scispace - formally typeset
Search or ask a question
Author

Michael J. Rycroft

Bio: Michael J. Rycroft is an academic researcher from International Space University. The author has contributed to research in topics: Space environment & Spacecraft design. The author has an hindex of 1, co-authored 3 publications receiving 1289 citations.

Papers

Cited by
More filters
Proceedings ArticleDOI
16 Jul 2001
TL;DR: A novel approach to the localization of sensors in an ad-hoc network that enables sensor nodes to discover their locations using a set distributed iterative algorithms is described.
Abstract: The recent advances in radio and em beddedsystem technologies have enabled the proliferation of wireless microsensor networks. Such wirelessly connected sensors are released in many diverse environments to perform various monitoring tasks. In many such tasks, location awareness is inherently one of the most essential system parameters. It is not only needed to report the origins of events, but also to assist group querying of sensors, routing, and to answer questions on the network coverage. In this paper we present a novel approach to the localization of sensors in an ad-hoc network. We describe a system called AHLoS (Ad-Hoc Localization System) that enables sensor nodes to discover their locations using a set distributed iterative algorithms. The operation of AHLoS is demonstrated with an accuracy of a few centimeters using our prototype testbed while scalability and performance are studied through simulation.

2,931 citations

Journal ArticleDOI
09 Dec 2002
TL;DR: Reference Broadcast Synchronization (RBS) as discussed by the authors is a scheme in which nodes send reference beacons to their neighbors using physical-layer broadcasts, and receivers use their arrival time as a point of reference for comparing their clocks.
Abstract: Recent advances in miniaturization and low-cost, low-power design have led to active research in large-scale networks of small, wireless, low-power sensors and actuators. Time synchronization is critical in sensor networks for diverse purposes including sensor data fusion, coordinated actuation, and power-efficient duty cycling. Though the clock accuracy and precision requirements are often stricter than in traditional distributed systems, strict energy constraints limit the resources available to meet these goals.We present Reference-Broadcast Synchronization, a scheme in which nodes send reference beacons to their neighbors using physical-layer broadcasts. A reference broadcast does not contain an explicit timestamp; instead, receivers use its arrival time as a point of reference for comparing their clocks. In this paper, we use measurements from two wireless implementations to show that removing the sender's nondeterminism from the critical path in this way produces high-precision clock agreement (1.85 ± 1.28μsec, using off-the-shelf 802.11 wireless Ethernet), while using minimal energy. We also describe a novel algorithm that uses this same broadcast property to federate clocks across broadcast domains with a slow decay in precision (3.68 ± 2.57μsec after 4 hops). RBS can be used without external references, forming a precise relative timescale, or can maintain microsecond-level synchronization to an external timescale such as UTC. We show a significant improvement over the Network Time Protocol (NTP) under similar conditions.

2,537 citations

Proceedings ArticleDOI
25 Oct 1998
TL;DR: A new routing protocol for ad hoc networks built around two novel observations, one of triggering the sending of location updates by the moving nodes autonomously, based on a node's mobility rate, and the other of minting the overhead used for maintaining routes using the two new principlw of update message frequency and distance.
Abstract: 1 Introduction h this paper we introduce a new routing protocol for ad hoc networks built around two novel observations. One, called the distance eflect, usw the fmt that the greater the distance separating two nodes, the slower they appear to be moving with respect to each other. Accor@gly, the location information in routing tables can be updated as a function of the distance separating nodes without compromising the routing accuracy. The second idea is that of triggering the sending of location updates by the moving nodes autonomously, based ody on a node's mobility rate. htuitively, it is clear that in a direction routing dgorithrn, routing information about the slower moving nodes needs to be updated less frequently than that about hig~y mobtie nodw. h this way e~ node can optimize the frequency at which it sends updates to the networks and correspondingly r~ duce the bandwidth and energy used, leading to a fully distributed and self-optimizing system. B~ed on thwe routing tablw, the proposed direction algorithm sends messages in the " recorded dwectionn of the destination node, guaranteeing detivery by following the direction with a given probability. We show by detailed simda-tion that our protocol always delivers more than 80% of the data messages by following the direction computed, without using any recovery procedure. In addition, it mintilzes the overhead used for maintaining routes using the two new principlw of update message frequency and distance. Lastly, the dgorithrn is fully distributed, provides loop-free paths, and is robust, since it suppfies multiple routes. Pemlissiontomakedigitalorhsrdcopiesof allorpartof this\vorkfor personal or classroom use is granted without fee provided that copies are not mzde or dis~.buted for prolit or commercial ad~arrtageand that copies bcwrthis notice and the full citation on the first page. To copy othm}tise, to republish, to post on senrers or to redistribute to lists, requires prior specific permission an&'ora fee. 76 Rom a routing perspective, an ad hoc network is a packet radio network in which the mobile nodes perform the routing functions. Generdy, routing is multi-hop since nodes may not be within the wireless transmission range of one another and thus depend on each other to forward packets to a given destination. Since the topology of an ad hoc network changes frequently, a routing protocol should be a distributed algorithm that computes multiple, cycle free routes while keeping the communication overhead to a minimum (see, e.g., [4]). One way to …

1,593 citations

Proceedings ArticleDOI
05 Nov 2003
TL;DR: The traffic-adaptive medium access protocol (TRAMA) is introduced for energy-efficient collision-free channel access in wireless sensor networks and is shown to be fair and correct, in that no idle node is an intended receiver and no receiver suffers collisions.
Abstract: The traffic-adaptive medium access protocol (TRAMA) is introduced for energy-efficient collision-free channel access in wireless sensor networks. TRAMA reduces energy consumption by ensuring that unicast, multicast, and broadcast transmissions have no collisions, and by allowing nodes to switch to a low-power, idle state whenever they are not transmitting or receiving. TRAMA assumes that time is slotted and uses a distributed election scheme based on information about the traffic at each node to determine which node can transmit at a particular time slot. TRAMA avoids the assignment of time slots to nodes with no traffic to send, and also allows nodes to determine when they can become idle and not listen to the channel using traffic information. TRAMA is shown to be fair and correct, in that no idle node is an intended receiver and no receiver suffers collisions. The performance of TRAMA is evaluated through extensive simulations using both synthetic- as well as sensor-network scenarios. The results indicate that TRAMA outperforms contention-based protocols (e.g., CSMA, 802.11 and S-MAC) as well as scheduling-based protocols (e.g., NAMA) with significant energy savings.

1,287 citations

BookDOI
24 Nov 2004
TL;DR: This book discusses the history and present situation of the GPS system, as well as some of the technologies and subjects related to receiver hardware and software used in the system.
Abstract: Preface. Preface to the First Edition. Chapter 1. Introduction. Chapter 2. Basic GPS Concept. Chapter 3. Satellite Constellation. Chapter 4. Earth-Centered, Earth-Fixed Coordinate System. Chapter 5. GPS C/A Code Signal Structure. Chapter 6. Receiver Hardware Considerations. Chapter 7. Acquisition of GPSb C/A Code Signals. Chapter 8. Tracking GPS Signals. Chapter 9. GPS Software Receivers. Chapter 10. Acquisition of Weak Signals. Chapter 11. Tracking Weak Signals. Chapter 12. GPS Receiver-Related Subjects. Index.

985 citations