MinXSS (Miniature X-ray Solar Spectrometer) nanosatellite
MinXSS is a 3U CubeSat solar physics mission of the CU (University of Colorado) at Boulder, designed to better understand the energy distribution of solar flare SXR (Soft X-ray) emissions and its impact on the Earth’s ITM (Ionosphere, Thermosphere, and Mesosphere). This project heavily involves in particular graduate student team members with scientists and engineers at the Laboratory for Atmospheric and Space Physics (LASP), located on the east campus at CU. 1) 2)
The peak solar energy in the SXR is expected to be emitted near 2 nm; however, only limited spectral measurements are available near that wavelength to verify this expectation. Energy from SXR radiation is deposited mostly in the ionospheric E-region, in the altitude range of ~80 to ~150 km, but the precise altitude is strongly dependent on the SXR spectrum because of the steep slope and structure of the photoionization cross sections of atmospheric gases in this wavelength range.
Despite many decades of solar SXR observations, almost all have been broadband measurements with insufficient spectral resolution to fully understand the varying contributions of emission lines amongst the underlying thermal and non-thermal continua. The MinXSS mission will improve the understanding of how highly variable solar X-rays affect the ITM, advance our knowledge of flare energetics in the SXR, and provide new spectral observations of the solar SXR near the maximum of solar cycle 24.
The educational objective of MinXSS is to train students as the next generation STEM ( Science, Technology, Engineering, and Mathematics) workforce. The students will learn – through formal university classes and with hands-on experience – about scientific instrumentation, satellite technology, and science data analysis and modeling techniques.
Measurement requirements: To address the MinXSS science objective, measurements of the solar SXR irradiance (full-disk, not imaging) with spectral resolution better than 1 nm and with accuracy better than 30%. While simple in concept, the technology to do this in the 1-5 nm range has traditionally been difficult. Grazing-incidence grating spectrometers are only effective longwards of 5-10 nm (e.g., SDO/EVE measures down to 6 nm with 0.1 nm resolution. Bragg crystal spectrometers, often used in the 1960s and 1970s for solar SXR measurements, have extremely high spectral resolution but a very narrow range of about 1 nm, and they are large and heavy instruments. Solid-state (semiconductor) photon-counting detectors work very well for obtaining HXR (Hard X-ray) and gamma ray spectra shorter than 0.5 nm (e.g. RHESSI measures up to 0.4 nm.
SDD (Silicon Drift Detectors) are the novel technology that enable new solar spectral measurements over a wider range in the SXR. The Amptek X123 SDD is a commercial off-the-shelf (COTS) instrument that the project acquired; it was flown on a NASA sounding rocket in 2012 as part of the project's calibration program for the SDO/EVE instrument.
The MinXSS mission is most relevant for the NSF Division of Atmospheric and Geospace Sciences (AGS). In particular, MinXSS directly supports goal #1 for the NSF Geospace Aeronomy CEDAR program: to study the “dynamics and energetics of the upper atmosphere, with particular emphasis on the hard to observe region between 80 and 150 km.”
In the current design, MinXSS will require approximately 6 W of orbit-averaged power to fulfill the mission goals. A 3U body-mounted (fixed) solar panel can provide, at most, 7 W of power with triple-junction solar cells, so a deployable solar panel is needed. The team has performed extensive trade studies on numerous solar panel configurations, and has decided to use a double deployed 3U panel array along with a fixed panel to provide about 20 W of power while on the dayside of the orbit, which will provide sufficient orbit-averaged power for any orbit and can adequately handle the peak power demands including charging the battery during the dayside (Ref. 2).
The structure of the nanosatellite is partitioned into three basic blocks (Figure 1): 1U for the instrument (COTS Amptek X123 spectrometer), 1.5U for system electronics, and 0.5U for the ADCS (Attitude Determination and Control Subsystem ). There are two solar arrays to provide 20 W when in sunlight; a body-mounted solar panel (3U surface area: 10 cm x 34 cm) fixed to the solar-oriented side, and a deployed double panel (6U surface area: 20 cm x 34 cm).
