1,720,962 research outputs found
The Air Traffic Flow Management Problem with Time Windows
No full textThis paper defines a set of temporal intervals,
called time windows, which are defined prior to flight departure
and constitute milestones to be met during the flight execution.
The size of the time windows is variable as it reflects all known
constraints, such as punctuality at destination, runway capacities
or congested en-route areas that the flight will cross. Once a
time window is defined, all the air traffic actors are committed
to guarantee that flight operations, e.g. enter an airspace sector,
depart from or arrive at an airport, are executed within the
time window. We propose a two-step approach based on a mixed
integer programming formulation. The first step determines
a set of time windows such that the overall cost of delay is
minimized. Then in the second step we choose the set of optimal
time windows which also maximizes the overall time window
size. In such a way, we provide to all air traffic stakeholders
the largest degree of flexibility to perform their operations
under the constraint that the minimum achievable delay is kept
constant. We also gain information on the critical flights of the
system: if the optimal width of a time window is equal to its
minimum available value, any disruption that may cause the
flight not to meet it may produce undesired downstream effects.
Our preliminary computational experience based on small-scale
random instances confirms that the flexibility granted to flights
increases with the capacity while the system delay simultaneously
decreases. We also show that when there is no congestion a
non negligible share of small size time windows may exist, thus
indicating the existence of bottlenecks and critical flights
Granting flexible operations in congested airspaces
No full textSeveral causes of delay are deterring the air transportation system from being efficient. System capacity reductions are the major cause of delay. However, there are also some other causes, directly imputable to airlines, which may produce, in combination with the capacity reductions, undesired downstream effects. Therefore, with the purpose of containing delays and disruptions in flight schedules, it is important to grant flexibility to flight operations. This paper presents a mathematical formulation that allows to determine the degree of flexibility given to flights by identifying through a set of temporal intervals, called time windows, those flights that have a larger impact on the air traffic system performances. A time window is a period of time during which a certain phase of the flight (e.g., take off, landing and entry into a sector) has to be executed. The size of the time windows is variable as it reflects system’s capacity constraints. The set of time windows, which maximizes the total width of the time windows, provides airline operators and air traffic control authorities with the largest degree of flexibility to perform their operations. Several formulations of the models are presented, which vary in the way of formulating the use of system capacity. However, by means of a computational analysis, we show that the solution of the time window model is insensitive to the formulation used for the capacity constraints
Reducing ATFM delays through strategic flight planning
Full text linkThis paper presents an integer programming model for strategic redistribution of flights so as to respect nominal sector capacities, in short computation times for large-scale instances. The main contribution lies in the combination of tackling large-scale strategic flight planning using hard capacity constraints, while considering the whole network (i.e., both airports and sectors). Real historic data for network and traffic description are used for our test instance. Strategic and tactical impact assessments show that early flight planning can lead to the reduction of delays and their costs, showing potential for actual implementation
Tradable Mobility Permits for the Strategic Allocation of Air Traffic
No full textCurrent strategic decisions involving air traffic flows are limited to the allocation of airport slots. In this work, a first strategic market-based mechanism for the allocation of en route resources, i.e., sector capacity, is proposed. Identifying en route capacity shortages at this phase can lead to strategic actions to reduce flight delays on the day of operations. Tradable mobility permits previously developed for roadway transportation are analyzed and adapted to the air transport case. A trading mechanism for the strategic alleviation of air traffic congestion that uses time-place specific permits is proposed, providing alternative implementations. We call this new approach Tradable Flight Permit System (TFPS). An example of its use is illustrated, showing how it can lead to cost reductions for the airlines competing for a single sector access permit. We also highlight the improvements it can provide compared to current practice and a variety of alternative methods
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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