1,720,957 research outputs found
Time-to-go weighted optimal guidance with impact angle constraints
No full textIn this paper, the optimal guidance law with terminal constraints of miss distance and impact angle is presented for a constant speed missile against the stationary target. The proposed guidance law is obtained as the solution of a linear quadratic optimal control problem with the energy cost weighted by a power of the time-to-go. Systematic selection of guidance gains and trajectory shaping are possible by adjusting the exponent of the weighting function. A new time-to-go calculation method taking account of the trajectory curve is also proposed for implementation of the proposed law. Nonlinear and adjoint simulations are performed to investigate the performance of the proposed guidance law and time-to-go calculation method
Implementation of optimal guidance laws using predicted missile velocity profiles
No full textA class of weighted control-effort minimizing guidance laws are derived for missiles of varying velocity. As a practical weighting function, we consider a function of air density and missile velocity parameterized by positive real numbers. The resulting optimal guidance problem can be interpreted as the drag minimization problem for subsonic or supersonic missiles, depending on what parameters are used. This approach is extended easily to solve the drag minimization of a typical antiaircraft missile system with an arbitrary velocity profile and arbitrary drag characteristics, as demonstrated by a simulation study, We also present analytical results on how the guidance gain of the optimal law varies according to the values of the parameters. Because the optimal guidance laws make use of the future missile velocity profile, one critical issue is how to implement the laws. To avoid the difficulty that an inaccurately predicted missile velocity profile causes the guidance command to blow up in the last part of the engagement, we suggest two simple on-line velocity-profile updating schemes, which considerably alleviate the problem
Line-of-sight guidance laws for formation flight
No full textTwo-dimensional formation guidance laws for formation flight using only line-of-sight angle information are addressed. The main idea is to use the line-of-sight angles to two nearby vehicles to maintain the formation position of the current vehicle. Such a formation guidance law is useful because measurement of the line-of-sight angles does. not require data communication between the vehicles. We propose two methods of using the line-of-sight angle information for formation guidance: the angle information is used to control the flight-path angle and the velocity of the formation vehicle. Approach guidance and formation guidance of the two leading vehicles are also proposed. Stability of the proposed formation guidance laws is analyzed by using the Jacobian at the equilibrium point. Multiple-vehicle formation flight is simulated to verify the guidance laws proposed
Recursive time-to-go estimation for homing guidance missiles
No full textAddressed here Is the problem of computing accurate time-to-go estimates, which is an important issue in implementing various optimal guidance laws developed for missiles of time-varying velocity. A recursive time-to-go computation method which updates the time-to-go in a noniterative way Is presented. The recursive method includes an error compensation feature which explicitly computes the time-to-go error produced by non-zero initial heading errors. The proposed method is simple and straightforward to implement for any missile velocity profiles. Various numerical examples show that the proposed method works effectively for optimal guidance laws as wen as proportional navigation and augmented proportional navigation
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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