the definitive difference plots.
MMS operation concepts:
Each spacecraft deploys 8 booms: 4 SDP (Spin-plane Double Probe) instruments on wire booms in the spin-plane for measuring the electric fields; 2 magnetometer instruments, also on booms in the spin-plane, for measuring the magnetic field; and 2 ADP (Axial Double Probe) instruments on rigid booms parallel to the spin-axis for measuring electric fields.
The propulsion system for each spacecraft consists of 8 radial 18 N thrusters oriented parallel to the spin plane, with sets of 4 on opposite sides of the spacecraft, and 4 axial 4.5 N thrusters. Each spacecraft is also equipped with a suite of guidance, navigation, and control sensors including a digital sun sensor, a star camera, an accelerometer, and a Navigator GPS receiver with GEONS navigation software built-in. Onboard controllers process data from this sensor suite to actuate the thrusters to the desired accuracy in the orbit and attitude maneuvers while ensuring the stability and safety of the deployed booms. Figure 15 shows a diagram of the spacecraft showing the deployed booms and a schematic of the propulsion system.
Figure 15: MMS spacecraft with deployed booms and with a propulsion system (image credit: NASA)
Legend to Figure 15: (a) MMS spacecraft showing the 8 booms for the electric and magnetic fields deployed and (b) MMS spacecraft with propulsion system with the location of the 18 N radial thrusters shown in red and the location of the 4.5 N axial thruster shown in yellow (with symmetric placement of the other 6 thrusters on the opposite faces).
From the flight dynamics perspective, the MMS operations concept is best understood by dividing the driving requirements into two categories. The first category, called the baseline, holds all of the requirements related to how the formation as a whole moves with respect to the magnetosphere. The second category, called formation flying, holds all of the requirements related to the relative orbital evolution of the formation.
• May 09, 2018: Though close to home, the space immediately around Earth is full of hidden secrets and invisible processes. In a new discovery reported in the journal Nature, scientists working with NASA's MMS (Magnetospheric Multiscale) spacecraft have uncovered a new type of magnetic event in our near-Earth environment by using an innovative technique to squeeze extra information out of the data. 32) 33)
Figure 16: In a turbulent magnetic environment, magnetic field lines become scrambled. As the field lines cross, intense electric currents (shown here as bright regions) form and eventually trigger magnetic reconnection (indicated by a flash), which is an explosive event that releases magnetic energy accumulated in the current layers and ejects high-speed bi-directional jets of electrons. NASA's Magnetospheric Multiscale mission witnessed this process in action as it flew through the electron jets the turbulent boundary just at the edge of Earth's magnetic environment (image credit: NASA's Goddard Space Flight Center's Conceptual Image Lab/Lisa Poje; Simulations by: Colby Haggerty (University of Chicago), Ashley Michini (University of Pennsylvania), Tulasi Parashar (University of Delaware))
- Magnetic reconnection is one of the most important processes in the space — filled with charged particles known as plasma — around Earth. This fundamental process dissipates magnetic energy and propels charged particles, both of which contribute to a dynamic space weather system that scientists want to better understand, and even someday predict, as we do terrestrial weather. Reconnection occurs when crossed magnetic field lines snap, explosively flinging away nearby particles at high speeds. The new discovery found reconnection where it has never been seen before — in turbulent plasma.
- "In the plasma universe, there are two important phenomena: magnetic reconnection and turbulence," said Tai Phan, a senior fellow at the University of California, Berkeley, and lead author on the paper. "This discovery bridges these two processes."
- Magnetic reconnection has been observed innumerable times in the magnetosphere — the magnetic environment around Earth — but usually under calm conditions. The new event occurred in a region called the magnetosheath, just outside the outer boundary of the magnetosphere, where the solar wind is extremely turbulent. Previously, scientists didn't know if reconnection even could occur there, as the plasma is highly chaotic in that region. MMS found it does, but on scales much smaller than previous spacecraft could probe.
- MMS uses four identical spacecraft flying in a pyramid formation to study magnetic reconnection around Earth in three dimensions. Because the spacecraft fly incredibly close together — at an average separation of just four-and-a-half miles, they hold the record for closest separation of any multi-spacecraft formation — they are able to observe phenomena no one has seen before. Furthermore, MMS's instruments are designed to capture data at speeds a hundred times faster than previous missions.
- Even though the instruments aboard MMS are incredibly fast, they are still too slow to capture turbulent reconnection in action, which requires observing narrow layers of fast moving particles hurled by the recoiling field lines. Compared to standard reconnection, in which broad jets of ions stream out from the site of reconnection, turbulent reconnection ejects narrow jets of electrons only a couple miles wide.
- "The smoking gun evidence is to measure oppositely directed electron jets at the same time, and the four MMS spacecraft were lucky to corner the reconnection site and detect both jets", said Jonathan Eastwood, a lecturer at Imperial College, London, and a co-author of the paper.
- Crucially, MMS scientists were able to leverage the design of one instrument, the Fast Plasma Investigation, to create a technique to interpolate the data — essentially allowing them to read between the lines and gather extra data points — in order to resolve the jets.
- "The key event of the paper happens in only 45 milliseconds. This would be one data point with the basic data," said Amy Rager, a graduate student at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the scientist who developed the technique. "But instead we can get six to seven data points in that region with this method, allowing us to understand what is happening."
- With the new method, the MMS scientists are hopeful they can comb back through existing datasets to find more of these events, and potentially other unexpected discoveries as well.
- Magnetic reconnection occurs throughout the universe, so that when we learn about it around our planet — where it's easiest for Earthlings to examine it — we can apply that information to other processes farther away. The finding of reconnection in turbulence has implications, for example, for studies on the Sun. It may help scientists understand the role magnetic reconnection plays in heating the inexplicably hot solar corona — the Sun's outer atmosphere — and accelerating the supersonic solar wind. NASA's upcoming Parker Solar Probe mission launches directly to the Sun in the summer of 2018 to investigate exactly those questions — and that research is all the better armed the more we understand about magnetic reconnection near home.
• January 3, 2018: The space high above Earth may seem empty, but it's a carnival packed with magnetic field lines and high-energy particles. This region is known as the magnetosphere and, every day, charged particles put on a show as they dart and dive through it. Like tiny tightrope walkers, the high-energy electrons follow the magnetic field lines. Sometimes, such as during an event called magnetic reconnection where the lines explosively collide, the particles are shot off their trajectories, as if they were fired from a cannon. 34)
- Since these acts can't be seen by the naked eye, NASA uses specially designed instruments to capture the show. The MMS (Magnetospheric Multiscale Mission) is one such looking glass through which scientists can observe the invisible magnetic forces and pirouetting particles that can impact our technology on Earth. New research uses MMS data to improve understanding of how electrons move through this complex region — information that will help untangle how such particle acrobatics affect Earth.
- Scientists with MMS have been watching the complex shows electrons put on around Earth and have noticed that electrons at the edge of the magnetosphere often move in rocking motions as they are accelerated. Finding these regions where electrons are accelerated is key to understanding one of the mysteries of the magnetosphere: How does the magnetic energy seething through the area get converted to kinetic energy — that is, the energy of particle motion. Such information is important to protect technology on Earth, since particles that have been accelerated to high energies can at their worst cause power grid outages and GPS communications dropouts.
