CubeRRT (CubeSat Radiometer Radio Frequency Interference Technology) Validation Mission
A NASA team at GSFC (Goddard Space Flight Center) in Greenbelt, Maryland, is collaborating with OSU (Ohio State University) and NASA/JPL (Jet Propulsion Laboratory) in Pasadena, California, to build and launch a new CubeSat mission that will test next-generation techniques for detecting and discarding RFI (Radio Frequency Interference). Funded by NASA's InVEST (In-Space Validation of Earth Science Technologies) program, the CubeRRT project specifically will evaluate a specialized digital-based spectrometer equipped with sophisticated algorithms that can detect and mitigate the radio interference that spills over and ends up as noise in scientific data. 1)
Goddard is charged with developing the instrument's front-end microwave electronics and overseeing the instrument's integration onto the spacecraft. JPL, meanwhile, is building the instrument's backend digital electronics. The Wallops Flight Facility on Virginia's Eastern Shore is handling ground-system design and operations, while Ohio State's Joel Johnson is leading this effort. In addition, Ohio State is implementing the dual-helical antenna and procuring the spacecraft bus from the Boulder, Colorado-based BCT (Blue Canyon Technologies). 2) 3)
RFI is a real problem for Earth science observations. To that end Blue Canyon Technologies will build the CubeRRT for Ohio State University to provide more accurate measurements and radiometers. The goal is to identify and mitigate RFI for spaceborne microwave radiometers. As spectrum usage has increased by the communications industry, RFI has become a problem for science missions. CubeRRT will build on the success of RFI mitigation techniques developed for NASA's SMAP (Soil Moisture Active/Passive) mission, but at higher frequencies, 6 - 40 GHz, instead of SMAP's 1.4 GHz. Mitigation at these higher frequencies enables scientists to continue using radiometry for high quality Earth science measurements. 4) 5)
Background: The cacophony of signals constantly at play on Earth - from radio traffic to cell phones and other communications systems – ultimately creates the white noise of human technology in action. In space, however, this cacophony is getting in the way of some important Earth science research - and it's only getting worse as human technology advances. Joel Johnson, chair and professor of the Department of Electrical and Computer Engineering at OSU, is now leading a NASA program to help navigate the noise. Since 2001, his team at OSU has focused on detecting and discarding man-made RFI from the Earth's naturally fluctuating microwave signals. The technology is imperative for future satellite missions using microwave radiometry to observe Earth's properties. 6)
Johnson's team contributed similar technologies for NASA's SMAP (Soil Moisture Active Passive) satellite, which launched in January 2015. SMAP is now helping scientists create the most detailed global maps of soil moisture to date, improving understanding of Earth's water and carbon cycles, as well as the ability to manage water resources.
RFI subsystems for higher frequency microwave radiometry over the range 6-40 GHz, however, require a larger bandwidth, so that the capabilities of RFI mitigation backends in terms of bandwidth and processing power must also increase. To date, no such wideband subsystem has been demonstrated in space for radiometers operating above 1413 MHz.
The enabling technology is a digital FPGA (Field Programmable Gate Array)-based spectrometer with a bandwidth of 1 GHz or more and capable of implementing advanced RFI mitigation algorithms such as the Kurtosis and cross-frequency methods. This technology has a strong ESTO heritage, with the algorithms developed and demonstrated via the IIP (Instrument Incubator Program) and wideband backends developed under other ESTO support. The digital backend is currently at TRL 5, having been successfully tested in an RFI environment, and can be ported easily to a flight-ready firmware. Though the technology can be demonstrated for any frequency band from 1 to 40GHz, the CubeRRT project will integrate the backend with a wideband radiometer operating over a 1 GHz bandwidth tunable from 6-40 GHz to demonstrate RFI detection and mitigation in important microwave radiometry bands. Along with a wideband dual-helical antenna, the payload will be integrated with a 6U CubeSat to demonstrate operation of the backend at TRL 7. The payload is expected to operate at a minimum duty-cycle of 25% to be compatible with spacecraft power capacity. Although the spatial resolution to be achieved will be coarse (due to the limited antenna size possible), the goal of demonstrating observation, detection, and mitigation of RFI is achievable in this configuration.
Figure 1 illustrates the 6-40 GHz portion of the spectrum, along with the frequency ranges used in several past radiometer missions; the sensitivity to environmental effects in each of these frequencies is also shown in the lower curves. Passive microwave observations are allocated primary-use only in a small number of bands (those shown as green vertical bars), with shared-use in those marked in yellow. Due to the high sensitivity of radiometer measurements, shared allocations. offer little protection from RFI corruption, as amply demonstrated in numerous past missions. As active sources expand over larger areas and occupy additional spectrum, it will be increasingly difficult to perform radiometry without an RFI mitigation capability. Co-existence in some cases may be possible provided that a subsystem for mitigating RFI is included in future systems (Ref. 5).
