RaInCube (Radar In a CubeSat) - A Precipitation Profiling Mission
The NASA/JPL project RaInCube (also spelled as RainCube) is proposing a nanosatellite constellation architecture capable of flying a miniaturized profiling radar instrument. The overall goal is to observe the short-time evolution of weather processes, which is necessary to validate and improve the current assumptions and skills of numerical weather models. 1) 2)
Radar instruments have often been regarded as unsuitable for small satellite platforms due to their traditionally large size, weight, and power. The JPL (Jet Propulsion Laboratory) has developed a novel radar architecture compatible with the 6U form factor. The RainCube mission will validate two key technologies in the space environment – a miniaturized Ka-band precipitation profiling radar that occupies ~3U and a 0.5 m Ka-band deployable parabolic antenna stowed within 1.5U. The spacecraft bus is developed by Tyvak Nanosatellite Systems of Irvine CA, responsible for integration and test of the flight system and mission operations. RainCube is funded through the NASA Science Mission Directorate's Research Opportunities in Space and Earth Science 2015 In-Space Validation of Earth Science Technologies solicitation with the goal of raising the instrument TRL (Technology Readiness Level) to 7.
Some background: Precipitation profiling capabilities were pioneered by the PR (Precipitation Radar) instrument of TRMM (Tropical Rainfall Measurement Mission) and further advanced by CloudSat's CPR (Cloud Profiling Radar) and the GPM/DPR (Global Precipitation Measurement Mission/Dual frequency Precipitation Radar) of NASA/JAXA. However, currently on ly 3 of these instruments are deployed in LEO. As such, their high quality observations are sparse in time with respect to the typical time-scale of weather phenomena (tens of seconds to hours). These missions therefore are generally unable to observe the short-time evolution of weather processes, which is necessary to validate and improve the current assumptions and skills of numerical weather models.
The synergy with larger numbers of wide-swath microwave radiometers and other passive sensors mitigated this observational gap, but only to the extent of coarse vertical profiling and increased uncertainties (especially over land and ice) which do not allow to actually observe large part of the processes driving the evolution of many types of weather systems. Projects to advance the technologies enabling the deployment of precipitation radars in GEO have been successful on several fronts, but the technological state of the art for very large lightweight deployable antennas and related cost considerations don't enable this approach quite yet.
The alternative is to deploy several radars in LEO (as convoy or constellation). This has not been realistically affordable for decades until the arrival of the SmallSat and CubeSat platforms, at which time the challenge moved to the capability to simultaneously miniaturize, reduce cost and preserve fundamental performance requirements for this type of radars.
RaInCube is a technology demonstration mission to enable Ka-band precipitation radar technologies on a low-cost, quick-turnaround platform. The proposed mission is to develop, launch, and operate a 35.75 GHz radar payload on a 6U CubeSat. This mission will validate a new architecture for Ka-band radars and an ultra-compact deployable Ka-band antenna in a space environment. RainCube will also demonstrate the feasibility of a radar payload on a CubeSat platform.
In February 2016, ESTO (Earth Science Technology Office) of the NASA InVEST (In-Space Validation of Earth Science Technologies) Program selected four new CubeSat missions for funding: CubeRRT, CIRiS, CIRAS and RaInCube. 3)
Numerical climate and weather models depend on measurements from space-borne satellites to complete model validation and improvements. Precipitation profiling capabilities are currently limited to a few instruments deployed in LEO (Low Earth Orbit), which cannot provide the temporal resolution necessary to observe the evolution of short time-scale weather phenomena and improve numerical weather prediction models. A constellation of precipitation profiling instruments in LEO would provide this essential capability, but the cost and timeframe of typical satellite platforms and instruments make this solution prohibitive. Thus, a new instrument architecture that is compatible with low-cost satellite platforms, such as CubeSats and SmallSats, will enable constellation missions and revolutionize climate science and weather forecasting.
The objective of the mission is to develop, launch, and operate a 35.75 GHz nadir-pointing precipitation profiling radar payload to validate a new architecture for Ka-band radars and an ultra-compact deployable Ka-band antenna design in the space environment. RainCube will also demonstrate the feasibility of a radar payload on a CubeSat platform. The radar payload is the evolution of two previous JPL research and development technologies – the miniaturized Ka-band atmospheric radar (miniKaAR) and the 0.5 m diameter KaPDA (Ka-band Parabolic deployable Antenna).