- New research, published in the Journal of Geophysical Research, found a novel way to help locate regions where electrons are accelerated. Until now, scientists looked at low-energy electrons to find these accelerations zones, but a group of scientists lead by Matthew Argall of the University of New Hampshire in Durham has shown it's possible, and in fact easier, to identify these regions by watching high-energy electrons. 35)
- This research is only possible with the unique design of MMS, which uses four spacecraft flying in a tight tetrahedral formation to give high temporal and spatial resolution measurements of the magnetic reconnection region. "We're able to probe very small scales and this helps us to really pinpoint how energy is being converted through magnetic reconnection," Argall said.
- The results will make it easier for scientists to identify and study these regions, helping them explore the microphysics of magnetic reconnection and better understand electrons' effects on Earth.
• May 18, 2017: You can't see them, but swarms of electrons are buzzing through the magnetic environment — the magnetosphere — around Earth. The electrons spiral and dive around the planet in a complex dance dictated by the magnetic and electric fields. When they penetrate into the magnetosphere close enough to Earth, the high-energy electrons can damage satellites in orbit and trigger auroras. Scientists with NASA's MMS (Magnetospheric Multiscale) mission study the electrons' dynamics to better understand their behavior. A new study, published in Journal of Geophysical Research revealed a bizarre new type of motion exhibited by these electrons. 36) 37)
- Electrons in a strong magnetic field usually exhibit a simple behavior: They spin tight spirals along the magnetic field. In a weaker field region, where the direction of the magnetic field reverses, the electrons go free style — bouncing and wagging back and forth in a type of movement called Speiser motion. New MMS results show for the first time what happens in an intermediate strength field. Then these electrons dance a hybrid, meandering motion — spiraling and bouncing about before being ejected from the region. This motion takes away some of the field's energy and it plays a key role in magnetic reconnection, a dynamic process, which can explosively release large amounts of stored magnetic energy.
- "MMS is showing us the fascinating reality of magnetic reconnection happening out there," said Li-Jen Chen, lead author of the study and MMS scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
- As MMS flew around Earth, it passed through an area of a moderate strength magnetic field where electric currents run in the same direction as the magnetic field. Such areas are known as intermediate guide fields. While inside the region, the instruments recorded a curious interaction of electrons with the current sheet, the thin layer through which the current travels. As the incoming particles encountered the region, they started gyrating in spirals along the guide field, like they do in a strong magnetic field, but in larger spirals. The MMS observations also saw signatures of the particles gaining energy from the electric field. Before long, the accelerated particles escaped the current sheet, forming high-speed jets. In the process, they took away some of the field's energy, causing it to gradually weaken.
- The magnetic field environment where the electrons' motions were observed was uniquely created by magnetic reconnection, which caused the current sheet to be tightly confined by bunched-up magnetic fields. The new results help the scientists better understand the role of electrons in reconnection and how magnetic fields lose energy.
- MMS measures the electric and magnetic fields it flies through, and counts electrons and ions to measure their energies and directions of motion. With four spacecraft flying in a compact, pyramid formation, MMS is able to see the fields and particles in three dimensions and look at small-scale particle dynamics, in a way never before achieved.
- "The time resolution of MMS is one hundred times faster than previous missions," said Tom Moore, senior project scientist for MMS at NASA's Goddard Space Flight Center. "That means we can finally see what's going on in such narrow layers and will be able to better predict how fast reconnection occurs in various circumstances."
- Understanding the speed of reconnection is essential for predicting the intensity of the explosive energy release. Reconnection is an important energy release process across the universe and is thought to be responsible for some shock waves and cosmic rays. Solar flares on the sun, which can trigger space weather, are also caused by magnetic reconnection.
- With two years under its belt, MMS has been revealing new and surprising phenomena near Earth. These discoveries enable us to better understand Earth's dynamic space environment and how it affects our satellites and technology.
- MMS is now heading to a new orbit which will take it through magnetic reconnection areas on the side of Earth farther from the sun. In this region, the guide field is typically weaker, so MMS may see more of these types of electron dynamics.
• March 31, 2017: Kinetic Alfvén waves have long been suspected to be energy transporters in plasmas — a fundamental state of matter composed of charged particles — throughout the universe. But it wasn't until now, with the help of NASA's MMS mission, that scientists have been able to take a closer look at the microphysics of the waves on the relatively small scales where the energy transfer actually happens. 38)
- A new finding, presented in a paper in Nature Communications, provides observational proof of a 50-year-old theory and reshapes the basic understanding of a type of wave in space known as a kinetic Alfvén wave. The results, which reveal unexpected, small-scale complexities in the wave, are also applicable to nuclear fusion techniques, which rely on minimizing the existence of such waves inside the equipment to trap heat efficiently. 39)
- "This is the first time we've been able to see this energy transfer directly," said Dan Gershman, lead author and MMS scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the University of Maryland in College Park. "We're seeing a more detailed picture of Alfvén waves than anyone's been able to get before."
- The waves could be studied on a small scale for the first time because of the unique design of the MMS spacecraft. MMS's four spacecraft fly in a compact 3-D pyramid formation, with just six kilometers between them — closer than ever achieved before and small enough to fit between two wave peaks. Having multiple spacecraft allowed the scientists to measure precise details about the wave, such as how fast it moved and in what direction it travelled.
- Alfvén waves are fundamental plasma wave modes that permeate the universe. At small kinetic scales, they provide a critical mechanism for the transfer of energy between electromagnetic fields and charged particles. These waves are important not only in planetary magnetospheres, heliospheres and astrophysical systems but also in laboratory plasma experiments and fusion reactors.
- As kinetic Alfvén waves move through a plasma, electrons traveling at the right speed get trapped in the weak spots of the wave's magnetic field. Because the field is stronger on either side of such spots, the electrons bounce back and forth as if bordered by two walls, in what is known as a magnetic mirror in the wave. As a result, the electrons aren't distributed evenly throughout: Some areas have a higher density of electrons, and other pockets are left with fewer electrons. Other electrons, which travel too fast or too slow to ride the wave, end up passing energy back and forth with the wave as they jockey to keep up.
- The wave's ability to trap particles was predicted more than 50 years ago but hadn't been directly captured with such comprehensive measurements until now. The new results also showed a much higher rate of trapping than expected.
- This method of trapping particles also has applications in nuclear fusion technology. Nuclear reactors use magnetic fields to confine plasma in order to extract energy. Current methods are highly inefficient as they require large amounts of energy to power the magnetic field and keep the plasma hot. The new results may offer a better understanding of one process that transports energy through a plasma.
- "We can produce, with some effort, these waves in the laboratory to study, but the wave is much smaller than it is in space," said Stewart Prager, plasma scientist at the Princeton Plasma Physics Laboratory in Princeton, New Jersey. "In space, they can measure finer properties that are hard to measure in the laboratory."
- This work may also teach us more about our sun. Some scientists think kinetic Alfvén waves are key to how the solar wind — the constant outpouring of solar particles that sweeps out into space — is heated to extreme temperatures. The new results provide insight on how that process might work.
- Throughout the universe, kinetic Alfvén waves are ubiquitous across magnetic environments, and are even expected to be in the extra-galactic jets of quasars. By studying our near-Earth environment, NASA missions like MMS can make use of a unique, nearby laboratory to understand the physics of magnetic fields across the universe.
