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)
ICIR (IceCube Cloud–Ice Radiometer)
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. 5)
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 ~4 kg.
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: IceCube was launched as a secondary payload on the ISS logistics mission of Orbital ATK (Cygnus OA-7 , also known as CRS-7) on April 18, 2017 for later deployment with a NRCSD (NanoRacks CubeSat Deployer) from the ISS. The launch vehicle was Atlas-5 401 of ULA and the launch site was Cape Canaveral (SLC-41), FL. 6) 7)
Orbit: Near-circular orbit, altitude of ~ 400 km, inclination of 51.6º (β angle variation: 0-75º).
Secondary payloads (CubeSats): 8)
• Altair-1, a 6U CubeSat technology demonstration mission of Millennium Space Systems, El Segundo, CA, USA.
• 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.
• Biarri-Point, a 3U CubeSat technology mission, a four nation defence related project involving Australia, the US, the UK and Canada. Biaari is an RF signal collection mission that can be related to the spot beam mapping mission through mutual use of GPS signals. 9) 10)
• QB50 x 28. Twentyeight 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, Belgium, will be flown to the ISS for subsequent deployment (atmospheric research). The 28 CubeSats of the QB50 constellation were integrated into 11 NanoRacks 6U deployers. 11) 12)
In addition, four Lemur-2 satellites, operated by Spire Global Inc. of San Francisco, were launched aboard the Cygnus OA-7 cargo craft to replenish and expand the company's constellation dedicated to obtaining global atmospheric measurements for operational meteorology and tracking ship traffic across the planet for various commercial applications. The four Lemur-2 CubeSats are mounted externally to the cargo ship. After Cygnus departs the station in July, it will climb to a higher altitude, around 500 km, and eject them into space.
Figure 14: IceCube communication system at NASA/GSFC Wallops Flight Facility (WFF), image credit: NASA, Brian Corbin
Figure 15: IceCube operations (image credit: NASA)
Figure 16: An artist's rendition of the IceCube small satellite in orbit (image credit: NASA)
• May 16, 2018: During a year in orbit, the IceCube ICIR (IceCube Cloud–Ice Radiometer) instrumentation has created a global map of ice clouds around the planet, which could someday help improve models and forecasts. 13)
- Deployed from the ISS in May 2017, IceCube is testing instruments for their ability to make space-based measurements of the small, frozen crystals that make up ice clouds. "Heavy downpours originate from ice clouds," said Dong Wu, IceCube principal investigator at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
- Ice clouds start as tiny particles high in the atmosphere. Absorbing moisture, the ice crystals grow and become heavier, causing them to fall to lower altitudes. Eventually, the particles get so heavy, they fall and melt to form rain drops. The ice crystals may also just stay in the air.
- Like other clouds, ice clouds affect Earth's energy budget by either reflecting or absorbing the Sun's energy and by affecting the emission of heat from Earth into space. Thus, ice clouds are key variables in weather and climate models.
- Measuring atmospheric ice on a global scale remains highly uncertain because satellites have been unable to detect the amount of small ice particles inside the clouds, as these particles are too opaque for infrared and visible sensors to penetrate. To overcome that limitation, IceCube was outfitted with a submillimeter radiometer that bridges the missing sensitivity between infrared and microwave wavelengths (Ref. 14).
- Despite its mass of ~ 4 kg and being about size of a loaf of bread, IceCube is a bona fide spacecraft, complete with three-axis attitude control, deployable solar arrays and a deployable UHF communications antenna. The CubeSat spins around its axis, like a plate spinning on a pole. It points at Earth to take a measurement then looks at the cold space to calibrate.
- Originally a 30-day technology-demonstration mission, IceCube is still fully operational in low-Earth orbit just about a year later, measuring ice clouds and providing data that's "good enough to do some real science," Wu said.
- "The hard part about developing the CubeSat is making the commercial parts durable in space," said Tom Johnson, Goddard's Small Satellite manager stationed at NASA's Wallops Flight Facility in Virginia. "We bought commercial components for IceCube and spent a lot of time testing the components making sure each part worked."
