Occurrence Of Collisions Research Articles

Random access schemes for satellite networks, from VSAT to M2M: a survey

SummaryIn this survey paper, we review the random access (RA) techniques with particular emphasis on the issues and the possible solutions applicable to satellite networks. RA dates back to the 1970s when the ALOHA protocol was developed to solve the problem of interconnecting university computers located in different Hawaiian islands. Since then, several evolutions of the ALOHA protocol have been developed. In particular, solutions were devised to mitigate the problem of packet collisions severely degrading the RA protocols performance. The approach followed for many years has been to avoid the occurrence of collisions rather than solving them. More recently, techniques tackling the RA packet collision problem have appeared triggered by the need of improving RA performance in satellite and terrestrial wireless networks. In particular, satellite networks large propagation delay does not allow the adoption of enhanced terrestrial RA techniques based on channel sensing. Adopting conventional demand assignment multiple access protocols is not suitable for supporting a large number of sensors or devices transmitting small‐size low duty cycle packets as required for machine‐to‐machine communications. This provided the stimulus to exploit successive interference cancelation schemes to solve packet collision issues. The use of successive interference cancelation in RA is relatively new and has opened up a promising research area. We provide an extensive review of recent high‐performance RA techniques achieving more than three orders of magnitude throughput increase compared with the original ALOHA at low packet loss rate. In this survey, we cover both slotted and unslotted techniques. Finally, we review the use of RA in satellite systems and related standards including recent proposals for machine‐to‐machine applications. Copyright © 2016 John Wiley & Sons, Ltd.

Scattering Parabolic Solutions for the Spatial N-Centre Problem

For the $N$-centre problem in the three dimensional space, $$ \ddot x = -\sum_{i=1}^{N} \frac{m_i \,(x-c_i)}{\vert x - c_i \vert^{\alpha+2}}, \qquad x \in \mathbb{R}^3 \setminus \{c_1,\ldots,c_N\}, $$ where $N \geq 2$, $m_i > 0$ and $\alpha \in [1,2)$, we prove the existence of entire parabolic trajectories having prescribed asymptotic directions. The proof relies on a variational argument of min-max type. Morse index estimates and regularization techniques are used in order to rule out the possible occurrence of collisions.

ON UNDERSTANDING THE NATURE OF COLLISIONS OF CORONAL MASS EJECTIONS OBSERVED BY STEREO

ABSTRACT We attempt to understand the collision characteristics of two coronal mass ejections (CMEs) launched successively from the Sun on 2013 October 25. The estimated kinematics, from three-dimensional (3D) reconstruction techniques applied to observations of CMEs by the SECCHI/Coronagraphic (COR) and Heliospheric Imagers, reveal their collision around 37 from the Sun. In the analysis, we take into account the propagation and expansion speeds, impact direction, and angular size as well as the masses of the CMEs. These parameters are derived from imaging observations, but may suffer from large uncertainties. Therefore, by adopting head-on as well as oblique collision scenarios, we have quantified the range of uncertainties involved in the calculation of the coefficient of restitution for expanding magnetized plasmoids. We show that the large expansion speed of the following CME compared with that of the preceding CME results in a higher probability of super-elastic collision. We also infer that a relative approaching speed of the CMEs lower than the sum of their expansion speeds increases the chance of a super-elastic collision. The analysis under reasonable errors in the observed parameters of the CME reveals a larger probability of occurrence of an inelastic collision for the selected CMEs. We suggest that the collision nature of two CMEs should be discussed in 3D, and the calculated value of the coefficient of restitution may suffer from a large uncertainty.

Analysis of Sideswipe Collision Precursors Considering the Spatial-Temporal Characteristics of Freeway Traffic

AbstractTo avoid bias caused by the high percentage of rear-end collisions in the generic models, only the observed sideswipe collisions on Highway I-5 in the central Puget Sound, Washington State, area are used to analyze the matched case-control logistic regression model. Considering the spatial-temporal characteristics of traffic flow and short-term variation of the sideswipe collision occurrence, a comprehensive analysis of sideswipe collision occurrence and its relationship with the freeway flow across lanes and detector locations are studied. The results imply that sideswipe collisions are more likely to occur at straight and level segments of multilane freeways in off-peak hours. High average occupancy and low average flow and speed variance upstream of collision location tend to increase the probability of sideswipe collision in congested scenarios. In contrast, high average speed and coefficient of variation in speed, low speed variance and coefficient of variation in occupancy, and high average ...

