Sensor webs are developmental collections of sensor pods that could be scattered over land or water areas or other regions of interest to gather data on spatial and temporal patterns of relatively slowly changing physical, chemical, or biological phenomena in those regions. Each sensor pod would be a node in a data-gathering/data-communication network that would span a region of interest. Each sensor pod would contain two modules: (1) a transducer module that would interact with the environment to gather the desired data and (2) a communication module.
The basic concept of a network of sensors is not new. The novelty of the sensor-web concept lies in exploitation of a confluence of advances in integrated circuits for radio communication, wireless-network communication technology, and cheap micromachined sensors. This exploitation takes the form of a design concept that affords flexibility of configuration and operation of networks while minimizing power consumption.
A sensor web would contain a few primary nodes and many secondary nodes. Data would be transferred from node to node within the network. The primary nodes would have the additional capability and task of communicating signals into and out of the sensor web: For example, at a primary node, sensory data gathered by the web could be transmitted to an overhead aircraft or satellite or to a local field computer.
Inasmuch as the power needed for intraweb transfer of data increases with the bandwidth of the sensor signals, the sensor output signals should be of low bandwidth to make it possible to minimize power consumption in the sensor pods. Fortunately, many natural phenomena that one might wish to monitor (e.g., temperature or concentrations of chemicals) are inherently of low bandwidth. In cases of phenomena that vary more rapidly, it could be necessary to compress the sensor data at the nodes before transmission.
The use of intraweb, node-to-node communication would reduce the power needed to transmit data out of a web. It would also make it possible to reduce the energy consumed by power-hungry sensors: Web nodes could query each other to track the movements of microclimatic or other fronts over the web, so that power-hungry sensors could be activated only when a front is known to be passing. In other words, intraweb communication enables a nonlinear increase in the value of the local data collected in much the same way that an aggregate of neurons exhibits more intelligence than does a single neuron. Moreover, the synergistic interaction among many separate node transducers would increase the value of the collected data by providing instantaneous correlation across the web.
A sensor web would not be inherently restricted to contain a specific number of nodes or to operate within a predefined area. The primary nodes could be located anywhere in the network, and multiple webs deployed in a given area would naturally mesh with one another. A sensor web could be regarded as instrument, the surveying area of which could be expanded by multiple deployments of nodes.
A sensor web would be a relatively cheap instrument because sensor pods could be mass-produced, taking advantage of economies of scale. Because of this cheapness, individual sensor pods could be regarded as expendable. As a further consequence, it would be possible to "reseed" a sensor web with fresh nodes to replace ones that have failed; thus, the sensor web could be repeatedly renewed, enabling it to operate for an indefinitely long time.
A sensor web could be made as redundant and/or as dense as desired by simply distributing as many nodes as desired over a given survey area. Redundancy would, of course, render the sensor web tolerant to failures of individual pods. High density could be utilized to achieve high spatial resolution and/or to obtain statistically significant numbers of data when surveying biological or other phenomena that are inherently stochastic.
Sensor webs could be useful in almost any endeavor in which there is a need for low-power, low-bandwidth, long-term monitoring over large areas. For example, they could be used to monitor microclimates and concentrations of nutrients for agricultural purposes, to track flows of toxins in ground water, to monitor traffic, or to monitor seismicity in a survey area.
This work was done by Kevin Delin, Shannon Jackson, and Raphael Some of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Electronic Components and Systems category. NPO-20616