- Over the past year, engineers tested the satellite's limits while on orbit. They wanted to see if the instrument's batteries stored enough power to run 24 hours. IceCube charges its batteries when the Sun shines on its solar arrays. During the test, safeguards prevented the satellite from losing all its power and ending the mission; however, the test was successful. The batteries operated the IceCube all night and recharged during the day. This change made the CubeSat more valuable for science data collection.
- While the IceCube team planned for the mission to operate for 30 days in space, "It does not cost very much to keep it going," Johnson said, "so we extended the mission due to the outstanding science that IceCube is performing. We download data eight to 10 times a week. Even if we miss a week, the CubeSat can hold a couple of weeks of data."
- The IceCube team built the spacecraft using funding from NASA's Earth Science Technology Office's (ESTO) In-Space Validation of Earth Science Technologies (InVEST) program and NASA's Science Mission Directorate CubeSat Initiative.
- Small satellites, including CubeSats, are playing an increasingly larger role in exploration, technology demonstration, scientific research and educational investigations at NASA. They have been used in planetary space exploration, fundamental Earth and space science, and developing precursor science instruments like cutting-edge laser communications, satellite-to-satellite communications and autonomous movement capabilities.
• January 30, 2018: The IceCube 3U CubeSat has produced the world's first map of the global distribution of atmospheric ice in the 883 GHz band, an important frequency in the submillimeter wavelength for studying cloud ice and its effect on Earth's climate. 14) 15)
- IceCube has demonstrated-in-space a commercial 883 GHz radiometer developed by VDI (Virginia Diodes Inc.) of Charlottesville, Virginia, under a NASA Small Business Innovative Research contract. It is capable of measuring critical atmospheric cloud ice properties at altitudes between 5 -15 km.
- NASA scientists pioneered the use of submillimeter wavelength bands, which fall between the microwave and infrared on the electromagnetic spectrum, to sense ice clouds. However, until IceCube, these instruments had flown only aboard high-altitude research aircraft. This meant scientists could gather data only in areas over which the aircraft flew.
- "With IceCube, scientists now have a working submillimeter radiometer system in space at a commercial price," said Dong Wu, a scientist and IceCube principal investigator at NASA/GSFC (Goddard Space Flight Center) in Greenbelt, Maryland. "More importantly, it provides a global view on Earth's cloud-ice distribution."
Figure 17: IceCube Principal Investigator Dong Wu set out to demonstrate a commercial 883 GHz radiometer in space, but ended up getting much more: the world's first ice-cloud map in that frequency. Here he is pictured holding the instrument (image credit: NASA)
- Sensing atmospheric cloud ice requires scientists deploy instruments tuned to a broad range of frequency bands. However, it's particularly important to fly submillimeter sensors. This wavelength fills a significant data gap in the middle and upper troposphere where ice clouds are often too opaque for infrared and visible sensors to penetrate. It also reveals data about the tiniest ice particles that can't be detected clearly in other microwave bands.
- The Technical Challenge: "IceCube's map is a first of its kind and bodes well for future space-based observations of global ice clouds using submillimeter-wave technology," said Wu, whose team built the spacecraft using funding from NASA's ESTO (Earth Science Technology Office) In-Space Validation of Earth Science Technologies (InVEST) program and NASA's Science Mission Directorate. The team's challenge was making sure the commercial receiver was sensitive enough to detect and measure atmospheric cloud ice using as little power as possible.
- Ultimately, the agency wants to infuse this type of receiver into an ice-cloud imaging radiometer for NASA's proposed ACE (Aerosol-Cloud-Ecosystems) mission. Recommended by the National Research Council, ACE would assess on a daily basis the global distribution of ice clouds, which affect the Earth's emission of infrared energy into space and its reflection and absorption of the Sun's energy over broad areas. Before IceCube, this value was highly uncertain.
