Small inexpensive satellite platforms, such as cubesats, offer opportunities for pathfinder experiments, space qualification of components and systems, and enhancements of larger assets. The Aerospace Corporation has been developing cubesats with lasercom transmitters for downlinking payload data to the ground from orbit. Recently we demonstrated a 200 Mbps link with a 1.5 U cubesat, AeroCube-7B, under NASA’s Optical Communications and Sensors Demonstration program . The nearly error free link was accomplished using on-board star trackers for attitude determination and control and without any forward error correction. The capability developed under this effort has been implemented in follow-on missions for two 3U AeroCube- 11 vehicles where camera recorded data has been downlinked to the ground. While very modest data volumes (~1.5 GB) have been transmitted (to date) in a single pass under non-ideal conditions, the utility of this architecture for certain types of missions has been demonstrated. Additional experiments involving the illumination of the ISARA cubesat with the laser from AeroCube-7B demonstrated the ability to locate and illuminate a remote satellite in orbit, a necessary step towards realizing intersatellite links with this class of vehicles.
In this presentation, we discuss the first demonstration of a lasercom downlink from a LEO 1.5U 2.3 kg CubeSat to our optical ground station at The Aerospace Corporation in El Segundo, CA. Two vehicles, AC7-B&C, built under NASA’s Optical Communications and Sensors Demonstration (OCSD) program and described in previous presentations, were launched in November 2017 and placed in a 450-km circular orbit. Following on-orbit checkouts and preliminary pointing calibration utilizing on-board star trackers, we have demonstrated (at the time of this manuscript submission) communications links up to 100 Mbps with bit error rates near 10-6 without any forward error correction. Further optimization of the vehicle pointing and detection electronics and operating the transmitter at its full power capacity should enable performance improvements and potential for higher data rates.
In this presentation, we discuss the first demonstration of a lasercom downlink from a LEO 1.5U CubeSat to our optical ground station at The Aerospace Corporation in El Segundo, CA. Two vehicles, AC7-B&C, were built under NASA’s Optical Communications and Sensors Demonstration (OCSD) which is a flight validation mission to test commercial-off-the-shelf components and subsystems that will enable new communications and proximity operations capabilities for CubeSats and other small spacecraft. As designed, the 1.5 U CubeSats weigh 2.3 kg and consume ~2 W during most of the mission life. During lasercom engagements, ~3 minutes, the spacecraft consumes an additional 10-20 W power depending on the set point of the laser transmitter, which yields 2-4 W at 1.06 m. The transmitter consists of a directly modulated laser diode followed by a Yb fiber amplifier and exhibits an overall wall-plug efficiency ~20%. The AC-7B&C vehicles were launched in November 2017 and placed in a 450-km circular orbit. Following on-orbit checkouts and preliminary pointing calibration utilizing on-board star trackers, we have demonstrated (at the time of this submission) first time communications downlinks up to 100 Mbps from the 7B vehicle using open loop pointing (beaconless) to our ground terminal, which is near sea level. The preliminary link experiments at 50 and 100 Mbps (OOK/PRBS23) using the AC-7B CubeSat were recorded at 100 ms intervals. At 50 Mbps, error rates near 1E-6 were observed with numerous error free intervals. At 100 Mbps we observed BERs approaching 1E-6. At the time of these collects, however, the B vehicle was still exercising a scan pattern since the final alignment had not been completed. Thus, the optical link was not continuous over the entire pass. Link budget estimates indicate that lower BERs should be achievable and we will continue to assess the link performance as the system is optimized.
A pair of 2.2 kg CubeSats using COTS hardware is being developed for a proof-of-principle optical communications demo from a 450-600 km LEO orbit to ground. The 10x10x15 cm platform incorporates a 25% wall-plug efficient 10-W Yb fiber transmitter emitting at 1.06 μm. Since there are no gimbals on board, the entire spacecraft is body-steered toward the ground station. The pointing accuracy of the LEO craft, which governs the data rate capability, is expected to be ~ 0.1-0.2 deg. Two optical ground stations, located at the Mt. Wilson observatory, have receiver apertures of 30 and 80 cm. Launch of the CubeSat pair is anticipated to be mid to late 2015.
This paper describes a multivariable controller design procedure that uses mixed-sensitivity H-infinity control theory. The design procedure is based on the assumption that structural noise can be modeled as entering a state-space system through a random input matrix. The design process starts with a full-order flexible state-space model that undergoes a frequency-weighted balanced truncation to obtain a reduced-order model with excellent low frequency matching.
Weighting functions are then created to specify the desired frequency range for disturbance rejection and controller bandwidth. A structural noise input matrix is also designed to identify system modes where maximal damping is desired. An augmented plant is then assembled using the reduced-order model, weighting functions and structural noise input matrix to create a mixed-sensitivity configuration. A state-space controller is then realized using an H-infinity design algorithm.
A two-input, three-output, doubly cantilevered beam system provides a design example. A 174th-order, discrete-time, state-space model of the cantilevered beam system was used to generate a reduced 40th- order model. A 55th-order Hinfinity controller was then designed with a controller bandwidth of approximately 300 Hz. This non-square modern
controller uses feedback signals from two piezoelectric sensors, each collocated with one of two piezoelectric actuators, and one highly non-collocated accelerometer. The two piezoelectric actuators provide the control actuation. Frequency analysis and time-domain simulations are utilized to demonstrate the damping performance.