Sensor Web

A Sensor Web is a type of sensor network or geographic information system (GIS) that is especially well suited for environmental monitoring. The term describes a specific type of sensor network: an amorphous network of spatially distributed sensor platforms (pods) that wirelessly communicate with each other. This amorphous architecture is unique since it is both synchronous and router-free, making it 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 at each measurement cycle.

Definition
The term "Sensor Web" was first used by Kevin Delin of NASA in 1997, the novelty of that architecture lies in the ability of the individual pieces to act and coordinate as a whole. Another feature of the JPL systems was their synchronous behaviour, which is also a desired feature of some other implementations. In addition, the individual pods/nodes of those JPL nodes are all equal with one another and Delin's architecture does not require special gateways or routing to have each of the individual pieces communicate with one another or an end user. The earliest definition of a Sensor Web was an autonomous, stand-alone, sensing entity that did not require the presence of the World Wide Web to function. This definition is currently evolving to encompass whole sensing systems from sensing elements to information systems relying on Web technology.

A Sensor Web pod is merely a physical platform for a sensor and thus can be orbital or terrestrial, fixed or mobile and might even have real time accessibility via the Internet. Pod-to-pod communication is both omni-directional and bi-directional where each pod sends out collected data to every other pod in the network. As a result, on-the-fly data fusion, such as false positive identification and plume tracking, can occur within the Sensor Web itself and the system subsequently reacts as a coordinated, collective whole to the incoming data stream. For example, instead of having uncoordinated smoke detectors, a Sensor Web can react as a single, spatially-dispersed, fire locator.

The term "Sensor Web" is sometimes used to refer to sensors connected to the Internet or World Wide Web. Such terms are occasionally used in conjunction with projects of the Open Geospatial Consortium (OGC). In this case, the network architecture requires the Internet to link together the individual sensing elements. The OGC architecture is very different than that of a true Sensor Web system and requires schemes to bring together vastly different datasets, in the same way that TCP/IP is used to tie together vastly different pieces of hardware and computing platforms. Note also that a single Sensor Web may be an individual sensing element inputing into an OGC-type network.

Some developments have a goal of allowing people to "publish" their sensor network data in such a way that other people's search and analysis systems can automatically find the information.

Fielded Applications
Sensor Webs have been deployed that have spanned miles and run continuously for years. Sensor Webs have been fielded in harsh environments (including deserts, mountain snowpacks, and Antarctica) for the purposes of environmental science and have also proved valuable in urban search and rescue and infrastructure protection.

The technology is not only monitoring the environment but sometimes also controlling the environment by actuating devices.

Pods
A sensor web is made of a certain number of pods. Each pod usually contains:
 * one or more sensor leading to one or more data channel,
 * a processing unit such as a micro-controller or microprocessor,
 * a two-way communication component such as a radio and antenna ,
 * an energy source such as a battery coupled with a solar cell,
 * a package to protect components against sometimes harsh environment,
 * a support such as a pole or tripod.

Some pods have been described as having a communication range of hundreds of meters.

Particular pods
In Delin's definition, one particular pod of a sensor web is called the portal pod and provides users access to the sensor web. The term of portal pod has also been used to describe a particular pod that sets the time reference for all other pods to use. In some case, this portal pod has a memory unit that stores collected data.

In some case, a "mother pod" downloads all of the sensor web's data to a large storage medium such as a laptop.

In some case, a few pods with no sensors can be deployed with the single purpose of facilitating communication between the other pods.

Characteristics of a sensor web
The number of pods may vary, with examples of sensor webs with 12 to 30 pods.

The shape of a sensor web may impact its usefulness, for instance a particular deployment made sure each pod was in range to communicate with at least two other pods.

Sensors data is typically shared with other pods every 5 minutes.