- On 30 December 2015, the four MMS observatories were near the dayside magnetopause, that is, the interface between the interplanetary magnetic field and the Earth's internal magnetic field, at [7.8, -6.9, 0.9] Re (1 Re=1 Earth radius=6,730 km). Magnetic reconnection at the magnetopause boundary generated a southward flowing exhaust at ~22:25 UT denoted by a -Vz jet, an increase in plasma density, and a decrease in plasma temperature (Figure 17). There was no discernable rotation in the magnetic field suggesting that the spacecraft constellation remained inside the Earth's magnetosphere throughout this interval. Low frequency (~1 Hz) waves were observed in the exhaust in a ~4 min interval localized to a region of strong proton temperature anisotropy . MMS partially crossed the magnetopause into the magnetosheath for the first time at ~22:35 UT at [8.0, -6.9, 0.9] Re. For the subsequent ~2 h, multiple magnetopause crossings resulted in the MMS spacecraft sampling both +Vz and -Vz jets, that is, above and below the reconnection site. However, ~1 Hz waves were only observed in the short interval shown in Figure 17. The MMS observatories were in a tetrahedron configuration (quality factor ~0.9) separated by ~40 km, a distance which corresponded to a local thermal ion gyroradius (ρi=35 km).
Figure 17: MMS observations of a reconnection exhaust (image credit: NASA)
Legend to Figure 17: (a) Illustration of the MMS constellation near the dayside magnetopause on 30 December 2015. MMS entered a southward flowing reconnection exhaust in the separatrix region on the magnetospheric (msp) side of the magnetopause. (b–i) Plasma parameters from MMS4 across the jet are shown from 22:23 to 22:30 UT. The density increased to ~10 cm-3 (d) and -Vz increased by ~200 km s-1 (e). No rotation in the magnetic field (B) indicated that the spacecraft remained inside the magnetosphere during this time period. Approximately 1 Hz waves (h,i) were observed to be localized in a region of enhanced ion temperature anisotropy. H+ dominated the ion composition during this time period.
• On February 9, 2017, NASA's MMS (Magnetospheric Multiscale) mission began a three-month long journey into a new orbit. MMS flies in a highly elliptical orbit around Earth and the new orbit will take MMS twice as far out as it has previously flown. In the new orbit, which begins the second phase of its mission, MMS will continue to map out the fundamental characteristics of space around Earth, helping us understand this key region through which our satellites and astronauts travel. MMS will fly directly through regions—where giant explosions called magnetic reconnection occur—never before observed in high resolution. 40)
- Launched on March 13, 2015, MMS uses four identical spacecraft to map magnetic reconnection – a process that occurs when magnetic fields collide and re-align explosively into new positions. NASA scientists and engineers fly MMS in an unprecedentedly close formation that allows the mission to travel through regions where the sun's magnetic fields interact with Earth's magnetic fields – but keeping four spacecraft in formation is far from easy.
- "This is one of the most complicated missions Goddard has ever done in terms of flight dynamics and maneuvers," said Mark Woodard, MMS mission director at NASA's Goddard Flight Space Center in Greenbelt, Maryland. "No one anywhere has done formation flying like this before."
- To form a three-dimensional picture of reconnection, the mission flies four individual satellites in a pyramid formation called a tetrahedron. While a previous joint ESA (European Space Agency)/NASA mission flew in a similar formation, MMS is the first to fly in such an extremely tight formation – only four miles apart on average. Maintaining this close separation allows for high-resolution mapping but adds an extra dimension of challenge to flying MMS, which is already a complex undertaking.
- Flying a spacecraft, as one would suspect, is nothing like driving a car. Instead of focusing on just two dimensions – left and right, forward and backwards – you also must consider up and down. Add on to that, keeping the four MMS spacecraft in the specific tetrahedral formation necessary for three-dimensional mapping, and you've got quite a challenge. And don't forget to avoid any space debris and other spacecraft that might cross your path. Oh, and each spacecraft is spinning like a top, adding another layer to the dizzying complexity.
- "Typically, it takes about two weeks to go through the whole procedure of designing maneuvers," said Trevor Williams, MMS flight dynamics lead at NASA Goddard. Williams leads a team of about a dozen engineers to make sure MMS's orbit stays on track. During a normal week of operations, the maneuvers, which have been carefully crafted and calculated beforehand, are finalized in a meeting at the start of the week.
- To calculate its location, MMS uses GPS, just like a smart phone. The only difference is this GPS receiver is far above Earth, higher than the GPS satellites sending out the signals. "We're using GPS to do something it wasn't designed for, but it works," Woodard said.
- Since GPS was designed with Earth-bound users in mind, signals are broadcast downwards, making it difficult to use from above. Fortunately, signals from GPS satellites are sent widely to blanket the entire planet and consequentially some from the far side of the planet sneak around Earth and continue up into space, where MMS can observe them. Using a special receiver that can pick up weak signals, MMS is able to stay in constant GPS contact. The spacecraft uses the GPS signals to automatically compute their location, which they send down to the flight control headquarters at Goddard. The engineers then use that positioning to design the maneuvers for the spacecraft's orbits.
- While the orbit for each MMS spacecraft is almost identical, small adjustments need to be made to keep the spacecraft in a tight formation. The engineers also rely on reports from NASA's Conjunction Assessment Risk Analysis, which identifies the locations of space debris and provides notification when objects, like an old communications satellite, might cross MMS's path. While nothing yet has been at risk for colliding with MMS, the crew has a prepared backup plan – a dodge maneuver – should the need arise.
- On scheduled Wednesdays, one or two per month, the commands are sent up to the spacecraft to adjust the tetrahedral formation and make any necessary orbit adjustments. These commands tell MMS to fire its thrusters in short bursts, propelling the spacecraft to its intended location.
- The new elliptical orbit will take MMS to within ~1000 km above the surface of Earth at its closest approach, and out to about 40 % of the distance to the moon. Previously, the spacecraft went out only one-fifth (20%) of the distance to the moon.
- In the first phase of the mission, MMS investigated the sun-side of Earth's magnetosphere, where the sun's magnetic field lines connect to Earth's magnetic field lines, allowing material and energy from the sun to funnel into near-Earth space. In the second phase, MMS will pass through the night side, where reconnection is thought to trigger auroras.
- In addition to helping us understand our own space environment, learning about the causes of magnetic reconnection sheds light on how this phenomenon occurs throughout the universe, from auroras on Earth, to flares on the surface of the sun, and even to areas surrounding black holes.
- While MMS will not maintain its tetrahedral formation as it moves to its new orbit, it will continue taking data on the environments it flies through. The operations crew expects MMS to reach its new orbit on May 4, 2017, at which point it will be back in formation and ready to collect new 3-D science data, as its elliptical orbit carries it through specific areas thought to be sites for magnetic reconnection.
