ELFIN (Electron Losses and Fields Investigation)
ELFIN is a 3U CubeSat mission under development by the Earth, Planetary, and Space Sciences department at UCLA (University of California Los Angeles). The objective of the mission is to study space weather, specifically to explore the mechanisms responsible for the loss of relativistic electrons in the radiation belts. ELFIN will complete this goal by measuring for the first time the full energy distribution and pitch angle resolution of precipitating electrons using a UCLA built Energetic Particle Detector. Additionally, ELFIN will fly a 3-axis Fluxgate Magnetometer to take sensitive measurements of Earth's magnetic field, allowing for the detection EMIC (Electromagnetic Ion Cyclotron) waves, thought to be the primary contributor to particle losses.1) 2)
Science objectives: ELFIN's primary science objective is to measure, for the first time, the angle and energy distribution of precipitating relativistic electrons within and near the loss cone and determine, for the first time, if these bear the characteristic signature of scattering by the dominant wave scatterer, Electromagnetic Ion Cyclotron (EMIC) waves. For purposes of modeling and predicting Earth's radiation environment, determining storm-time radiation belt electron loss rates and mechanisms is just as important as determining electron acceleration from the far more expensive, equatorial satellites flying through the radiation belts. The STAR option means that ELFIN-B will pass over the same location of the ELFIN-A on a time-scale between 0-60 minutes. From that vantage point, the data will allow us to determine if the precipitation rate has changed spatially (in latitude extent), temporally (in intensity only but not in extent), or both. ELFIN's instrumentation and dual satellite approach will advance our understanding of dominant wave-loss mechanism of relativistic "killer" electrons and help build better radiation belt models to characterize and predict storm-time radiation belt fluxes. 3)
ELFIN's secondary science objective is to identify the magnetospheric source location of ionospheric field aligned currents (FACs), in relation to tail boundaries (dipole region, magnetotail, tail boundary). By measuring multiple 100-500 keV ion and 0.5-5 MeV electron isotropy boundaries, ELFIN will adjust mapping models and constrain the location of FAC sources in the magnetosphere. Multiple satellites at the same or different local times provide much better constraints to the mapping than a single satellite, thus ELFIN's second satellite improves this capability by a factor of two. Given that ionospheric Joule heating (which is driven by FACs) is a significant, perhaps dominant repository of storm and substorm energy, it is imperative to understand and model the flow of that energy in our space weather models. ELFIN's secondary objective will thus make significant contributions to the area of Magnetosphere-ionosphere coupling and allow us to build better models of the Sun-Earth interaction, moving us closer to the point of predictability.
Some background: ELFIN got its start in 2012 as a participant in the UNP (University Nanosatellite Program), funded by the AFRL(Air Force Research Laboratory). The mission was then picked up by CSLI/ELaNa (CubeSat Launch Initiative/Education Launch of Nanosatellite) and awarded joint funding from NASA/NSF in 2014, helping in providing the means to prepare ELFIN for development and launch.
The ELFIN team is made up of around 40 UCLA undergraduates, with a few graduate students and three staff members serving in mentorship roles. Students take responsibility as subsystem leads and the vast majority of the spacecraft are designed, manufactured, and tested in-house. Students get experience in a real-life engineering environment and interact with multiple disciplines to accomplish project goals. The experience is invaluable for their future careers, as nearly 250 students have passed through the project since its inception in 2009. They have formed a powerful alumni network with ELFINers in various technical industries. ELFIN is currently in position to be UCLA's first fully built satellite. While UCLA staff have provided instruments on other satellites in the past, this is the first spacecraft to be built, managed, and operated by UCLA. The ELFIN mission is a collaboration with the Aerospace Corporation.
The ELFIN-STAR (Spatio-Temporal Ambiguity Resolution) option was awarded and executed in November 2017, which added an additional identical CubeSat. Having two satellites will help us build a more precise picture of electron behavior by determining time dependence of these mechanisms.
Figure 1: Artist's rendition of the ELFIN nanosatellite in orbit (image credit: UCLA) 4)
ELFIN is a spin-stabilized 3U+ CubeSat at 20 rpm. The nanosatellite spins like a hammer spinning head over handle to be able to resolve the full range of pitch angles. 5) The spacecraft is largely being developed by a student team with student leadership. It is currently in position to be UCLA's first fully built satellite. While UCLA staff have provided instruments on other satellites in the past, this is the first project to combine UCLA instruments and a UCLA bus on the same spacecraft.