Figure 1: Frequency ranges allocated for microwave radiometer observations (green=primary, yellow=shared) in the 6-40 GHz range. Spectral ranges used by several past missions also indicated, as well as curves of sensitivity to environmental parameters vs. frequency (CubeRRT collaboration)
Objectives of the CubeRRT mission:
• Demonstrate wideband RFI (Radio Frequency Interference) mitigating backend technology for future spaceborne microwave radiometers operating 6 to 40 GHz
• Crucial to maintain US national capability for spaceborne radiometry and associated science goals
• Demonstrate successful real-time on-board RFI detection and mitigation in 1 GHz instantaneous bandwidth
• Demonstrate reliable CubeSat mission operations, include tuning to EESS (Earth Exploration Satellite Service) allocated bands in the 6-40 GHz region.
• Build upon heritage of airborne and spaceborne (SMAP) digital backends for RFI mitigation in microwave radiometry
• Apply existing RFI mitigation strategies onboard spacecraft; downlink additional RFI data for assessment of onboard algorithm performance
• Integrate radiometer front end, digital backend, and wideband antenna systems into 6U CubeSat
• NASA CSLI (CubeSat Launch Initiative) launch from the ISS into 400 km orbit; ~ 120-300 km Earth footprint for RFI mitigation validation
• Operate for one year at 25% duty cycle to acquire adequate RFI data.
Table 1: Challenges in current and future missions
CubeRRT is a 6U CubeSat of size 20 cm x 30 cm x 10 cm to be built by BCT (Blue Canyon Technologies) of Boulder, CO for OSU. BCT will integrate the CubeRRT payloads with the 6U spacecraft bus and perform environmental testing of the complete spacecraft. The spacecraft will be operated from BCT's Mission Operations Center located in Boulder, Colorado. BCT's 6U spacecraft is a high-performance CubeSat that includes an ultra-precise attitude control system that allows for accurate knowledge and fine-pointing of the satellite payload (Ref. 4).
Table 2: Summary of CubeRRT properties
Figure 2: Illustration of CubeRRT on-orbit configation (image credit: CubeRRT Team)
CubeRRT is expected to operate with a duty-cycle of 30%. CubeRRT will mostly be operational over landmasses where the occurrence of RFI is expected to be significantly higher. CubeRRT is designed for 12 months of commissioning and operations. Once the CubeRRT mission has met all of its validation goals, the proven technology can be included in NASA's future radiometry missions to ensure they can continue to collect high-quality Earth-observing data – data that will help researchers answer scientific questions about our planet and better understand our home.
Figure 3: Artist's view of the deployed CubeRRT satellite (image credit BCT)
Figure 4: The CubeRRT satellite and Blue Canyon Technologies team members with Principal Investigator Joel Johnson (far left) of The Ohio State University (image credit: Blue Canyon Technologies) 7)
Launch: The CubeRRT satellite was launched on 21 May 2018 (08:44 UTC) on the Cygnus CRS-9 flight of Orbital ATK (OA-9E), ELaNa-23 flight of NASA to the ISS. The launch vehicle was Antares 230 and the launch site was MARS (Mid-Atlantic Regional Spaceport) LP-0A, Wallops Island, VA, USA. 8)
Orbit: Near circular orbit, altitude of ~400 km, inclination = 51.6º.
The ELaNa 23 (Education Launch of Nanosatellites 23) initiative payloads of NASA on OA-9 are: 9)
• HaloSat (Soft X-ray Surveyor), a 6U CubeSat of the University of Iowa, Iowa City, Iowa.
• TEMPEST-D1 (Temporal Experiment for Storms and Tropical Systems Technology - Demonstration 1) , a 6U CubeSat of CSU (Colorado State University), Fort Collins, CO.
• EQUISat, a 1U CubeSat of Brown University, Providence, R.I.
• MemSat, a 1U CubeSat of Rowan University, Glassboro, N.J.
• CaNOP (Canopy Near-IR Observing Project), a 3U CubeSat of Carthage College, Kenosha, WIS, USA.
• RadSat, (Radiation-tolerant SmallSat Computer System), a 3U CubeSat of MSU (Montana State University), Bozeman, Montana.