The deployable UHF antenna, UHF communication, and EPS (Electrical Power System) are inherited from the CSSWE (Colorado Student Space Weather Experiment), an NSF nanosatellite at CU, which was launched on Sept. 13, 2012.
The ADCS will maintain the spacecraft orientation such that the solar panels and instruments are always sun-pointed, and the radiator (on the opposite side from the deployable solar array) is oriented roughly towards zenith (Figure 2).
Figure 1: Mechanical model of MinXSS (image credit: CU)
Figure 2: Nominal orbital attitude configuration (image credit: CU)
Table 1: Hardware component TRL
ADCS (Attitude Determination and Control Subsystem): The MinXSS bus is a 3-axis-controlled nanosatellite to observe the solar SXR spectrum between 0.04 and 3 nm. Use of the XACT (fleXible ADCS Cubesat Technology) unit of BCT (Blue Canyon Technologies). XACT is a standalone 0.5U 3-axis stabilized ADCS unit, featuring a star tracker, coarse sun sensor, IMU (Inertial Measurement Unit), reaction wheels, and torque rods.
Table 2: Specification of the XACT performance
Figure 3: Functional block diagram of XACT (image credit: CU)
Figure 4: System interconnect diagram of MinXSS (image credit: CU)
C&DH (Command and Data Handling subsystem): The C&DH is designed and developed by students. The core processor is the microchip DSPic32 (40 MIPS 16 bit µcontroller). The core processor has a flight history on the SDO/EVE (Solar Dynamics Observatory/EUV Variability Experiment) rocket flight (June 23, 2012) of LASP (Laboratory for Atmospheric and Space Physics) at the University of Colorado. The processor runs on RTOS (slot architecture).
Figure 5: Photo of the C&DH board design (image credit: CU)
EPS (Electrical Power Subsystem): The nanosatellite uses 28 PV triple junction solar cells distributed among its components. A BCR (Battery Charge Regulator) is used for the battery assembly. Two Li-ion batteries have a storage capacity of 14.8 Whr. The battery cells are connected in series to supply between 6 and 8.4 V (nominally 7.4 V) to the power system.
COMM (Communications subsystem): A half-duplex communication scheme is used with two deployable steel-tape monopole antennas connected together with a 180º hybrid for RF communications. This dual monopole configuration has been tested, and exhibits a gain pattern very close to that of a dipole. A GMSK (Gaussian Minimum Shift Keying) modulation chip has been selected to be the heart of the communications board, the SX1231.
The COMM microcontroller (µC) interfaces the C&DH subsystem with a GMSK modem and sends all data received by the C&DH to the GMSK modem to be modulated and transmitted to the GN (Ground Network). The nanosatellite uses a MSP430 microcontroller for the COMM and C&DH subsystems, which enables a common programming environment and reduces interface complexity.
The transceiver operates in the UHF band at 433 MHz (amateur band), which allows cooperation with any number of amateur ground stations across the globe. The transmission data rate is 9.6 kbit/ in downlink and uplink. 3) 4)
The ground station antenna is a M2 MCP436CP30 cross polarized 42 element yagi antenna with a gain of 14.15dBdc.
Figure 6: Block diagram of the COMM subsystem (image credit: CU-Boulder)
The MinXSS nanosatellite has a mass of ~4 kg and a power consumption of 7.3/18.1 W(average/peak).
Launch: A launch of MinXSS as a secondary payload is planned for November 2014.
Orbit: The mission requires an orbit with an inclination of ≥40º.
Figure 7: Artist's view of the MinXSS spacecraft orbit (image credit: CU-Boulder)
Sensor complement: (X123)
X123 (X-ray Spectrometer)
The MinXSS science measurements will be achieved using the commercially available X123 advanced X-ray spectrometer of Amptek which has an active area of 25 mm2, an effective Si thickness of 0.5 mm, an 8 µm thick Be (beryllium) filter on the detector vacuum housing, an active 2 stage TEC (Thermoelectric Cooler) on the detector, and sophisticated MCA (Multichannel Analyzer) detector electronics. This commercially available handheld X-ray spectrometer was recently flown on a sounding rocket and demonstrated its ability to operate in space.