The key objectives of RaInCube are: 4)
• Demonstrate new radar technologies in Ka-band (35.75 GHz)
• Demonstrate a Ka-band precipitation radar on a 6U CubeSat
• Identify and burn down technical risks for radar payload and CubeSat bus
• Enable precipitation profiling Earth science missions.
Successful demonstration of the RainCube technology may lead to advances in numerical climate and weather models using small satellites like CubeSats. Precipitation profiling capabilities are currently limited to a few instruments deployed on large LEO vehicles. While these instruments are sensitive, they cannot provide the temporal resolution necessary to observe the evolution of weather phenomena at short time-scale and ultimately improve weather models. RainCube is the first demonstration of what could become a constellation of precipitation profiling instruments in small satellite form-factors.
A novel architecture compatible with the 6U class has been developed at JPL (size of 10 x 20 x 30 cm). The key lies in the simplification and miniaturization of the radar subsystems. The RaInCube architecture reduces the number of components, power consumption and mass by over one order of magnitude with respect to the existing spaceborne radars, therefore it opens up a new realm of options for low-cost spacecraft platforms such as CubeSats and SmallSats, with obvious savings not only on the instrument implementation (especially beyond the first unit), but also on the spacecraft and launch costs. In the timeframe 2015, it is now possible to actually consider deploying a constellation of identical copies of the same instrument in various relative positions in LEO to address specific observational gaps left open by the current missions that require high- resolution vertical profiling capability. 5)
JPL has contracted Tyvak to develop the spacecraft bus, integrate the payload, and operate the spacecraft — with JPL involvement for radar commissioning activities and subsequent technical support and guidance. The first month of flight is allocated for the commissioning phase where the vehicle will deploy its solar panels and UHF antennas (seen in Figure 1), undergo subsystem checkouts, and power on and exchange telemetry with the radar in standby mode. About two weeks into the commissioning phase, the radar's Ka-band antenna will be deployed (Figures NO TAG# and 1). This deployment is the mission's only planned critical event. Deployment of the antenna will take about three minutes and could be done during an operator-in-the-loop ground pass. To supplement the antenna's own deployment sensors, Tyvak has integrated a camera on the spacecraft bus to image the antenna during and after deployment. With the antenna deployed, the remaining commissioning time will be used to check out the radar in both receive and transmit modes Ref. 1).
With a healthy spacecraft and payload, RainCube will transition to a one-month payload operations phase. During this phase, the radar will be operated with a 25% transmit-mode (active) duty cycle and with continuous operation in this mode for at least a full, 90-minute orbit. This allows the spacecraft bus to use the next three orbits to recharge the batteries, dissipate payload thermal energy, and downlink payload data and spacecraft telemetry via UHF or S-band. When in transmit-mode, the radar will collect vertical precipitation profiles between 0 and 18 km altitude above Earth's surface, with a horizontal resolution <10 km and a vertical resolution <250 m. Even with onboard compression of the raw measurements, the payload will generate up to 1.73 Gbit of data per day, excluding bus telemetry. Tyvak will downlink and deliver this data to JPL for processing within seven days of onboard collection.
With the 25% transmit-mode duty cycle, there is a 6σ certainty that the mission will fly over precipitation within the first two days of payload operations. Including commissioning and decommissioning, the mission expects to have no more than twelve months of flight time before atmospheric drag causes the spacecraft to reenter into the atmosphere. The radar can continue to collect data down to an altitude of about 310 km, where the CubeSat will be imminently close to reentering the atmosphere.
Figure 1: The RainCube's umbrella-like parabolic mesh antenna deploys out of its 1.5U canister (image credit: NASA/JPL-Caltech)
RainCube is made up of two main sections, the radar payload and the spacecraft bus. The radar payload (shown assembled in Figure 2) consists of the radar electronics, miniKaAR-C (miniaturized Ka-band Atmospheric Radar for CubeSats), and the deployable Ka-band antenna, KaRPDA (Ka-band Radar Parabolic Deployable Antenna). The spacecraft bus is developed using Tyvak's Endeavour avionics platform and provides power, data, and thermal interfaces to the payload.
miniKaAR-C: The miniKaAR-C is demonstrating an enabling radar technology that can fit within a small satellite form factor. Radar instruments are typically not suitable for small satellite platforms due to their large size, weight, and power (SWaP). A novel architecture for a Ka-band precipitation profiling radar has been developed at JPL, the miniKaAR, which reduces the number of components, power consumption, and mass by over an order of magnitude with respect to the existing spaceborne radars and is compatible with the capabilities of low-cost satellite platforms such as SmallSats or CubeSats.
