1,427 research outputs found
Scalability and Evolutionary Dynamics of Air Transportation Networks in the United States
With the growing demand for air transportation and the limited ability to increase capacity at key points in the air transportation system, there are concerns that, in the future, the system will not scale to meet demand. This situation will result in the generation and the propagation of delays throughout the system, impacting passengers’ quality of travel and more broadly the economy. There is therefore the need to investigate the mechanisms by
which the air transportation system scaled to meet demand in the past and will do so in the
future. In order to investigate limits to scale of current air transportation networks, theories of scale free and scalable networks were used. It was found that the U.S. air transportation network is not scalable at the airport level due to capacity constraints. However, the results of a case study analysis of multi-airport systems that led to the aggregation of these multiple airports into single nodes and the analysis of this network showed that the air transportation network was scalable at the regional level. In order to understand how the network evolves,
an analysis of the scaling dynamics that influence the structure of the network was
conducted. Initially the air transportation network scales according to airport level
mechanisms –through the addition of capacity and the improvement of efficiency- but as
infrastructure constraints are reached; higher level scaling mechanisms such as the
emergence of secondary airports and the construction of new high capacity airports are
triggered. These findings suggest that, given current and future limitations on the ability to
add capacity at certain airports, regional level scaling mechanisms will be key to
accommodating future needs for air transportation.This work was supported by NASA Langley under grant NAG-1-2038 and by the FAA under contract DTFA01-01-C-00030’D.0#16
Interpreting the photometry and spectroscopy of directly imaged planets: a new atmospheric model applied to beta Pictoris b and SPHERE observations
We aim to interpret future photometric and spectral measurements from these
instruments, in terms of physical parameters of the planets, with an
atmospheric model using a minimal number of assumptions and parameters.
We developed Exoplanet Radiative-convective Equilibrium Model (Exo-REM) to
analyze the photometric and spectro- scopic data of directly imaged planets.
The input parameters are a planet's surface gravity (g), effective temperature
(Teff ), and elemental composition. The model predicts the equilibrium
temperature profile and mixing ratio profiles of the most important gases.
Opacity sources include the H2-He collision-induced absorption and molecular
lines from eight compounds (including CH4 updated with the Exomol line list).
Absorption by iron and silicate cloud particles is added above the expected
condensation levels with a fixed scale height and a given optical depth at some
reference wavelength. Scattering was not included at this stage.
We applied Exo-REM to photometric and spectral observations of the planet
beta Pictoris b obtained in a series of near-IR filters. We derived Teff = 1550
+- 150 K, log(g) = 3.5 +- 1, and radius R = 1.76 +- 0.24 RJup (2-{\sigma} error
bars from photometric measurements). These values are comparable to those found
in the literature, although with more conservative error bars, consistent with
the model accuracy. We were able to reproduce, within error bars, the J- and
H-band spectra of beta Pictoris b. We finally investigated the precision to
which the above parameterComment: 15 pages, 14 figures, accepted by A&
Investigation of the Potential Impacts of the Entry of Very Light Jets in the National Airspace System
Very Light Jets (VLJs) constitute a class of three to eight passenger turbofan-powered aircraft that will enter service in 2006 and will need to be integrated into the National Airspace System. An aircraft performance analysis showed similarities between the predicted performance and capability of Very Light Jets and the performance of existing Light Jets. Based on this an analysis of operating patterns of existing Light Jets was used to predict how Very Light Jets will be operated. Using 396 days of traffic data from the FAA Enhanced Traffic Management System (ETMS), the operating patterns of existing Light Jets were analyzed. It was found that 64% of all the flights flown by Light Jets had their origin, destination or both within the top 23 regional airport systems in the continental United States. This concentration of LJ traffic was found in areas of the air transportation system that are currently exhibiting dense traffic and capacity constraints. The structure of the network of routes flown by existing Light Jets was also studied and a model of network growth was developed. It is anticipated that this concentration will persist with emerging Very Light Jet traffic. This concentration of traffic at key areas in the system will have implications for air traffic control management and airport activity. For regional airport systems, core airports are expected to saturate and, reliever airports will become critical for accommodating traffic demand. The entry of Very Light Jets will significantly increase the traffic load at the terminal airspace; Terminal Radar Approach Control (TRACON). These impacts need to be taken into account to allow a successful integration of these aircraft in the National Airspace System
Emergence of Secondary Airports and Dynamics of Regional Airport Systems in the United States
With the growing demand for air transportation and limited capacity at major airports, there is a need to increase the capacity of airport systems at the metropolitan area level. The increased use of secondary airports has been and is expected to be one of the key mechanisms by which future demand is met in congested metropolitan areas.
