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 spaceborne 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. 4)
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: 5)
• 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. 6)
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.
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. 7)
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. 8) 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.
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.
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) 9)
Figure 8: Artist's rendition of the deployed RainCube 6U CubeSat (image credit: Tyvak/Jonathan Sauder/NASA/JPL-Caltech) 10)
Launch: The RaInCube mission was launched on 21 May 2018 for an ISS deployment on the ELaNa-23 flight on the Cygnus CRS-9 Antares-230 vehicle of Orbital ATK (OA-9). The launch site was MARS (Mid-Atlantic Regional Spaceport), Wallops Island, VA. 11) 12) 13) 14)
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 ELaNa 23 (Education Launch of Nanosatellites 23) initiative payloads of NASA on OA-9 are: 15)
• 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) Boulder, CO.
• TEMPEST-D 1 (Temporal Experiment for Storms and Tropical Systems - Demonstrator 1), 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 demonstration mission of NASA/JPL, Pasadena, CA.
• 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.
• Lemur-2, four 3U CubeSats (4.6 kg each) of Spire Global Inc., San Francisco,CA.
• February 8, 2021: After nearly 2 1/2 years in orbit, a shoebox-size weather satellite phoned home one last time before plunging into Earth's atmosphere and burning up on Dec. 24, 2020. RainCube (Radar in a CubeSat) was a technology demonstration meant to show that shrinking a weather radar into a low-cost, miniature satellite called a CubeSat could provide science-quality data. 16)
- RainCube was deployed on July 13, 2018, from the International Space Station and had a primary mission of three months. The CubeSat's instrument "saw" rain and other kinds of precipitation by bouncing radar signals off of raindrops, ice, and snowflakes, and measuring the strength and the time it took for the signals to return to the satellite. It provided scientists with pictures of what was happening inside of storms around the world.
- Radar instruments on full-size Earth-observing satellites have carried out such measurements for years. "But the key thing with RainCube wasn't bringing in new science," said Simone Tanelli, RainCube principal scientist at NASA's Jet Propulsion Laboratory in Southern California. "Instead, it was showing that we could give you similar data with a box that's roughly 100 times smaller in volume than a full-size satellite."
- RainCube lasted far longer than the initial three months it was scheduled for, allowing researchers to collect data on hurricanes Marco and Laura in 2020 at the same time as another CubeSat called TEMPEST-D. The two CubeSats used different types of instruments to collect disparate, but complementary, observations that provided researchers with a 3D look inside these churning storms.
Figure 9: Fleets of miniature satellites like RainCube could one day study the rapid development and evolution of storms like this supercell thunderstorm over Nebraska (image credit: Mike Coniglio/NOAA NSSL)
- "That opened the door to something that Earth scientists are getting really excited about, which is using multiple CubeSats at the same time to study our planet," said Shannon Statham, RainCube project manager at JPL.
Figure 10: Two CubeSats, RainCube and TEMPEST-D, were intended to demonstrate that small satellites could collect science-quality data of precipitation in Earth's atmosphere (video credit: NASA/JPL-Caltech, NASA Scientific Visualization Studio)
Filling in the Blanks
- Earth's atmosphere is in constant motion, and some phenomena – like storms – can change from minute to minute. Current satellites in low-Earth orbit can observe a storm once or twice a day depending on the storm's location. That means many hours can pass between observations of a single storm. Flying a fleet of satellites spaced minutes apart could provide researchers with fine-grained temporal data to help to fill in those coverage gaps.
- But a full-size Earth-observing satellite can cost hundreds of millions of dollars to build, launch, and operate, and many are as large as cars or buses. "It'd be impossible to fly a fleet of these full-size satellites because it wouldn't be affordable," said Tanelli.
- CubeSats, on the other hand, can range from something about the size of a cereal box to a toaster oven, and their build, deployment, and operations can cost less than $10 million. This lower price tag could give researchers the chance to fly several of these tiny satellites at the same time.
Big Things in Little Packages
- However, a CubeSat's diminutive stature requires extensive engineering to shrink down an instrument while preserving its ability to collect and transmit scientific data. Other equipment, such as the radar antenna that receives signals, must also be revamped.
