268,463 research outputs found
Planet Signatures in Collisionally Active Debris Discs: scattered light images
Planet perturbations are often invoked as a potential explanation for many
spatial structures that have been imaged in debris discs. So far this issue has
been mostly investigated with collisionless N-body numerical models. We
numerically investigate how the coupled effect of collisions and radiation
pressure can affect the formation and survival of radial and azimutal
structures in a disc perturbed by a planet. We consider two set-ups: a planet
embedded within an extended disc and a planet exterior to an inner debris ring.
We use the DyCoSS code of Thebault(2012) and derive synthetic images of the
system in scattered light. The planet's mass and orbit, as well as the disc's
collisional activity are explored as free parameters.
We find that collisions always significantly damp planet-induced structures.
For the case of an embedded planet, the planet's signature, mostly a density
gap around its radial position, should remain detectable in head-on images if
M_planet > M_Saturn. If the system is seen edge-on, however, inferring the
presence of the planet is much more difficult, although some planet-induced
signatures might be observable under favourable conditions.
For the inner-ring/external-planet case, planetary perturbations cannot
prevent collision-produced small fragments from populating the regions beyond
the ring: The radial luminosity profile exterior to the ring is close to the
one it should have in the absence of the planet. However, a Jovian planet on a
circular orbit leaves precessing azimutal structures that can be used to
indirectly infer its presence. For a planet on an eccentric orbit, the ring is
elliptic and the pericentre glow effect is visible despite of collisions and
radiation pressure, but detecting such features in real discs is not an
unambiguous indicator of the presence of an outer planet.Comment: Accepted for Publication in A&A (NOTE: Abridged abstract and
(very)LowRes Figures. Better version, with High Res figures and full abstract
can be found at http://lesia.obspm.fr/perso/philippe-thebault/planpapph.pdf
Three-body dynamics in single ionization of atomic hydrogen by 75 keV proton impact
Doubly differential cross sections (DDCS) for single ionization of atomic hydrogen by 75 keV proton impact have been measured as a function of the projectile scattering angle and energy loss. This pure three-body collision system represents a fundamental test case for the study of the reaction dynamics in few-body systems. A comparison between theory and experiment reveals that three-body dynamics is important at all scattering angles, and that an accurate description of the role of the projectile-target nucleus interaction as well as the second order projectile-electron interaction remains a major challenge to theory. However, progress is being made in understanding these higher order interactions and a better understanding of the collision dynamics seems possible --Abstract, page iii
Radar/electro-optical data fusion for non-cooperative UAS sense and avoid
Abstract This paper focuses on hardware/software implementation and flight results relevant to a multi-sensor obstacle detection and tracking system based on radar/electro-optical (EO) data fusion. The sensing system was installed onboard an optionally piloted very light aircraft (VLA). Test flights with a single intruder plane of the same class were carried out to evaluate the level of achievable situational awareness and the capability to support autonomous collision avoidance. System architecture is presented and special emphasis is given to adopted solutions regarding real time integration of sensors and navigation measurements and high accuracy estimation of sensors alignment. On the basis of Global Positioning System (GPS) navigation data gathered simultaneously with multi-sensor tracking flight experiments, potential of radar/EO fusion is compared with standalone radar tracking. Flight results demonstrate a significant improvement of collision detection performance, mostly due to the change in angular rate estimation accuracy, and confirm data fusion effectiveness for facing EO detection issues. Relative sensors alignment, performance of the navigation unit, and cross-sensor cueing are found to be key factors to fully exploit the potential of multi-sensor architectures
Collision Helps - Algebraic Collision Recovery for Wireless Erasure Networks
Current medium access control mechanisms are based on collision avoidance and
collided packets are discarded. The recent work on ZigZag decoding departs from
this approach by recovering the original packets from multiple collisions. In
this paper, we present an algebraic representation of collisions which allows
us to view each collision as a linear combination of the original packets. The
transmitted, colliding packets may themselves be a coded version of the
original packets.
