ExoCube, also known as CP10 (Cal Poly 10), is a space weather nanosatellite developed by the California Polytechnic State University (Cal Poly), San Luis Obispo and sponsored by NSF (National Science Foundation). The objective is to measure the density of hydrogen, oxygen, helium, and nitrogen in the Earth's lower exosphere and upper ionosphere. It will characterize (O, H, He, N2, O+, H+, He+, NO+), as well as the total ion density above the ground stations, ISR (Incoherent Scatter Radar) stations, and periodically throughout the entire orbit.
Figure 1: Photo of the ExoCube nanosatellite (image credit: ExoCube Team)
ExoCube is a 3U CubeSat designed and built by PolySat, a research group at California Polytechnic State University, in collaboration with NASA/GSFC, SSI (Scientific Solutions Inc.) of North Chelmsford, MA, the University of Wisconsin of Madison, WI, and the University of Illinois - funded by the NSF. 1) 2)
ADCS (Attitude Determination Control Subsystem): To achieve accurate measurements, there are pointing and orientation requirements for the mission. The satellite must maintain a nadir pointing of ±10o and a ram knowledge of ±5o . To maintain these requirements, an ADCS was developed by PolySat and integrated on the satellite. The control system has the following components:
• Gravity Gradient system with deployable booms
- Provides passive control of the system with no power
- Helps maintain pointing and rate of change of the satellite with ADCS turned off
- Provides stability during momentum wheel spin up
- Booms developed in-house
• Sinclair momentum wheel
- 10 mNm momentum wheel
- Orientated on pitch axis to provide rigidity and stability of pointing
- Couples roll and yaw axes together to provide gyroscopic stability.
• Kalman filter
- Calculates orientation based on SGP4 propagator, solar array measurements, and magnetometers
- Filters out noise from sensors, actuators, and other system disturbances to achieve accurate orientation
- Acquires initial orientation using TRIAD algorithm
- Stays on for duration of mission
- Works for eccentricities < .1 and rates < 10 degrees/second
• PD controller
- PD control law developed by Bong Wie for a gravity gradient ADCS
- Chosen for simplicity and power efficiency
- Proven global stability by Bong Wie.
Figure 2: Left: ExoCube orientation on-orbit. Right: With updated orbital parameters, ExoCube's observatory overpasses can be predicted for timing coincident ground based airglow observations (image credit: ExoCube Team)
• ADCS sensors and actuators
- Magnetometers for reading magnetic field
- Solar array sensors for calculating orientation in reference to the sun
- Magnetorquers, designed by PolySat, for torquing and controlling the satellite
- Gyroscope for verification of rates
• Z-panel camera
- OmniVision 3 Mpixel camera with 25o FOV (Field of View)
- On both z-panels
- Used for verification of boom deployment
- Can calculate rates and orientation of satellite with several successive pictures
- Derivative imaging scales the images down to make download from satellite quicker and easier
- Cameras were tested on CP8 balloon launch and are on CP8 which is scheduled for launch at the end of 2013.
EPS (Electrical Power Subsystem):
RF communications: Use of UHF communication (437.27 MHz), FSK modulation, output power of ~ 1W. The UHF Marconi ground station is located at Cal Poly (California Polytechnic State University), San Luis Obispo, CA.
The 3U CubeSat has a mass of ~4 kg and complies to the CubeSat standard in size of 10 cm x 10 cm x 30 cm.
Launch: ExoCube is a secondary payload on ELaNa-X and was launched on January 31, 2015 (14:22:00 UTC) from VAFB, CA. The SMAP (Soil Moisture Active/Passive) spacecraft of NASA is the primary mission on this flight. The launch provider is ULA using a Delta-2 7320-10C vehicle. 3) 4) 5)
Orbit of primary payload: Sun-synchronous dawn/dusk orbit, altitude = 685 km, inclination = 98o, period = 98.5 minutes, LTAN (Local Time of Ascending Node) = 18:00 hours, exact repeat cycle = 8 days after 117 orbits, (near-global coverage of Earth can be obtained every three days, 44 orbits).