This thesis provides an analysis of the factors influencing the emergence of secondary airports and the dynamics of multi-airport systems. The congestion of the core airport, the distribution of population at the regional level, the existence and the proximity of a secondary basin of population close to secondary airports were identified as major factors. Ground access and airport infrastructure, the low level of connecting passengers at the core airport were also identified as a contributing factors. The entry of an air carrier –generally a low-cost carrier- was determined to be an essential stimulus in the emergence phenomenon impacting fares and airport competition levels resulting in market stimulation. But the emergence of secondary airports imposes new constraints that need to be taken into account in the national air transportation system improvements. By providing an identification of the factors that influence the emergence of secondary airports and an understanding of the dynamics of regional airport systems this research provides useful support for the planning and the future development of multi-airport systems.NASA Langley, NAG-1-203
Dynamics of Implementation of Mitigating Measures to Reduce CO2 Emissions from Commercial Aviation
Increasing demand for air transportation and growing environmental concerns motivate the need to implement measures to reduce CO2 emissions from aviation. Case studies of historical changes in the aviation industry have shown that the implementation of changes generally followed S-curves with relatively long time–constants. This research analyzed the diffusion characteristics of a portfolio of CO2 emission mitigating measures and their relative contribution to cumulative system wide improvements. A literature review identified 41 unique measures, including (1) technological improvements, (2) operational improvements, and (3) the use of alternative fuels. It was found that several operational changes can be implemented in the short term but are unlikely to significantly reduce CO2 emissions. Technology retrofits and some operational changes can be implemented in the medium term. 2nd and 3rd generation biofuels can significantly reduce carbon emissions but are likely to have long diffusion times and may not be available in sufficient quantities to the aviation industry. Technology measures in the form of next generation aircraft have the highest CO2 reduction potential, but only in the long term due to slow fleet turnover.
An Aircraft Diffusion Dynamic Model (ADDM) was developed using System Dynamics modeling techniques to understand how the fleet efficiency will be influenced by the entry of various generations of aircraft with different levels of emissions performance. The model was used to evaluate effects of several future potential scenarios on the US narrow body jet fleet as well as their sensitivity to S-curve parameters.
Results from the model showed that strategies that emphasize the early entry into service of available technology, as opposed to waiting and delaying entry for more fuel- efficient technology, have greater potential to improve fleet fuel-burn performance. Also, strategies that incentivize early retirement of older aircraft have marginal potential for reducing fuel burn.
Future demand scenarios showed that the infusion of fuel-efficient aircraft alone is unlikely to reduce emissions below 2006 levels. Instead, a portfolio of measures that also include demand reduction mechanisms, operational improvements, and adoption of alternative fuels will be required in order to limit the growth of CO2 emissions from aviation.This work was supported by the MIT/Masdar Institute of Science and Technology under grant number Mubadala Development Co. Agreement 12/1/06
Hunting for brown dwarf binaries and testing atmospheric models with X-Shooter
The determination of the brown dwarf binary fraction may contribute to the
understanding of the substellar formation mechanisms. Unresolved brown dwarf
binaries may be revealed through their peculiar spectra or the discrepancy
between optical and near-infrared spectral type classification.
We obtained medium-resolution spectra of 22 brown dwarfs with these
characteristics using the X-Shooter spectrograph at the VLT.
We aimed to identify brown dwarf binary candidates, and to test if the
BT-Settl 2014 atmospheric models reproduce their observed spectra.
To find binaries spanning the L-T boundary, we used spectral indices and
compared the spectra of the selected candidates to single spectra and synthetic
binary spectra. We used synthetic binary spectra with components of same
spectral type to determine as well the sensitivity of the method to this class
of binaries.
We identified three candidates to be combination of L plus T brown dwarfs. We
are not able to identify binaries with components of similar spectral type. In
our sample, we measured minimum binary fraction of .