Figure 18: Over three months in 2017, the MMS spacecraft transitions from the dayside magnetopause, to a new, larger orbit on the nightside, as shown in this visualization. This image shows the four satellites' orientation on March 15, 2017 (image credit: NASA/GSFC)
• November 4, 2016: NASA's MMS (Magnetospheric Multiscale) mission now holds the Guinness World Record for highest altitude fix of a GPS signal. Operating in a highly elliptical orbit around Earth, the MMS satellites set the record at 70,000 km above the surface. The four MMS spacecraft incorporate GPS measurements into their precise tracking systems, which require extremely sensitive position and orbit calculations to guide tight flying formations. 41)
- Earlier this year, MMS achieved the closest flying separation of a multi-spacecraft formation with only four-and-a-half miles between the four satellites. When the satellites are closest to Earth, they move at up to 35,400 km/hour, making them the fastest known operational use of a GPS receiver.
- When MMS is not breaking records, it conducts ground-breaking science. Still in the first year of its prime mission, MMS is giving scientists new insight into Earth's magnetosphere. The mission uses four individual satellites that fly in a pyramid formation to map magnetic reconnection – a process that occurs as the sun and Earth's magnetic fields interact. Precise GPS tracking allows the satellites to maintain a tight formation and obtain high resolution three-dimensional observations.
• May 12, 2016: A team led by SwRI (Southwest Research Institute) has made the first direct detection of the source of magnetic reconnection. Analyzing data from NASA's MMS (Magnetospheric Multiscale) mission, scientists have observed how this explosive physical process converts stored magnetic energy into kinetic energy and heat. - With MMS a new ‘microscope' has opened a new window to clearly see reconnection," said James L. Burch, vice president of SwRI's Space Science and Engineering Division and MMS principal investigator. "The four MMS satellites hone in on the role of electrons in magnetic reconnection, which allows two magnetic fields to interconnect. MMS measures the process 100 times faster than previously possible to clearly visualize the rapidly evolving process." 42) 43) 44)
- In late summer 2015, the four identically instrumented MMS spacecraft began to survey the magnetopause. In this boundary between the solar wind and Earth's magnetosphere, the scientists are searching for locations where the solar wind magnetic field and the terrestrial magnetic field reconnect. In half of the over 4,000 magnetopause encounters, MMS has seen evidence of reconnection, but most often the spacecraft did not pass through the reconnection sites themselves. Then on October 16, 2015, MMS flew through the heart of a reconnection region. "We hit the jackpot," says Roy Torbert, MMS deputy principal investigator and director of SwRI's Earth, Oceans, and Space office at the University of New Hampshire. "The spacecraft passed directly through the electron dissipation region, and we were able to perform the first-ever physics experiment in this environment."
- MMS measures plasmas, hot ionized gases consisting of approximately equal numbers of positively charged ions and negatively charged electrons. The solar wind and Earth's magnetospheric plasmas are both magnetized. For reconnection to occur, the plasmas become "demagnetized" — that is, the plasma and the magnetic field become decoupled. The critical and final stage in this process occurs in a relatively small region in space known as the "electron dissipation region." As the electrons become demagnetized, the magnetic fields of the Sun and the Earth interconnect and the solar wind and magnetospheric plasmas mix.
- Magnetic reconnection — which occurs in both natural plasma environments and laboratory plasma and fusion experiments — is at the heart of space weather. Reconnection is responsible for explosive solar events, such as solar flares and coronal mass ejections, and drives disturbances in Earth's space environment. Such disturbances produce spectacular auroras, but can also shut down electrical power grids and disrupt satellite-based communication and navigation systems.
- "We've studied it theoretically, and we've simulated it with supercomputers. But up to now we haven't known what controls the conversion of magnetic energy into particle energy," said Burch. "We designed the MMS mission to use Earth's magnetosphere as a giant laboratory to perform the definitive experiment on reconnection." Examining the data from the encounter, the MMS team saw a drop in the magnetic field to near zero, oppositely directed ion flows, accelerated electrons, an enhanced electric field, and a strong electrical current — all indications that the spacecraft had entered the dissipation region. The tell-tale signature of reconnection, however, was a spike observed in the electric power generated by the electrons. "This was the ‘smoking gun' for reconnection," explains Burch. "It was theoretically predicted but never seen until MMS."
- Another feature observed for the first time by MMS as it traversed the dissipation region was a rapid change in the electrons as they streamed into the dissipation region and were accelerated outward along field lines opened during reconnection. This observation was the first definitive measurement of the interconnection of the solar and terrestrial magnetic fields.
• On March 13, 2016, the MMS constellation of NASA was 1 year on orbit. 45)
Figure 19: Artist's rendition of the MMS mission to study how magnetic fields release energy in a process known as magnetic reconnection (image credit: NASA)
• Dec. 18, 2015: Just under four months into the science phase of the mission, MMS is delivering promising early results on a process called magnetic reconnection — a kind of magnetic explosion that's related to everything from the northern lights to solar flares. The unprecedented set of MMS measurements will open up our understanding of the space environment surrounding Earth, allowing us to better understand what drives magnetic reconnection events. These giant magnetic bursts can send particles hurtling at near the speed of light and create oscillations in Earth's magnetic fields, affecting technology in space and interfering with radio communications. Scientists from the Southwest Research Institute, NASA, the University of Colorado Boulder and the Johns Hopkins University Applied Physics Laboratory presented an overview of MMS science and early results on Dec. 17, 2015, at the American Geophysical Union's Fall Meeting in San Francisco. 46)
Figure 20: The four identical MMS spacecraft (one of which is illustrated here) fly through the boundaries of Earth's magnetic field to study an explosive process of magnetic reconnection (image credit: NASA/GSFC)
- MMS' four instrument suites and incredible measurement rates — a hundred times faster than ever before on certain instruments — is giving scientists their best look ever at magnetic reconnection. In fact, the mission's high resolution produces so much data it requires a scientist on duty during every MMS contact to prioritize which data is sent down from the spacecraft.
- One of the key features of MMS is its scaling ability. The four spacecraft fly in a four-sided, pyramid-shaped formation called a tetrahedron, allowing them to build up three-dimensional views of the regions and events they fly through. Because the four spacecraft are controlled independently, the scale of their formation — and their observations — can be zoomed in or out by a factor of ten.
- "We can see the effects of reconnection on the sun in the form of coronal mass ejections and solar flares," said Michael Hesse, lead co-investigator for theory and modeling on the MMS mission at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "But with MMS, we're finally able to observe the process of magnetic reconnection directly."
- Magnetic reconnection is a process in which magnetic fields reconfigure suddenly, releasing huge amounts of energy. When magnetic field lines snap and join back together in new formations, some of the energy that was stored in the magnetic field is converted to particle energy in the forms of heat and kinetic energy. "Reconnection is a fundamental energy release process," said Hesse. "It impacts both the temperature and speed of particles in a plasma, two of the defining characteristics."
- Katherine Goodrich, a graduate student at the University of Colorado Boulder, is working with measurements from a suite of six instruments to characterize the behavior of electric and magnetic fields at magnetic reconnection sites. This suite of instruments, the FIELDS suite — duplicated on each of the four MMS spacecraft — contains six sensors that work together to form a three-dimensional picture of the electric and magnetic fields near the spacecraft. This suite has a very high accuracy, in part due to the very long booms on each sensor. "The long booms allow us to measure the fields with minimal contamination from the electronics aboard the spacecraft," said Goodrich. Along the spin plane, the booms measure 400 feet from end to end — longer than a regulation soccer field. The booms on the axis of spin measure 100 feet from end to end.
