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At least 253 records · Page 14Linked to original sources
PubMed2005

A roadmap for optimal control: the right way to commute.

Optimal control theory is the foundation for many problems in astrodynamics. Typical examples are trajectory design and optimization, relative motion control of distributed space systems and attitude steering. Many such problems in astrodynamics are solved by an alternative route of mathematical analysis and deep physical insight, in part because of the perception that an optimal control framework generates hard problems. Although this is indeed true of the Bellman and Pontryagin frameworks, the covector mapping principle provides a neoclassical approach that renders hard problems easy. That is, although the origins of this philosophy can be traced back to Bernoulli and Euler, it is essentially modern as a result of the strong linkage between approximation theory, set-valued analysis and computing technology. Motivated by the broad success of this approach, mission planners are now conceiving and demanding higher performance from space systems. This has resulted in new set of theoretical and computational problems. Recently, under the leadership of NASA-GRC, several workshops were held to address some of these problems. This paper outlines the theoretical issues stemming from practical problems in astrodynamics. Emphasis is placed on how it pertains to advanced mission design problems.

Journal Article↗
PubMed2006

Flight and echolocation behaviour of whiskered bats commuting along a hedgerow: range-dependent sonar signal design, Doppler tolerance and evidence for 'acoustic focussing'.

Echolocating bats obtain three-dimensional images of their surroundings in complete darkness by emitting sonar signals and evaluating returning echoes. When flying close to objects, bats risk collision and therefore depend on the accuracy of images--particularly in the perceived distance of obstacles, which is coded by the time delay between call and echo. Yet, during flight, such accuracy is perturbed first because bats call and receive echoes at different positions and second because echoes are modified by Doppler shifts. Certain call designs avoid both sources of ranging error, but only for a limited range of distances [the 'distance of focus' (DOF)]. Here, we show that whiskered bats (Myotis mystacinus) using broadband echolocation calls adjust call design in a range-dependent manner so that nearby obstacles are localised accurately. Such behaviour is adaptive because it reduces collision risk. The bats also reduced call duration to some extent as they approached obstacles so that most returning echoes arrived after they finished calling. This reduction in call duration during the approach to obstacles was neither the only nor the main factor that influenced DOF. Indeed, both duration and bandwidth of calls influenced DOF independently, with lower bandwidths and longer durations giving greater DOF. Our findings give a new perspective on the adaptive significance of echolocation call design in nature and have implications for sonar engineering.

Animals↗
PubMed2006

The role of affect in UK commuters' travel mode choices: an interpretative phenomenological analysis.

Previous research has suggested that the choice between public transport and private car use is not solely based on utility considerations, such as time and cost. However, affective considerations tend not to be targeted in policy interventions to reduce car use. This may be due, in part, to a lack of clarity about which affective responses to car use are important and how they may affect willingness to switch to public transport. This study sought to clarify the role of affective responses in transport mode choice. An interpretative phenomenological analysis (IPA) of car users' accounts was conducted to (i) explore affect associated with decisions to drive or use public transport to get to work; and (ii) describe the role of affect on such transport decisions, and its relationship to utility considerations. Semi-structured interviews were conducted with 18 car users employed at a medium-sized UK university. Four affect themes were identified: These were journey-based affect (JBA), personal space, autonomy and identity. Typical 'utility' factors such as time, cost and reliability had important affective effects, and these were considered alongside utility components (e.g. getting to work on time). However, these effects were not always additive, and the role of affect depended on participants' own assessment of their circumstances. Implications for interventions are discussed.

Adult↗
PubMedDate not supplied

Suitability of commuting by bicycle to Arizona elementary schools.

PURPOSE: To determine the biking suitability (i.e., bikeability) of and prevalence of biking in 14 elementary schools representing two extremes of bused students (2.4% vs. 53.6%). METHODS: Street segments (within 0.25-mile radius of school) were scored for bikeability. Bikes in racks per school student population established biking prevalence. Mann-Whitney U-test compared bikeability and prevalence of biking between groups. RESULTS: A total of 12.5 +/- 2.2 streets per school were assessed. Thirteen schools scored very good (< 3.0) and one scored fair (4.0-4.9). Median bikeability score was 0.69 for the low-busing schools and 0.53 for the high-busing schools (nonsignificant). Median biking prevalence was 3.1% in the low-busing schools and 1.3% in the high-busing schools (p < .05). CONCLUSION: Streets surrounding schools were adequate for biking. Biking prevalence was significantly higher in low-busing schools but was relatively low in both low- and high-busing schools. Other factors, including intraindividual, social, school, and community, likely contribute to choice of biking to school.

Arizona↗