The key enabler to reduce SWaP in miniKaAR is the modulation technique: offset IQ (in-phase and quadrature) with pulse compression. Previous spaceborne cloud and precipitation radars have adopted high power short monochromatic pulses to achieve the required sensitivity with low range sidelobes (to avoid contamination of the tropospheric echoes by the surface response). This requires high-power amplifiers and either high-voltage power supplies or large power-combining networks, precluding small-size/low-power platforms. Pulse compression is used to achieve the required sensitivity with a custom amplifier fabricated with off-the-shelf GaAs solid-state pHEMT chips. Optimal selection of the pulse shape minimizes the range side lobes.
Figure 2: The radar electronics (yellow) and deployable antenna (green) integrated into the flight chassis. The payload has undergone qualification vibration and TVAC testing at JPL (image credit: NASA/JPL RainCube Team)
The digital subsystem consists of a single board that includes low power CMOS DAC (Digital-to-Analog Conversion), ADC (Analog-to-Digital Conversion), telemetry ADC chips providing 24 channels of telemetry, and a single commercial-grade flash-based FPGA performing all control, timing, and OBP (On-Board Processing). The radar OBP algorithm consists of data filtering, range compression, power computation and along-track averaging. Triple mode redundancy is used for all critical functions and most non-critical functions. In addition, rad-hard interlock circuits are used for all critical signals that could result in radar damage in the event of a single-event-upset.
These advances make the miniKaAR-C practical with the inherently limited resources of the CubeSat form factor. When operating in transmit-mode, the radar requires 22 W of power (with up to 10 W peak, 1 W average RF out) and produces a data stream of up to 50 kbit/s. Receive-only and standby modes only consume 10 W and 3 W, respectively, and have lower data rates (<10 kbit/s). Including the antenna, the radar (Figure 2) has a flight mass of 5.5 kg and the bolted interface allows heat to be transferred to thermal radiating faces on the spacecraft bus.
Radar Antenna KaRPDA (Ka-band Radar Parabolic Deployable Antenna): The radar's resolution is directly related to the aperture of the antenna. RainCube is using an antenna that is larger than the spacecraft's longest dimension. KaRPDA) is a 0.5 m antenna that stows in 1.5U. This antenna is optimized for the radar frequency of 35.75 GHz and is measured to produce a gain of 42.6 dBi (over 50% efficiency) in the flight configuration. The antenna uses a Cassegrain architecture as it places the sub-reflector below the focal point of the antenna, allowing the antenna to stow in a tight volume. 6) The design for KaRPDA is shown in Figure 3. The mesh antenna surface is supported by deep ribs, which provide high structural rigidity to stretch the mesh to a precise parabolic shape. These ribs provide another advantage by allowing the hinges to have precision stops located approximately one half inch from the pivot point. This ultimately minimizes the influence of manufacturing tolerances. The tip ribs are tapered near the end, where stiffness is not required, to maximize stowed space. The ribs are connected at the bottom of the root rib to a hub, which also supports the horn and secondary reflector. The hub supports all 30 ribs on the antenna.
Figure 3: Illustration of key KaRPDA components (image credit: NASA/JPL RainCube Team)
To deploy, the hub is driven upwards by four lead screws attached to nuts in the hub, pushing the antenna upwards (images A/B in Figure 4). As the hub begins to reach the top, the spring ring, which is attached to the root rib hinges, catches on a detent in the top of the antennas stowed canister and the ribs begin to deploy (B/C). The tip ribs reach a point where they no longer interfere with the horn and they are deployed by the constant force springs located in the mid rib hinge (C). After the ribs have cleared the horn the sub-reflector is released by a latching feature on the root rib hinges and is held in place by a spring (C/D). The hub continues to travel upwards until the root ribs have fully deployed (D). After the antenna is fully deployed, it is locked in place with the lead screws and the root ribs are preloaded by the spring ring.
