IceCube — 874 GHz submillimeter wave radiometer mission on a 3U CubeSat
IceCube is a NASA/GSFC (Goddard Space Flight Center) nanosatellite mission with the objective to validate a new 874 GHz submillimeter wave radiometer for cloud ice observations. The measurements of ice clouds and their processes are important for climate models and for cloud precipitation processes. - In July 2014, NASA's SMD (Science Mission Directorate) selected a team at GSFC, led by PI Dong L. Wu, to build its first Earth science-related CubeSat mission. The IceCube project is being managed and co-funded by ESTO (Earth Science Technology Office) of NASA. 1) 2) 3) 4)
Background: Ice clouds play a key role in Earth's climate system, primarily through regulating atmospheric radiation and interacting with dynamic, energetic and precipitation processes. Sub-millimeter wave remote sensing offers a unique capability for improving cloud ice measurements from space, due to its great depth of cloud penetration and volumetric sensitivity to cloud ice mass. At 874 GHz, ice cloud scattering produces a larger brightness temperature depression than at lower frequencies, which can be used to retrieve vertically-integrated cloud IWP (Ice Water Path) and ice particle size. This effect was measured with the CoSSIR (Compact Scanning Sub millimeter wave Imaging Radiometer), an airborne instrument developed at NASA/GSFC. CoSSIR is a conical and cross-track imager with six receivers and eleven channels centered at 183, 220, 380, 640 V&H, and 874 GHz (Figure 1). CoSSIR measurements from NASA's ER-2 aircraft showed that the selected channel set is capable of accurately retrieving IWP in a wide dynamic range between ~10 g/m2 and 10,000 g/m2 after validation against cloud radar and lidar, with large brightness temperature depression centered at 874 GHz (Figure 2).
Figure 1: CoSSIR has been flown in several NASA airborne field campaigns (image credit: NASA)
The objective of the IceCube project is to retire risks of the 874 GHz receiver technology for future Earth and space remote sensing instruments by raising its TRL (Technology Readiness Level) from 5 to 7. The project will demonstrate, on a 3U CubeSat in a LEO (Low Earth Orbit) environment, a receiver system with a NEDT (Noise Equivalent Differential Temperature) of ~0.15 K over 1 second integration, and <2 K calibration uncertainty.
IceCube is scheduled for launch and subsequently release from the ISS (International Space Station) in mid-2016 for a nominal operation of 28+ days. In addition to its technology demonstration, IceCube will acquire the first 874 GHz cloud map from cloud-induced scattering radiance, thereby accelerating scientific exploration through efficient and frequent access to space using CubeSats. The receiver technology used here was initially developed by VDI (Virginia Diodes Inc.), under NASA's SBIR Phase II program. Communication with the CubeSat will be through NASA's WFF (Wallops Flight Facility) UHF station. Mission Operations and data processing and validation will be conducted at GSFC.
Figure 2: Brightness temperature vs frequency spectrum: CoSSIR measurements of ice clouds were used to successfully demonstrate retrieval of IWP (Ice Water Path) and the ice particle Dme (median mass-equivalent diameter). The 874 GHz data proved to have the greatest sensitivity to ice (image credit: NASA)
Figure 3: The first ever 874 GHz cloud measurements acquired by CoSSIR in 2008 (image credit: NASA)
The new 874 GHz radiometer of IceCube is based on a previous design for the airborne instrument CoSSIR ( Compact Scanning Submillimeter-wave Imaging Radiometer), a 12 channel (183 - 874 GHz) total power imaging radiometer that was mainly developed for the measurements of ice clouds.
Figure 4: Schematic view of sub-mm wave wave radiometry for ice cloud remote sensing in the upper atmosphere (image credit: NASA, Ref. 1)
Ice cloud scattering at sub-mm waves:
• Higher sensitivity to cloud scattering at sub-mm waves
• Cloud-induced radiance, Tcir, is proportional to CIWP (Cloud Ice Water Path)
• Cloud microphysical properties (i.e., particle size) from different frequencies
• Simultaneous retrievals with T, H2O.
