7 research outputs found
Recommended from our members
Stochastic Optimal Control to Minimize State Uncertainty
In many spacecraft control scenarios, the standard design procedure begins with a deterministic propellant minimization that does not consider random errors due to navigation uncertainty, control noise, unknown parameters, or mismodeled dynamics. However, these random errors can render a deterministic trajectory infeasible or result in large state deviations that are resource-intensive to correct. Therefore, a deterministic design is followed by feasibility studies to ensure that potential errors will not compromise the mission performance. Alternatively, this dissertation investigates the more direct approach of stochastic optimal control. Stochastic control techniques include uncertainty within the optimization process such that error statistics can be minimized or constrained directly. Two novel stochastic control problems are investigated in this dissertation: 1) open-loop multi-objective trajectory optimization to minimize state error covariance and control energy using indirect methods and 2) closed-loop guidance to minimize mean squared error using dynamic programming. These techniques may be employed independently or in tandem to improve mission robustness. Both approaches consider control-dependent noise that is proportional to the magnitude of the nominal control (a structure that is common in spacecraft thrusters). The stochastic control methods in this dissertation are demonstrated for various mission scenarios including asteroid orbits in microgravity and multi-body orbits in the Earth-Moon system.
Moreover, the growth of state uncertainty over time is dependent on the dynamical system in question, with the degree of nonlinearity playing an important role. This dissertation also studies the relationship between uncertainty and nonlinearity and proposes a semianalytical measure of nonlinearity that is based on the eigenpairs of higher-order tensors. The nonlinearity measure can be used to identify regions of strong nonlinearity, estimate the size of the ``linear'' region about a nominal trajectory, and inform navigation and control algorithms. Collectively, this research results in a generalized guidance and control framework that facilitates robust performance.</p
Recommended from our members
Risk of COVID-19 after natural infection or vaccinationResearch in context
Background: While vaccines have established utility against COVID-19, phase 3 efficacy studies have generally not comprehensively evaluated protection provided by previous infection or hybrid immunity (previous infection plus vaccination). Individual patient data from US government-supported harmonized vaccine trials provide an unprecedented sample population to address this issue. We characterized the protective efficacy of previous SARS-CoV-2 infection and hybrid immunity against COVID-19 early in the pandemic over three-to six-month follow-up and compared with vaccine-associated protection. Methods: In this post-hoc cross-protocol analysis of the Moderna, AstraZeneca, Janssen, and Novavax COVID-19 vaccine clinical trials, we allocated participants into four groups based on previous-infection status at enrolment and treatment: no previous infection/placebo; previous infection/placebo; no previous infection/vaccine; and previous infection/vaccine. The main outcome was RT-PCR-confirmed COVID-19 >7–15 days (per original protocols) after final study injection. We calculated crude and adjusted efficacy measures. Findings: Previous infection/placebo participants had a 92% decreased risk of future COVID-19 compared to no previous infection/placebo participants (overall hazard ratio [HR] ratio: 0.08; 95% CI: 0.05–0.13). Among single-dose Janssen participants, hybrid immunity conferred greater protection than vaccine alone (HR: 0.03; 95% CI: 0.01–0.10). Too few infections were observed to draw statistical inferences comparing hybrid immunity to vaccine alone for other trials. Vaccination, previous infection, and hybrid immunity all provided near-complete protection against severe disease. Interpretation: Previous infection, any hybrid immunity, and two-dose vaccination all provided substantial protection against symptomatic and severe COVID-19 through the early Delta period. Thus, as a surrogate for natural infection, vaccination remains the safest approach to protection. Funding: National Institutes of Health
Risk of COVID-19 after natural infection or vaccinationResearch in context
Summary: Background: While vaccines have established utility against COVID-19, phase 3 efficacy studies have generally not comprehensively evaluated protection provided by previous infection or hybrid immunity (previous infection plus vaccination). Individual patient data from US government-supported harmonized vaccine trials provide an unprecedented sample population to address this issue. We characterized the protective efficacy of previous SARS-CoV-2 infection and hybrid immunity against COVID-19 early in the pandemic over three-to six-month follow-up and compared with vaccine-associated protection. Methods: In this post-hoc cross-protocol analysis of the Moderna, AstraZeneca, Janssen, and Novavax COVID-19 vaccine clinical trials, we allocated participants into four groups based on previous-infection status at enrolment and treatment: no previous infection/placebo; previous infection/placebo; no previous infection/vaccine; and previous infection/vaccine. The main outcome was RT-PCR-confirmed COVID-19 >7–15 days (per original protocols) after final study injection. We calculated crude and adjusted efficacy measures. Findings: Previous infection/placebo participants had a 92% decreased risk of future COVID-19 compared to no previous infection/placebo participants (overall hazard ratio [HR] ratio: 0.08; 95% CI: 0.05–0.13). Among single-dose Janssen participants, hybrid immunity conferred greater protection than vaccine alone (HR: 0.03; 95% CI: 0.01–0.10). Too few infections were observed to draw statistical inferences comparing hybrid immunity to vaccine alone for other trials. Vaccination, previous infection, and hybrid immunity all provided near-complete protection against severe disease. Interpretation: Previous infection, any hybrid immunity, and two-dose vaccination all provided substantial protection against symptomatic and severe COVID-19 through the early Delta period. Thus, as a surrogate for natural infection, vaccination remains the safest approach to protection. Funding: National Institutes of Health