- That's where technology demonstrations like RainCube come in. For this particular mission, engineers whittled down the guts of a full-size radar instrument to only the essentials and redesigned how the parts fit together. The antenna – inspired by an antenna developed by the University of Southern California for their Aeneas CubeSat – went from being a rigid structure to something akin to an umbrella with collapsible components that could fold into an ultra-compact volume and unfurl once in space. RainCube engineers performed this mechanical origami, built their creation, and then launched the CubeSat within three years.
- "RainCube is my baby," said Statham, who – along with Tanelli and JPL Principal Investigator Eva Peral – has been with the project since its inception. "So its ending is bittersweet because we were hoping to have a little more time with it, but we've shown that science missions with CubeSats are possible, which is what we set out to do."
More About the Mission
- RainCube is a technology-demonstration mission to enable Ka-band precipitation radar technologies on a low-cost, quick-turnaround platform. It is sponsored by NASA's Earth Science Technology Office through the InVEST-15 program. JPL worked with Tyvak Nanosatellite Systems, Inc. in Irvine, California, to fly the RainCube mission. Caltech in Pasadena, California, manages JPL for NASA.
- For more information, visit: https://www.jpl.nasa.gov/cubesat/missions/raincube.php
• June 2019: To fit a science-grade radar within the tight 6U CubeSat volume, RainCube takes advantage of a new architecture: the key innovation in this architecture is the modulation technique: offset IQ (in-phase and quadrature) with pulse compression. 17)
RainCube's radar payload: Pulse compression is a well-known radar technique that reduces the peak power needed to achieve a certain sensitivity without losing resolution by sending a long pulse with a frequency modulation or chirp. However, pulse compression is rarely used for atmospheric radars and has never been used before in a spaceborne precipitation radar. The reason for that is the clutter introduced by the surface (that is, the surface backscatter spread in time t by the pulse compression filter response and the antenna pattern), which at near-nadir, especially over ocean, is highly reflective. Using standard pulse compression, the radar surface response exhibits high-range sidelobes that contaminate the echoes that arrive from the troposphere in the lowest layer that is within the pulse length above the surface (which may mean from a few km to the entire troposphere, depending on the length of the pulse).
Previous spaceborne cloud and precipitation radars have adopted high power short monochromatic pulses to achieve the required sensitivity and range resolution such that measurements of clouds and precipitation are affected by surface clutter only within the time of the short pulse response (typically a few hundred meters to less than 2 km when pointing off nadir). This requires high-power amplifiers and either high-voltage power supplies or large power-combining networks, precluding small-size/lowpower platforms. RainCube uses pulse compression with a long pulse (up to 166 µs) to achieve high sensitivity with off-the-shelf GaAs solid-state amplifiers. Through an optimal selection of the pulse shape and subsequent digital processing, the range sidelobes are suppressed to less than -55 dB at 500 m and -70 dB at 1 km above the surface, which is sufficient to accurately measure most relevant precipitation processes near the surface.
Figure 11 shows the typical range compressed pulse response in dB computed from flight data using the calibration path. The low-near range sidelobes enable precipitation detection close to the surface, and the suppressed far range sidelobes (-90 dB beyond 2 km) ensure a clutter-free measurement for the complete troposphere. RainCube pulse bandwidth of 2.5 MHz and amplitude apodization with a Hanning window result in a range resolution of ~120 m, which is then averaged to 240 m to improve sensitivity and for consistency with state-of-the-art spaceborne precipitation mission data products.
The IQ (in-phase and quadrature) modulation scheme selected for RainCube reduces the number of RF components with direct up/downconversion from baseband to Ka-band as shown in Figure 12. However, IQ mixers are not ideal, and the residual LO (Local Oscillator) leakage and signal image can severely impact the radar response sidelobes. The range sidelobes produced by these spurious products are roughly equal to the image or LO suppression minus the range compression gain, which is defined as the pulse width times the bandwidth. State-of-the-art IQ mixers can easily achieve approximately -20 dB suppression for both LO leakage and image. However, with the 26 dB range compression gain of RainCube, the sidelobe level at -56 dB is not acceptable.
There are techniques to suppress the image and LO even further, 18) 19) but these are temperature and aging sensitive, and they are not sufficiently robust for a low-cost flight mission. RainCube uses an offset IQ technique, in which the baseband signal is offset in frequency sufficiently to avoid contamination from the LO leakage and image by a combination of analog and digital techniques. It has been demonstrated that RainCube's performance is not impacted even if the image/LO suppression is highly degraded; therefore, it is a very robust modulation technique that does not require adjustments or tuning.