- Ian Cohen, a postdoctoral fellow at JHU/APL (Johns Hopkins University /Applied Physics Laboratory), uses a different instrument suite to identify and study the telltale particle behaviors that come with magnetic reconnection. Cohen works with two particle detectors aboard MMS: the FEEPS ( Fly's Eye Energetic Particle Sensor), and the Energetic Ion Spectrometer. The measurements are providing evidence for a mechanism by which particles can escape the Earth system and join the interplanetary medium.
- When magnetic reconnection happens on the day-side, magnetic field lines from the sun connect directly to Earth's magnetic field. "The linking of these magnetic fields means that particles can drift from within the magnetosphere to the boundary between Earth's magnetic field and the solar wind," said Cohen. "Once they get to that boundary, further reconnection events allow them to escape and float along the interplanetary magnetic field."
• Sept. 18, 2015: The MMS Navigator system exceeded all of the team's expectations. At the farthest point of the MMS orbit of 70,000 km, Navigator was able to receive signals from the GPS satellites and perform onboard navigation solutions. At the lowest point of the MMS orbit, Navigator traveled at velocities over 35,000 km/hr. In comparison, GPS satellites orbit at ~20100 km away from the earth and travel at 13,840 km/hr, and most satellites using GPS receivers are in LEO (Low Earth Orbits) at altitudes between 180 and 2000 km. 47)
- The Navigator system will be even more important during the second phase of the MMS mission when the orbit will double in size and travel all the way out to 153,000 miles from Earth.
• Sept. 4, 2015: The MMS mission successfully completed the commissioning activities on Sept. 1, 2015, and is now in full science mode. During the next six months, the four MMS spacecraft will fly in a tetrahedral formation that will skim Earth's magnetopause near every apogee, sampling that boundary and the myriad of plasma processes that occur there. The primary focus for the MMS science team will be one of the most important and least understood of those processes: Magnetic Reconnection. Magnetic reconnection occurs when magnetic fields around Earth connect and disconnect, explosively releasing energy. 48)
• July 29, 2015: The four spacecraft of NASA's MMS mission began flying in a pyramid shape for the first time on July 9, 2015 . The four-sided pyramid shape—called a tetrahedron—means that scientists' observations will be spread out over three dimensions. 49)
Figure 21: This image shows the pyramid-shaped formation of the four MMS spacecraft (image credit: NASA)
• July 9, 2015: MMS instruments have been activated and commissioning activities are resuming following exit of the long eclipse period. 50)
- MMS successfully executed all procedures during the leap second event introduced on June 30, 2015.
- MMS completed a major milestone with the planned execution of maneuvers to start MMS formation flying in a 160 km spaced tetrahedron.
- At the highest point of the MMS orbit, at more than 70,000 km above the surface of the Earth, the Navigator set a record for the highest-ever reception of signals and onboard navigation solutions by an operational GPS receiver in space.
- At the lowest point of the MMS orbit, the Navigator set a record as the fastest operational GPS receiver in space, at velocities over 34,500 km/hr.
A precise tracking system is crucial for MMS, which requires extremely sensitive position and orbit calculations. The four spacecraft must fly in a tight pyramid formation to gather science data as they move through Earth's magnetic environment. The formation is required to obtain 3-dimensional observations of a phenomenon called magnetic reconnection that occurs when magnetic fields from the sun connect and disconnect with magnetic fields of Earth, which can allow energy and solar material to funnel into near-Earth space. With its instrument booms deployed, each spacecraft is the size of a baseball field — while flying as close as 10 km from each other.
Figure 22: The red ellipses show the MMS orbit paths during the first and second phases of the mission. Each spacecraft uses GPS signals – which come from satellites situated along the green circle shown surrounding Earth — from the far side of Earth to track its position (image credit: NASA, MMS)
Tracking spacecraft can be done by radar stations from the ground, but it's much more expensive and takes longer than an inflight system. However, using GPS as is typically done on Earth by such things as cars, boats and smart phones isn't nearly as simple for something like MMS. For one thing, the bulk of its highly-elliptical orbit occurs above where the GPS transmitters orbit. So MMS must have specialized, extremely sensitive receivers to capture GPS signals transmitted from the far side of Earth. In addition the MMS spacecraft spin; each one makes three revolutions per minute.
In the first month after launch, the MMS team began turning on and testing each instrument and deploying booms and antennas. During this time, the team compared the Navigator system with ground tracking systems and found it to be even more accurate than expected. At the farthest point in its orbit, some 70,000 km away from Earth, the Navigator can determine the position of each spacecraft with an uncertainty of better than 15 m.
What's more, the receivers on MMS have turned out to be strong enough that they consistently track transmissions from eight to 12 GPS satellites – excellent performance when compared to pre-flight predictions that there might be frequent drop outs during each orbit.
Even if the receiver were to lose all GPS signals for part of the orbit, Navigator is specifically designed to handle such dropouts. By gathering as many observations as possible, integrated software called GEONS (Goddard Enhanced Onboard Navigation System) can still compute the orbit by incorporating additional information including drag force, gravity, and solar radiation pressure.
This system will be even more important during the second phase of the MMS mission when the orbit will double in size and travel all the way out to 153,000 km from Earth.
While Navigator technology and GPS receivers were previously flown for testing and to help navigate a low-earth-orbit mission, this is the first time that the complete Navigator package has been used to actively navigate a high-altitude mission. Now that the team knows it works so well, Navigator can be used for other missions that travel in similar high orbits (Ref. 51).
- The Spin-plane Double Probe 60 m wire boom deployments were completed on all Observatories on April 23. The MMS Fields (electric and magnetic field instruments) investigation is now fully operational in the spin-plane and all - the data looks good.
- The Axial Double Probe 12.5 m boom deployments are scheduled to begin the evening of April 26. These carry electric field instruments and will complete the 3-dimensional Fields investigation sensor deployments.
- The data downlink sequence issue has been resolved, with repeated error-free performance during Deep Space Network passes.
• April 07, 2015: The MMS constellation's orbit, spin rates and attitudes are all nominal. 55)
- All spacecraft subsystems continue to perform well. Final perigee raising maneuvers have been completed.
- Low voltage checkout has been completed on all MMS instruments. High Voltage activation of Fields Electron Drift Instruments is nearing completion. Instrument activation is proceeding as planned. Spin plane Double Probe wire boom deployments began April 4.
• March 19, 2015: The four MMS observatories were inserted with perfect accuracy and attitude into our initial orbit by the Atlas-Centaur AV-53. The separations and spin rates were exactly as planned. All spacecraft systems are operating nominally with no problems. The Navigator GPS system is exceeding its required performance, tracking more GPS vehicles at greater distances than the baseline mission required. This has potential benefits to the MMS science mission. During the past week attitude and spin rate maneuvers were performed as planned and the system performance (spin rate changes, nutation, damping) matched pre-flight predications extremely well. This indicates that the Observatory mass properties, spin- balancing, and control system design are all essentially perfect. The Instrument Suite command and control electronics were powered-on and are operating nominally with no significant problems, instrument activation is proceeding along the planned commissioning timeline. The 8 Axial Double Probe (ADP) Receiving Elements (RE) were successfully deployed although the ADP Booms remain stowed as planned. All 8 Magnetometer Booms were successfully deployed and the Magnetometers are performing nominally. The magnetometer data shows that the MMS observatories achieved the desired level of magnetic cleanliness with the observatories being nearly invisible to the deployed magnetometers. 56)
• March 13, 2015: After reaching orbit, each spacecraft deployed from the rocket's upper stage sequentially, in five-minute increments, beginning at 05:16 UTC , with the last separation occurring at 05:31 UTC. NASA scientists and engineers were able to confirm the health of all separated spacecraft at 05:40 UTC. Craig Tooley, the project manager at NASA/GSFC said: "I am speaking for the entire MMS team when I say we're thrilled to see all four of our spacecraft have deployed and data indicates we have a healthy fleet."
