40,785 research outputs found
An experimental study on a motion sensing system for sports training
In sports science, motion data collected from athletes is
used to derive key performance characteristics, such as stride length
and stride frequency, that are vital coaching support information. The
sensors for use must be more accurate, must capture more vigorous
events, and have strict weight and size requirements, since they must
not themselves affect performance. These requirements mean each
wireless sensor device is necessarily resource poor and yet must be
capable of communicating a considerable amount of data, contending
for the bandwidth with other sensors on the body. This paper analyses
the results of a set of network traffic experiments that were designed
to investigate the suitability of conventional wireless motion sensing
system design � which generally assumes in-network processing - as
an efficient and scalable design for use in sports training
Behavior modelling and individual recognition of sonar transmitter for secure communication in UASNs
It is necessary to improve the safety of the underwater acoustic sensor networks (UASNs) since it is mostly used in the military industry. Specific emitter identification is the process of identifying different transmitters based on the radio frequency fingerprint extracted from the received signal. The sonar transmitter is a typical low-frequency radiation source and is an important part of the UASNs. Class D Power Amplifier, a typical non-linear amplifier, is usually used in sonar transmitters. The inherent nonlinearity of
power amplifiers provides fingerprint features that can be distinguished without transmitters for specific emitter recognition. Firstly, the non-linearity of the sonar transmitter is studied in depth, and the nonlinearity of the power amplifier is modeled and its non-linearity characteristics are analyzed. After obtaining the nonlinear model of an amplifier, a similar amplifier in practical application is obtained by changing its model parameters as the research object. The output signals are collected by giving the same input of different models, and then the output signals are extracted and classified. In this paper, the memory polynomial model is used to model the amplifier. The power spectrum features of the output signals are extracted as fingerprint features. Then the dimensionality of the high-dimensional features is reduced. Finally, the classifier is used to recognize the amplifier. The experimental results show that the individual sonar transmitter can be well identified by using the non-linear characteristics of the signal. By this way, this method can enhance the communication safety of UASNs
An unexpectedly low-redshift excess of Swift gamma-ray burst rate
Gamma-ray bursts (GRBs) are the most violent explosions in the Universe and
can be used to explore the properties of high-redshift universe. It is believed
that the long GRBs are associated with the deaths of massive stars. So it is
possible to use GRBs to investigate the star formation rate (SFR). In this
paper, we use Lynden-Bell's method to study the luminosity function and
rate of \emph{Swift} long GRBs without any assumptions. We find that the
luminosity of GRBs evolves with redshift as with
. After correcting the redshift evolution through
, the luminosity function can be expressed as
for dim GRBs and for bright GRBs, with the break point
. We also find that the formation
rate of GRBs is almost constant at for the first time, which is
remarkably different from the SFR. At , the formation rate of GRB is
consistent with the SFR. Our results are dramatically different from previous
studies. Some possible reasons for this low-redshift excess are discussed. We
also test the robustness of our results with Monte Carlo simulations. The
distributions of mock data (i.e., luminosity-redshift distribution, luminosity
function, cumulative distribution and distribution) are in good
agreement with the observations. Besides, we also find that there are
remarkable difference between the mock data and the observations if long GRB
are unbiased tracers of SFR at .Comment: 33 pages, 10 figures, 1 table, accepted by ApJ
The dependence of the pairwise velocity dispersion on galaxy properties
(abridged) We present measurements of the pairwise velocity dispersion (PVD)
for different classes of galaxies in the Sloan Digital Sky Survey. For a sample
of about 200,000 galaxies, we study the dependence of the PVD on galaxy
properties such as luminosity, stellar mass (M_*), colour (g-r), 4000A break
strength (D4000), concentration index (C), and stellar surface mass density
(\mu_*). The luminosity dependence of the PVD is in good agreement with the
results of Jing & B\"orner (2004) for the 2dFGRS catalog. The value of
\sigma_{12} measured at k=1 h/Mpc decreases as a function of increasing galaxy
luminosity for galaxies fainter than L*, before increasing again for the most
luminous galaxies in our sample. Each of the galaxy subsamples selected
according to luminosity or stellar mass is divided into two further subsamples
according to colour, D4000, C and \mu_*. We find that galaxies with redder
colours and higher D4000, C, and \mu_* values have larger PVDs on all scales
and at all luminosities/stellar masses. The dependence of the PVD on parameters
related to recent star formation(colour, D4000) is stronger than on parameters
related to galaxy structure (C, \mu_*), especially on small scales and for
faint galaxies. The reddest galaxies and galaxies with high surface mass
densities and intermediate concentrations have the highest pairwise peculiar