UCLA students are working on all subsystems, including ADCS (Attitude Determination and Control Subsystem), Communication, C&DH (Command and Data Handling), Flight Software, Payload, Power, and Structures. The majority of the final spacecraft will be constructed in-house by UCLA students, with avionics and bus programming by UCLA students and structure fabrication in an in-house machine shop.
Structures: ELFIN's chassis is a modified C2B customized to use the stacer can and torquer coils as structural elements. Vibration simulations are conducted in SolidWorks. The spacecraft antennas are stowed, coiled in the P-POD 3U+ Plus "tuna can" volume. All aluminum and plastic components of ELFIN are fabricated in-house, including the spacecraft chassis and the EPD (Energetic Particle Detector) shells. ELFIN uses a Haas TM-1 CNC mill, and directly export SolidWorks CAD files using the CAMWorks plugin.
The ELFIN spacecraft are 3U CubeSats, measuring 10 cm x 10 cm x 30 cm with a mass of about 3.5 kg. The satellite chassis is built from 6061 aluminum rails and "top hats" which are fastened at the corners with non-magnetic steel screws. The sensitivity of the magnetic instruments onboard the spacecraft necessitate a magnetically clean design; while most of the fasteners used for ELFIN are brass, non-magnetic A286 and 316 SS screws were used where brass was deemed structurally insignificant.
ADCS (Attitude Determination and Control Subsystem): Two torquer coils, comprised of aluminum wire on plastic spools, provide spin and precession capability to ELFIN. Periodic (daily/weekly) scheduled maneuvers are executed with these coils using onboard control laws and a magnetoresistive magnetometer. Coarse sun sensors and a fine sun sensor, as well as the fluxgate magnetometer, provide supplementary data for ground based attitude determination. 6)
Power: The magnetically clean solar panels contain 20 total UTJ (Ultra Triple Junction) cells that are arranged in opposing pairs and distributed along the 3U faces. The body mounted panels mean only some cells will be illuminated at a time for a given attitude. Two power boards, equipped with PIC microcontrollers, manage the four Lithium-ion batteries (18650, 2.2 Ah each) and provide +5 V for the spacecraft bus. 7)
Figure 2: Antenna mounting scheme of ELFIN (image credit: UCLA)
C&DH (Command and Data Handling) subsystem: The flight computer monitors the satellite, collects housekeeping data, executes scheduled tasks, and commands the Watchdog, ADCS Main PIC, two Power PICs (Peripheral Interface Controllers), and the radio. The Watchdog provides a layer of redundancy by heartbeating the flight computer and conducting scheduled resets of the entire spacecraft.
Communications: ELFIN is a 20 rpm spinner, with its spin axis orbit normal (as dictated by science requirements). The antennas are positioned in between the spin axis and the spin plane to mitigate spin fading in all orientations. ELFIN-A is IARU coordinated within the amateur satellite service's 435-438 MHz band so that we may beacon globally. We use an AstroDev Helium radio to provide a 19200 baud GFSK AX.25 UI downlink. Beacons will be transmitted at 9600 baud via the same radio & frequency. 8)
ELFIN-A transmits 9600 baud beacons on 437.450 MHz with the FCC experimental call sign - WJ2XNX.
ELFIN-B transmits 9600 baud beacons on a 437.475 MHz with the FCC experimental call sign - WJ2XOX.
Figure 3: The VHF and UHF antenna elements are stowed in the 3U+ bonus volume ("tuna can"), image credit: UCLA
TCS (Thermal Control Subsystem): Passive thermal stabilization is particularly important for the EPDs (Energetic Particle Detectors), the FGM (Fluxgate Magnetometer) sensor, and the Li-ion batteries. The FGM sensor is thermally stabilized with MLI (Multi-Layered Insulation) blankets, and overall heat input into the spacecraft will be reduced with aluminized Kapton. In addition to an in-house MATLAB simulation, ELFIN is beginning to use Thermal Desktop for validation and improved fidelity.
Deployable mechanisms: There are two main deployable systems on the ELFIN spacecraft: the "tuna can" antenna and the FGM boom. The tuna can antenna system makes use of the additional volume allowed in the 3U+ CubeSat design. The antenna elements are rolled and stowed prior to launch. Once the spacecraft is successfully deployed from the P-POD, resistors will burn through the lines constraining the elements, allowing them to unfurl. The FGM is housed inside the stacer can, and will likewise be deployed once ELFIN is out of the P-POD. To induce a separation between the two ELFIN satellites, one of the satellites will deploy its stacer boom before the other in order to experience greater atmospheric drag. Both mechanisms underwent a series of deployment tests, including stowing the tuna can antennas for over 6 months and testing their deployment in our thermal vacuum.
