Development of CubeSat Spacecraft-to-Spacecraft Optical Link Detection Chain for the CLICK B/C Mission

The growing interest in and expanding applications of small satellite constellation networks necessitates effective and reliable high-bandwidth communication between spacecraft. The applications of these constellations (such as navigation or imaging) rely on the precise measurement of timing offset between the spacecraft in the constellation. The CubeSat Laser Infrared CrosslinK (CLICK) mission is being developed by the Massachusetts Institute of Technology (MIT), the University of Florida (UF), and NASA Ames Research Center. The second phase of the mission (CLICK-B/C) will demonstrate a crosslink between two CubeSats (B and C) that each host a \u3c 2U laser communication payload. The terminals will demonstrate full-duplex spacecraft-to-spacecraft communications and ranging capability using commercial components. As part of the mission, CLICK will demonstrate two-way time-transfer for clock synchronization and data transfer at a minimum rate of 20 Mbps over separation distances ranging from 25 km to 580 km. The payloads of CLICK B and C include a receiver chain with a custom photodetector board, a Time-to-Digital Converter (TDC), a Microchip Chip-Scale Atomic Clock (CSAC), and a field-programmable gate array (FPGA). The payloads can measure internal propagation delays of the transmitter and the receiver, and cancel environmental effects impacting timing accuracy. The photodetector board is 2.5 cm x 2.5 cm and includes an avalanche photodiode (APD) and variable-gain amplifiers through which the detected signal is conditioned for the TDC to be time-stamped. This design has been developed from the UF and NASA Ames CubeSat Handling Of Multisystem Precision Time Transfer (CHOMPTT) project and associated MOCT (Miniature Optical Communication Transceiver) demonstration. The TDC samples the signal at four points: twice on the rising edge at set thresholds, and twice at the falling edge at those same thresholds. These four time-offset samples are sent to the FPGA, which combines the measurements for a reported timestamp of the detected laser pulse. These timestamps can then be used in a pulse-position modulation (PPM) demodulation scheme to receive data at up to 50 Mbps, to calculate range down to 10 cm, and for precision time-transfer with \u3c 200 ps resolution. In this paper, we will discuss the designed capabilities and noise performance of the CLICK TDC-based optical receiver chain

Sensorless Wavefront Correction Algorithms for Free-Space Optical Communications

Free-space optical communications (FSOC) technology facilitates high-throughput wireless links across large distances with low size, weight, and power (SWaP) terminals. However, it is difficult to design reliable, low-cost FSOC terminals for long-range links through the atmosphere. Even in clear conditions, the effects of air turbulence along such links usually necessitate active wavefront correction via adaptive optics (AO). Conventional AO algorithms rely on direct wavefront sensing, an approach that is high in cost and SWaP and usually degrades in strong atmospheric scintillation. Sensing methods that are more tolerant to scintillation have been developed, but they are often more challenging to implement and further increase cost and SWaP. Sensorless wavefront correction algorithms, such as stochastic parallel gradient descent (SPGD), are preferable in terms of cost and SWaP, and have been used in FSOC terminals as methods to optimize the received signal strength indicator (RSSI). A key challenge with such algorithms, however, is that their convergence rate degrades as more atmospheric modes are optimized. This can lead to an inadequate correction rate due to limited bandwidth of the AO element and cause link interruptions. To maintain a sufficient link margin in such conditions, correction algorithms with better convergence properties are needed. This thesis focuses on the development and testing of a new non-stochastic algorithm for multimodal wavefront correction and a more general analysis of the circumstances where sensorless algorithms attain adequate performance for FSOC, including in strong scintillation. An end-to-end simulation environment is built to compare SPGD with the developed non-stochastic algorithm over a range of atmospheric conditions and hardware configurations. We show that in identical conditions, the non-stochastic algorithm either improves the link margin by 2--3 dB or relaxes the AO element bandwidth requirement by a factor of 2--3 compared to SPGD. Finally, the simulation results are validated in the laboratory under simulated atmospheric turbulence and compiled into a useful design tool for predicting sensorless wavefront correction performance.Ph.D

Precision Closed-Loop Laser Pointing System for the Nanosatellite Optical Downlink Experiment

The use of advanced small-satellite platforms has become increasingly more popular in the recent years. Several private companies are investing enormous capital into constellations of small satellites that are designed to provide highly data-intensive global services, such as rapid Earth imaging or fast worldwide Internet access. The scientific community is also interested in the development of miniature and high throughput platforms, for instance in the area of microwave radiometry or hyperspectral imaging. The current state of the art nanosatellite radio frequency (RF) communications systems struggle to keep up with the increasing downlink demand and satellite data processing capabilities. Laser communications (lasercom) offers various advantages: increased bandwidth, smaller size, weight, power consumption, and a license-free spectrum. While the narrow beamwidths allow lasercom to achieve higher data rates than RF, they, however, also result in higher pointing requirements for the spacecraft. Precision laser pointing systems have been successfully demonstrated on bigger satellites, but not on a nanosatellite scale, where the size and weight constraints are so severe. The Nanosatellite Optical Downlink Experiment (NODE) developed at MIT is a lasercom terminal designed to demonstrate the technologies required for a high-speed optical downlink using commercial off-the-shelf components within the constraints of a typical 3U CubeSat. NODE augments the bus attitude control system with a compact fine laser pointing stage to compensate for the spacecraft body pointing error. This thesis focuses on the development and laboratory verification of the laser pointing system for NODE. A control scheme utilizing a miniature fast steering mirror (FSM) used to track a beacon uplink signal from the ground station is presented. An on-orbit FSM calibration algorithm is developed to improve the control robustness and precision. A novel sampling approach that enables closed-loop FSM control is proposed and implemented. The method focuses on simultaneous sampling of the beacon and an internal feedback signal on a single detector. Finally, a hardware-in-the-loop testbed is built in the laboratory with components that were selected for NODE, and the system is functionally verified and analyzed with regards to pointing accuracy. Experimental results show that the pointing requirements given by the mission link budget are met, and that the system performs reliably under various laboratory-simulated conditions