Figure 12: RainCube's radar block diagram. Abbreviations: S/C, spacecraft; Osc, oscillator; DAC (Digital-to-Analog Converter); ADC (Analog-to-Digital Converter); FPGA (Field Programmable Gate Array); Pre-amp (Preamplifier); LNA (Low Noise Amplifier); Tel (Telemetry), image credit: NASA/JPL Team
RainCube's digital subsystem is highly simplified compared to similar radars, and it consists of a single board that includes low power complementary metal-oxide-semiconductor DAC (Digital-to-Analog Conversion),ADC (Analog-to-Digital Conversion ) and telemetry ADC chips, and a single commercial-grade flash-based FPGA (Field-Programmable Gate Array) performing all control, timing, telemetry acquisition, data formatting, S/C bus communications, and OBP (On-Board Processing). The radar raw data capture rate is ~425 Mbit/s and includes a science window, as well as noise and calibration windows, to capture receive only noise and a replica of the transmit chirp. Since it would be prohibitively expensive to downlink this amount of data, RainCube's radar relies on extensive OBP to reduce the data rate by almost four orders of magnitude to ~50 kbit/s in transmit mode, which is in line with current CubeSat technology capabilities.
The radar also supports a receive-only mode (radar is not transmitting), and a standby mode, where only basic health and telemetry functions are available at lower data rates (<10 kbit/s). The radar OBP algorithm includes data filtering, range compression, power computation, and along-track averaging for a given integration time. It also performs averaging of the calibration signal. Given the simplicity of the algorithm, and the large amount of resources in commercial FPGAs, TMR (Triple Mode Redundancy) is used for all critical functions and most noncritical functions, including most of the OBP, and error detection and correction (EDAC) is used for critical memory functions. In addition, rad-hard hardware interlock circuits are implemented for all critical signals that could result in radar damage in the event of a single-event upset. The digital board reports telemetry faults in the interlock circuits, TMR voter logic and EDAC, both correctable and uncorrectable.
The RainCube radar commanding interface was designed to support variable pulse repetition interval, pulse width, and pulse amplitude parameters. In order to simplify the testing, a fixed pulse repetition interval of 1660 µs was selected. However, the pulse width, pulse chirp bandwidth, and amplitude, in addition to various parameters that determine the pulse shape, peak power, and image/LO rejection, remain programmable flight parameters.
When operating in transmit-mode, the radar requires 22 W of power (up to 1 W average RF output power with a 10% transmit duty cycle). Receive-only and standby modes only consume 10 and 3 W, respectively. Including the antenna, the radar has a flight mass of 5.5 kg.
Even with this modest power consumption for a radar, thermal management has been a significant part of RainCube's design since the beginning of the mission. RainCube has a passive thermal design (except for survival heaters) with the subsystems mounted on a single plate that allows heat to be transferred to thermal radiating faces on the spacecraft bus. Detailed analyses have been performed on all analog, RF, digital boards, and their components following standard practices of higher-class flight missions, to ensure appropriate margins exist for the components and materials given the expected mission environment.
RainCube's radar payload uses mostly commercial off-the-shelf components that have been carefully selected to withstand an LEO radiation environment. For critical functionality, such as where single event upsets could result in damage to the radar or unpredictable behavior, radiation hardened components are used.
RainCube's Flight System
RainCube is a real aperture nadir pointing radar, that is, the horizontal resolution is directly related to the aperture of the antenna. RainCube uses an antenna whose aperture size is larger than the bus longest dimension. The KaRPDA (Ka-band Radar Parabolic Deployable Antenna) 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 subreflector below the focal point of the antenna. This enables the antenna to be stowed in a tight volume.
Figure 2 shows a picture of the as-built radar payload prior to integration with the spacecraft (S/C) bus. The radar's RF, analog, and digital electronics occupy a volume of ~2.5U of the spacecraft's 6U volume. These components are tightly packaged around the mission's 0.5 m deployable antenna, which is stowed in a canister that occupies about 1.5U volume.
The flight system was designed to have enough power generation and energy storage capability to operate the radar in transmit mode for a continuous, 90-min orbit. Though designed for a 35 W power consumption, the final radar consumption was significantly lower. In addition, the flight system had to achieve an overall transmit-mode duty cycle of 25%, effectively allocating one orbit for the radar in transmit mode and the following three orbits for the spacecraft bus to recharge the batteries, radiate waste heat, and downlink the expected 1.7 Gbit daily payload data volume with the radar in standby mode.