From the best fit of the BT-Settl models 2014 to the observed spectra, we
derived the atmospheric parameters for the single objects. The BT-Settl models
were able to reproduce the majority of the SEDs from our objects, and the
variation of the equivalent width of the RbI (794.8 nm) and CsI (852.0 nm)
lines with the spectral type. Nonetheless, these models did not reproduce the
evolution of the equivalent widths of the NaI (818.3 nm and 819.5 nm) and KI
(1253 nm) lines with the spectral type.Comment: Accepted for publication in MNRA
High-Cadence, High-Contrast Imaging for Exoplanet Mapping: Observations of the HR 8799 Planets with VLT/SPHERE Satellite Spot-Corrected Relative Photometry
Time-resolved photometry is an important new probe of the physics of
condensate clouds in extrasolar planets and brown dwarfs. Extreme adaptive
optics systems can directly image planets, but precise brightness measurements
are challenging. We present VLT/SPHERE high-contrast, time-resolved broad
H-band near-infrared photometry for four exoplanets in the HR 8799 system,
sampling changes from night to night over five nights with relatively short
integrations. The photospheres of these four planets are often modeled by
patchy clouds and may show large-amplitude rotational brightness modulations.
Our observations provide high-quality images of the system. We present a
detailed performance analysis of different data analysis approaches to
accurately measure the relative brightnesses of the four exoplanets. We explore
the information in satellite spots and demonstrate their use as a proxy for
image quality. While the brightness variations of the satellite spots are
strongly correlated, we also identify a second-order anti-correlation pattern
between the different spots. Our study finds that PCA-based KLIP reduction with
satellite spot-modulated artificial planet-injection based photometry (SMAP)
leads to a significant (~3x) gain in photometric accuracy over standard
aperture-based photometry and reaches 0.1 mag per point accuracy for our
dataset, the signal-to-noise of which is limited by small field rotation.
Relative planet-to-planet photometry can be compared be- tween nights, enabling
observations spanning multiple nights to probe variability. Recent high-quality
relative H-band photometry of the b-c planet pair agree to about 1%.Comment: Astrophysical Journal, in pres
Pulsar Prospects for the Cherenkov Telescope Array
In the last few years, the Fermi-LAT telescope has discovered over a 100
pulsars at energies above 100 MeV, increasing the number of known gamma-ray
pulsars by an order of magnitude. In parallel, imaging Cherenkov telescopes,
such as MAGIC and VERITAS, have detected for the first time VHE pulsed
gamma-rays from the Crab pulsar. Such detections have revealed that the Crab
VHE spectrum follows a power-law up to at least 400 GeV, challenging most
theoretical models, and opening wide possibilities of detecting more pulsars
from the ground with the future Cherenkov Telescope Array (CTA). In this
contribution, we study the capabilities of CTA for detecting Fermi pulsars. For
this, we extrapolate their spectra with "Crab-like" power-law tails in the VHE
range, as suggested by the latest MAGIC and VERITAS results.Comment: 4 pages, 3 figures. In Proceedings of the 2012 Heidelberg Symposium
on High Energy Gamma-Ray Astronomy. All CTA contributions at arXiv:1211.184
An upper limit on the mass of the circumplanetary disk for DH Tau b
DH Tau is a young (1 Myr) classical T Tauri star. It is one of the few
young PMS stars known to be associated with a planetary mass companion, DH Tau
b, orbiting at large separation and detected by direct imaging. DH Tau b is
thought to be accreting based on copious H emission and exhibits
variable Paschen Beta emission. NOEMA observations at 230 GHz allow us to place
constraints on the disk dust mass for both DH Tau b and the primary in a regime
where the disks will appear optically thin. We estimate a disk dust mass for
the primary, DH Tau A of , which gives a disk-to-star
mass ratio of 0.014 (assuming the usual Gas-to-Dust mass ratio of 100 in the
disk). We find a conservative disk dust mass upper limit of 0.42
for DH Tau b, assuming that the disk temperature is dominated by irradiation
from DH Tau b itself. Given the environment of the circumplanetary disk,
variable illumination from the primary or the equilibrium temperature of the
surrounding cloud would lead to even lower disk mass estimates. A MCFOST
radiative transfer model including heating of the circumplanetary disk by DH
Tau b and DH Tau A suggests that a mass averaged disk temperature of 22 K is
more realistic, resulting in a dust disk mass upper limit of 0.09
for DH Tau b. We place DH Tau b in context with similar objects and discuss the
consequences for planet formation models.Comment: accepted for publication in A
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