Figure 5: Schematic of cloud-induce radiance (image credit: NASA)
Key performance parameters of the IceCube radiometer are shown in Table 1. The RF (Radio Frequency) receiver is comprised of an offset parabola reflector with feedhorn, mixer, stable oscillator, RF multiplier chain, IF (Intermediate Frequency) chain, video amplifier, and detector. There are also supporting circuit boards including the iPDU (instrument Power Distribution Unit) and C&DH (Command and Data Handling), which is shared with the spacecraft. The radiometer will have a noise figure of 15 dB with an NEDT of ~0.15 K for a 1 second dwell time. The instrument is both externally and internally calibrated using views of deep space and an internal IF noise source and reference state.
Table 1: Key IceCube Radiometer parameters
Figure 6: Simplified radiometer block diagram (image credit: NASA)
The radiometer simplified block diagram is shown in Figure 6 and the instrument layout is illustrated in Figure 7 . The radiometer front-end is comprised of an 874 GHz LO (Local Oscillator). Intermediate frequency (6-12 GHz) calibration by noise injection provides the means of discriminating the calibration state of front-end components, referenced to extended observations of space. The RF input to the mixer is a GSFC-designed antenna, which is a straightforward offset-fed paraboloid yielding a 1.7º half-power beam-width. At nadir, the main beam will cover a ~10 to 20 km 3 dB footprint for an orbital altitude in the range of 400-800 km, respectively. With a ground track velocity of approximately 7 km/s, a 1 second output sampling period will provide 0.7 to 1.4 times the Nyquist sampling rate of the antenna mainbeam.
Figure 7: IceCube radiometer layout (image credit: NASA)
Calibration of the radiometer is achieved by both internal electronic and external natural target means. Externally, the primary target is space, which is viewable by pointing the antenna beam above the Earth's limb and provides the absolute offset of the system. Internal calibration of the receiver is carried out by the IF stage, which is used during and between external views of space. The noise source coupled into the IF path is used to estimate IF section gain. An illustration of the vehicle observations over an orbit is shown in Figure 8.
• In-flight operation 28 days
• Periodical views of Earth (science) and space (calibration) within an orbit
• Science data 30+% (8+h /day)
• Pointing knowledge < 25 km.
Figure 8: Typical operations over one orbit, with alternate Earth/Space views for calibration (image credit: NASA)
Figure 9: IceCube challenge No 1: 874-GHz radiometric calibration (image credit: NASA)
Figure 10: Spinning CubeSat: In-flight calibration (image credit: NASA)
IceCube challenge No 2: Large orbital thermal variations
- Preferred instrument operations temperatures: 20-30ºC
- Low CubeSat/instrument mass, or thermal inertia, for thermal stability
- Wide range of β-angles: ±75°
- Day-on and night-off operations
- Spin around the Sun-pointing axis.
Figure 11: Daily average temperature variations as measured by the CSSWE (Colorado Student Space Weather Experiment) mission (image credit: Colorado University, NASA)
The IceCube mission is to demonstrate and space-qualify a commercially available 874 GHz submillimeter-wave receiver developed by VDI (Virginia Diodes Inc.), of Charlottesville, Virginia. This instrument is capable of providing an accurate assessment of the distribution of atmospheric ice.
The 1.3U instrument is accommodated within a 3U CubeSat, with internal volume and mass margins adequate to fit within the required CubeSat specifications standards (CubeSat Design Specification Rev. 12, Cal Poly SLO). The CubeSat bus uses COTS components with proven flight heritage (Figure 12). The ADCS (Attitude Determination and Control Subsystem) and UHF antenna are attached to the end opposite the radiometer. The overall system length is 340.5 mm, including 6.5 mm posts on both ends, with a 100 mm by 100 mm body frame. The mass of the nanosatellite is ~4kg.
IceCube is a 3U customized CubeSat based on the model built by Pumpkin Inc. (with solar panels and batteries from Clyde Space and ADCS and GPS from Blue Canyon). Only one bus system card will be custom manufactured at GSFC, and is necessary to provide a data interface to the instrument and other bus components. The electrical block diagram in shown in Figure 13.