RainCube Mission Operations
Tyvak is responsible for operating the RainCube satellite in orbit, with JPL involved for radar commissioning activities and subsequent technical support and guidance. Tyvak maintains UHF ground stations at their Mission Operations Center (MOC) in California and in Italy. These ground stations provide both uplink and downlink support for RainCube and are primarily used for spacecraft command and telemetry. To handle the radar's daily data volume of 1.73 Gbit, Tyvak is interfacing with the KSAT ground station network for S-band downlink. KSAT's large and distributed ground station network affords RainCube many potential downlink passes per day. All spacecraft operations and data are routed through servers in Tyvak's MOC, and the radar payload data are delivered to JPL using a secure VPN (Virtual Private Network).
For launch, RainCube was part of the ELaNa-23 mission, which manifested seven CSLI (CubeSat Launch Initiative) missions on the Orbital OA-9 ISS resupply mission. The RainCube satellite was integrated into a NanoRacks "doublewide" (2U x 6U) CubeSat dispenser and shared the volume with the HaloSat satellite. RainCube was launched to ISS from Wallops Island on May 21, 2018 and ejected to LEO on July 13, 2018 at the scheduled time of 08:05:00 GMT (Figure 13).
Figure 13: RainCube (left) and HaloSat (right) following ISS deployment. Solar panel deployment occurred 5 minutes after ejection followed by UHF antenna deployment 30 minutes later and first beaconing (image credit: NASA/JPL Team)
Tyvak detected RainCube energy on the first pass over the Irvine station in California about 1 h after deployment from ISS and verified that the spacecraft was responding to commands. After a few passes, it was confirmed that the UHF and solar panel had successfully deployed and that all subsystems including communications, power and attitude control, and determination were healthy.
The first weeks after deployment were used to commission the spacecraft, including attitude determination and control subsystem calibration, deployment camera verification, and first S-band downlink. The first radar operation in standby mode was completed with ground-in- the-loop during a UHF pass window on July 25, 2018. Radar telemetry confirmed nominal temperatures and power draw. The radar antenna deployment occurred on July 28, 2018 after a short commissioning phase of the antenna motor controller to confirm health telemetry. Immediate feedback from four deployment switches on the radar antenna indicated successful deployment. In addition to the deployment switches, a payload camera on the spacecraft bus captured images of the antenna during and after deployment, and the photos were downlinked on July 29, 2018 (Figure 14).
Figure 14: On-orbit pictures captured during the 3-minute deployment sequence: (a) the ribs unfurl as the antenna is nearly extended, (b) ribs open, and (c) final dish shape tensioning and subreflector separation (image credit: NASA/JPL Team)
Radar commissioning was completed after the successful antenna deployment. Standby mode telemetry was verified on August 1, 2018 followed by receive-only mode operation on August 3, 2018 while the spacecraft was in coarse Sun-pointing mode, where the S/C solar panels are steered toward the Sun. Receive-only mode data verified nominal values in voltages, currents, and temperatures, but it also provided the first experimental confirmation that the radar RF receiver chain was functioning as expected. The radiometric signal showed an excursion of about 0.5 dB, consistent with the predicted change in brightness temperature between an oceanic scene and a land scene, and the transitions were coarsely correlated with crossing of Sumatra and Malaysia (Figure 15).
Figure 15: First receiver noise only measurements from RainCube (in logarithmic uncalibrated units). Purple arrows indicate approximate estimated look directions in coarse Sun-pointing mode, and the approximate Sun subpoint area is indicated by the yellow circle. Over a limited region the radar antenna was pointing close to nadir and it was verified that the receiver RF chain of the radar was functioning nominally (image credit: NASA/JPL Team)
The first radar echo returns from Earth's surface were obtained on August 5, 2018 while the spacecraft was still in coarse Sun-pointing mode. As the antenna slewed toward nadir, the strongest returns were obtained, and, as expected, faded as the incidence angle, slant range, and Doppler shift increased (Figure 16). These measurements validated the RF transmit chain and the overall health of all the radar subsystems.
