Separated spacecraft interferometry : system architecture design and optimization

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1999.Includes bibliographical references (p. 123-126).Through a process of system design, analysis, and optimization, the trade space for future optical separated spacecraft interferometers (SSI's) is explored. Using Distributed Satellite System principles, eleven separate architectures ranging in size from three to five spacecraft are compared on the basis of four metrics: 1) capability, 2) performance, 3) adaptability, and 4) cost per function. The independent SSI architecture variables include the total number of spacecraft, the type of spacecraft, the number of combiner payloads, and the number of collector payloads. Architecture variables held constant in this study, but which may be varied in future studies, include the array's geometric configuration, the maximum baseline, the mass and power requirements of each payload, the spacecraft propulsion system, the component failure rates, and the mission design life. Three models are developed and coupled to analyze each architecture. A capability model calculates the instantaneous u-v coverage, image quality, angular resolution, and imaging rate of each proposed architecture. The reliability model uses combinatorial analysis and Markov techniques to determine both the probability that the system will continue to function over a given amount of time and the likelihood with which the system will function in different partially failed states throughout the mission. The reliability model is then coupled with the capability model to calculate total performance over the mission lifetime. The cost model, which is divided into payload, spacecraft bus, launch, and operations costs, estimates the total lifecycle cost of each architecture. On the basis of the cost per synthesized image metric, two architectures were identified as providing the greatest value for the money. The design features in order of importance are imaging rate, total system cost, and total system reliability.v by Cyrus D. Jilla.S.M

A multiobjective, multidisciplinary design optimization methodology for the conceptual design of distributed satellite systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2002.Includes bibliographical references (p. 427-438).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.A multiobjective, multidisciplinary design optimization methodology for mathematically modeling the distributed satellite system (DSS) conceptual design problem as an optimization problem has been developed to advance the state-of-the-art in complex distributed satellite network design. An increasing number of space missions are utilizing DSS architectures in which multiple satellites work in a coordinated fashion to improve system performance, cost, and survivability. The trade space for distributed satellite systems can be enormous - too large to enumerate, analyze, and compare all possible architectures. The seven-step methodology enables an efficient search of the trade space for the best families of architectures, and explores architectures that might not otherwise be considered during the conceptual design phase, the phase of a DSS program in which the majority of lifecycle cost gets locked in. Four classes of multidisciplinary design optimization (MDO) techniques are investigated - Taguchi, heuristic, gradient, and univariate methods. The heuristic simulated annealing (SA) algorithm found the best DSS architectures with the greatest consistency due to its ability to escape local optima within a nonconvex trade space. Accordingly, this SA algorithm forms the core single objective MDO algorithm in the methodology. The DSS conceptual design problem scope is then broadened by expanding from single objective to multiobjective optimization problems, and two variant multiobjective SA algorithms are developed.(cont.) The utility in knowing the global Pareto boundary of a DSS trade space is presented, and several methods are explored for approximating the true global Pareto boundary with only a limited knowledge of the full DSS trade space. Finally, methods for improving the performance of the SA algorithm are tested, and it was found that the 2-DOF variant of the SA algorithm is most effective at both single objective and multiobjective searches of a DSS trade space. The versatility of the methodology is demonstrated through its application to the conceptual design of three separate distributed satellite systems - the civil NASA Origins Terrestrial Planet Finder mission, the military TechSat 21 GMTI space-based radar mission, and the commercial broadband satellite communications mission. In each case, the methodology identifies more cost-effective system architectures than those previously considered for the single objective optimization problem, and a Pareto optimal set of architectures for the multiobjective optimization problem. In this manner, the methodology serves as a powerful, versatile systems engineering tool for the conceptual design of distributed satellite systems.by Cyrus D. Jilla.Ph.D

Patterns of geohelminth infection, impact of albendazole treatment and re-infection after treatment in schoolchildren from rural KwaZulu-Natal/South-Africa

BACKGROUND: Geohelminth infection is a major health problem of children from rural areas of developing countries. In an attempt to reduce this burden, the Department of Health of the province of KwaZulu-Natal (KZN) established in 1998 a programme for helminth control that aimed at regularly treating primary school children for schistosomiasis and intestinal helminths. This article describes the baseline situation and the effect of treatment on geohelminth infection in a rural part of the province. METHODS: Grade 3 schoolchildren from Maputaland in northern KZN were examined for infections with hookworm, Ascaris lumbricoides, and Trichuris trichiura, treated twice with 400 mg albendazole and re-examined several times over one year after the first treatment in order to assess the impact of treatment and patterns of infection and re-infection. RESULTS: The hookworm prevalence in the study population (83.2%) was considerably higher than in other parts of the province whereas T. trichiura and especially A. lumbricoides prevalences (57.2 and 19.4%, respectively) were much lower than elsewhere on the KZN coastal plain. Single dose treatment with albendazole was very effective against hookworm and A. lumbricoides with cure rates (CR) of 78.8 and 96.4% and egg reduction rates (ERR) of 93.2 and 97.7%, respectively. It was exceptionally ineffective against T. trichiura (CR = 12.7%, ERR = 24.8%). Re-infection with hookworm and A. lumbricoides over 29 weeks after treatment was considerable but still well below pre-treatment levels. CONCLUSION: High geohelminth prevalences and re-infection rates in the study population confirm the need for regular treatment of primary school children in the area. The low effectiveness of single course albendazole treatment against T. trichiura infection however demands consideration of alternative treatment approaches

A Reliability Model for the Design and Optimization of Separated Spacecraft Interferometer Arrays

The concept of using identical spacecraft for space-based optical interferometry is introduced. The built in redundancy of such a separated spacecraft interferometer (SSI) design not only improves the reliability of the system, but also improves system performance by placing the redundant components where they can be used during nominal operations. Five metrics have been developed to compare SSI designs. These include 1) total system reliability, 2) specific system reliability, 3) cost per image, 4) time to produce an image, and 5) reduced mission effectiveness due to partial system failure. The reliability model incorporates both combinatorial analysis and Markov modeling to evaluate different SSI designs on the basis of these five metrics. The results indicate that the modular and multifunctional spacecraft (MAMS/C) design rates higher than the single function spacecraft design (SFD) for all five metrics under the assumed mission parameters. These parameters include the number of small satellites in the array, the failure rate of the three components within the array, and the mission design life. For small arrays with extremely reliable components and short mission design lives, the current NASA SFD array with only one combiner suffices. This is because the intended design life of the system is shorter than the mean-time-to failure of the system. For future larger arrays with more realistic component failure rates and longer mission design lives, designs that incorporate both collector and combiner functions on each small satellite bus rate higher. On the basis of these results, rules of thumb have been developed for the design and optimization of SSI small satellite arrays