Integrated Test Facility for Nanosat Assessment and Verification

SDL has developed an advanced test facility to characterize and verify the performance of small satellites up to 10 kg, thus reducing pre-flight risk. NOVA (Nanosat Operation, Verification, and Assessment), has been in operation since Aug. 2010, and has been used to test and calibrate a number of CubeSats and payloads, including DICE, PEARL, Falling Sphere, and STAROP. This testing has included determining the attitude determination accuracy of spinning sun sensor magnetometer systems. The test stations are: mass properties, sun sensor calibration and testing, reaction wheel testing and characterization, magnetometer testing and calibration, torquer coil testing and characterization, solar array power generation, and end-to-end system-level testing. Horizon sensor testing is currently being developed. Such a facility is warranted by the anticipated national development rate of dozens of CubeSats per year and an international projection of hundreds of CubeSats per year. Due to the small size of CubeSats and their sensitivity to disturbances, novel testing approaches are needed. For example, a spherical air bearing that was previously used on a larger spacecraft was not an option due to the alignment inaccuracies. A single-axis air bearing with an integrated optical encoder is used to provide truth data for closed loop attitude control testing. The facility considers three classes of nanosats, depending on their accuracy requirements: Class 1 spinning satellites with sun sensors and magnetometers with pointing knowledge of 1° and control of 5°, Class 2 non-spinning spacecraft with sun sensors and magnetometers with pointing knowledge of 0.2° and control of 0.2°, and Class 3 non-spinning spacecraft using star trackers with pointing knowledge of 0.01° and control of 0.02°. Class 1 can currently be tested with sufficient accuracy using a combination of a spinning attitude determination test and a hardware-in-the-loop (HWIL) sensor/dynamics/actuator “flatsat” emulator. Class 2 can also be tested, but with some loss of fidelity, with the HWIL system. Magnetic cleanliness for this class of pointing is a challenge. For Class 3, the test facility will use a highly accurate Stewart platform for motion simulation, and a star simulator source. These upgrades are planned for the future when warranted by the maturity of nanosat-sized star trackers. SDL has a Stewart platform available in its Albuquerque, NM, laboratory. Since a key component of the system-level testing is the HWIL simulation, the component test stations must provide high-fidelity models. The sun sensor test cell uses a simulated sun source and a 2-axis gimbal with 0.01° repeatability. The horizon sensor test uses an earth simulator plate in front of a liquid nitrogen cooled background representing space. The magnetic test cell for torquer coils and magnetometers uses a 3-axis Helmholtz cage with a field which can be rotated in real time using closed-loop control to within 10 nanotesla. Dual NIST-traceable differential magnetometers provide highly accurate results. A zero-gauss chamber is also available for magnetometer calibration and determination of nanosat magnetic dipole moment. The reaction wheel test cell measures wheel speed very accurately using 400 MHz sampling and derives the torque analytically. The solar panel test cell provides a continuous AM0 light source to verify the power output of the solar arrays. A NIST-traceable pyranometer is used to measure the light intensity in the target area. The mass properties test cell features load cells and kinematic mounts to obtain the measurements needed to verify and refine the calculations from the CAD models and to statically and dynamically balance the spacecraft. Mass can be measured within 2 grams (3σ) and center of gravity to within 1 mm (3σ). The resulting laboratory provides a single location for evaluating a large set of systems and properties for completed nanosats