Orbit of secondary payloads: The CubeSats will be deployed after separation of the SMAP (Soil Moisture Active Passive) observatory, into a sun-synchronous elliptical orbit of 440 km x 670 km of 99.12o inclination.
Secondary (auxiliary) payloads:
ELaNa-X (Educational Launch of Nanosatellite-X), which consists of three P-PODs (Poly Picosatellite Orbital Deployers) containing a total of four CubeSats (representing three CubeSat missions). The three CubeSat projects on ELaNa X include (Ref. 5): 6)
• GRIFEX (GEO-CAPE ROIC In-Flight Performance Experiment), a 3U CubeSat flight test experiment and a collaborative mission of the University of Michigan with NASA ESTO (Earth Science Technology Office) and JPL (Jet Propulsion Laboratory.
• ExoCube,a space weather nanosatellite (3U CubeSat) developed by the California Polytechnic State University (Cal Poly), San Luis Obispo. The payload is developed by NASA/GSFC.
• FIREBIRD-2 A and B (Focused Investigations of Relativistic Electron Burst Intensity, Range, and Dynamics), each a 1.5U CubeSat collaborative mission of the University of New Hampshire, Montana State University, LANL (Los Alamos National Laboratory), and the Aerospace Corporation.
• Since deployment ExoCube has been a little quiet. Unfortunately, the transmit power has been lower than expected. Thankfully, there is been great support in the community. SRI International has been generous and has allowed us to use their 45 m dish. Members of the Cal Poly team drove to SRI on Feb. 8, 2015 to use the dish for the night and the following morning. The team received a significant amount of health and telemetry data from the spacecraft! 7) 8)
- The Cal Poly team reviewed the telemetry data and determined that the antenna unfortunately hasn't deployed. Multiple deploy commands have been sent to the antenna. At some point during shock and vibes testing and the ride up the mechanism broke. - The team has been investigating options for long term solutions. The team has investigated ways to still receive important data from the payload with limited data capacity from the satellite. The mission will move on!
Figure 3: Photo of the large SRI dish radio antenna in the Stanford foothills (image credit: PolySat)
Legend to Figure 3: Constructed by SRI (Stanford Research Institute) , now SRI International, in the early 1960s for the U.S. Department of Defense, and located on land leased from Stanford University, the 45 m diameter radio reflector antenna has become known locally as "the Dish." The Dish, which is owned by the U.S. government, is operated and maintained by SRI. It is used for satellite calibrations, spacecraft command and telemetry, radio astronomy measurements, and weak signal detection and the related diagnosis of spacecraft conditions.
• All four ELaNa-X CubeSats were ejected from the second stage per the mission timeline (1 hr and 45 minutes after liftoff), and are flying free. Prior to the deployment, the second stage of the Delta-2 rocket performed an 8 second retrograde maneuver to lower the orbit of the vehicle for the release of the CubeSats. Three P-PODs were installed on the second stage, filled with two 3U CubeSats and two 1.5U satellites.
Sensor complement: (INMS)
The goal of the ExoCube mission is to measure the densities of all significant neutral and ionized atom species in the ionosphere, the outer region of the atmosphere where incoming solar radiation ionizes a large fraction of atoms. Knowledge of the composition of that volatile region of the atmosphere is vital to the modeling and forecasting of space weather; the conditions of the upper regions of the atmosphere (above approximately 150 km in altitude) that have a significant effect on satellite communications and spacecraft operation and performance. 9)
INMS (Ion Neutral Mass Spectrometer)
INMS uses a gated time-of-flight spectrometer. Atomic oxygen and helium have not been measured in situ since the early 1980s during the era of the Dynamics Explorer and then only for 18 months. Atomic hydrogen has never been directly measured in situ in this region. By providing benchmark measurements over Arecibo, Wisconsin, Kitt Peak, and Cerro Tololo, ExoCube aids in the validation and inter-comparison of ground-based observations from these sites using passive optical interferometry and photometry of neutral airglow emissions as well as active ISR to characterize the local ionosphere.