Safety in numbers for cyclists beyond national-level and city-level data: a study on the non-linearity of risk within the city of Hong Kong

ObjectiveThis paper examines the relationship between bicycle collisions and the amount of cycling at the local level. Most previous research has focused on national and city comparisons, little is known...

Massive impact-induced release of carbon and sulfur gases in the early Earth's atmosphere

Recent revisions to our understanding of the collisional history of the Hadean and early-Archean Earth indicate that large collisions may have been an important geophysical process. In this work we show that the early bombardment flux of large impactors (>100 km) facilitated the atmospheric release of greenhouse gases (particularly CO2) from Earth's mantle. Depending on the timescale for the drawdown of atmospheric CO2, the Earth's surface could have been subject to prolonged clement surface conditions or multiple freeze-thaw cycles. The bombardment also delivered and redistributed to the surface large quantities of sulfur, one of the most important elements for life. The stochastic occurrence of large collisions could provide insights on why the Earth and Venus, considered Earth's twin planet, exhibit radically different atmospheres.

A full Bayes before-after study accounting for temporal and spatial effects: Evaluating the safety impact of new signal installations

Recently, important advances in road safety statistics have been brought about by methods able to address issues other than the choice of the best error structure for modeling crash data. In particular, accounting for spatial and temporal interdependence, i.e., the notion that the collision occurrence of a site or unit times depend on those of others, has become an important issue that needs further research.Overall, autoregressive models can be used for this purpose as they can specify that the output variable depends on its own previous values and on a stochastic term. Spatial effects have been investigated and applied mostly in the context of developing safety performance functions (SPFs) to relate crash occurrence to highway characteristics. Hence, there is a need for studies that attempt to estimate the effectiveness of safety countermeasures by including the spatial interdependence of road sites within the context of an observational before-after (BA) study. Moreover, the combination of temporal dynamics and spatial effects on crash frequency has not been explored in depth for SPF development.Therefore, the main goal of this research was to carry out a BA study accounting for spatial effects and temporal dynamics in evaluating the effectiveness of a road safety treatment. The countermeasure analyzed was the installation of traffic signals at unsignalized urban/suburban intersections in British Columbia (Canada). The full Bayes approach was selected as the statistical framework to develop the models.The results demonstrated that zone variation was a major component of total crash variability and that spatial effects were alleviated by clustering intersections together. Finally, the methodology used also allowed estimation of the treatment’s effectiveness in the form of crash modification factors and functions with time trends.

Effects of Practice and Psychological Pressure on Interpersonal Coordination Failures.

Although sports players in the same team try to manage their interpersonal coordination for improved performance, failures such as hesitations and collisions are often seen in interpersonal coordination between teammates. However, it is unclear what factors influence the occurrence of such hesitations and collisions. The purpose of this study was to examine the effects of practice and psychological pressure on the occurrence of hesitations and collisions. A total of 80 right-handed university students (aged 19.1 years ± 0.8; 32 males and 48 females) were randomly assigned into pairs and were instructed to perform a serial-tapping task cooperatively. An apparatus had five buttons in a row, which flashed once in each trial in a quasi-random order. When a flashing button was hit, a corresponding light went off and another button flashed. The participants were instructed that the task was to hit a flashing button as quickly and accurately as possible, and either member of the pair could hit the button. They performed 80 practice trials, 10 trials as a control test, and 10 trials as a pressure test. Before the pressure test, pressure was added by informing them about audience and confiscation of the prize if they could not fulfill a criterion. As a result, the occurrence rates of hesitations and collisions and the performance time significantly decreased from the first 40 trials to the next 40 trials of the practice session. Under pressure, state anxiety, the intention to cooperate, and the occurrence rates of hesitations and collisions increased, though heart rate and performance time did not change. These results suggested that interpersonal coordination improved with practice but deteriorated under pressure.

Effect of auditory in-vehicle warning information on drivers’ brake response time to red-light running vehicles during collision avoidance

There is usually a high red-light running (RLR) collision rate at signalized intersections because of the unpredictability of dangerous RLR vehicles traversing intersections illegally. The RLR collision warning systems can help drivers be aware of the red-light running (RLR) vehicles and take collision avoidance actions timely. The objective of this paper is to evaluate the effects of auditory warning information on brake response time (RT) to red-light running vehicles during the collision avoidance process based on a driving simulator experiment. Warning status (e.g., warning vs. no warning), warning lead time (3s vs. 5s) and warning content (directional information vs. undirectional information) were considered to examine the effectiveness of the RLR collision warning system under different conditions. The experiment results showed that the auditory warning information can effectively reduce brake RT and collision occurrence rate and RT is the most influential variable on the collision occurrence. Although early warning is more effective than late warning in reducing brake RT and collision rate, the late warning can still lower the collision rate by 37%. However, the directional warning information did not have apparent advantages over the undirectional warning information. The findings of this study are helpful for the designers of RLR collision warning systems to enhance the effectiveness of the systems’ application on traffic safety.

