Comparison of two on-orbit attitude sensor alignment methods

Compared here are two methods of on-orbit alignment of vector attitude sensors. The first method uses the angular difference between simultaneous measurements from two or more sensors. These angles are compared to the angular differences between the respective reference positions of the sensed objects. The alignments of the sensors are adjusted to minimize the difference between the two sets of angles. In the second method, the sensor alignment is part of a state vector that includes the attitude. The alignments are adjusted along with the attitude to minimize all observation residuals. It is shown that the latter method can result in much less alignment uncertainty when gyroscopes are used for attitude propagation during the alignment estimation. The additional information for this increased accuracy comes from knowledge of relative attitude obtained from the spacecraft gyroscopes. The theoretical calculations of this difference in accuracy are presented. Also presented are numerical estimates of the alignment uncertainties of the fixed-head star trackers on the Extreme Ultraviolet Explorer spacecraft using both methods

The Cacophony of Politics: Northern Democrats and the American Civil War

J. Matthew Gallman delights in the chaos, offering a dizzying litany of anecdotes and personalities to give readers a sense of the opacity of partisanship in the early 1860s. “One chief goal here is to demonstrate how fundamentally messy, and often inconsistent, Civil War partisan politics really was,” he warns at the start. (3

Records recovery and terrorism

A recent disaster at Oregon State University did not occur the day after Christmas, on a Sunday, or even on campus. It was not the result ofa water pipe accidentally breaking, an earthquake, or a fire. Instead, it was caused by an act ofterrorism. Early in the morning of Monday, June 10, members ofthe Animal liberation Front (ALF), a radical animal- rights group, vandalized the University's mink research farm. A storage barn was completely destroyed by fire, graffiti was spray-painted on the farm's office and laboratory walls, research records in the office were dumped on the floor, and color slides were stolen. An unidentified chemical agent was poured on a small amount of record material, and a nearby bathroom fixture was broken, flooding the office and the strewn records. What faced the disaster recovery team at the mink research facility was not the usual disaster situation, as we have come to think of it in the archival and library community

NASA Innovative Advanced Concepts (NIAC) Phase 1 Final Report: Venus Landsailer Zephyr

Imagine sailing across the hot plains of Venus! A design for a craft to do just this was completed by the COncurrent Multidisciplinary Preliminary Assessment of Space Systems (COMPASS) Team for the NASA Innovative Advanced Concepts (NIAC) project. The robotic craft could explore over 30 km of surface of Venus, driven by the power of the wind

Probabilistic Graphical Model Representation in Phylogenetics

Recent years have seen a rapid expansion of the model space explored in statistical phylogenetics, emphasizing the need for new approaches to statistical model representation and software development. Clear communication and representation of the chosen model is crucial for: (1) reproducibility of an analysis, (2) model development and (3) software design. Moreover, a unified, clear and understandable framework for model representation lowers the barrier for beginners and non-specialists to grasp complex phylogenetic models, including their assumptions and parameter/variable dependencies. Graphical modeling is a unifying framework that has gained in popularity in the statistical literature in recent years. The core idea is to break complex models into conditionally independent distributions. The strength lies in the comprehensibility, flexibility, and adaptability of this formalism, and the large body of computational work based on it. Graphical models are well-suited to teach statistical models, to facilitate communication among phylogeneticists and in the development of generic software for simulation and statistical inference. Here, we provide an introduction to graphical models for phylogeneticists and extend the standard graphical model representation to the realm of phylogenetics. We introduce a new graphical model component, tree plates, to capture the changing structure of the subgraph corresponding to a phylogenetic tree. We describe a range of phylogenetic models using the graphical model framework and introduce modules to simplify the representation of standard components in large and complex models. Phylogenetic model graphs can be readily used in simulation, maximum likelihood inference, and Bayesian inference using, for example, Metropolis-Hastings or Gibbs sampling of the posterior distribution

RevBayes: Bayesian Phylogenetic Inference Using Graphical Models and an Interactive Model-Specification Language.

Programs for Bayesian inference of phylogeny currently implement a unique and fixed suite of models. Consequently, users of these software packages are simultaneously forced to use a number of programs for a given study, while also lacking the freedom to explore models that have not been implemented by the developers of those programs. We developed a new open-source software package, RevBayes, to address these problems. RevBayes is entirely based on probabilistic graphical models, a powerful generic framework for specifying and analyzing statistical models. Phylogenetic-graphical models can be specified interactively in RevBayes, piece by piece, using a new succinct and intuitive language called Rev. Rev is similar to the R language and the BUGS model-specification language, and should be easy to learn for most users. The strength of RevBayes is the simplicity with which one can design, specify, and implement new and complex models. Fortunately, this tremendous flexibility does not come at the cost of slower computation; as we demonstrate, RevBayes outperforms competing software for several standard analyses. Compared with other programs, RevBayes has fewer black-box elements. Users need to explicitly specify each part of the model and analysis. Although this explicitness may initially be unfamiliar, we are convinced that this transparency will improve understanding of phylogenetic models in our field. Moreover, it will motivate the search for improvements to existing methods by brazenly exposing the model choices that we make to critical scrutiny. RevBayes is freely available at http://www.RevBayes.com [Bayesian inference; Graphical models; MCMC; statistical phylogenetics.]

Building an Institutional Repository in Hard Times

This poster presents an overview of an exploratory research initiative to examine and assess the viability of developing an institutional repository system at a teaching-oriented four-year university with minimal monetary commitment. A need has been identified for an institutional repository and necessary steps have been taken to implement it. Several departments worked together to create a prototype Institutional Repository using DSpace, an open source repository software. This repository represents a unique endeavor, in that it has been instituted at a non-research based university and has chosen to involve students in the planning, design, implementation, and documentation stages of the project. In addition, the university\u27s Library Science students will also be involved in creating and maintaining collections. This poster focuses on the steps taken to set up and the plans to maintain a quality Institutional Repository at Valdosta State University without placing a large demand on the institution\u27s resources

Phase-field modeling of hydraulic fracture

This study demonstrates how phase-field methods can be applied to the problem of hydraulic fracture. Hydraulic fracturing involves coupling of fluid flow with material failure, which must account for the interactions of several cracks, both natural and man-made. The authors present a continuum thermodynamics framework for fluid flow through a damaging porous medium to represent the processes and interactions that occur during hydraulic fracturing. The model presented is capable of simulating both Stokesian flow through cracks and Darcy flow through the porous medium. The flow is coupled to the deformation of the bulk solid medium and the evolution of cracks within the material. Thus, fractures are propagated via the pressurization of fluid injected into the cracks. In this study, several benchmark solutions are presented to verify the expected behavior and accuracy of the method. In addition, a number of interesting problems are presented that demonstrate the ability of the method to respond to various complexities like material inhomogeneity and multiple cracks interacting under various loading conditions