Miklós Maróti

The flooding time synchronization protocol

Wireless sensor network applications, similarly to other distributed systems, often require a scalable time synchronization service enabling data consistency and coordination. This paper describes the Flooding Time Synchronization Protocol (FTSP), especially tailored for applications requiring stringent precision on resource limited wireless platforms. The proposed time synchronization protocol uses low communication bandwidth and it is robust against node and link failures. The FTSP achieves its robustness by utilizing periodic flooding of synchronization messages, and implicit dynamic topology update. The unique high precision performance is reached by utilizing MAC-layer time-stamping and comprehensive error compensation including clock skew estimation. The sources of delays and uncertainties in message transmission are analyzed in detail and techniques are presented to mitigate their effects. The FTSP was implemented on the Berkeley Mica2 platform and evaluated in a 60-node, multi-hop setup. The average per-hop synchronization error was in the one microsecond range, which is markedly better than that of the existing RBS and TPSN algorithms.

Smart Dust: communicating with a cubic-millimeter computer

W hat do Rational Rose, Simulink, and LabVIEW have in common? At first, these tools seem very different. Rational Rose (http://www.rational.com) is a visual modeling tool, Simulink (http:// www.mathworks.com) is a hierarchical block-diagram design and simulation tool, and LabVIEW (http:// www.ni.com) is a graphical programming development environment. Despite the different terminology, these three tools share a common underlying theme: Each is an integrated set of modeling, model analysis, simulation, and code-generation tools that help design and implement computer-based systems (CBSs) in a specific, well-defined engineering field. These tools and other popular domain-specific integrated development environments can help capture specifications in the form of domain models. They also support the design process by automating analysis and simulating essential system behavior. In addition, they can automatically generate, configure, and integrate target application components. These environments translate the verified design—expressed in a domainspecific, primarily graphical modeling formalism—into a variety of artifacts that constitute a CBS implementation. These artifacts can include glue code, database schema, and configuration tables. These tools use domain-specific modeling languages that allow developers to represent essential design views and to both formally express and automatically enforce integrity constraints. These tools also support model composition that is synergistic with the design process in the particular engineering domain. Other benefits include having integrated models as opposed to relying merely on source code. In addition, the common input—that is, the shared design model—guarantees the consistency of different analysis results as long as all of the applied generators are correct. While the industry understands the welldocumented benefits of domain-specific, integrated modeling, analysis, and application-generation environments, their high cost represents a significant block to wide acceptance and application. Consequently, these tools are available only for domains with large markets in which high volume offsets the substantial initial investment cost. For CBSs in smaller, specialized domains, or even for single projects, the industry needs technology that can help rapidly and efficiently compose these environments from reusable components.Domain-specific integrated development environments can help capture specifications in the form of domain models. These tools support the design process by automating analysis and simulating essential system behavior. In addition, they can automatically generate, configure, and integrate target application components. The high cost of developing domain-specific, integrated modeling, analysis, and application-generation environments prevents their penetration into narrower engineering fields that have limited user bases. Model-integrated computing (MIC), an approach to model-based engineering that helps compose domain-specific design environments rapidly and cost effectively, is particularly relevant for specialized computer-based systems domains-perhaps even single projects. The authors describe how MIC provides a way to compose such environments cost effectively and rapidly by using a metalevel architecture to specify the domain-specific modeling language and integrity constraints. They also discuss the toolset that implements MIC and describe a practical application in which using the technology in a tool environment for the process industry led to significant reductions in development and maintenance costs.

Sensor network-based countersniper system

An ad-hoc wireless sensor network-based system is presented that detects and accurately locates shooters even in urban environments. The system consists of a large number of cheap sensors communicating through an ad-hoc wireless network, thus it is capable of tolerating multiple sensor failures, provides good coverage and high accuracy, and is capable of overcoming multipath effects. The performance of the proposed system is superior to that of centralized countersniper systems in such challenging environment as dense urban terrain. In this paper, in addition to the overall system architecture, the acoustic signal detection, the most important middleware services and the unique sensor fusion algorithm are also presented. The system performance is analyzed using real measurement data obtained at a US Army MOUT (Military Operations in Urban Terrain) facility.

Radio interferometric geolocation

We present a novel radio interference based sensor localization method for wireless sensor networks. The technique relies on a pair of nodes emitting radio waves simultaneously at slightly different frequencies. The carrier frequency of the composite signal is between the two frequencies, but has a very low frequency envelope. Neighboring nodes can measure the energy of the envelope signal as the signal strength. The relative phase offset of this signal measured at two receivers is a function of the distances between the four nodes involved and the carrier frequency. By making multiple measurements in an at least 8-node network, it is possible to reconstruct the relative location of the nodes in 3D. Our prototype implementation on the MICA2 platform yields an average localization error as small as 3 cm and a range of up to 160 meters. In addition to this high precision and long range, the other main advantage of the Radio Interferometric Positioning System (RIPS) is the fact that it does not require any sensors other than the radio used for wireless communication.