Collision of Almost Parallel Vortex Filaments

AbstractWe investigate the occurrence of collisions in the evolution of vortex filaments through a system introduced by Klein, Majda, and Damodaran and Zakharov . We first establish rigorously the existence of a pair of almost parallel vortex filaments, with opposite circulation, colliding at some point in finite time. The collision mechanism is based on one of the self‐similar solutions of the model, described by the authors in an earlier work. In the second part of this paper we extend this construction to the case of an arbitrary number of filaments, with polygonal symmetry, that are perturbations of a configuration of parallel vortex filaments forming a polygon, with or without its center, rotating with constant angular velocity.© 2016 Wiley Periodicals, Inc.

A time-series analysis of motorway collisions in England considering road infrastructure, socio-demographics, traffic and weather characteristics

Traffic injuries on motorways are a public health problem worldwide. Collisions on motorways represent a high injury rate in comparison to the entire national network. Furthermore, collisions that occur on the hard–shoulder are even more severe than those that happen on the main carriageway. The purpose of this paper is to explore motorway safety through the identification of patterns in the sequence of monthly hard–shoulder and main carriageway collisions separately over a long period of time (1993–2011) by using reported collision data from British motorways. In order to examine the trends of hard–shoulder and motorway collisions over the same period, a Vector Autoregressive (VAR) model is developed; this allows the inclusion of two time-series in the same model and the examination of the effect of one series on the other and vice-versa. Exogenous variables are also added in order to explore the long-term factors that might affect the occurrence of collisions. The factors considered are related to the infrastructure (e.g. length of motorways), socio-demographics (e.g. percentage of young drivers), traffic (e.g. percentage of vehicle-miles travelled by Heavy Goods Vehicles) and weather (e.g. precipitation). The results suggest different patterns in the sequences in terms of the lingering effects of preceding observations for the two time-series. In terms of the significance of exogenous variables, it is suggested that main carriageway collision frequency is affected by weather conditions and the presence of Heavy Goods Vehicles, while hard–shoulder collisions are decreased by the presence of Motorway Service Areas, which allow a safe exit off the motorway to stop and rest in case of fatigue.

Analysis of the effect of older drivers’ driving behaviors on traffic flow based on a modified CA model

As the global population ages, there are more and more older drivers on the road. The decline in driving performance of older drivers may influence the properties of traffic flow and safety. The purpose of this paper is to investigate the effect of older drivers’ driving behaviors on traffic flow. A modified cellular automaton (CA) model which takes driving behaviors of older drivers into account is proposed. The simulation results indicate that older drivers’ driving behaviors induce a reduction in traffic flow especially when the density is higher than 15 vehicles per km per lane and an increase in Lane-changing frequency. The analysis of stability shows that a number of disturbances could frequently emerge, be propagated and eventually dissipate in this modified model. The results also reflect that with the increase of older drivers on the road, the probability of the occurrence of rear-end collisions increases greatly and obviously. Furthermore, the value of acceleration influences the traffic flow and safety significantly. These results provide the theoretical basis and reference for the traffic management departments to develop traffic management measure in the aging society.

A CSMA/CA-Based Media Access Protocol Using the Registered Backoff Time Vector (RBTv) Mechanism for M2M Communications

This paper proposes an enhanced IEEE 802.11 CSMA/CA media access scheme designated as CSMA/CA-RBTv, in which RBT denotes the Registered Backoff Time and v denotes vector for Machine-to-Machine (M2M) communication. In the proposed CSMA/CA-RBTv, when a mobile node transmits data to the AP, it attaches a random number called the RBT, which denotes the next time slot the mobile node would like to transmit its next data packet, to the packet. Once the AP receives the packet, it extracts the RBT and adds it to a Registered-Time-Slot-List (RTSL) used to conduct channel access scheduling. Before transmitting the ACK packet to the mobile node, the AP (1) selects the node with the smallest RBT from the RTSL list as the next one to access the wireless channel and (2) uses RTSL to make a bit vector marking which time slots have been registered for data transmission, i.e., channel access. Then, the AP appends the aforementioned two pieces of channel access information to the ACK packet. Since the ACK packet is transmitted in a broadcast manner, all of the mobile nodes which overhear the transmission know which node has the next right to access the channel and which time slots are still available for channel access. Consequently, the occurrence of collisions can be reduced, which also results in decreasing power consuming. The simulation results show that CSMA/CA-RBTv has significant improvement in both transmission performance and energy efficiency of the M2M network as the number of mobile nodes increases.

