R. S. Wolff

Detection of X-ray polarization of the Crab Nebula.

Two different types of X-ray polarimeters were used in a sounding rocket to search for X-ray polarization of the Crab Nebula. Polarization was detected at a statistical confidence level of 99.7%. If the X-ray polarization is assumed to be independent of energy, the results of this and a previous experiment lead to a polarization of 15.4 (plus or minus 5.2) percent at a position angle of 156 (plus or minus 10) deg. This result confirms the synchrotron model for X-ray emission from the Crab Nebula.

Measurement of the X-ray polarization of the Crab Nebula

The graphite crystal X-ray polarimeters aboard the OSO-8 satellite were used to observe the Crab Nebula for six days from March 11 through March 17, 1976 (UT). Analysis of 15 orbits of quick-look data shows that the polarization and position angles at 2.6 and 5.2 keV are 15.7%(+ or - 1.5%) at 161.1(+ or - 2.8) deg and 18.3%(+ or - 4.2%) at 155.5(+ or - 6.6) deg, respectively. These results confirm the previous measurement and the hypothesis of synchrotron X-ray emission.

Routing protocols for vehicular Ad Hoc networks in rural areas

Research on vehicular ad hoc networks has focused mainly on efficient routing protocol design under conditions where there are relatively large numbers of closely spaced vehicles. These routing protocols are designed principally for urban areas with high node density and fully connected networks and are not suitable for packet delivery in a sparse, partially connected VANET. In this article, we examine the challenges of VANETs in sparse network conditions, review alternatives including epidemic routing, and propose a border node-based routing protocol for partially connected VANETs. The BBR protocol can tolerate network partition due to low node density and high node mobility. The performance of epidemic routing and BBR are evaluated with a geographic and traffic information- based mobility model that captures typical highway conditions. The simulation results show that under rural network conditions, a limited flooding protocol such as BBR performs well and offers the advantage of not relying on a location service required by other protocols proposed for VANETs.

Measurement of the gas constants for various proportional-counter gas mixtures

Abstract The relationship between proportional-counter gain and the properties of the gas can be expressed in terms of a semi-empirical relationship. Measurements have been made to verify the validity of this expression, and values of the gas constants for various mixtures have been measured.

Crossing the digital divide: cost-effective broadband wireless access for rural and remote areas

The use of wireless, DSL, and cable for broadband access has become increasingly prevalent in metropolitan areas. While these technologies are being successfully utilized in terms of both service quality and economics in densely populated areas, there are still vast geographic regions where broadband services are either prohibitively expensive or simply unavailable at any price. The article examines several alternatives for using 2.4 GHz 802.11b (WiFi) technology to provide fixed broadband access in rural areas consisting of towns, smaller remote communities, clusters of subscribers separated by large intercluster distances, as well as widely scattered users. Our approach is to model a network based on realistic demographics, equipment and operations costs, service revenues, and customer demand and usage, and calculate the lifecycle economics in terms of capital investment and profitability. We consider the cost benefits of several emerging technologies and architectures, including high gain antennas, dynamically steerable beamforming antennas, and multihop routing. Our results show that cost-effective affordable high-speed wireless Internet access can be provided in rural and remote areas using nontraditional and innovative approaches, bridging the so-called digital divide.

Reliable Routing for Roadside to Vehicle Communications in Rural Areas

The demands for vehicular Internet access are proliferating. Access points (APs) can be deployed along the roadside to provide wireless coverage and network access for mobile vehicles. However, high mobility may cause frequent link breakages, which will seriously impact quality of service (QoS). In this paper, we study reliable routing for roadside to vehicle (R2V) communications in rural areas where rough terrain poses additional challenges. We propose a novel routing protocol where the stationary APs play a key role in route maintenance. The protocol includes a prediction algorithm which can predict the lifetimes of wireless links with consideration for terrain effects, as well as routing algorithms which can find stable paths for packet forwarding based on the prediction. Simulation results based on OPNET modeler and the rural roadways in the Yellowstone National Park show that the proposed protocol substantially outperforms existing ad-hoc routing protocols.

A Border Node Based Routing Protocol for Partially Connected Vehicular Ad Hoc Networks

Research on vehicular ad hoc networks (VANETs) has focused primarily on efficient routing protocol design under conditions where there are relatively large numbers of closely spaced vehicles, typical of major highways and urban areas. These routing protocols are designed principally for fully connected networks and are not suitable for packet delivery in a sparse, partially connected VANET. In rural areas, vehicle densities are low and roadway communication infrastructure is scarce, leading to long periods where vehicle-to-vehicle or vehicleto- roadside communications is infrequent, interrupted, or simply not possible. These attributes characterize a sparse VANET, and are characteristic of delay tolerant networks (DTNs). In this paper, we examine the challenges of VANETs in sparse network conditions, review alternatives including epidemic routing and propose a Border node Based Routing (BBR) protocol for partially connected VANETs. Unlike many VANET protocols that assume location awareness or mobility patterns to aid in routing decisions, BBR is designed to function in domains where location and mobility information is not available, as is typical in rugged terrain conditions. The BBR protocol can tolerate network partition due to low node density and high node mobility. The performance of this protocol is evaluated in OPNET TM with a Random Waypoint mobility model and a Geographic and Traffic Information (GTI) based mobility model that captures typical highway conditions. The simulation results are compared with those obtained using the Dynamic Source Routing (DSR) protocol and with an epidemic routing protocol. The simulation results show that BBR performs well for partially connected VANETs where other protocols fail and provides the advantage of not relying on a location service required by other protocols proposed for VANETs.

A focusing gas scintillation proportional counter

Abstract An improved large-area gas scintillation proportional counter has been built, suitable for use in X-ray astronomy. This counter employs electron focusing in a drift region to achieve uniform response. We show that we can obtain a window area of 60 cm 2 and a spectral resolution of 10% fwhm or better at 5.9 keV.

A graphite crystal polarimeter for stellar X-ray astronomy.

The first crystal x‐ray polarimeter to be used for x‐ray astronomy is described. Polarization is measured by modulation of the x rays diffracted at an average 45° glancing angle from large, curved graphite crystal panels as these rotate about an axis parallel to the incident x‐ray flux. Arrangement of the crystal panels, the design of the detector, and the signal‐processing circuitry were optimized to minimize systematic effects produced by off‐axis pointing of the rocket and cosmic ray induced events. The in‐flight performance of the instrument in relation to the observed background signal is discussed.

A Markov-Based Reservation Algorithm for Wavelength Assignment in All-Optical Networks

Most routing and wavelength assignment algorithms for all-optical networks do not consider the potential problem of reservation connection, which occurs when two or more reservation requests compete for the same wavelength. Confliction can happen even if the network is only lightly loaded. In this paper, we propose a new reservation protocol Markov-based backward reservation (MBR) based on Markov modeling of the network traffic on optical links for resolving wavelength reservation confliction. Our simulations show that this new protocol can improve network performance as measured by decreased blocking probability. The MBR algorithm works best in small-hop-count networks with a comparatively small number of wavelengths per fiber compared to other frequently considered algorithms.