Figure 16: First radar echo reflectivity measurements from RainCube (in logarithmic uncalibrated units) as a function of along-track distance and range. The spacecraft is in coarse Sun-pointing mode. The data gaps were a temporary issue that was resolved, and data collected after this data set has no gaps (image credit: NASA/JPL Team)
While encouraging, these first radar measurements were not sufficient to fully validate the RainCube radar performance. The next three weeks were spent completing the radar and spacecraft commissioning to obtain the attitude control calibrations necessary for fine nadir-pointing and to improve the robustness of the data downlinks. Finally, on August 27, 2018, RainCube observed precipitation for the first time when it acquired a thunderstorm in fine nadir pointing mode over the Sierra Madre Oriental, near Monterrey, Mexico. A fast-growing orographic precipitation developed shortly before RainCube's pass, which overflew its north-eastern edge as later confirmed by analyzing visible satellite imagery.
Figure 17 illustrates several aspects typical of precipitation radar returns. The radar reflectivity as a function of acquired radar profile number and altitude is shown calibrated to be expressed in dBZ, the weather radar standard measure of precipitation reflectivity. The background image is a Google Maps image of the terrain, with the white line showing RainCube satellite's track. The brightest return (in red colors) comes from Earth's surface and the white line follows the peak of the return, and as such it tracks the topography. The surface response jaggedness is explained by the peculiar orography in this particular region, with high peaks corresponding to mountains. The returns above the surface are due to precipitation. For strong precipitation, the signal is attenuated and the surface return is no longer visible in the data. The visible satellite imagery confirmed not only the main storm but also the isolated rain cell that was observed before that.
During the first few weeks of the mission, the RainCube team self-imposed a requirement to only acquire data between +30 deg latitude to limit operations in low radiation regions. This set of data has been used to validate the radar performance, which is in solid agreement with on-the-ground predictions (see Table 1). Since then, RainCube's operations have been expanded to encompass the full orbit, including the South Atlantic Anomaly. The on-board fault telemetry has not reported any errors in the interlock logic, the TMR voting logic or the EDAC, which demonstrates the radar design's robustness to radiation.
Figure 17: First RainCube radar precipitation measurements on August 27, 2018 over Mexico in fine nadir pointing mode. The radar reflectivity, calibrated to be expressed in dBZ and as a function of radar profile number and altitude, is shown over a Google Maps image of the terrain. (image credit: NASA/JPL/Caltech—Google)
In summary, RainCube has validated in-space two new technologies: a miniaturized architecture for Ka-band atmospheric radars and an ultracompact deployable Ka-band antenna. As such, RainCube has introduced a new paradigm to observe weather processes as it opens up the possibility of a constellation of small precipitation radars, enabling unique science. This paper reports on the path to RainCube's successful mission, the demonstration of its performance requirements, and the first measurements ever acquired by a radar in space on a CubeSat platform.
At the time of writing (Ref. 17), RainCube has collected more than 90 h of radar data, including near simultaneous measurements with Tempest-D, 20) a multifrequency radiometer in a CubeSat, and measurements collocated with the GPM (Global Precipitation Measurement) mission. 21) These results are outside the scope of this paper and will be the subject of future publications. The RainCube spacecraft and radar remain healthy. The radar power and thermal subsystems continue operating within expected limits with steady voltages and temperatures, and the radar RF performance remains nominal. RainCube is expected to stay in a stable orbit until July 2020 (2 years after ISS deployment).
• April 2019: Infographic of the RainCube mission. 22)
• September 25, 2018: A satellite no bigger than a shoebox may one day help. Small enough to fit inside a backpack, the aptly named RainCube (Radar in a CubeSat) uses experimental technology to see storms by detecting rain and snow with very small instruments. The people behind the miniature mission celebrated after RainCube sent back its first images of a storm over Mexico in a technology demonstration in August. Its second wave of images in September caught the first rainfall of Hurricane Florence. 23)
Figure 18: NOAA's GOES (Geoweather Operational Environmental Satellite). The same storm captured by RainCube is seen here in infrared from a single, large weather satellite (image credit: NOAA)
- The small satellite is a prototype for a possible fleet of RainCubes that could one day help monitor severe storms, lead to improving the accuracy of weather forecasts and track climate change over time.
- "We don't have any way of measuring how water and air move in thunderstorms globally," said Graeme Stephens, director of the Center of Climate Sciences at NASA's Jet Propulsion Laboratory in Pasadena, California. "We just don't have any information about that at all, yet it's so essential for predicting severe weather and even how rains will change in a future climate."