Practical Spectrum Aggregation for Secondary Networks With Imperfect Sensing

We investigate a collision-sensitive secondary network that intends to opportunistically aggregate and utilize spectrum of a primary network to achieve higher data rates. In opportunistic spectrum access with imperfect sensing of idle primary spectrum, secondary transmission can collide with primary transmission. When the secondary network aggregates more channels in the presence of the imperfect sensing, collisions could occur more often, limiting the performance obtained by spectrum aggregation. In this context, we aim to address the following fundamental query: How much spectrum aggregation is worthy with imperfect sensing? For collision occurrence, we focus on two different types of collision: One is imposed by asynchronous transmission, and the other is imposed by imperfect spectrum sensing. The collision probability expression has been derived in closed form with various secondary network parameters: primary traffic load, secondary user transmission parameters, spectrum sensing errors, and number of aggregated subchannels. In addition, the impact of spectrum aggregation on data rate is analyzed under the constraint of collision probability. Then, we solve an optimal spectrum aggregation problem and propose the dynamic spectrum aggregation approach to increase the data rate subject to practical collision constraints. Our simulation results clearly show that the proposed approach outperforms the benchmark that passively aggregates subchannels with lack of collision awareness.

Creation of collision data base through the “bottom-up” approach

The present epoch of knowledge economy, research and innovation poses unpredictable challenges for high-tech enterprises in aerospace industry. Knowledge and technology have mostly become the key source of creating competitive advantage. However production of successful aerospace products and service is impossible without collisions. The aim of the paper is to elaborate a bottom-up approach that allows reducing collisions by setting up a database of collisions (CDB) that occurs in the life cycle process. The introduced definition of “collision” determines any breach in the implementation process, leading to a deviation in product quality. The classification of collision types is given and the stages of collision management system in aerospace enterprises are described. By means of Risk Priority Numbers (RPN) method the authors have proposed to evaluate the significance of probable production collisions. According to conclusions of the analysis, collision database allows to accumulate, formalize and store invaluable experience of skilled practitioners, especially in unique production process. Other research outcome deals with the understanding that collision database allows detecting business processes where collision occurrence is not always obvious.

Contact efflorescence as a pathway for crystallization of atmospherically relevant particles

Inadequate knowledge of the phase state of atmospheric particles represents a source of uncertainty in global climate and air quality models. Hygroscopic aqueous inorganic particles are often assumed to remain liquid throughout their atmospheric lifetime or only (re)crystallize at low relative humidity (RH) due to the kinetic limitations of efflorescence (salt crystal nucleation and growth from an aqueous solution). Here we present experimental observations of a previously unexplored heterogeneous nucleation pathway that we have termed "contact efflorescence," which describes efflorescence initiated by an externally located solid particle coming into contact with the surface of a metastable aqueous microdroplet. This study demonstrates that upon a single collision, contact efflorescence is a pathway for crystallization of atmospherically relevant aqueous particles at high ambient RH (≤80%). Soluble inorganic crystalline particles were used as contact nuclei to induce efflorescence of aqueous ammonium sulfate [(NH4)2SO4], sodium chloride (NaCl), and ammonium nitrate (NH4NO3), with efflorescence being observed in several cases close to their deliquescence RH values (80%, 75%, and 62%, respectively). To our knowledge, these observations represent the highest reported efflorescence RH values for microdroplets of these salts. These results are particularly important for considering the phase state of NH4NO3, where the contact efflorescence RH (∼20-60%) is in stark contrast to the observation that NH4NO3 microdroplets do not homogeneously effloresce, even when exposed to extremely arid conditions (<1% RH). Considering the occurrence of particle collisions in the atmosphere (i.e., coagulation), these observations of contact efflorescence challenge many assumptions made about the phase state of inorganic aerosol.