Figure 4: The twin assembled Flight Model units, ELFIN and ELFIN-STAR (image credit: UCLA)
Figure 5: Exploded view of the ELFIN nanosatellite showing all subsystems within the chassis (image credit: UCLA) 9)
Launch: The two ELFIN 3U CubeSats were launched as secondary payloads on 15 September 2018 (13:02 UTC). ELFIN was selected for NASA's CLSI (CubeSat Launch Initiative) No 5. The primary payload on this mission was the ICESat-2 spacecraft of NASA. The launch service was provided by ULA's Delta-2 vehicle from VAFB, CA. 10) 11)
Orbit: Near polar circular orbit, altitude of ~496 km, inclination = 92º.
The secondary payloads on IceSat-2 are:
• SurfSat (Surface charging Satellite), a 2U CubeSat mission developed at the UCF (University of Central Florida), Orlando, FL.
• CP-7 (CalPoly-7) or DAVE (Damping And Vibrations Experiment), a 1U CubeSat, a collaboration of Northrop Grumman Aerospace Systems and CalPoly.
Status of mission
• February 11, 2019: According to Vassilis Angelopoulos, the ELFIN mission is doing fine. The instruments are being commissioned, and the project expects science collections to start on the first satellite in a couple of weeks, and on the other satellite in a month. We expect the mission to last through mid-January 2020 (reentry).
- "We've just received our first downlink of data from ELFIN-A," reports Ryan Caron, Development Engineer at UCLA's Department of Earth, Planetary, and Space Sciences.
- "Sensors onboard our two cubesats detect electrons in the energy range 50 keV to 4.5 MeV," says Caron. "These are the so-called ‘killer electrons,' which can damage spacecraft and cause electrical disruptions on the ground. They also give rise to the majestic aurora borealis."
- "ELFIN is doing something new," says Vassilis Angelopoulos, a UCLA space physicist who got his doctorate at UCLA and serves as ELFIN's principal investigator. "No previous mission was able to measure the angle and energy of killer electrons as they rain down on Earth's atmosphere. ELFIN will help us investigate how disturbances called ‘Electromagnetic Ion Cyclotron waves' knock these electrons out of the Van Allen Belts and scatter them down toward Earth."
Figure 6: UCLA sends student-built ELFIN CubeSat into space (video credit: John Vande Wege/UCLA Broadcast Studio)
Sensor complement: (EPD, FGM)
The science mission of ELFIN is complementary to larger NASA missions (THEMIS, MMS, DSX, etc). The conjunctions with equatorial spacecraft will reveal the full significance of wave-particle dynamics in the magnetosphere.
Charged particles from the Sun interact with Earth's magnetic field and travel along field lines in a spiral or helical fashion, and the angle between a particle's velocity vector and the direction of the field line is known as a pitch angle. Those that travel within a characteristic range of pitch angles, known as a loss cone, can collide with atmospheric particles and get lost in the atmosphere to create phenomenon such as auroras.
Particles sometimes come close enough to Earth such that the stronger magnetic field causes them to reverse direction, and particles that continuously oscillate due to these mirror points are said to be trapped and become highly energetic. When these trapped particles precipitate into the loss cone, damage towards critical assets can occur, ranging from single event upsets, losses of satellites, and even terrestrial blackouts.
Modeling suggests that equatorial electromagnetic ion cyclotron (EMIC) waves may be the primary cause of trapped electron losses, but the contribution from other effects have not been determined observationally. The ELFIN mission will address this contentious issue by determining whether electron losses bear the characteristic signatures of EMIC wave scattering.
The spacecraft have two Energetic Particle Detectors, one for Electrons (EPD-E) and one for Ions (EPD-I), as well as a Fluxgate Magnetometer (FGM ) deployed at the end of a 75cm stacer boom. The science instruments were developed by UCLA staff at the Institute for Geophysics and Planetary Physics (IGPP) and the Earth, Planetary, and Space Sciences department (EPSS). ELFIN will explore the key mechanisms responsible for the loss of relativistic electrons from the radiation belts and will provide a unique perspective into modeling and predicting Earth's radiation environment.
ELFIN will also be flying a Switching Instrument Power Supply (SIPS) and an Instrument Data Processing Unit (IDPU) which support the primary instruments in their operation. Students are extensively involved in the design and layout for these instrument boards.
Figure 7: Schematic overview of the ELFIN science mission (image credit: UCLA)
EPD (Energetic Particle Detector)
Two EPDs will resolve pitch angle distributions of charged particles in the loss cone of Earth's radiation belts. One detector is dedicated to detecting electrons (EPD-E) and the other for ions (EPD-I). They are made of aluminum and tantalum, and their design was driven by Geant4 simulations.
