RACE (Radiometer Atmospheric CubeSat Experiment)
RACE is a technology demonstration nanosatellite mission of NASA/JPL and UTA (University of Texas, Austin). The goal is to demonstrate state-of-the-art microwave radiometer receiver technology on a 3U CubeSat platform. RACE will fly a water vapor radiometer selected for implementation under the NASA HOPE-3 (Hands-On Project Experience-3) small-scale program. In this setup, JPL is developing the radiometer, while UTA is providing the CubeSat bus. 1)
The mission objectives are:
• To advance the technology of the 35 nm indium phosphide (InP) receiver subsystem of the radiometer instrument.
• To advance the technology of a 183 GHz water vapor radiometer CubeSat system.
• To reduce the risk for future users of the technology.
• To enhance the hands-on training for the RACE project team members within the Phaeton Program platform.
• To explore possibilities for smaller missions with distributed risks.
A radiometer within a CubeSat platform has the potential to revolutionize systems by moving from the traditional large scale missions (risky and expensive) to various smaller missions. The SDL (Satellite Design Laboratory) at the University of Texas, Austin is designing, building, and testing the CubeSat for the RACE mission. The RACE CubeSat is a 3U nanosatellite (10 cm x 10 cm x 34 cm), and the radiometer is designed to fit within a 1.5U volume. 4) 5) 6)
Figure 1: Illustration of the 3U RACE CubeSat (image credit: NASA/JPL, UTA/SDL)
RACE will utilize 4 deployable solar panels and will be spinstabilized by means of an active attitude determination and control system to maintain the orbit rotation requirements. Communications will be performed using an L3 UHF Cadet radio with the ground segment at NASA Wallops.
The system block diagram shown in Figure 3 depicts the interaction of the RACE functional components. The majority of the components are of >TRL6 (Technology Readiness Level 6).
Figure 2: RACE concept of operations with relevant instrument, CubeSat and mission parameters (image credit: NASA)
Figure 3: Block diagram of the RACE bus (image credit: NASA/ARC)
Modifications include the addition of 4 deployable solar panels, replacement of the existing radio with the L3 UHF Cadet radio and addition of an active ADCS. Flight system technical margins are shown in Table 1, in which only the system mass and volume margins are under 30%. However, given the high TRL of the bus and its components, as well as ARC's experience in building nanosats, this is not expected to be an issue. The mass and volume margin allocated for the radiometer instrument is in excess of 30%.
Table 1: Flight system technical margins
Figure 4: Photo of the RACE nanosatellite (upside down) after integration tests at UTA/SDL (Spacecraft Design Laboratory) on Feb. 25, 2014 (image credit: UTA)
Launch: The RACE nanosatellite was launched as a secondary payload on October 28, 2014 on the Cygnus CRS Orb-3 mission from MARS (Mid-Atlantic Regional Spaceport), NASA's Wallops Flight Facility in Eastern Virginia. — RACE had been selected for launch by the NASA CLI (CubeSat Launch Initiative) in 2012.
Table 2: Cygnus cargo 7)
Unfortunately, a launch failure occurred 6 seconds into the flight. On Oct. 29, 2014, NASA's Wallops Incident Response Team completed an initial assessment of Wallops Island, VA. It will take many more weeks to further understand and analyze the full extent of the effects of the event. 8) 9) 10) 11)
The secondary payloads on this flight were: 12)
1) 26 Flock-1D imaging nanosatellite constellation (3U CubeSats) of Planet Labs, San Francisco that markets the Earth Observation data products to a range of customers for a variety of applications.
2) RACE (Radiometer Atmospheric CubeSat Experiment), a demonstration mission of NASA/JPL and UTA (University of Texas, Austin).
3) Arkyd-3, a 3U CubeSat of Planetary Resources Inc. of Bellevue, WA (4 kg). 13)
4) GOMSpace-2, a 2U CubeSat (2 kg) of GomSpace, Aalborg,Denmark
Orbit: The nominal obit will be attainable by an ISS (International Space Station) resupply launch with an inclination of 51.6° and a nominal altitude of ~350 km. However, other orbits are also compatible. The primary mission goal is 2 months; the secondary mission goal is 1 year.
After deployment from the P-POD (Poly-Picosatellite Orbital Deployer), RACE will extend the solar panels and enter a high drag, quasi-stable configuration, with the ADCS aiding in the initial detumble.
Once the communications link has been established, the payload will be activated and the spacecraft will then spinup to the required rotation rate. The payload antenna, positioned on one of the long faces of the spacecraft, will then perform repeated cross-track measurements of the Earth, Earth limb, and cold space. A nominal spin rate of ~30 rpm is required for continuous on the ground measurements.
Figure 5: Team RACE: Fifteen JPL Early Career Hires (recently graduated engineers and scientists) worked closely together to get the payload of the RACE nanosatellite ready for flight (image credit: NASA/JPL, Caltech) 17)
The technology that the RACE team pushed from concept, to test bed, to launch pad, lives on. The lessons learned developing the radiometer, the instrument that was the heart of the RACE mission, are being applied to a new CubeSat proposal called TEMPEST-D (Temporal Experiment for Storms and Tropical Systems - Demonstrator).