Figure 4: The deployment sequence unfurls the 0.5m antenna from a 0.1m diameter cylinder (image credit: NASA/JPL RainCube Team)
The radar payload is integrated into a spacecraft bus provided by Tyvak. The majority of hardware, and all the software used for RainCube is designed, developed, and tested by Tyvak in their Irvine, CA facility. This ‘one-stop-shop' approach simplifies the overall development and gives greater control of design accommodations, schedule, cost, and risk management by eliminating third party interfaces and troubleshooting. RainCube is based off of Endeavour technology base, which includes C&DH (Command and Data Handling), EPS (Electrical Power System), ADCS (Attitude Determination and Control), and Communication Systems. To meet the specific requirements of the mission, the bus is tailored around the payload with minimal non-recurring engineering. Only small changes are needed to the structure, deployable solar panels, and electronics backplane between programs to accommodate internal mounting, instrument aperture locations, and payload interface thermal requirements.
The Endeavour avionics board provides a data recorder and processing for the C&DH and ADCS systems, along with interfaces to the inertial reference module that contains two star cameras, three orthogonal reactions wheels, and three torque rods, highlighted in Figure 5. The battery module is scalable and has been configured into a single 70 W hr pack that supports RainCube's high peak charge currents and 90-minute payload operations in transmit-mode. The spacecraft's 1U x 3U faces provide area for electronics routing, GPS antennas, the S-band patch antenna, and coarse sun sensors. The broad 2U x 3U faces of the structure act as primary radiating surfaces for thermal management and have a silver Teflon coating. The radar operational temperature range lead to a thermal design that includes survival heaters and tuned optical properties of the structure to ensure the flight system stays within allowable flight temperatures over the duration of the mission.
The communication systems has both a UHF (Rx/Tx) and S-band (Tx) link. Lastly, the large, fixed-angle deployable solar arrays close the power budget with the vehicle operating in LVLH (Local Vertical Local Horizontal) attitude for one continuous orbit of radar operations and three orbits of sun-pointing to recharge. Operations of the vehicle will utilize Tyvak's C2D2 ground software in their local MOC (Mission Operations Center) in Irvine, CA. The bus also hosts a 3MP color imager with a fish-eye lens mounted at an optimal angle to capture images of the Ka-band radar antenna deployment.
Figure 5: An exploded view of the Tyvak avionics shows the core C&DH and and ADCS. For RainCube, the torque coils are replaced with torque rods for improved torque authority at lower altitudes (image credit: NASA/JPL RainCube Team)
• June 2017: The assembled payload of 5.5 kg was delivered to Tyvak. Integration and testing of the full RainCube vehicle, illustrated in Figure 6, will be completed at Tyvak. Upon delivery, the radar payload is mounted to one of the 2U x 3U structural walls, which also act as a radiator. After subsystems are tested together on the flight flat-sat, they are installed into the flight vehicle with planned functional checkouts at specific assembly steps. The flight system will then undergo end-to-end hardware performance characterization, including the first flight system-commanded deployment of the radar antenna, and EMI self-compatibility testing. Following this, the final flight build of the software will be loaded and the flight system will complete a random vibration test, with a test dispenser provided by NanoRacks, and thermal-vacuum to bake-out the system and perform a thermal balance test. The solar panels, UHF antennas, and radar antenna will deployed one final time before the flight system is placed into a planned storage in September 2017.
• March 2017: Integration and test of the radar was completed in March 2017 with all payload requirements verified and validated. Radar integration began in January 2017 with a ‘flat-sat' to connect and operate the flight radar subsystems. The radar electronics were then assembled in the flight configuration and calibrated over temperature in thermal-ambient testing. The stowed flight antenna was installed in February, and the fully assembled radar payload went through workmanship vibe and thermal-vacuum testing (including antenna deployment) in March. The assembled payload is 5.5 kg and will be delivered to Tyvak in June.
Figure 6: No extra volume is wasted in the final pre-fabrication CAD rendering! The spacecraft is assembled by first installing the integrated radar (left), followed by the bus avionics (right), and finally closing out the system with remaining structure and deployable solar panels (image credit: NASA/JPL RainCube Team, Ref. 1)
Figure 7: RainCube, a Ka-band precipitation radar in a 6U CubeSat (image credit: (image credit: NASA/JPL RainCube Team) 7)
Figure 8: Artist's rendition of the deployed RainCube 6U CubeSat (image credit: Tyvak/Jonathan Sauder/NASA/JPL-Caltech) 8)
Launch: A launch of the RainCube mission is manifested for an ISS deployment on the ELaNa-23 flight, scheduled for Q2 of 2018 on the Cygnus CRS-9 Antares-230 vehicle of Orbital ATK. The launch site is MARS (Mid-Atlantic Regional Spaceport) in VA.