ExoCube density measurements will be used to characterize the climatology of the upper ionospheric and lower exospheric composition. The combination of orbital inclination and precession will enable a robust assessment of diurnal density and composition variations, while the expected minimum six-month mission lifetime will facilitate comparisons between equinox and solstice conditions. Key scientific objectives include investigation of upper atmospheric global, diurnal, and seasonal variability, charge exchange processes, atmospheric response to geomagnetic storms, and validation of empirical and climatological atmospheric models (e.g. MSIS, TIE-GCM). ExoCube measurements of in-situ neutral hydrogen also help constrain retrievals of aeronomical parameters of interest from airglow observations and forward radiative transfer modeling.
Instrument: A compact (miniaturized) INMS has been developed at the Heliophysics Science Division of GSFC (Goddard Space Flight Center) for the ExoCube mission. The INMS occupies the central 1.5U on the Cal Poly ExoCube CubeSat, with both apertures facing the RAM direction. The INMS is fully redundant for ions and neutrals. 10)
Figure 4: Photo of the INMS (image credit: ExoCube Team)
Figure 5: Schematic of the neutral static energy angle analyzer (image credit: ExoCube Team)
Figure 6: The INMS sensor (half configuration), image credit: ExoCube Team
Legend to Figure 6: Gated time of flight with deflectors / collimator, retarding grid, post acceleration, electrostatic analyzer, detector/time of flight electronics.
Principle of operation:
• Neutral particles are first ionized with compact thermionic filament ionizer
• Post acceleration by voltage V gives all ions same energy E=qV, much greater than initial energy dispersions dE
• Thus,the ions are ordered in velocity according to their mass on the basis of the simple formula E=1 mv2
• Measuring the velocity of each ion - with TOF (Time of Flight) over a distance d - gives the mass of the ion according to M/q=2 x E/q x TOF2/d2
• Ions are focused through an electric gate into an ESA (Electrostatic Analyzer)
• Ions are normally blocked by the gate and can pass through only during a short pulse duration t,marking a start in the time of flight measurement.
• Ions are detected at the output of the ESA by a CEM detector marking the stops for the time of flight measurements.
• The ESA is tuned to the proper energy pass band blocking out of band particles, as well as attenuating any UV.
• The mass resolution is limited by uncertainties in energy dispersion, time resolution and time of flight path.
1) "Composition Variations in the Exosphere, Thermosphere, and Topside Ionosphere (EXOCUBE)," NSF CubeSat-based science missions for geospace and atmospheric research, Oct. 2013, pp:44-47, URL: http://www.nsf.gov/geo/ags/uars/cubesat/nsf-nasa-annual-report-cubesat-2013.pdf
2) "CP10 (ExoCube), " Cal Poly, URL: http://polysat.calpoly.edu/in-development/cp10-exocube/
3) Steve Cole, Alan Buis, NASA Launches Groundbreaking Soil Moisture Mapping Satellite ," NASA, Release 15-016, Jan. 31, 2015, URL: http://www.nasa.gov/press/2015/january/nasa-launches-groundbreaking-soil-moisture-mapping-satellite/#.VM3oYC7-Y_c
5) Steve Cole, Alan Buis, Rani Gran, Jessica Rye, George Diller, "Soil Moisture Active Passive Launch," NASA Press Kit, January 9, 2015, URL: http://www.jpl.nasa.gov/news/press_kits/smaplaunch.pdf
6) "ELaNa X, Educational Launch of Nanosatellites," NASA, URL: http://www.nasa.gov/mission_pages/smallsats/elana/#.VLex1HvGX_V
8) "EXOCube and SRI 150 foot Dish," Feb. 11, 2015, URL: http://www.pe0sat.vgnet.nl/2015/exocube-and-sri-150-foot-dish/
10) N. Paschalidis, S. Jones, M. Rodriguez, E. Sittler, D. Chornay, P. Uribe, T. Cameron, G. Nanan, J. Noto, L. Waldrop, C. Taylor, D. Gardner, S. Nosal, E. Mierkiewicz, "A Compact Ion Neutral Mass Spectrometer for the ExoCube Mission," 6th European CubeSat Symposium, Estavayer-le-Lac, Switzerland, October 14-16, 2014
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).