Revisiting the determination of hydromechanical stresses encountered by microcarriers in stem cell culture bioreactors

Background Expansion of mesenchymal stem cells (MSC) is one of the key steps for their use in tissue engineering or cell therapies. Today, expansion processes are mainly based on the use of microcarriers to allow large interfacial adherence areas [1]. However, this culture technology is known to be practically limited to low agitation intensity and microcarrier concentrations due to possible cell damage arising from particle hydromechanical stress or collisions between microcarriers [2]. Unfortunately, the description of the relationship between bioreactor hydrodynamics, microcarrier suspension and occurrence of collisions was neither clearly established in the case of stem cell cultures, nor based on a local description of the bioreactor hydrodynamics heterogeneity. Thus, in the present study, it is proposed to use numerical simulations to describe not only the liquid phase but also the microcarrier dispersion and the occurrence of hydromechanical stress encountered by the microcarriers. Two kinds of hydromechanical stress can be distinguished: (i) fluid-solid interactions (fluid shear stress) arising from turbulent eddies and (ii) solid-solid interactions arising from collisions between microcarriers or between microcarriers and bioreactor walls [2].

Puritanism vs. Syncretism: An Islamic Cultural Collision in the Rural Farmer Community in Trucuk, Indonesia

This writing presents a view on the change in the rural farmer community in Trucuk from being abangan-syncretic to being santri-puritan. After becoming puritan, it is radical in style. The radical style becomes a social phenomenon causing the occurrence of cultural collision of puritan Islamic cultural values versus syncretic ones in the region of the rural farmer community in Trucuk. This research is done in a rural area in Trucuk district, Klaten regency, Central Java provence, Indones. The major of collecting data methods are participant observer, deep interview, and study documentation. This research uses ethnography approach. The results show that the change in farmer’s society in a rural are from syncretism to Puritanism influenced by education rationalization and religion up gradation. After being Puritanism, the characteristics changes to plural those are racialism and moderate. Particularly for radicalism that is to be a social phenomenon caused by kept of syncretism. In addition the other factor of Muhammadiyah movement to do the radicalism is the background of Klaten as an imbalance and seethed place since period Hinduism. The radicalism is done as s effort to return to Al-Qur’an emerge the perception and religious behavior that is believed as the truest thing. This perception and behavior emerge the feeling that Puritanism Islamic is the truest and perfect religion, and it can save them, and considered other religion is the polluted or mixed with other unsure, especially syncretism. So, they think that syncretism is misleading, through radicalism, Puritanism try to reduce the practice of syncretism.

Predicting Risk of Motor Vehicle Collisions in Patients with Glaucoma: A Longitudinal Study.

PurposeTo evaluate the ability of longitudinal Useful Field of View (UFOV) and simulated driving measurements to predict future occurrence of motor vehicle collision (MVC) in drivers with glaucoma.DesignProspective observational cohort study.Participants117 drivers with glaucoma followed for an average of 2.1 ± 0.5 years.MethodsAll subjects had standard automated perimetry (SAP), UFOV, driving simulator, and cognitive assessment obtained at baseline and every 6 months during follow-up. The driving simulator evaluated reaction times to high and low contrast peripheral divided attention stimuli presented while negotiating a winding country road, with central driving task performance assessed as “curve coherence”. Drivers with MVC during follow-up were identified from Department of Motor Vehicle records.Main Outcome MeasuresSurvival models were used to evaluate the ability of driving simulator and UFOV to predict MVC over time, adjusting for potential confounding factors.ResultsMean age at baseline was 64.5 ± 12.6 years. 11 of 117 (9.4%) drivers had a MVC during follow-up. In the multivariable models, low contrast reaction time was significantly predictive of MVC, with a hazard ratio (HR) of 2.19 per 1 SD slower reaction time (95% CI, 1.30 to 3.69; P = 0.003). UFOV divided attention was also significantly predictive of MVC with a HR of 1.98 per 1 SD worse (95% CI, 1.10 to 3.57; P = 0.022). Global SAP visual field indices in the better or worse eye were not predictive of MVC. The longitudinal model including driving simulator performance was a better predictor of MVC compared to UFOV (R2 = 0.41 vs R2 = 0.18).ConclusionsLongitudinal divided attention metrics on the UFOV test and during simulated driving were significantly predictive of risk of MVC in glaucoma patients. These findings may help improve the understanding of factors associated with driving impairment related to glaucoma.

Renal Collision and Composite Tumors: Imaging and Pathophysiology

To illustrate the imaging appearances of a spectrum of renal collision and composite tumors. Occurrence of collision and composite tumors in the genitourinary tract is rare compared to the usual occurrence of synchronous tumors. Case studies were chosen that represent the different tumors. Analysis was made on both the imaging and the pathology if excision was performed. Presence of 2 different cell types can lead to confusing imaging findings, and biopsy or excision is typically needed for final diagnosis. Some composite tumors have a characteristic appearance on imaging based on their pathologic features. Familiarity with imaging findings may help radiologists include these tumors in their differential diagnosis.