- Over the next several weeks, NASA scientists and engineers will deploy booms and antennas on the spacecraft, and test all instruments. The observatories will later be placed into a pyramid formation in preparation for science observations, which are expected to begin in early September (Ref. 17) .
Sensor complement: (ADP, AFG, ASPOC, CEB, CIDP, DES, DFG, DIS, EDI, EIS, FEEPS, HPCA, IDPU, SCM SDP)
SMART (Solving Magnetospheric Acceleration, Reconnection, and Turbulence) is the name of the MMS science investigation program headed by James L. Burch of SWRI as PI (Principal Investigator) in cooperation with researchers from other institutions. 57) 58) 59) 60) 61) 62) 63) 64)
The SMART payload comprises three instrument groups: Hot Plasma, Energetic Particles, and Fields. In addition, the payload includes two ASPOC (Active Spacecraft Potential Control Devices) and a CIDP (Central Instrument Data Processor). The ASPOCs neutralize the electrical potential of the spacecraft, allowing measurement of low-energy ions and electrons by the plasma instruments and eliminating spurious electric fields that can contaminate double-probe measurements. The CIDP provides the interface between the instruments and the spacecraft C&DH subsystem. The ASPOCs are being developed at the IWF (Institut fuer Weltraumforschung) of the Austrian Academy of Sciences; the CIDP is being developed at Southwest Research Institute (SwRI).
Identical in situ instruments on each satellite measure:
• Hot plasma composition.
• Energetic particles
• Electric and magnetic fields
Figure 23: SMART instrument suite organization (image credit: NASA)
Legend to Figure 23: The organization was developed to give team PMs (Project Management) cost and schedule ownership.
Figure 25: MMS SMART instrument suite architecture (image credit: SwRI)
Hot Plasma composition suite: (FPI, DIS, DES, HPCA)
• FPI (Fast Plasma Instrument): The FPI consists of DIS (Dual Ion Sensors) and DES (Dual Electron Sensors) and measures 3D ion and electron flux distributions over the energy range ~10 eV to 30 keV with an energy resolution of 20%. Electrons will be measured with a time resolution of 30 ms, ions with a time resolution 150 ms. The FPI development is led by Co-I Tom E. Moore (GSFC). FPI partners: SwRI, JAXA/ISAS, CESR, MSFC, Meisei Electric Co., SPEI. 67)
FPI observes the fast-moving plasma. Incoming particles pass through a filter which cherry picks certain particle speeds and directions and allows them to pass through to a sensor plate. When an incoming particle hits the sensor plate, some million electrons come out the other side, so the instrument can detect the event. The whole process takes several nanoseconds. By separately measuring electrons and ions, and by filtering for specific energies, FPI can count the number of particles of each kind entering the instrument from a range of directions at different energies during any given time span.
Each of the MMS spacecraft will carry four FPIs, so there are 16 DIS instruments total. They will be paired with 16 DES (Dual Electron Spectrometers) and 6 IDPUs (Instrument Data Processing Units) that are being built at Goddard to complete the full FPI.
In combination, FPI – consisting of the four dual electron spectrometers, the four dual ion spectrometers (DIS), and one data processing unit — will produce a three-dimensional picture of the ion plasma every 150 ms and of the electron plasma every 30 milliseconds. These frame rates are similar to those used in video and a factor of 100 times faster than what has been accomplished before for electrons.
The dual electron spectrometers (DES) and the processing unit, or IDPU, were built at NASA Goddard. The dual ion spectrometers were built by Meisei Electric in Gunma, Japan, under the direction of the Institute of Space and Aeronautical Sciences, a part of JAXA.
• HPCA (Hot Plasma Composition Analyzer): The HPCA employs a novel RF technique to measure minor ions such as oxygen and helium in regions of high flux. Energy range = ~10 eV to 30 keV; energy resolution = 20%; time resolution = 15 s. HPCA development is led by Co-I David T. Young (SwRI).
The HPCA is a toroidal top-hat electrostatic analyzer coupled to a TOF (Time-of-Flight) mass spectrometer. The instrument will take quick measurements at the mass resolution that can accurately separate and identify the minute amounts of hydrogen, helium, and oxygen ions from the magnetosphere. Furthermore, the instrument will also have several features essential to space travel: robustness, low-power requirements, and minimal mass.
The HPCA also has a unique capability never before flown. There are usually so many solar wind protons compared to, for example, magnetospheric oxygen that mass spectrometers flown in the past were overwhelmed — and the oxygen signal was masked. HPCA uses radio frequency oscillations to sweep the majority of solar wind protons away from the detector, without affecting the magnetospheric oxygen, resulting in a 10- to 100-fold improvement in detection.
EPD (Energetic Particles Detector Suite): (FEEPS, EIS)
Magnetic reconnection causes both a bulk flow of plasma – not unlike a blowing wind — and also can pump up a small subset of the charged plasma particles to incredibly high speeds and energies. The details of this latter process remain undetermined, although many theories have been suggested. The Energetic Particles Detector Suite will help distinguish between the theories and help determine whether this acceleration only works for electrons or also for heavier particles, the charged atoms known as ions. Short-term bursts of incredibly fast ions have been observed in the magnetic tail trailing behind Earth, and it is possible that these are due to magnetic reconnection as well. The electrons and ions detected by the EPD suite have energies far exceeding those detectable by FPI and HPCE. Together, the three sets of instruments are necessary to observe the full range of charged particles associated with magnetic reconnection.
EPD also remotely senses the structure of the larger space environment surrounding reconnection sites. Many of the sensors on MMS measure the material its flying through directly, but EPD can observe particles coming in from far away, as it tracks ions that move along giant circles, often larger than 3000 km in diameter, and very fast electrons that move up to 80% the speed of light.
EPD observes these high-speed particles through two instruments:
• FEEPS (Fly's Eye Energetic Particle Sensor): The two FEEPS devices will measure 3D energetic ion and electron flux distributions over the energy ranges ~25 keV to 500 keV (electrons) and ~45 keV to 500 keV (ions). Time resolution = 10 s. FEEPS development is led by Co-I J. Bernard Blake (The Aerospace Corporation).
The primary goal of FEEPS is to obtain nearly instantaneous all-sky measurements of how many electrons of different energies and different arrival directions are present. The instrument relies on solid state detectors made of silicon, a semiconductor much like those used in computer electronic systems. Whenever a charged particle hits the detector, it initiates a current that can be used to measure the energy of the original particle. There are two FEEPS instruments per spacecraft, and together they provide 18 views in different directions simultaneously, giving rise to the "fly's eye" in the instrument's name. FEEPS has two sets of sensors, one for electrons and one for ions. The solid state detectors within each of the electron "eyes" are covered by a 2 µm aluminum foil, which keeps out the ions. The detectors for the ion views, on the other hand, have no aluminum foil and are exceedingly thin so that electrons generally pass through without leaving a detectable signal.