We propose a new acknowledgment (ACK) mechanism for collisions based on the
idea that if a set of packets collide, the receiver can afford to ACK exactly
one of them and still decode all the packets eventually. We analytically
compare delay and throughput performance of such collision recovery schemes
with other collision avoidance approaches in the context of a single hop
wireless erasure network. In the multiple receiver case, the broadcast
constraint calls for combining collision recovery methods with network coding
across packets at the sender. From the delay perspective, our scheme, without
any coordination, outperforms not only a ALOHA-type random access mechanisms,
but also centralized scheduling. For the case of streaming arrivals, we propose
a priority-based ACK mechanism and show that its stability region coincides
with the cut-set bound of the packet erasure network
FireFly Mosaic: A Vision-Enabled Wireless Sensor Networking System
Abstract — With the advent of CMOS cameras, it is now possible to make compact, cheap and low-power image sensors capable of on-board image processing. These embedded vision sensors provide a rich new sensing modality enabling new classes of wireless sensor networking applications. In order to build these applications, system designers need to overcome challanges associated with limited bandwith, limited power, group coordination and fusing of multiple camera views with various other sensory inputs. Real-time properties must be upheld if multiple vision sensors are to process data, com-municate with each other and make a group decision before the measured environmental feature changes. In this paper, we present FireFly Mosaic, a wireless sensor network image processing framework with operating system, networking and image processing primitives that assist in the development of distributed vision-sensing tasks. Each FireFly Mosaic wireless camera consists of a FireFly [1] node coupled with a CMUcam3 [2] embedded vision processor. The FireFly nodes run the Nano-RK [3] real-time operating system and communicate using the RT-Link [4] collision-free TDMA link protocol. Using FireFly Mosaic, we demonstrate an assisted living application capable of fusing multiple cameras with overlapping views to discover and monitor daily activities in a home. Using this application, we show how an integrated platform with support for time synchronization, a collision-free TDMA link layer, an underlying RTOS and an interface to an embedded vision sensor provides a stable framework for distributed real-time vision processing. To the best of our knowledge, this is the first wireless sensor networking system to integrate multiple coordinating cameras performing local processing. I
Perfect tag identification protocol in RFID networks
Radio Frequency IDentification (RFID) systems are becoming more and more
popular in the field of ubiquitous computing, in particular for objects
identification. An RFID system is composed by one or more readers and a number
of tags. One of the main issues in an RFID network is the fast and reliable
identification of all tags in the reader range. The reader issues some queries,
and tags properly answer. Then, the reader must identify the tags from such
answers. This is crucial for most applications. Since the transmission medium
is shared, the typical problem to be faced is a MAC-like one, i.e. to avoid or
limit the number of tags transmission collisions. We propose a protocol which,
under some assumptions about transmission techniques, always achieves a 100%
perfomance. It is based on a proper recursive splitting of the concurrent tags
sets, until all tags have been identified. The other approaches present in
literature have performances of about 42% in the average at most. The
counterpart is a more sophisticated hardware to be deployed in the manufacture
of low cost tags.Comment: 12 pages, 1 figur
Cellular automaton supercolliders
Gliders in one-dimensional cellular automata are compact groups of
non-quiescent and non-ether patterns (ether represents a periodic background)
translating along automaton lattice. They are cellular-automaton analogous of
localizations or quasi-local collective excitations travelling in a spatially
extended non-linear medium. They can be considered as binary strings or symbols
travelling along a one-dimensional ring, interacting with each other and
changing their states, or symbolic values, as a result of interactions. We
analyse what types of interaction occur between gliders travelling on a
cellular automaton `cyclotron' and build a catalog of the most common
reactions. We demonstrate that collisions between gliders emulate the basic
types of interaction that occur between localizations in non-linear media:
fusion, elastic collision, and soliton-like collision. Computational outcomes
of a swarm of gliders circling on a one-dimensional torus are analysed via
implementation of cyclic tag systems
- …