682 research outputs found
Apparatus and method for control of a solid fueled rocket vehicle Patent
Solid propellant rocket vehicle thrust control method and apparatu
Speeding up of microstructure reconstruction: I. Application to labyrinth patterns
Recently, entropic descriptors based the Monte Carlo hybrid reconstruction of
the microstructure of a binary/greyscale pattern has been proposed (Piasecki
2011 Proc. R. Soc. A 467 806). We try to speed up this method applied in this
instance to the reconstruction of a binary labyrinth target. Instead of a
random configuration, we propose to start with a suitable synthetic pattern
created by cellular automaton. The occurrence of the characteristic attributes
of the target is the key factor for reducing the computational cost that can be
measured by the total number of MC steps required. For the same set of basic
parameters, we investigated the following simulation scenarios: the
biased/random alternately mixed #2m approach, the strictly biased #2b and the
random/partially biased #2rp one. The series of 25 runs were performed for each
scenario. To maintain comparable accuracy of the reconstructions, during the
final stages the only selection procedure we used was the biased one. This
allowed us to make the consistent comparison of the first three scenarios. The
purely random #2r approach of low efficiency was included only for completeness
of the approaches. Finally, for the conditions established, the best single
reconstruction and the best average tolerance value among all the scenarios
were given by the mixed #2m method, which was also the fastest one. The
slightly slower the alternative #2b and #2rp variants provided comparable but
less satisfactory results.Comment: 11 pages, 3 figures, 2 tables, published versio
Search for universality in one-dimensional ballistic annihilation kinetics
We study the kinetics of ballistic annihilation for a one-dimensional ideal
gas with continuous velocity distribution. A dynamical scaling theory for the
long time behavior of the system is derived. Its validity is supported by
extensive numerical simulations for several velocity distributions. This leads
us to the conjecture that all the continuous velocity distributions \phi(v)
which are symmetric, regular and such that \phi(0) does not vanish, are
attracted in the long time regime towards the same Gaussian distribution and
thus belong to the same universality class. Moreover, it is found that the
particle density decays as n(t)~t^{-\alpha}, with \alpha=0.785 +/- 0.005.Comment: 8 pages, needs multicol, epsf and revtex. 8 postscript figures
included. Submitted to Phys. Rev. E. Also avaiable at
http://mykonos.unige.ch/~rey/publi.html#Secon
Earth-Facing Antenna Characterization in Complex Ground Plane/Multipath Rich Environment
The Space Communications and Navigation (SCAN) Testbed was a Software Defined Radio (SDR)-based payload launched to the International Space Station (ISS) in July of 2012. The purpose of the SCAN Testbed payload was to investigate the applicability of SDRs to NASA space missions in an operational environment, which means that a proper model for system performance in said operational space environment is a necessary condition. The SCAN Testbed has line-of-sight connections to various ground stations with its S-Band Earth-facing Near-Earth-Network Low Gain Antenna (NEN-LGA). Any previous efforts to characterize the NEN-LGA proved difficult, therefore, the NASA Glenn Research Center built its own S-Band ground station, which became operational in 2015, and has been used successfully to characterize the NEN-LGA's in-situ pattern measurements. This methodology allows for a more realistic characterization of the antenna performance, where the pattern oscillation induced by the complex ISS ground plane, as well as shadowing effects due to ISS structural blockage are included into the final performance model. This paper describes the challenges of characterizing an antenna pattern in this environment. It will also discuss the data processing, present the final antenna pattern measurements and derived model, as well as discuss various lessons learne
Earth-Facing Antenna Characterization in a Complex Ground Plane/Multipath Rich Environment
The Space Communications and Navigation (SCAN) Testbed was a Software Defined Radio (SDR)-based payload launched to the International Space Station (ISS) in July of 2012. The purpose of the SCAN Testbed payload was to investigate the applicability of SDRs to NASA space missions in an operational space environment, which means that a proper model for system performance in said operational space environment is a necessary condition. The SCAN Testbed has line-of-sight connections to various ground stations with its S-Band Earth-facing Near-Earth Network Low Gain Antenna (NEN-LGA). Any previous efforts to characterize the NEN-LGA proved difficult, therefore, the NASA Glenn Research Center built its own S-Band ground station, which became operational in 2015, and has been successfully used to characterize the NEN-LGAs in-situ pattern measurements. This methodology allows for a more realistic characterization of the antenna performance, where the pattern oscillation induced by the complex ISS ground plane, as well as shadowing effects due to ISS structural blockage are included into the final performance model. This paper describes the challenges of characterizing an antenna pattern in this environment. It will also discuss the data processing, present the final antenna pattern measurements and derived model, as well as discuss various lessons learned
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