Use of automated rendezvous trajectory planning to improve spacecraft operations efficiency

The current planning process for space shuttle rendezvous with a second Earth-orbiting vehicle is time consuming and costly. It is a labor-intensive, manual process performed pre-mission with the aid of specialized maneuver processing tools. Real-time execution of a rendezvous plan must closely follow a predicted trajectory, and targeted solutions leading up to the terminal phase are computed on the ground. Despite over 25 years of Gemini, Apollo, Skylab, and shuttle vehicle-to-vehicle rendezvous missions flown to date, rendezvous in Earth orbit still requires careful monitoring and cannot be taken for granted. For example, a significant trajectory offset was experienced during terminal phase rendezvous of the STS-32 Long Duration Exposure Facility retrieval mission. Several improvements can be introduced to the present rendezvous planning process to reduce costs, produce more fuel-efficient profiles, and increase the probability of mission success

Tests of an International Capital Asset Pricing Model with Stocks and Government Bonds and Regime Switching Prices of Risk and Intercepts

The paper tests a conditional multivariate International Capital Asset Pricing Model for US, Japanese and European stocks and government bonds, covering the period 1993-2001. Time variation in the prices of market and currency risk is modelled by means of synchronous regime switching. The paper also explores the statistical significance and time variation of asset specific intercept terms, again using synchronous regime switching. The prices of risk are found to be highly time varying. The price of market risk is statistically significant, and the international CAPM risk premia are validated, although currency risk premia are not statistically significant. However, the intercept terms are typically large and significant, implying an overall rejection of the international CAPM, and suggesting that additional, unidentified pricing factors contribute to return expectations.CAPM; Multivariate GARCH-in-Mean

Income dynamics and the life cycle

This paper argues that our understanding of income and poverty dynamics benefits from taking a life cycle perspective. A person¿s age and family circumstances ¿ the factors that shape their life cycle ¿ affect the likelihood of experiencing key life events, such as partnership formation, having children, or retirement; this in turn affects their probability of experiencing rising, falling, or other income trajectories. Using ten waves of the British Household Panel Survey, we analyse the income trajectories of people at different stages in their lives in order to build a picture of income dynamics over the whole life cycle. We find that particular life events are closely associated with either rising or falling trajectories, but that there is considerable heterogeneity in income trajectories following these different events. Typically, individuals experiencing one of these life events are around twice as likely to experience a particular income trajectory, but most individuals will not follow the trajectory most commonly associated with that life event. This work improves our understanding of the financial impact of different life events and provides an indication of how effectively the welfare state cushions people against the potentially adverse impact of certain events

Quasar H II Regions During Cosmic Reionization

Cosmic reionization progresses as HII regions form around sources of ionizing radiation. Their average size grows continuously until they percolate and complete reionization. We demonstrate how this typical growth can be calculated around the largest, biased sources of UV emission, such as quasars, by further developing an analytical model based on the excursion set formalism. This approach allows us to calculate the sizes and growth of the HII regions created by the progenitors of any dark matter halo of given mass and redshift with a minimum of free parameters. Statistical variations in the size of these pre-existing HII regions are an additional source of uncertainty in the determination of very high redshift quasar properties from their observed HII region sizes. We use this model to demonstrate that the transmission gaps seen in very high redshift quasars can be understood from the radiation of only their progenitors and associated clustered small galaxies. The fit sets a lower limit on the redshift of overlap at z = 5.8 +/- 0.1. This interpretation makes the transmission gaps independent of the age of the quasars observed. If this interpretation were correct it would raise the prospects of using radio interferometers currently under construction to detect the epoch of reionization.Comment: 6 pages, 3 figures, accepted by MNRAS, revised to match published versio

The direct perception hypothesis: perceiving the intention of another’s action hinders its precise imitation

We argue that imitation is a learning response to unintelligible actions, especially to social conventions. Various strands of evidence are converging on this conclusion, but further progress has been hampered by an outdated theory of perceptual experience. Comparative psychology continues to be premised on the doctrine that humans and nonhuman primates only perceive others’ physical ‘surface behavior’, while mental states are perceptually inaccessible. However, a growing consensus in social cognition research accepts the Direct Perception Hypothesis: primarily we see what others aim to do; we do not infer it from their motions. Indeed, physical details are overlooked – unless the action is unintelligible. On this basis we hypothesize that apes’ propensity to copy the goal of an action, rather than its precise means, is largely dependent on its perceived intelligibility. Conversely, children copy means more often than adults and apes because, uniquely, much adult human behavior is completely unintelligible to unenculturated observers due to the pervasiveness of arbitrary social conventions, as exemplified by customs, rituals, and languages. We expect the propensity to imitate to be inversely correlated with the familiarity of cultural practices, as indexed by age and/or socio-cultural competence. The Direct Perception Hypothesis thereby helps to parsimoniously explain the most important findings of imitation research, including children’s over-imitation and other species-typical and age-related variations