Sensor complement: RACE (Radiameter Atmospheric CubeSat Experiment)
The payload name is identical to the mission name. Initially, the payload was called CHARM (Hydrometric Atmospheric Radiometer Mission). Water vapor is important on a global scale as it helps us understand the water cycle, which is turn is a crucial component of the Earth's energy balancing system. Understanding and monitoring of water vapor will help in the understanding of both weather and climate processes (both near and long term impacts). Specifically, 183 GHz is useful as a water vapor line as we are able to profile different layers of the atmosphere with multiple channels, this allow the project to investigate the flow between regions, both vertically and horizontally. The higher frequency also allows for smaller components, especially with regards to the antenna aperture which affords smaller footprints/pixels on the ground, which is crucial as water vapor features tend to vary on small spatial scales. The technology proposed is cutting edge which will improve the performance of future instruments developed. Further, utilizing CubeSats allows for cheaper missions, which is increasingly important with limited science budgets.
CHARM is a 183 GHz radiometer and ideal for the nanosatellite platform which has stringent mass, power and volume requirements. The objective is to measure microwave radiation at the 183 GHz water vapor line, which is relevant to the water cycle and Earth energy budget. Key to the instrument development is a low noise amplifier front end that utilizes the 35 nm Indium Phosphide High Electron Mobility Transfer process.
Figure 6: Illustration of the 183 GHz radiometer instrument (image credit: NASA/JPL)
The enabling technology in this effort is the NGC (Northrop Grumman Corporation) indium phosphide (InP) 35 nm HEMT (High Electron Mobility Transistor) process, which was initially developed under a DARPA (Defense Advanced Research Projects Agency) program aimed at applications above 300 GHz. NASA's ESTO (Earth Science Technology Office) has made significant investment into the technology and related instruments, and applied specifically to 183 GHz at JPL has resulted in a reduction in power while improving noise performance (Ref. 2). Boon H. Lim of JPL is the PI of the mission.
Figure 7: Schematic view of the RACE radiometer payload (image credit: NASA/JPL)
Figure 8: a) InP amplifier, 900 x 560 µm2, b) 2 amplifiers and a subharmonic mixer in a package, c) packaged receiver with a penny (image credit: JPL)
Figure 8 shows the physical scale of the technology starting from the smallest building block at 900 x 560 µm2, to an intermediate stage with 2 amplifiers and a sub-harmonic mixer wire bonded, to a fully integrated receiver block that was developed internally at JPL for cosmic microwave background measurements. 18) 19) 20)
The proposed radiometer configuration is shown in Figure 9. Calibration of the radiometer will be performed by external cold space/Earth limb looks, and vicarious scene comparison combined with an extensive pre-launch testing campaign to determine the receiver characteristics over temperature. The dash-outlined block represents a custom JPL packaging that would house the InP LNAs (Low-Noise Amplifiers) and subharmonic mixer, similar to that in Figure 8c. The intermediate frequency (IF) subsystem feeds a four way splitter with band definition filters. Nominally the radiometer will measure around 183.31 GHz at ±1, 3, 4.5 and 7 GHz [4 of the 5 ATMS (Advanced Technology Microwave Sounder) 183 GHz channels on Suomi NPP].
Figure 9: Proposed 183 GHz receiver chain with four channel output (the outlined parts represent the JPL developed frontend), image credit: JPL
Minimal development is required for the configuration shown in Figure 2 as the majority of components are available COTS (Commercially off-the-Shelf) except the antenna and the JPL-developed receiver. The antenna will be based on the mature offset-parabolic design with careful attention paid to the mass and volume. The JPL developed receiver requires only minor changes to the current housing. The radiometer payload is expected to utilize a 1.5 U volume of the 3U CubeSat. The primary effort will be packaging the components into the available volume with careful consideration of the thermal environment for radiometric stability considerations.
Comparative technology assessment:
Current 183 GHz radiometer systems utilize mixer frontends with a typical noise figure of 9 dB. The CHARM receiver, utilizing the 35 nm InP MMICs (Monolithic Microwave Integrated Circuits), will at minimum reduce the noise figure level of the 183 GHz receiver to 6 dB. This represents an approximate improvement to the radiometer sensitivity or the NE?T (Noise-Equivalent Differential Temperature) by a factor of 2. CHARM will be the first mission to fly the state-of-the-art InP MMIC low power and low mass 183 GHz low noise amplifiers.
Table 4: Summary of several RACE spacecraft parameters with the 2 recent spaceborne instruments carrying 183 GHz channels
Table 4 compares the CHARM spacecraft to two different microwave radiometer instruments currently in use. The MHS (Microwave Humidity Sounder) is a water vapor specific instrument; CHARM has 60% comparable functionality (3 of 5 shared channels) at 1% of the volume, 11% of the mass, and 16% of the power. The ATMS (Advanced Technology Microwave Sounder) is the most recent microwave radiometer instrument suite (launch Oct. 28, 2001, on Suomi NPP) with a wider range of frequencies; CHARM has 18% comparable functionality (4 of 22 shared channels) at 2% of the volume, 7% of the mass, and 10% of the power. CHARM compares favorably in performance even when comparing the entire CHARM flight system to the respective microwave instruments.
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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).