- "It speaks volumes that our scientists are doing science with a mission that primarily was supposed to demonstrate technology," said Jared Lucey, one of IceCube's instrument engineers. Lucey, along with a handful of scientists and engineers at Goddard and the Wallops Flight Facility, developed IceCube in just two years with a limited budget. "We met our mission goals and now everything else is bonus."
- Multiple Lessons Learned: In addition to demonstrating submillimeter-wave observations from space using a commercially available instrument and producing the ice-cloud map, the team gained important insights into how to efficiently develop a CubeSat mission, determining which systems to make redundant and which tests to forgo because of limited funds and a short schedule, said Jaime Esper, IceCube's mission systems designer and technical project manager at Goddard.
- To keep down costs, the team used commercial off-the-shelf components, including, of course, VDI's radiometer. The components came from multiple commercial providers and didn't always work together harmoniously, requiring engineering rework on the part of team members. The team not only integrated the radiometer to the spacecraft, but also built spacecraft ground-support systems and conducted thermal-vacuum, vibration, and antenna testing at Goddard's Greenbelt campus and Wallops' facilities in Virginia.
- "IceCube isn't perfect," Wu conceded, referring to noise or slight errors in the radiometer's data. "However, we can make a scientifically useful measurement. We came away with a lot of lessons learned from this CubeSat project, and next time engineers can build it much more quickly. But under this program, we had to complete the project within a limited budget and schedule, and therefore, we had to take and balance the risks."
- "This is a different mission model for NASA," Wu continued. "Our principal goal was to show that this small mission could be done at NASA. The question was, could we get useful science and advance space technology with a low-cost CubeSat developed under an effective government-commercial partnership? I believe the answer is yes."
- Small satellites, including CubeSats, are playing an increasingly larger role in exploration, technology demonstration, scientific research and educational investigations at NASA, including: planetary space exploration; Earth observations; fundamental Earth and space science; and developing precursor science instruments like cutting-edge laser communications, satellite-to-satellite communications and autonomous movement capabilities.
• August 2017: First-light measurements from ICIR were obtained on June 6 with a regular technology-demonstration mode starting on June 16 for daytime-only observations. The first 883-GHz cloud radiance map from IceCube (Figure 18) covers the period from June 20 through July 2. As of this writing, IceCube continues to operate normally with the CubeSat spinning around the sun vector in daytime. The spin produces periodical views between Earth and space, allowing radiometric calibration of ICIR. 16)
Figure 18: The first map of 883-GHz cloud-induced radiance (Tcir) from the ICIR (IceCube Cloud–Ice Radiometer). Tcir is defined as the difference between observed and modeled clear-sky radiances, and it is roughly proportional to cloud ice amount above ~11 km (Tcir is negative because cloud scattering acts to reduce the upwelling radiation at submm-wave frequencies), image credit: NASA's ICECube team
• On May 16-17, 2017, NanoRacks began the first of two airlock cycles for the 11th and 12th NanoRacks CubeSat Deployer Missions (NRCSD-11, NRCSD-12). NRCSD-11 and NRCSD-12 were brought to the International Space Station on the Orbital ATK-7 mission, which launched on April 18, 2017 from the Kennedy Space Center in Cape Canaveral, Florida. This launch was the largest CubeSat mission of NanoRacks to date, bringing 34 satellites into the Space Station, plus four CubeSats mounted on externally on the Cygnus spacecraft. 17)
A total of 17 CubeSats were deployed on NRCSD-11 by the NanoRacks Team:
- 11 CubeSats of the QB50 mission were deployed in this first airlock cycle:
SOMP2 – TU Dresden, Germany
- 3 CubeSats were deployed of the NASA ELaNa XVII Sponsored CubeSats:
- Additionally, 3 CubeSats were deployed:
Figure 19: In this photo taken by NASA astronaut Peggy Whitson from inside the International Space Station cupola, the NanoRacks deployer (foreground) is clearly visible as the CXBN-2 and IceCube CubeSats deploy (image credit: NASA)
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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.