Elapsed time on arrival: a simple and versatile primitive for canonical time synchronisation services

Time synchronisation is one of the most important and fundamental middleware services for wireless sensor networks. However, there is an apparent disconnect between existing time synchronisation implementations and the actual needs of current typical sensor network applications. To address this problem, we formulate a set of canonical time synchronisation services distilled from actual applications and propose a set of general application programming interfaces for providing them. We argue that these services can be implemented using a simple time-stamping primitive called Elapsed Time on Arrival (ETA) and we provide two such implementations. The Routing Integrated Time Synchronisation (RITS) is an extension of ETA over multiple hops. It is a reactive time synchronisation protocol that can be used to correlate multiple event detections at one or more locations to within microseconds. Rapid Time Synchronisation (RATS) is a proactive timesync protocol that utilises RITS to achieve network-wide synchronisation with microsecond precision and rapid convergence. Our work demonstrates that it is possible to build high-performance timesync services using the simple ETA primitive and suggests that more complex mechanisms may be unnecessary to meet the needs of many real world sensor network applications.

Countersniper system for urban warfare

An ad-hoc wireless sensor network-based system is presented that detects and accurately locates shooters even in urban environments. The localization accuracy of the system in open terrain is competitive with that of existing centralized countersniper systems. However, the presented sensor network-based solution surpasses the traditional approach because it can mitigate acoustic multipath effects prevalent in urban areas and it can also resolve multiple simultaneous shots. These unique characteristics of the system are made possible by employing novel sensor fusion techniques that utilize the spatial and temporal diversity of multiple detections. In this article, in addition to the overall system architecture, the middleware services and the unique sensor fusion algorithms are described. An analysis of the experimental data gathered during field trials at US military facilities is also presented.

Shooter localization in urban terrain

Detecting and accurately locating snipers has been an elusive goal of the armed forces and law enforcement agencies for a long time. Most successful sniper-detecting systems are based on acoustic measurements. We develop an acoustic system that works well even in complex urban environments. Funded through the Network Embedded Systems Technology program of the US Defense Advanced Research Projects Agencys Information Exploitation Office, the PinPtr system uses a wireless network of many low-cost sensors to determine both a shooters location and the bullets trajectory by measuring both the muzzle blast and the shock wave. The PinPtr sensor-fusion algorithm, which runs on a base station, performs a search on a hyper-surface defined by a consistency function. This function provides the number of sensor measurements that are consistent with hypothetical shooter positions and shot times. The algorithm automatically classifies measurements and eliminates those that result from multipath effects or are otherwise erroneous. A fast search algorithm finds the global maximum of the surface, which corresponds to the shooter position.

Node density independent localization

This paper presents an enhanced version of a novel radio interferometric positioning technique for node localization in wireless sensor networks that provides both high accuracy and long range simultaneously. The ranging method utilizes two transmitters emitting radio signals at almost the same frequencies. The relative location is estimated by measuring the relative phase offset of the generated interference signal at two receivers. Here, we analyze how the selection of carrier frequencies affects the precision and maximum range. Furthermore, we describe how the interplay of RF multi-path and ground reflections degrades the ranging accuracy. To address these problems, we introduce a technique that continuously refines the range estimates as it converges to the localization solution. Finally, we present the results of a field experiment where our prototype achieved 4 cm average localization accuracy for a quasi-random deployment of 16 COTS motes covering the area of two football fields. The maximum range measured was 170 m, four times the observed communication range. Consequently, node deployment density is no longer constrained by the localization technique, but rather by the communication range

Composition and cloning in modeling and meta-modeling

The Generic Modeling Environment (GME) is a configurable tool suite that facilitates the rapid creation of domain-specific model-integrated program synthesis environments. There are three characteristics of the GME that make it a valuable tool for the construction of domain-specific modeling environments. First, the GME provides generic modeling primitives that assist an environment designer in the specification of new graphical modeling environments. Second, these generic primitives are specialized to create the domain-specific modeling concepts through meta-modeling. The meta-models explicitly support composition enabling the creation of composite modeling languages supporting multiple paradigms. Third, several ideas from prototype-based programming languages have been integrated with the inherent model containment hierarchy, which gives the domain expert the ability to clone graphical models. This paper explores the details of these three ideas and their implications.

Sensor node localization using mobile acoustic beacons

We present a mobile acoustic beacon based sensor node localization method. Our technique is passive in that the sensor nodes themselves do not need to generate an acoustic signal for ranging. This saves cost, power and provides stealthy operation. Furthermore, the beacon can generate much more acoustic energy than a severely resource constrained sensor node, thereby significantly increasing the range. The acoustic ranging method uses a linear frequency modulated signal that can be accurately detected by matched filtering. This provides longer range and higher accuracy than the current state-of-the-art. The localization algorithm was especially designed to work in such acoustically reverberant environment, as urban terrain. The algorithm presented handles non-Gaussian ranging errors caused by echoes. Node locations are computed centrally by solving a global non-linear optimization problem in an iterative and incremental fashion