• EIS (Energetic Ion Spectrometer): EIS uses a time-of-flight/pulse-height sensor to provide ion composition measurements (protons vs. oxygen ions) and angular distributions over the energy range ~45 keV to 500 keV and with a temporal resolution of 30 s. EIS development is led by Co-I Barry H. Mauk (Applied Physics Laboratory), who also heads the Energetic Particle investigation as a whole.
The EIS device also gathers all-sky measurements of the energetic ions, gathering information about their energy, their arrival direction and their mass. EIS can determine the mass of these particles by measuring their velocity and total energy. The mass information helps determine how many protons, helium and oxygen ions are present at energies above those reachable by HPCE.
To measure the energy, EIS uses a solid state detector like the one on FEEPS. Velocity is measured using two very thin foils and a microchannel plate sensor. When an ion travels through the first foil it knocks a few electrons off. These electrons are deflected toward the microchannel plate, which can amplify the signal, sending 1 million electrons out the other side — just like the detectors used in the plasma suite. The ion continues traveling to the second foil, where a similar process occurs. By determining the time of flight between electron detection at the first and second foils, the instrument can determine the velocity of the original incoming particle.
Combining the comprehensive ion measurements of EIS with the simpler ion measurements on FEEPS allows researchers to determine the ion properties at a faster rate of 1/3 of a spacecraft spin, a cadence that will sometimes be needed in the vicinity of fast changing reconnection sites.
Figure 26: MMS instrument suite components (image credit: NASA)
FIELDS sensor suite: (AFG, DFG, EDI, SPD, ADP, SCM, CEB)
The FIELDS investigation is an advanced suite of six sensors to measure critical electric and magnetic fields in and around reconnection regions. The investigation is led by Co-I Roy B. Torbert of UNH (University of New Hampshire). 68) 69)
• AFG (Analog Fluxgate) and DFG (Digital Fluxgate) Magnetometers: The AFG and DFG sensors are provided by UCLA and the Technical University of Braunschweig, respectively. C. T. Russell (UCLA) has overall responsibility for fluxgate development. The two different kinds of magnetometers will provide redundant measurements of the magnetic field and current structure in the diffusion region.
The fluxgate magnetometers provide two sets of similar measurements. The fluxgates carry a permeable material that changes properties in response to the presence of magnetic fields. Measuring how they change can be correlated to strength of the field down to a half a nanotesla – typical fields in the regions of interest will be about 50 nanotesla.
• EDI (Electron Drift Instrument): The EDI determines the electric and magnetic fields by measuring the drift of ~1 keV electrons emitted from the GDU (Gun Detector Unit). Each GDU sends (receives) a coded beam to (from) the other EDI-GDU. The EDI gun is being developed at the Institut fuer Weltraumforschung of the Austrian Academy of Sciences; EDI optics are being developed at the University of Iowa.
The EDI instrument measures both the electric and magnetic fields by tracking the path of electron beams through space. EDI sends a beam of electrons out into space using each of its two Gun Detector Units. In the presence of magnetic fields, electrons travel in orbits that are nearly circles, so over the course of about half a mile, the electron beam curves around on itself until it comes back in to the second Gun Detector Unit. By measuring how long it takes the electrons to circle back, one can calculate the strength of the magnetic fields through which the beam traveled.
When electric fields are present as well, the electron beam will not make a perfect circle, but will drift in a predictable way as it returns. By measuring the size of that sideways drift, one can calculate the strength of the electric fields.
This technique of correctly capturing the electron beam was successfully demonstrated by the joint ESA/NASA Cluster mission. On MMS, the EDI will take faster measurements than on Cluster. Its strength, however, is not in its speed but in its precision. Knowing the displacement of the particles in space due to an electron field is crucial for accurate measurements by other instruments aboard MMS.
If needed, EDI can also be used solely as a detector, measuring all incoming electrons from space as opposed to just tracking its own specialized electron beam. In this case, EDI can make observations at rates of up to 1,000 times a second.
• Double Probe (SDP/ADP): MMS requires two sets of double-probe instruments:
- The SPD (Spin-plane Double Probe ) consists of four 48 m wire booms with spherical sensors at the end. The SDP assembly is provided by KTH (Royal Institute of Technology) Stockholm and by IRF-U (Swedish Institute of Space Physics-Upsalla), Sweden (Ref. 62).
Table 2: Parameters of SDP
Figure 27: Illustration of the SDP components (image credit: KTH, IRF-U)
- The ADP (Axial Double Probe), developed at LASP of the University of Colorado and the University of New Hampshire, consists of two 10 m antennas deployed axially near the spacecraft spin axis. The SDP and ADP provide full 3D electric field measurements over a range from DC to 100 kHz with an accuracy of 1 mV/m.
MMS carries two sets of double-probe instruments. Each measures the voltage between two electrodes to determine the electric field. As the field changes are quite small, the electrodes must be set as widely apart as possible to provide a robust signal. Thus the double probes sensors reside at the ends of very long booms that deploy away from the main body of each observatory after they are launched.
• SCM (Search Coil Magnetometer): The SCM will measure the 3-axis AC magnetic field up to 6 kHz and will be used together with the ADP and SDP to determine the contribution of plasma waves to the turbulent dissipation that occurs in the diffusion region. The SCM is being developed at CETP (Centre d'etude des Environnements Terrestre et Planetaires) Velizy/ Saint-Maur, France.
The SCM provides direct measurements of changes in the magnetic fields, using something called an induction magnetometer. The magnetometer contains a coil of wire around a ferromagnetic material. It is a basic law of physics that a changing magnetic field near such a coil will induce a voltage. This voltage, in turn, can be used to measure how the magnetic field changes.
• CEB (Central Electronics Box): The CEB provides power, control and data processing for the Fields sensor suite. KTH (Royal Institute of Technology) provides the power supply. UNH provides the CEB and the software with contributions from the sensor team institutions: KTH, LASP, UCLA and IWF.
All the FIELDS measurements are coordinated, collected, and transmitted from a central electronics system. This set of electronics was the responsibility of the University of New Hampshire, the Royal Institute of Technology, the University of California in Los Angeles, the University of Colorado, and the Space Research Institute of the Austrian Academy of Sciences in Graz, Austria.
ASPOC (Active Spacecraft Potential Control Device):
The ASPOC instrument on MMS has technical and scientific heritage from ESA's Cluster and joint ESA/CNSA (Chinese National Space Administration) Double Star space missions. On each MMS spacecraft are two ASPOCs, the generated indium ion beams (energy range ~ 4-12 keV, currents up to 70 µA, antiparallel direction) limit the spacecraft potential to several volts positive. An instrument consists of the electronics box with the digital-, low voltage-, and high voltage boards (stacked frames) and two cylindrical emitter modules, all units are mechanically connected.