- RainCube is a type of "tech demo," an experiment to see if shrinking a weather radar into a low-cost, miniature satellite could still provide a real-time look inside storms. RainCube "sees" objects by using radar, much as a bat uses sonar. The satellite's umbrella-like antenna sends out chirps, or specialized radar signals, that bounce off raindrops, bringing back a picture of what the inside of the storm looks like.
Figure 19: A photo from Google Earth of the mountainous area over Mexico where RainCube measured its first storm. The white line shows RainCube's flight path. The colorful graph in the bottom right shows the amount of rain produced by the storm, as seen by RainCube's radar (image credit: NASA/JPL-Caltech/Google)
- Engineers like Principal Investigator Eva Peral had to figure out a way to help a small spacecraft send a signal strong enough to peer into a storm. "The radar signal penetrates the storm, and then the radar receives back an echo," said Peral. "As the radar signal goes deeper into the layers of the storm and measures the rain at those layers, we get a snapshot of the activity inside the storm."
- "There's a plethora of ground-based experiments that have provided an enormous amount of information, and that's why our weather forecasts nowadays are not that bad," said Simone Tanelli, the co-investigator for RainCube. "But they don't provide a global view. Also, there are weather satellites that provide such a global view, but what they are not telling you is what's happening inside the storm. And that's where the processes that make a storm grow and/or decay happen."
- But RainCube is not meant to fulfill a mission of tracking storms all by itself. It is just the first demonstration that a mini-rain radar could work.
- Because RainCube is miniaturized, making it less expensive to launch, many more of the satellites could be sent into orbit. Flying together like geese, they could track storms, relaying updated information on them every few minutes. Eventually, they could yield data to help evaluate and improve weather models that predict the movement of rain, snow, sleet and hail.
- "We actually will end up doing much more interesting insightful science with a constellation rather than with just one of them," Stephens said. "What we're learning in Earth sciences is that space and time coverage is more important than having a really expensive satellite instrument that just does one thing."
- And that future seems closer now that RainCube and other Earth-observing CubeSats like it have proved they can work.
- "What RainCube offers on the one hand is a demonstration of measurements that we currently have in space today," said Stephens. "But what it really demonstrates is the potential for an entirely new and different way of observing Earth with many small radars. That will open up a whole new vista in viewing the hydrological cycle of Earth."
- RainCube is a technology-demonstration mission to enable Ka-band precipitation radar technologies on a low-cost, quick-turnaround platform. It is sponsored by NASA's Earth Science Technology Office through the InVEST-15 program. JPL is working with Tyvak Nanosatellite Systems, Inc. in Irvine, California, to fly the RainCube mission.
• July 13, 2018: NanoRacks successfully completed the 14th CubeSat Deployment mission from the Company's commercially developed platform on the International Space Station. Having released nine CubeSats into low-Earth orbit, this mission marks NanoRacks' 185th CubeSat released from the Space Station, and 217th small satellite deployed by NanoRacks overall. 24)
Figure 20: RainCube was deployed into low-Earth orbit from the International Space Station in July, where it has been measuring rain and snowfall from space. A closer look reveals there are two CubeSats in these images- RainCube is the bottom CubeSat closer to Earth, while the one above it is HaloSat (image credit: NASA)
- The CubeSats deployed were launched to the Space Station on the ninth contracted resupply mission for Orbital ATK (now Northrop Grumman Innovation Systems) from Wallops Island, Virginia in May 2018.
- NanoRacks offered an affordable launch opportunity, payload manifesting, full safety reviews with NASA, and managed on-orbit operations in order to provide an end-to-end solution that met all customer needs.
- The satellites deployed were: CubeRRT, EQUiSat, HaloSat, MemSat, RadSat-g, RainCube, TEMPEST-D, EnduroSat One, Radix (the last two entries are commercial CubeSats).
- The CubeSats mounted externally to the Cygnus spacecraft from the May 2018 launch are scheduled to be deployed on Sunday, July 15th, pending nominal operations.
Climate and weather models depend on measurements from spaceborne satellites to complete model validation and improvements. RainCube is a technology demonstration mission enabling precipitation radar technologies on a low-cost, quick-turnaround platform, demonstrating a small radar and ultra-compact deployable antenna and providing a profile of the Earth's vertically falling precipitation, such as rain and snow. The RainCube mission enables future Earth science missions to improve weather and climate models.
Figure 21: RainCube mission operations concept (image credit: (image credit: NASA/JPL RainCube Team) 25)
Figure 22: 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)
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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 (email@example.com).