8,330 research outputs found
Predictive Duty Cycle Adaptation for Wireless Camera Networks
Wireless sensor networks (WSN) typically employ dynamic duty cycle schemes to efficiently handle different patterns of communication traffic in the network. However, existing duty cycling approaches are not suitable for event-driven WSN, in particular, camera-based networks designed to track humans and objects. A characteristic feature of such networks is the spatially-correlated bursty traffic that occurs in the vicinity of potentially highly mobile objects. In this paper, we propose a concept of indirect sensing in the MAC layer of a wireless camera network and an active duty cycle adaptation scheme based on Kalman filter that continuously predicts and updates the location of the object that triggers bursty communication traffic in the network. This prediction allows the camera nodes to alter their communication protocol parameters prior to the actual increase in the communication traffic. Our simulations demonstrate that our active adaptation strategy outperforms TMAC not only in terms of energy efficiency and communication latency, but also in terms of TIBPEA, a QoS metric for event-driven WSN
A Parallel Histogram-based Particle Filter for Object Tracking on SIMD-based Smart Cameras
We present a parallel implementation of a histogram-based particle filter for object tracking on smart cameras based on SIMD processors. We specifically focus on parallel computation of the particle weights and parallel construction of the feature histograms since these are the major bottlenecks in standard implementations of histogram-based particle filters. The proposed algorithm can be applied with any histogram-based feature sets—we show in detail how the parallel particle filter can employ simple color histograms as well as more complex histograms of oriented gradients (HOG). The algorithm was successfully implemented on an SIMD processor and performs robust object tracking at up to 30 frames per second—a performance difficult to achieve even on a modern desktop computer
Seasonal LAI in slash pine estimated with LANDSAT TM
The leaf area index (LAI, total area of leaves per unit area of ground) of most forest canopies varies throughout the year, yet for logistical reasons it is difficult to estimate anything more detailed than a seasonal maximum LAI. To determine if remotely sensed data can be used to estimate LAI seasonally, field measurements of LAI were compared to normalized difference vegetation index (NDVI) values derived using LANDSAT Thematic Mapper (TM) data, for 16 fertilized and control slash pine plots on 3 dates. Linear relationships existed between NDVI and LAI with R(sup 2) values of 0.35, 0.75, and 0.86 for February 1988, September 1988, and March, 1989, respectively. This is the first reported study in which NDVI is related to forest LAI recorded during the month of sensor overpass. Predictive relationships based on data from eight of the plots were used to estimate the LAI of the other eight plots with a root-mean-square error of 0.74 LAI, which is 15.6 percent of the mean LAI. This demonstrates the potential use of LANDSAT TM data for studying seasonal dynamics in forest canopies
Metamagnetism and soliton excitations in the modulated ferromagnetic Ising chain CoV2O6
We report a combination of physical property and neutron scattering
measurements for polycrystalline samples of the one-dimensional spin chain
compound CoV2O6. Heat capacity measurements show that an effective S = 1/2
state is found at low temperatures and that magnetic fluctuations persist up to
6.Tn. Above Tn = 6.3 K, measurements of the magnetic susceptibility as a
function of T and H show that the nearest neighbour exchange is ferromagnetic.
In the ordered state, we have discovered a crossover from a metamagnet with
strong fluctuations between 5 K and Tn to a state with a 1/3 magnetisation
plateau at 2 < T < 5 K. We use neutron powder diffraction measurements to show
that the AFM state has incommensurate long range order and inelastic time of
flight neutron scattering to examine the magnetic fluctuations as a function of
temperature. Above Tn, we find two broad bands between 3.5 and 5 meV and
thermally activated low energy features which correspond to transitions within
these bands. These features show that the excitations are deconfined solitons
rather than the static spin reversals predicted for a uniform FM Ising spin
chain. Below Tn, we find a ladder of states due to the confining effect of the
internal field. A region of weak confinement below Tn, but above 5 K, is
identified which may correspond to a crossover between 2D and 3D magnetic
ordering.Comment: Expanded version, includes results from arXiv:0804.2966 and neutron
powder diffraction. To appear in PR
A Super-Earth Orbiting the Nearby Sun-like Star HD 1461
We present precision radial velocity data that reveal a Super-Earth mass
planet and two probable additional planets orbiting the bright nearby G0V star
HD 1461. Our 12.8 years of Keck HIRES precision radial velocities indicate the
presence of a 7.4M_Earth planet on a 5.77-day orbit. The data also suggest, but
cannot yet confirm, the presence of outer planets on low-eccentricity orbits
with periods of 446.1 and 5017 days, and projected masses (M sin i) of 27.9 and
87.1M_Earth, respectively. Test integrations of systems consistent with the
radial velocity data suggest that the configuration is dynamically stable. We
present a 12.2-year time series of photometric observations of HD 1461, which
comprise 799 individual measurements, and indicate that it has excellent
long-term photometric stability. However, there are small amplitude variations
with periods comparable to those of the suspected 2nd and 3rd signals in the
radial velocities near 5000 and 446 days, thus casting some suspicion on those
periodicities as Keplerian signals. If the 5.77-day companion has a
Neptune-like composition, then its expected transit depth is of order ~0.5
millimags. The geometric a priori probability of transits is ~8%. Phase-folding
of the ground-based photometry shows no indication that transits of the
5.77-day companion are occurring, but high-precision follow-up of HD 1461
during upcoming transit phase windows will be required to definitively rule out
or confirm transits. This new system joins a growing list of solar-type stars
in the immediate galactic neighborhood that are accompanied by at least one
Neptune- (or lower) mass planets having orbital periods of 50 days or less.Comment: 33 pages, 7 figure
Stellar Activity and its Implications for Exoplanet Detection on GJ 176
We present an in-depth analysis of stellar activity and its effects on radial
velocity (RV) for the M2 dwarf GJ 176 based on spectra taken over 10 years from
the High Resolution Spectrograph on the Hobby-Eberly Telescope. These data are
supplemented with spectra from previous observations with the HIRES and HARPS
spectrographs, and V- and R-band photometry taken over 6 years at the Dyer and
Fairborn observatories. Previous studies of GJ 176 revealed a super-Earth
exoplanet in an 8.8-day orbit. However, the velocities of this star are also
known to be contaminated by activity, particularly at the 39-day stellar
rotation period. We have examined the magnetic activity of GJ 176 using the
sodium I D lines, which have been shown to be a sensitive activity tracer in
cool stars. In addition to rotational modulation, we see evidence of a
long-term trend in our Na I D index, which may be part of a long-period
activity cycle. The sodium index is well correlated with our RVs, and we show
that this activity trend drives a corresponding slope in RV. Interestingly, the
rotation signal remains in phase in photometry, but not in the spectral
activity indicators. We interpret this phenomenon as the result of one or more
large spot complexes or active regions which dominate the photometric
variability, while the spectral indices are driven by the overall magnetic
activity across the stellar surface. In light of these results, we discuss the
potential for correcting activity signals in the RVs of M dwarfs.Comment: Accepted for publication in Ap
- …