Figure 28 shows the instrument with electrically equivalent dummies replacing the real emitter modules. Each emitter module contains two ion emitters connected to a common high voltage supply. The cable harness is routed outside the electronics box and during operation only one ion emitter is switched active. In the final configuration the MLI (Multi-Layer Insulation) of the spacecraft is connected to the ion emitter modules via a plate, i.e. except the emitters the instrument is inside the spacecraft envelope mounted on the instrument deck. 70)
Figure 28: Close-up view of the ASPOC EQM instrument (image credit: ESA, IWF Graz)
EMC strategy: During development of the instrument – in order to get an in-house electromagnetic emission baseline – routine EMC pre-compliance measurements and sniff testing where performed in the frequency range up to 1 GHz (the digital clock of the instrument is 12 MHz). The ASPOC instruments for MMS — 8 flight models (FMs), and the engineering-qualification model (EQM) — have to be verified in EMC and magnetic tests. The test matrix (major items: structural / mechanical, EMC and magnetics, thermal / vacuum, calibration and general verifications) is slightly different for the EQM, FM1 and FM2-8, the basic principle is that all tests are in flight-like conditions. - The ASPOC EQM instrument for the MMS mission fulfills the EMC requirements.
Figure 29: Block diagram with the major components electronics box (digital-, low voltage-, and high voltage boards) and the two emitter modules (image credit: ESA, IWF Graz)
Figure 30: Overview of the MMS instruments suite 71)
The MMS ground system supports on-orbit operations of the MMS observatories, as well as the production, storage, management, and dissemination of MMS science data products. The MMS ground system consists of the following functional elements:
• MOC (Mission Operations Center), located at GSFC (Goddard Space Flight Center) in Greenbelt, MD. Responsible for spacecraft operations and telemetry capture.
• FDOA (Flight Dynamics Operations Area), located at GSFC. Responsible for orbit and attitude determination and control.
• SOC (Science Operations Center), located at LASP (Laboratory for Atmospheric and Space Physics) of the University of Colorado, Boulder, CO. Responsible for IS (Instrument Suite) operations, instrument data processing, archiving, and distribution.
• SMART ITF (Instrument Team Facilities). Instrument teams are responsible for data analysis and validation; instrument monitoring and special operations requests; software for producing Quicklook and Level-2 data products; Level-2 data processing; analysis tools for publicly available data products.
• EPO (Education and Public Outreach), located at Rice University, Houston, TX. Responsible for dissemination of educational materials to schools and the general public.
Figure 31: Overview of MMS ground data system responsibilities (image credit: SwRI)
AGS (Attitude Ground System)
The AGS is one of the components within the MOC (Mission Operations Center) providing flight dynamics support to the MMS mission. The AGS is responsible for determining and predicting the spacecraft attitude, providing support to the mission operations team to keep the spin axis orientation within the target tolerance. The AGS is also essential for validation of the onboard attitude and body rate estimates, sensor interference predictions, and sensor and inertia tensor calibrations to improve the accuracy of the ground and onboard attitude solutions. 72)
The AGS functionalities required and designed for MMS support include:
• Estimation and validation of the spacecraft attitude and body rates using an extended Kalman filter.
• Attitude and sensor interference prediction with the use of a smart targeting algorithm.
• Maneuver planning and scheduling support to stay within the science attitude requirements.
• Calibration of mass properties (inertia tensor and center of mass) to improve ground and flight attitude determination solutions.
• Sensor alignment calibrations, such as DSS to Star Sensor and relative Star Sensor Camera Head alignments.
Although the core of the AGS could either be the MTASS (Multi-mission Three-Axis Stabilized Spacecraft) system , or the MSASS (Multi-mission Spin Stabilized Spacecraft) system, the former was selected as the baseline for the MMS AGS with features and capabilities added to support spin-stabilized spacecraft and satisfy MMS mission requirements. The MMS spacecraft are spinners with an autonomous quaternion-output STS (Star Tracker System) that make possible the determination of full three-axis attitudes, instead of just the single spin-axis direction.
This wide variety of features place the AGS at the heart of a powerful and invaluable ground support system that has been used for nominal operations, calibrations, data analyses, as well as anomaly resolution of over two dozen missions. The oldest parts of the AGS were written in FORTRAN in the 1970s and 1980s. The generalized multi-mission version was created in the early 1990s and was used to support, e.g., the UARS (Upper Atmosphere Research Satellite), EUVE (Extreme Ultraviolet Explorer), and RXTE (Rossi X-ray Timing Explorer) missions. The AGS software was ported to MATLAB in the late 1990s.
The MMS spacecraft attitude requirement is 0.1 deg (3σ) per axis for both phases of the mission. The resulting history of the spacecraft attitude and body rates is called the definitive attitude. This definitive attitude is generated using quaternion solutions from each of the four sensor heads output by the STS at 0.5 to 4 Hz, depending on the telemetry downlink rate. The STS heads are mounted in pairs on two separate stable optical benches and are oriented roughly 10º off the body -Z axis. This orientation was chosen to minimize star motion through the FOVs (Fields of View) and to avoid interference from an axial sensor boom deployed along the –Z axis direction. An alignment transformation is applied to the output from each STS camera head so that the final output quaternions represent the GCI (Geocentric Inertial) frame to spacecraft body frame transformation.
In summary, the existing functionalities and new utilities implemented in the AGS have proven to be essential for MMS mission support in many ways. The support has included validation of the onboard spacecraft attitude and rate estimates, calibration of the star camera head alignments, calibration of the direction of the major principal axis of inertia, and attitude maneuver planning of "smart targets" to stay within the mission science target box. The AGS capabilities have enabled the proper identification of error sources for improving the attitude and body rate solutions, including thermal variations and sensor interferences that affect the attitude solutions.
The AGS was also used to perform some special analyses to support the mission. Namely, a long-term planning tool was developed in order to cope with the long shadow period near the beginning of the mission, and a method of sensor interference prediction was developed after observing unexpected sensor noise in the STS. It was found that the existing functionalities of the AGS and its library routines could be readily extended to solve both of these special-case problems.
Although there is high dependency of the attitude and rate solutions on the accuracy of the STS alignments and the knowledge of the inertia tensor, the SpinKF has performed very well to meet the MMS estimation tolerances. The dependency arises first from a need for self-consistent observations. These are provided by the relative STS alignment calibrations. Secondly, the use of Euler's equations of motion to propagate the state between observations requires a good inertia tensor. The MPA calibrations performed after every burn ensure the AGS always is using the best available inertia tensor and coning angle estimate. With these calibrations, the definitive attitude reports generated using the SpinKF have been delivered to the scientists daily, covering every orbit perigee-to-perigee, with the best available attitude accuracy.
It was known from early tests that there is coupling between the estimates for the STS alignments and the MPA direction. This was dealt with by iterating between these two calibrations. Once a set of STS alignments were found that made the four STS camera head measurements consistent, these alignments have been used both onboard and in the AGS. These alignments, in effect, define the MMS body frame, and the MPA calibrations have been performed relative to this frame ever since the alignments were uplinked.
The AGS requirement to support maneuver planning drove the need to predict the precession of the angular momentum for up to several weeks at a time. The AGS utility to predict the attitude relies on good knowledge of the inertia tensor along with an environmental torque model. This model has been shown to predict the spin axis direction very accurately, with an error of only 0.01º after a one-month propagation.
In summary, the AGS suite of utilities has been crucial to satisfying the MMS mission requirements, and has been used to provide good ground attitude determination and support to the mission during spacecraft commissioning and nominal operations.
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