• RaInCube (Radar In a CubeSat), a 6U CubeSat of NASA/JPL (Jet Propulsion Laboratory), Pasadena, CA.
• SORTIE (Scintillation Observations and Response of the Ionosphere to Electrodynamics), a 6U CubeSat of ASTRA (Atmospheric & Space Technology Research Associates), Boulder, CO.
• CubeRTT (CubeSat Radiometer Radio Frequency Interference Technology) Validation Mission , a 6U CubeSat of OSU (Ohio State University), Columbus, Ohio.
• AeroCube-12A and -12B, a pair of 3U CubeSats of the Aerospace Corporation, El Segundo , CA, to demonstrate a the technological capability of new star-tracker imaging, a variety of nanotechnology payloads, advanced solar cells, and an electric propulsion system on on one of the two satellites (AC12-B).
• EnduroSat One, a 1U CubeSat of Bulgaria, developed by Space Challenges program and EnduroSat collaborating with the Bulgarian Federation of Radio Amateurs (BFRA) for the first Bulgarian Amateur Radio CubeSat mission.
The CubeRRT payload has three critical pieces of technology, a wideband antenna unit, a radiometer front-end (RFE) unit, and a radiometer digital back-end (RDB) that performs the on-board detection and filtering of RFI. The main objective of the CubeRRT mission is to demonstrate the RFI mitigation technology on a flight-ready hardware in space, increasing the technology readiness level (TRL) from 6 to 7. CubeRRT is designed to make wideband measurements over the whole 6 to 40 GHz range, but the prime mission objective is to demonstrate RFI mitigation over ten "golden" frequency bands that are allocated to Earth observation bands. The Ohio State University leads the CubeRRT mission. The algorithm validating back end technology is built at Jet Propulsion Laboratory, California Institute of Technology and the radiometer front end is built at NASA Goddard Space Flight Center. 10)
The dual-element antenna system is composed of a conical antenna (three circularly polarized tapered helical antennas, 6-40 GHz). The antennas are being designed, developed and tested at The Ohio State University. The series of antenna are necessary to provide sufficient gain over a wide range of frequencies from 6 to 40 GHz. The current design provides a gain of 12 dBi at 6 GHz and 21 dBi at 40 GHz.
The CubeRRT RFE (Radiometer Front-End) is designed to sweep from 6 to 40 GHz with a 1 GHz bandwidth being injected into the RDB. The radiometer is a single tunable superheterodyne receiver. At the front-end of the radiometer the RFE has a four-position switch to choose between the three helical antennas as well as a reference load for calibration. The RFE contains front-end wideband coaxial low noise amplifiers for pre-amplification. The RFE also contains a coupled wideband noise-source to for full internal calibration of the radiometer. The RFE achieves frequency tuning via a phased-locked oscillator (PLO) and sub-harmonic image rejection (IR) mixer. The design allows flexibility between choosing upper and lower side-bands to completely cover the 6 to 40 GHz regime. The architecture sacrificed radiometer performance to meet within the size, mass and power requirements of the 6U system.
The RDB (Radiometer Digital Back-end ) of NASA is designed to digitize a 1 GHz bandwidth signal and perform advanced digital signal processing algorithms on an on-board FPGA for RFI mitigation. The RDB ADC is capable of ingesting the IF signal produced by the RFE from 1-2 GHz aliased region. The FPGA proceeds to produce a 128 frequency spectra of the incoming signal using a front-end polyphase filter-bank. The output of the 128 channel spectra then undergoes gain adjustment to account for the non-uniform pass-band shape of the RFE signal. The higher order statistical moments of the data per channel are calculated (2nd and 4th moment) as a pre-cursor to RFI detection and mitigation. The second moment is uncalibrated power of the signal. The first RFI detection algorithm applied is a simple threshold detection algorithm across the spectra to detect frequency outliers. The second RFI detection algorithm is more advanced and uses the fourth and second moments to calculate kurtosis of the signal as a test of normality. Any signal that deviates from normality is flagged as being corrupted by RFI. The flags of the two algorithms are combined and the power in each frequency bin is summed to produce mitigated and unmitigated accumulated power. CubeRRT will downlink all 128 channels to verify performance of mitigated and unmitigated uncalibrated power outputs. Most of the thresholds, coefficients, gain-adjustment values within the RDB are updatable from the user perspective.