ELFIN will measure, for the first time, if the angle and energy distribution of precipitating electrons bear the characteristic signature of scattering by EMIC (Electromagnetic Ion Cyclotron) waves.
• EPD- E (Energetic Particle Detector – Electrons): 50 keV – 4 MeV
•EPD-I (Energetic Particle Detector – Ions): 50 keV – 300 keV
• Capable of 10,000 to 50,000 counts/s
• Field of View < 28°
Figure 8: Left: The EPD instrument; Right: Cross-section of the EPD instrument with annotations (image credit: UCLA)
Figure 9: Illustration of the EPD component accommodation within the ELFIN nanosatellite (image credit: UCLA)
FGM (Fluxgate Magnetometer)
The FGM enables correlation of pitch angle information with energetic particle spectra by making 3-axis magnetic field measurements. UCLA has a long history with magnetometer design, and the FGM is based on a previously delivered magnetometer. The following parameters apply to the FPM:
• Dynamic range: ±55,000 nT
• Resolution: 6.5 pT
• Noise resolution: 0.2 nT/√Hz
• Digitization: 24 bits
• Sample rate: 80 samples/s.
Figure 10: Left: FGM electronics of size 90 x 90 x 25 mm and a mass of 100 g; Right: FGM sensor of size 48 x 48 x 25 mm and a mass of 58 g (image credit: UCLA)
Figure 11: Illustration of the FGM component accommodation within the ELFIN nanosatellite (image credit: UCLA)
Knudsen North serves as UCLA's primary Earth Station for ELFIN, with VHF (2 m) uplink and UHF (70 cm) downlink antennas. It provides a higher gain than the traditional OSCAR-class amateur radio station to allow ELFIN's transmitter to be operated at a lower level.
The Knudsen station supports other university nanosatellite missions. This helps demonstrate technical capability, improves and maintains our proficiency, and fosters relationships within the nanosatellite and amateur radio communities that will be beneficial for ELFIN's launch and early orbit operations. 16)
The main features of the Knudsen Station include:
• Quad array 436CP42UG cross polarized UHF Yagis
• AlphaSpid's RAS rotor with a Green Heron RT-21 Az-El controller
• Two FunCube Dongles Pro+'s with power splitter for RX (this is pending test results, may go back to USRP N210)
• Icom IC-910H for TX
• Hamlib to control rotors/ custom tracking script using PyEphem and TLEs.
Figure 12: Block diagram of the ground station (image credit: UCLA)
Figure 13: Photo of the Knudsen station (image credit: UCLA)
Knudsen South Earth Station
The Knudsen south station is the newest addition. Knudsen South serves as UCLA's backup uplink station, with the planned upgrade path allowing it to serve as a full uplink/downlink backup station. 17)
The main features of the Knudsen Station include:
• Two phased 2MCP8A circularly polarized VHF yagis with polarizer switching capabilities
• AlphaSpid's Big RAS rotor with a Green Heron RT-21 Az-El controller
• Shared Icom IC-910H for TX
• 500 W Gemini 2-500 Linear Amplifier
• Hamlib to control rotors/ custom tracking script using PyEphem and TLEs
• Quad array 436CP42UG cross polarized UHF Yagis (planned for future)
• Two FunCube Dongles Pro+'s with power splitter for RX (planned for future).
1) Chris Shaffer, "ELFIN - An Update on UCLA's Electron Losses and Fields Investigation," Proceedings of the 11th Annual CubeSat Developers' Workshop - The Edge of Exploration," San Luis Obispo, CA, USA, April 23-25, 2014, URL: http://mstl.atl.calpoly.edu/~bklofas/Presentations/
2) Lydia Bingley, Vassilis Angelopoulos, Ryan Caron, "ELFIN - Electron Losses and Fields Investigation," Proceedings of the 30th Annual AIAA/USU SmallSat Conference, Logan UT, USA, August 6-11, 2016, paper: SSC16-VIII-5, URL: http://digitalcommons.usu.edu/cgi/viewcontent
10) "NASA, ULA Launch Mission to Track Earth's Changing Ice," NASA Release 18-078, 15 September 2018, URL: https://www.nasa.gov/press-release/nasa
11) "2018: A Big Year for NASA's Launch Services Program," NASA, 1 Feb. 2018, URL: https://www.nasa.gov/feature/2018-a-big-
14) Tony Phillips, "Student-Built Space Weather Satellite Targets Killer Electrons," 20 September 2018, URL: https://spaceweatherarchive.com/2018/09/20/student
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).