RainCube was selected in 2016 by NASA's CubeSat Launch Initiative (CSLI) program to be launched as part of the ELaNa (Education Launch of Nanosatellite) program.
Orbit: Near circular orbit, altitude of ~400 km, inclination = 51.6º.
The manifested payloads on the Cygnus CRS-9 flight are:
• CubeRRT (CubeSat Radiometer Radio Frequency Interface Technology), a technology demonstration mission of OSU (Ohio State University) of Columbus, Ohio. A 6U CubeSat with a mass of 6 kg.
• HaloSat, an X-rax astronomy 6U CubeSat mission of the University of Iowa to study the Hot Galactic Halo.
• SORTIE (Scintillation Observations and Response of The Ionosphere to Electrodynamics), a 6U CubeSat mission, led by ASTRA LLC (Atmospheric & Space Technology Research Associates).
• TEMPEST-D (Temporal Experiment for Storms and Tropical Systems - Demonstrator), a 6U CubeSat mission of CSU (Colorado State University).
• CaNOP (Canopy Near-IR Observing Project), a 3U CubeSat science investigation mission (4 kg) of Carthage College, Kenosha, Wisconsin.
• RadSat (Radiation Satellite), a 3U CubeSat demonstration mission (4 kg) of MSU (Montana State University), Bozeman, MT.
• EQUiSat, an educational 1U CubeSat mission of Brown University, Providence, RI (Rhode Island).
• MemSat (Memristor Satellite), a technology 1U CubeSat demonstration mission of Rowan University of Glassboro, NJ.
• RainCube, a 6U CubeSat (12 kg) demonstration mission of NASA/JPL, Pasadena, CA.
Figure 9: RainCube mission operations concept (image credit: (image credit: NASA/JPL RainCube Team) 9)
Figure 10: Proposed RaInCube constellation configuration. The vertical sections in this picture are actual W-band data from CloudSat, here used only as graphic aid (image credit: NASA/JPL RainCube Team)
1) Eva Peral, Travis Imken, Jonathan Sauder, Shannon Statham, Simone Tanelli, Douglas Price, Nacer Chahat, Austin Williams, "RainCube, a Ka-band Precipitation Radar in a 6U CubeSat," Proceedings of the 31st Annual AIAA/USU Conference on Small Satellites, Logan UT, USA, Aug. 5-10, 2017, paper: SSC17-III-03, URL: http://digitalcommons.usu.edu/cgi/viewcontent
2) Eva Peral, Simone Tanelli, Eastwood Im, Graeme Stephens, Ziad Haddad, "RaInCube: a proposed constellation of precipitation profiling Radars In CubeSat," Proceedings of the Advanced RF Sensors and Remote Sensing Instruments-Ka-band Earth Observation Radar Missions," (ARSI'14 & KEO'14), ESA/ESTEC, Noordwijk, The Netherlands, Nov. 4-7, 2014
3) Jim Wilson, "New CubeSats To Test Earth Science Tech in Space," NASA, Feb. 22, 2016, URL: http://www.nasa.gov/feature/new-cubesats
4) "Radar in a CubeSat (RainCube)," URL: http://www.jpl.nasa.gov
5) Eva Peral, Simone Tanelli, Ziad Haddad, Ousmane Sy, Graeme Stephens, Eastwood Im, "RaInCube: A proposed constellation of Precipitation Profiling Radat in CubeSat," Proceedings of the IGARSS (International Geoscience and Remote Sensing Symposium) 2015, Milan, Italy, July 26-31, 2015
6) N. Chahat, R. Hodges, J. Sauder, M. Thomson, E. Peral, E., Y. Rahmat-Samii, "CubeSat Deployable Ka-Band Mesh Reflector Antenna Development for Earth Science Missions," IEEE Transactions on Antennas and Propagation, 64(6), 2083-2093, 2016, DOI 10.1109/TAP.2016.2546306, IEEE, URL: http://tinyurl.com/y8j47dj9
7) Travis Imken, "RainCube, a Ka-band precipitation radar in a 6U CubeSat," 2017 CubeSat Developers Workshop, San Lois Obispo, CA, April 26, 2017, URL: http://mstl.atl.calpoly.edu/~workshop/archive/2017
8) "A Box of 'Black Magic' to Study Earth from Space," NASA/JPL, Nov. 7, 2016, URL: https://www.jpl.nasa.gov
9) "Radar in a CubeSat (RainCube)," NASA/JPL, URL: https://www.jpl.nasa.gov
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).