Figure 12: Illustration of the IceCube nanosatellite in flight configuration (image credit: NASA)
Figure 13: Block diagram with custom payload and spacecraft interface card (image credit: NASA)
Launch: A NASA CSLI (CubeSat Launch Initiative) launch of IceCube is scheduled as a secondary payload on the ISS logistics mission of Orbital ATK (Cygnus OA-7 , also known as CRS-7) in March 2017 and deployment with a NRCSD (NanoRacks CubeSat Deployer) from the ISS. The launch vehicle is Atlas-5 401 of ULA and the launch site is Cape Canaveral (SLC-41), FL. The total mass of the Cygnus cargo is ~3500 kg. 5)
Orbit: Near-circular orbit, altitude of ~ 400 km, inclination of 51.6º (β angle variation: 0-75º).
Secondary payloads (CubeSats): 6)
• IceCube, a NASA/GSFC 3U CubeSat technology demonstration mission.
• HARP (HyperAngular Rainbow Polarimeter), a 3U CubeSat of UMBC (University of Mayland, Baltimore County)
• CSUNSat-1, a 2U CSUN (CubeSat of California State University Northridge).
• CXBN-2 (Cosmic X-Ray Background-2), a 2U CubeSat of Morehead State University, Morehead, Kentucky.
• OPEN (Open Prototype for Educational NanoSats), a 1U CubeSat of UND (University of North Dakota).
• Violet, a 1U CubeSat of Cornell University, Ithaca, N.Y.
• QB50 x 40. Forty CubeSats of the international QB50 constellation, a European FP7 (7th Framework Program) Project for Facilitating Access to Space and managed by the Von Karman Institute for Fluid Dynamics in Brussels, will be flown to the ISS for subsequent deployment. 7)
Figure 14: IceCube communication system at NASA/GSFC Wallops Flight Facility (WFF), image credit: NASA, Brian Corbin
Figure 15: IceCube operations (image credit: NASA)
1) D. L. Wu, J. Esper, N. Ehsan, T. E. Johnson, W. R. Mast, J. R. Piepmeier, P. E. Racette, "IceCube: Spaceflight Validation of an 874 GHz Submillimeter Wave Radiometer for Cloud Ice Remote Sensing," ESTF 2014 (Earth Science Technology Forum), Leesburg, VA, USA, Oct. 28-30, 2014, URL: http://esto.nasa.gov/forum/estf2014/presentations/B1P5_Wu.pdf
2) Lori Keesey, "NASA's IceCube No Longer On Ice," NASA, July 30, 2014, URL: http://www.nasa.gov/content/goddard/nasas-icecube-no-longer-on-ice/#.VMpRnC7-b-Y
3) Jaime Esper, Dong Wu, Jeffrey Piepmeier, Negar Ehsan, Paul Racette, "Ice-Cube: Spaceflight Validation of an 874 GHz Sub-millimeter Wave Radiometer for Ice Cloud Remote Sensing," 10th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 20-24, 2015, paper: IAA-B10-0303, URL of presentation: http://www.dlr.de/iaa.symp/Portaldata/49/Resources/dokumente/archiv10/pdf/0303.pdf
4) K. Franklin Evans, James R. Wang, Paul E. Racette, Gerald Heymsfield, Lihua Li, "Ice Cloud Retrievals and Analysis with the Compact Scanning Submillimeter Imaging Radiometer and the Cloud Radar System during CRYSTAL FACE," Journal of Applied Meteorology and Climatology, Volume 44, Issue 6,June 2005), pp: 839–859, URL: http://journals.ametsoc.org/doi/pdf/10.1175/JAM2250.1
5) Jeff Foust, "Orbital to launch next Cygnus mission on Atlas 5," Space News, Nov. 4, 2016, URL: http://spacenews.com/orbital-to-launch-next-cygnus-mission-on-atlas-5/
6) "United States Commercial ELV Launch Manifest," Dec. 28, 2016, URL: http://www.sworld.com.au/steven/space/uscom-man.txt
7) Davide Masutti, "QB50-ISS CubeSats ready to be launched," Dec. 9, 2016, URL: https://www.qb50.eu/index.php/news/78-qb50-iss-ready-to-be-launched
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 (email@example.com).