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Networking & Communications

The Sensor Web: Distributed Sensing for Collective Action

July 1, 2006 By: Kevin A. Delin, SensorWare Systems Inc., PhD Sensors

The Sensor Web—a macroinstrument of individual sensing elements that can act as a collective whole—has already been deployed in a number of challenging environments. Here's how its unique properties come into play in real-world applications.


Wireless sensor networks seem to be everywhere. Technology magazines talk about them. Universities offer courses on the topic. Companies, both large and small, are working aggressively to push the development of these systems. Despite this interest and activity, such systems have not yet achieved the broad adoption envisioned by pundits and anticipated by engineers. Before this technology can attain its expected ubiquity, more effort is required to identify and satisfy real-world needs.

Pods with Potential

The Sensor Web (see the April 2004 article for an in-depth description, www.sensorsmag.com/sensors/article/articleDetail.jsp?id=328918) is a type of wireless network of sensors that acts as a single, autonomous macroinstrument. It is a temporally synchronous, spatially amorphous network, creating an embedded, distributed monitoring presence. This provides a dynamic infrastructure for distributed sensing and collective action. Because its architecture is both synchronous and router-free, the Sensor Web is distinct from the more typical TCP/IP-like network schemes. The architecture allows every pod to know what is going on with every other pod throughout the Sensor Web within every measurement cycle. A portal pod provides the system's master clock to which all other pods synchronize. Sensor Web networking is more akin to a data bus inside a computer that allows for interactive communication and reaction among various components.

By design, the Sensor Web spreads collected data and processed information throughout the entire network. As a result, there is no design criterion for routing as in more typical wireless systems since routing, by definition, is a focused moving of information from one point to another. Instead, the communication architecture is relatively simple and structured for both omni-and bidirectional information flows. Omnidirectional communication implies no directed information flow, while bidirectional communication allows individual pods (and end users) to command other pods, as well as receive information from them. Consequently, information on the Sensor Web can result from four types of data: (a) raw data sensed at a specific pod, (b) post-processed sensed data from a pod or group of pods, (c) commands entered into the distributed instrument by an external end user via the portal pod, and (d) commands entered into the distributed instrument by a component pod. The Sensor Web processes this internal, continuous data stream, draws knowledge from it, and reacts to that knowledge.

To date, more than 25 Sensor Webs have been fielded, with systems spanning up to 6 miles and running continuously for >3 years. The systems have been extensively tested in challenging environments, ranging from the ice slopes of Antarctica, to the searing heat of the central New Mexico desert, and to the corrosive salt air of the Florida coast. Real-time, streaming output of some of these systems is available over the Internet in a variety of presentation formats (e.g., http://caupanga.Huntington.org/swim/). It is also possible to remotely command the Sensor Web via the same Internet connection. Originally developed at the NASA/Jet Propulsion Laboratory, the technology is ready to be customized for a variety of practical uses, including environmental resource management and life safety applications.

Because of its architecture, this distributed instrument is particularly well-suited to macroscopic, on-the-fly data fusion and reaction, including statistical analysis and vector identification, allowing the entire system to actuate as a collective whole to the incoming data stream. For example, instead of having a collection of uncoordinated smoke detectors, a Sensor Web can organize the sensor information into a single, spatially-dispersed, fire locator. Previous discussions of the Sensor Web have often focused on botanical issues (Figure 1, page 18). In this article we'll examine two other case studies of real-world applications—snowpack monitoring and urban search and rescue—that illustrate the power of the Sensor Web's unique properties.

Figure 1. Researchers from NASA/Jet Propulsion Laboratory install a Sensor Web 5.0 pod at the Huntington Botanical Gardens in San Marino, CA
Figure 1. Researchers from NASA/Jet Propulsion Laboratory install a Sensor Web 5.0 pod at the Huntington Botanical Gardens in San Marino, CA

Pods as Pixels 

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