NanoRacks CubeSats are delivered to the International Space Station (ISS) already integrated within a NRCSD (NanoRacks CubeSat Deployer) or NanoRacks DoubleWide Deployer (NRDD). A crew member transfers each NRCSD/NRDD from the launch vehicle to the Japanese Experiment Module (JEM). Visual inspection for damage to each NRCSD is performed. When CubeSat deployment operations begin, the NRCSD/NRDDs are unpacked, mounted on the JAXA Multi-Purpose Experiment Platform (MPEP) and placed on the JEM airlock slide table for transfer outside the ISS. A crew member operates the JEM Remote Manipulating System (JRMS) – to grapple and position for deployment. CubeSats are deployed when JAXA ground controllers command a specific NRCSD. 11)
CubeRRT operations are commanded based on a scheduler simulation tool developed by OSU (Ohio State University). This tool may be used to develop algorithms for power cycling and frequency tuning, which propagates over the orbital lifetime to predict and optimize CubeRRT measurement results. The scheduler provides information such as the duration until mission-level requirements are fulfilled, radiometry coverage maps, long-term battery depth-of-discharge (DOD), and payload data. In addition, the scheduler may be used to automate the process of generating payload command sequences uplinked regularly to the spacecraft during operations. To properly model CubeRRT's operations, the scheduler simulates the power system, telemetry, RFI coordinates, and orbital propagation models. The power system state is modelled with knowledge of the available energy from CubeRRT's solar cells, the known power draw from the satellite bus and payload subsystems, and the battery capacity. Telemetry and payload data buffers are monitored and downlinked at a known rate to the Wallops Flight Facility to predict and prevent buffer overflow conditions.
1) "Four Projects Awarded Funding Under the In-Space Validation of Earth Science Technologies (InVEST) Program," NASA, 2015, URL: https://esto.nasa.gov/files/solicitations/INVEST_15/ROSES2015_InVEST_A42_awards.html
2) Lori Keesey, "NASA's next CubeSat mission to test ways of detecting and discarding Radio-Frequency Interference," Feb. 24, 2016, URL: http://www.clarksvilleonline.com/2016/02/24/nasas-next-cubesat-mission-to-test-ways-of-detecting-and-discarding-radio-frequency-interference/
3) "CubeRRT approved for launch," Feb. 23, 2016, URL: https://ece.osu.edu/news/2016/02/cuberrt-approved-launch
4) "Blue Canyon Technologies (BCT) selected by the Ohio State University to provide spacecraft for CubeRRT mission," BCT, April 12, 2016, URL: http://bluecanyontech.com/blue-canyon-technologies-selected-by-the-ohio-state-university-to-provide-spacecraft-for-cuberrt-mission/
5) J. T. Johnson, C. C. Chen, A. O'Brien, G. E. Smith, C. McKelvey, M. Andrews, C. Ball, Sidharth Misra, Shannon Brown, Jonathan Kocz, Robert Jarnot, Damon C. Bradley, Priscilla N. Mohammed, Jared F. Lucey, Jeffrey R. Piepmeier, "The CubeSat Radio Frequency Interference Technology Validation (CubeRRT) Mission," Proceedings of the IEEE IGARSS (International Geoscience and Remote Sensing Symposium) Conference, Beijing, China, July 10-15, 2016,
7) "Small Packages to Test Big Space Technology Advances," NASA, 17 May 2018, URL: https://www.nasa.gov/feature/small-packages-to-test-big-space-technology-advances
8) "NASA Sends New Research on Orbital ATK Mission to Space Station," NASA/JPLRelease 18-037, 21 May 2018, URL: https://www.jpl.nasa.gov/news/news.php?feature=7134
9) "Upcoming ELaNa CubeSat Launches," NASA CubeSat Launch Initiative, URL: https://www.nasa.gov/content/upcoming-elana-cubesat-launches
10) Sidharth Misra, C. Ball, J. T. Johnson, C. C. Chen, A. O'Brien, G. Smith, C. McKelvey, M. Andrews, L. Garry, R. Bendig, C. Felten, S. T. Brown, R. Jarnot, J. Kocz, D. Bradley, P. Mohammed, J. Lucey, K. Horgan, M. Fritts, Q. Bonds, C. Duran-Aviles, M. Solly, J. R. Piepmeier, D. Laczkowski, M. Pallas, E. Krauss, "The CubeSat Radiometer Radio Frequency Interference Technology Validation (CUBERRT) Mission," 98th American Meteorological Society Annual Meeting, Austin, TX, USA, 8-11 January, 2018, URL: https://ams.confex.com/ams/98Annual/webprogram/Paper336562.html
11) "CubeSat Radiometer Radio Frequency Interference Technology Validation (CubeRRT)," NASA News, 14 March, 2018, URL: https://www.nasa.gov/mission_pages/station/research/experiments/2575.html
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).