Key to achieving our science goals is accurate pointing control and knowledge. With the wide field of view of the X123 spectrometer (±4°), the pointing requirements for MinXSS are only 2° (3σ) accuracy and 0.05° (3σ) knowledge.
Features of the X123 science instrument: 5)
• Observes the soft X-ray solar spectrum from <1 keV (>12 A) to >5 keV (<2.5 A)
- Resolution: < 0.5 keV
- Accuracy: < 30%
- Measures in a ‘gap’ of current knowledge.
• Complete package: silicon-drift detector, thermoelectric cooler, preamp, digital pulse processor and power supplies
• Photon counter, pulse pile-up rejection, multi-channel analyzer (creates spectra).
Figure 8: Photo of the X123 X-ray spectrometer (image credit: Amptek)
Figure 9: Illustration of the detector system (image credit: Amptek)
Background: The Amptek X123 (X-ray Spectrometer), a payload flown on the NASA SDO/EVE (Solar Dynamics Observatory/Extreme ultraviolet Variability Experiment) calibration sounding rocket in June 2012 (an underflight of the SDO satellite mission), will be flown again in October 2013; it is the same one that is being integrated into MinXSS. This X123 instrument has also been taken to the SURF (Synchrotron Ultraviolet Radiation Facility) of NIST (National Institute of Standards and Technology) for calibration. Hence, the primary science instrument onboard MinXSS will be fully calibrated and proven in flight. 6)
1) S. Palo, T. Woods, X. Li, J. Mason, M. Carton, A. Caspi, A. Jones, R. Kohnert, S. Solomon, “MinXSS: A Three-Axis Stabilized CubeSat for Conducting Solar Physics,” 5th European CubeSat Symposium, Royal Military Academy, VKI (Von Karman Institute), Brussels, Belgium, June 3-5, 2013
2) Thomas N. Woods, Amir Caspi, Xinli Li, Scott Palo, Stanley Solomon, Andrew Jones, “MinXSS - CubeSat: Miniature X-ray Solar Spectrometer (MinXSS),” May 3, 2012, URL: http://www.google.de/url?sa=t&rct=j&q=minxss%20(miniature%20x-ray%20solar%20spectrometer)%2C%20launch&source=web&cd=2&ved=0CDsQFjAB&url=httjVz-z0l22Sj59RCg&cad=rja
3) Xinlin Li, Scott Palo, Shri Kanekal, Rick Kohnert, Gail Tate, Vaughn Hoxie, David Gerhardt, Lauren Blum, Quintin Schiller, “The Colorado Student Space Weather Experiment (CSSWE) - CSSWE COM System,” 9th Annual Spring CubeSat Developer's Workshop, Cal Poly State University, San Luis Obispo, CA, USA, April 18-20, 2012, URL: http://mstl.atl.calpoly.edu/~bklofas/Presentations/DevelopersWorkshop2012/CHDC_Palo_CSSWE.pdf
4) Xinlin Li, Scott Palo, Rick Kohnert, David Gerhardt, Lauren Blum, Quintin Schiller, Drew Turner, Weichao Tu, Nathan Sheiko, Christopher S. Cooper, “Colorado Student Space Weather Experiment: Differential flux measurements of energetic particles in a highly inclined low Earth orbit,” Dynamics of the Earth’s Radiation Belts and Inner Magnetosphere, Geophysical Monograph Series, Vol. 199, edited by D. Summers et al., pp. 385-404, 2012, AGU, Washington, D. C., doi:10.1029/2012GM001313.
6) Information provided by James Mason, PhD candidate at the University of Colorado/LASP, Boulder, CO.
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.