Date of Award
Master of Science (MS)
Mechanical & Aerospace Engineering
In recent years, there has been a growing interest in CubeSats and very low Earth orbit (VLEO) space missions. Mission SeaLion, a collaborative CubeSat mission between Old Dominion University, the U.S. Coast Guard Academy, and U.S. Air Force Institute of Technology, planned to launch a 3U CubeSat into VLEO. The VLEO mission is a particularly challenging environment for navigation and orbit propagation because drag introduces a significant perturbation for orbit models such as SGP4. Additionally, mission requirements left no capacity for attitude determination or control, further reducing knowledge of drag behavior of the satellite in flight. This deficiency is a common problem for CubeSats due to their small form factor. The mission requires the onboard computer to maintain ephemerides of the satellite orbit so it can predict communication intervals with the ground station network and downlink its orbital state. The downlinked data enables parallel orbit propagation on the ground. The time interval before initial downlink is thus of critical importance. The satellite design included a GPS receiver to obtain an orbit fix after launch and periodically maintain orbit information throughout the mission lifetime. Due to the short lifetime of the mission, solar panels were not included in the design, resulting in system batteries being the sole power source. Continuous operation of the GPS receiver for ephemerides was therefore not feasible as it would deplete the batteries too quickly.
To address this issue, an orbit propagation tool is developed to be onboard the CubeSat. The tool is based on the Vinti oblate spheroidal method with the addition of a drag routine. The propagator is named drag equipped Vinti oblate spheroidal propagator (DEVS). The tool utilizes the GPS receiver data to obtain an initial state estimate, then propagates the CubeSat motion using the efficient DEVS routine. The propagator logic then periodically obtains another state fix from GPS, drastically reducing power usage. Analysis of accuracy of the model without GPS, shows a moderate preference toward DEVS compared to SGP4 for VLEO; in the span of 5 hours, SGP4’s error is over 2 times that of DEVS’s. Case studies are then performed for various GPS pinging frequencies, representing different mission requirements. The resulting effect on power draw, accuracy, and communications with the ground station networks via S-band is examined. Predicted total access time with the ground network differs from the truth model by only 1% for a 4-orbit GPS period and retains accuracy of access start and end times within 11 seconds. Link budget analysis predicts maximum error in link margin to be 0.15 decibels relative to the truth model. A state estimator is implemented to further increase accuracy of the propagator. Positional error, in terms of root mean square, does not exceed: 70 meters when the GPS is used every 5 minutes, and 1.1 kilometers when the GPS is used once per orbit. The resulting tool show cases the applicability of Vinti’s solution in the modern space environment.
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Senecal, Ethan M..
"A Comparative Study of Vinti-Based Orbit Propagation and Estimation for CubeSats in Very Low Earth Orbits"
(2023). Master of Science (MS), Thesis, Mechanical & Aerospace Engineering, Old Dominion University, DOI: 10.25777/p5v5-0x22
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