This is the second of three articles covering NASA’s “State of the Art of Small Spacecraft Technology” report.
NASA recently released an update to its “State of the Art of Small Spacecraft Technology” report, which aims to provide the most current information on the state of small spacecraft technology — mainly focused on the CubeSat market. The report is divided into multiple sections.
NASA uses the term “TRL,” which stands for technology readiness level, to indicate the current state of the technology. The TRL scale ranges from 1 to 9. Anything below a 4 is considered “on the horizon,” meaning the technology is theorized, but that no out-of-laboratory tests have been conducted. A TRL above 5 generally indicates state-of-the-art technologies that are in the testing phase, either in space or on Earth. A TRL of 8 or 9 means the system is currently or in the process of being flight qualified for active use.
Below are a few key highlights from the report.
Guidance, Navigation, and Control (GNC)
The field of GNC is well established, but CubeSats have been able to take advantage of certain systems only recently, due to miniaturization. Course-corrective reaction wheels and magnetorquers, for instance, take advantage of gyroscopic forces and external magnetic forces, respectively, to orient the spacecraft.
Star trackers, magnetometers, sun sensors, horizon sensors, gyros, GPS receivers, deep space navigation, and atomic clocks all contribute to the CubeSats’ ability to navigate in space. All of these systems have TRLs of 9.
The only on-the-horizon system in this field is the high-speed, magnetically levitated reaction wheel being developed by the Swiss Federal Institute of Technology. The wheel will take advantage of Lorentz forces in a levitating self-bearing magnetic motor to provide torque to the spacecraft, with the main advantage being less wear and tear on internal systems.
Structures, Materials, and Mechanisms
The CubeSat structure is the frame upon which all other components are mounted. The structure serves multiple purposes, including bearing the load of the components, radiation shielding, and thermal regulation.
The main innovation thus far has been standardization of structures and sizes across the industry, and the main challenge moving forward is the requirement that the structure protects the electronic components from solar radiation.
Most CubeSats operate in low Earth orbit right now, where most solar radiation is filtered out by the Earth’s atmosphere and magnetosphere. But if the CubeSat market ever wants to reach higher altitudes or deep space, structure manufacturers will have to integrate radiation shielding into their designs.
Proper heat regulation is a crucial design element for any spacecraft. Without the benefit of the air to cool the skin of the spacecraft, it must instead rely on radiative and conductive processes to carry away waste heat. Larger spacecraft generally rely upon deployable radiators or thermal louvers to radiate away heat.
However, implementation on CubeSats had proven difficult due to size constraints. A new on-the-horizon technology is the phase-changing thermal storage unit, which, as the name suggests, takes advantage of matter phase changing from solid to liquid to store heat for future use. The current TRL for this system is 7.
CryoCube-1 is expected to launch in 2019 and will carry two CubeSat cooling experiments: the sunshield and the cryocooler. The deployable sunshield will be the smallest ever implemented on a spacecraft. The cryocooler will actively cycle supercool fluid throughout the CubeSat to cool its systems.
Command and Data Handling
The onboard computing technology field has enjoyed the most benefits from commercial off-the-shelf (COTS) products, particularly from smartphones. The consumer smartphone market, which has driven manufacturers to create incredibly small, efficient, and high-processing power components, has simultaneously benefitted the small spacecraft market.
Many spacecraft are now using phone hardware — such as the Google Nexus One on NASA’s PhoneSat-1.0 — to provide a high-performance central processing unit (CPU), camera, radio link, and accelerometer capability.
Open-source platforms are also being leveraged, including Arduino boards, Raspberry Pi, and Intel’s Edison system. Many of these systems support Linux development, allowing spacecraft programmers to code in many developer environments, including Android and Python. NASA itself has embraced open-source solutions and has employed them on many of its own missions.
Satellite communication is a well-developed and established field. Advances in radio frequency technology have allowed CubeSats to operate on many frequencies, ranging from ultra high frequency (UHF) to the S-band.
Recent developments have also allowed CubeSats to operate in the X-band, and ongoing research is aiming to allow them to operate in the Ku to Ka bands, which is currently only possible on large spacecraft.
One on-the-horizon technology of note is the development of asymmetric laser communication. The aim of this technology is to lower the requirements on the CubeSat for signal transmission. This works by sending a laser beam from the ground toward the satellite, which then is reflected off the satellite back down to the surface, but now with signal-encoded data (achieved by interrupting the beam at specific intervals). This reduces the need for onboard radio equipment on the CubeSat.
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