velocities, i.e. these move in the strongest gravitational fields. We conclude
that the faint red population located in rich clusters is responsible for the
high PVD values that are measured for low-luminosity galaxies on small scales.Comment: 14 pages, 13 figures; reference updated and text slightly changed to
match the published version; data of measurements of power spectrum and PVD
available at http://www.mpa-garching.mpg.de/~leech/papers/clustering
Beaming Effects in Gamma-Ray Bursts
Based on a refined generic dynamical model, we investigate afterglows from
jetted gamma-ray burst (GRB) remnants numerically. In the relativistic phase,
the light curve break could marginally be seen. However, an obvious break does
exist at the transition from the relativistic phase to the non-relativistic
phase, which typically occurs at time 10 to 30 days. It is very interesting
that the break is affected by many parameters, especially by the electron
energy fraction (xi_e), and the magnetic energy fraction (xi_B^2). Implication
of orphan afterglow surveys on GRB beaming is investigated. The possible
existence of a kind of cylindrical jets is also discussed.Comment: Minor changes; 10 pages, with 9 eps figures embedded. Talk given at
the Sixth Pacific Rim Conference on Stellar Astrophysics (Xi'an, China, July
11-17, 2002). A slightly revised version will appear in the proceeding
Semiclassical Time Evolution of the Holes from Luttinger Hamiltonian
We study the semi-classical motion of holes by exact numerical solution of
the Luttinger model. The trajectories obtained for the heavy and light holes
agree well with the higher order corrections to the abelian and the non-abelian
adiabatic theories in Ref. [1] [S. Murakami et al., Science 301, 1378(2003)],
respectively. It is found that the hole trajectories contain rapid oscillations
reminiscent of the "Zitterbewegung" of relativistic electrons. We also comment
on the non-conservation of helicity of the light holes.Comment: 4 pages, 5 fugure
The Luminosity - E_p Relation within Gamma--Ray Bursts and Implications for Fireball Models
Using a sample of 2408 time-resolved spectra for 91 BATSE gamma-ray bursts
(GRBs) presented by Preece et al., we show that the relation between the
isotropic-equivalent luminosity (L_iso) and the spectral peak energy (E_p) in
the cosmological rest frame, L_iso \propto E_p^2, not only holds within these
bursts, but also holds among these GRBs, assuming that the burst rate as a
function of redshift is proportional to the star formation rate. The possible
implications of this relation for the emission models of GRBs are discussed. We
suggest that both the kinetic-energy-dominated internal shock model and the
magnetic-dissipation-dominated external shock model can well interpret this
relation. We constrain the parameters for these two models, and find that they
are in a good agreement with the parameters from the fittings to the afterglow
data (abridged).Comment: 3 pages plus 5 figures, emulateapj style, accepted for publication in
ApJ Letter
Measuring dark energy with the correlation of gamma-ray bursts using model-independent methods
In this paper, we use two model-independent methods to standardize long
gamma-ray bursts (GRBs) using the correlation, where
is the isotropic-equivalent gamma-ray energy and is
the spectral peak energy. We update 42 long GRBs and try to make constraint on
cosmological parameters. The full sample contains 151 long GRBs with redshifts
from 0.0331 to 8.2. The first method is the simultaneous fitting method. The
extrinsic scatter is taken into account and assigned to the
parameter . The best-fitting values are ,
, and in the flat
CDM model. The constraint on is at the
1 confidence level. If reduced method is used, the best-fit
results are , and . The
second method is using type Ia supernovae (SNe Ia) to calibrate the correlation. We calibrate 90 high-redshift GRBs in the redshift
range from 1.44 to 8.1. The cosmological constraints from these 90 GRBs are
for flat CDM, and
and for non-flat
CDM. For the combination of GRB and SNe Ia sample, we obtain
and for the flat CDM, and
for the non-flat CDM, the results are ,
and . These results from
calibrated GRBs are consistent with that of SNe Ia. Meanwhile, the combined
data can improve cosmological constraints significantly, comparing to SNe Ia
alone. Our results show that the correlation is
promising to probe the high-redshift universe.Comment: 10 pages, 6 figures, 4 table, accepted by A&A. Table 4 contains
calibrated distance moduli of GRB
A rapid cosmic-ray increase in BC 3372-3371 from ancient buried tree rings in China
Cosmic rays interact with the Earth's atmosphere to produce C, which
can be absorbed by trees. Therefore, rapid increases of C in tree rings
can be used to probe previous cosmic-ray events. By this method, three C
rapidly increasing events have been found. Plausible causes of these events
include large solar proton events, supernovae or short gamma-ray bursts.
However, due to the lack of measurements of C by year, the occurrence
frequency of such C rapidly increasing events is poorly known. In
addition, rapid increases may be hidden in the IntCal13 data with five-year
resolution. Here we report the result of C measurements using an ancient
buried tree during the period between BC 3388 and 3358. We find a rapid
increase of about 9\textperthousand~ in the C content from BC 3372 to BC
3371. We suggest that this event could originate from a large solar proton
event.Comment: 23 pages, 3 figures, 2 tables, published in Nature Communication
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