| dc.description.abstract | This thesis proposes a communications system that utilizes the benefits of CubeSats to provide jam-resistant communications. The growth of CubeSats within educational communities has prompted their use in industry; both industry and academia have contributed towards making CubeSats much more capable. CubeSats can now perform many advanced missions, from technology demonstrations to Earth observation missions and science missions. Meanwhile, military satellite communications (MILSATCOM) continues to rely primarily on large, highly-capable satellites. CubeSats could augment MILSATCOM by providing many low-cost space terminals with short development times as a means to create a more robust communications suite. The CubeSat communications architecture proposed in this thesis aims to support mobile users in hostile environments who need to relay information to a command center. Jam-resistant communications are achieved by performing ground-based beam-forming (GBBF) on a radio-frequency (RF) uplink and relaying the information to a ground station via a laser communications (lasercom) downlink: each CubeSat acts as an element of a sparse antenna array. With the growth of free-space lasercom in the last decade, lasercom is now a reality on CubeSat-scale platforms. Lasercom systems have lower size, weight, and power (SWaP) compared to RF systems with similar data rates, making them a good fit for CubeSat platforms. GBBF is a special case of beamforming where each element of an antenna array relays its signal to a ground station for processing, minimizing complexity on the space terminal. Beam-forming provides anti-jamming capabilites due to the spacings between elements in the array, also known as spatial diversity. This spatial diversity allows spatial filtering to occur, which modifies the array's radiation pattern to mitigate interference, add gain to the main lobe, or add multiple beams. The system is designed with the goal of minimizing cost and development time, and two ways of accomplishing this are by supporting currently fielded handheld RF transmitters and by utilizing a lasercom downlink which is being developed as part of the Nanosatellite Optical Downlink Experiment (NODE) in MIT's Space, Telecommunications, Astronomy, and Radiation Lab (STARLab). This thesis builds on previous work done on the NODE project, specifically the waveform design for NODE. NODE is a 3U CubeSat demonstrating a lasercom down-link while in low Earth Orbit (LEO). NODE uses 200mW transmit power to obtain data rates from 8 Mbps to 80 Mbps. The Optical Communications Telescope Laboratory (OCTL) at the Jet Propulsion Laboratory (JPL) and an amateur telescope will be used as optical ground stations. In order to send information to the ground station, NODE uses a waveform that provides forward error correction (FEC) and interleaving to mitigate channel effects. This thesis develops the channel coding, interleaving, modulation, and framing approach employed in the NODE waveform to provide error-free communications. A Reed-Solomon code, selected because of its performance and the existence of open-source implementations, provides error-correction capabilities. NODE uses a one-second interleaver to combat the effects of channel fading when the laser beam passes through the atmosphere. The transmitter uses pulse position modulation (PPM), an intensity modulation scheme that uses the delay of a single pulse within a symbol time to transmit information, due to the advantages in using a duty-cycled waveform with an average-power limited optical amplifier. Since the delay of the pulse conveys information for PPM, the transmitter clock must be recovered in order to properly demodulate the received waveform, and NODE uses inter-symbol guard times to encode the transmitter clock onto the waveform. Python simulations are presented showing that the channel coding, interleaving, and modulation are sufficient to obtain error-free communications with a target channel bit error rate (BER) of 1 x 10-⁴. The modulator is implemented within a field programmable gate array (FPGA), and the design, validation, and testing of the modulator are described. The feasibility of performing GBBF on RF uplinks to CubeSats in LEO, where each CubeSat acts as an element of an adaptive array, is examined. The high Doppler and large spacing between CubeSats requires the use of a space-time-frequency adaptive processor (STFAP). The STFAP consists of Doppler and delay taps, complex weights, an adaptive processor, a polyphase filter bank, and a polyphase combiner. The STFAP becomes infeasible as the Doppler and delay spread between different CubeSats increases, and analysis is used to identify scenarios where the Doppler and delay spreads seen in LEO are acceptable. Systems Tool Kit (STK) simulations are performed to analyze the Doppler and delay environment in LEO. Two CubeSat formations and multiple orientations between a user and jammer are examined to determine cases where null-forming, a special case of beamforming, is effective. A constellation is necessary to provide global coverage and maximize the effectiveness of null-forming, and two possible constellations are discussed. | en_US |
