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NASA Creates Thinking RF Sensors

Low-cost wireless sensor networks developed by NASA can detect environmental changes and take action in response to what they detect. Now RFID is set to make them even more effective.
By Jonathan Collins
At a water recharge basin just west of Tucson, Delin’s group deployed 16 pods in a Sensor Web to measure the movement of water across the basin’s surface, as well as to follow the penetration of the water into the ground. Sensors in each pod measure ambient air temperature, relative humidity and light level. In addition, the pods inside the recharge basin are equipped with a soil temperature sensor (at surface level) and two soil moisture sensors, one buried just below the surface and the second buried at a depth of half a meter (1.5 feet). These soil sensors are attached to the pods via long leads, which allow the pods to stay above water level so they can continue to communicate wirelessly.

NASA Sensor Web team members Dave Johnson and Kevin Delin prepare a pod for field deployment at a water recharge basin west of Tucson, Ariz.

Thanks to this deployment, NASA’s partners in the project—the University of Arizona and Tucson Water (the city’s water department)—now have a way to monitor activity in the water recharge basin 20 miles outside the city. And NASA had a way to test the Sensor Web’s ability to detect an environmental change—such as the water moving in and out of the basin—that takes place at a predictable and measurable rate. The Sensor Web’s results were also used as a ground-truth check against remote measurements taken from orbiting satellites. (To verify the accuracy of satellite observations, NASA takes ground-based measurements of terrestrial objects at the same time the satellite is measuring them and compares the data.)

For each of its Sensor Web deployments, Delin’s team installed from 12 to 30 network pods, also known as nodes. Each pod consists of a microprocessor or microcontroller connected to whatever sensors may be required, depending on the application; wireless networking technology; and network operating software (developed by the Sensor Webs Project) that ensures that each pod in the web can communicate automatically with any other pod within its hundreds-of-meters transmission range. The pod is enclosed in a protective casing and powered by a rechargeable battery (like the ones used in cordless phones) that is often continually recharged through solar panels.

The Sensor Web is a distributed network, meaning that all the intelligence and data gathered by one pod is shared and used by the other pods. Different applications require different details, such as what data is collected and how frequently that data is recorded and shared with other pods. The software is loaded onto each pod before deployment, along with each pod’s unique ID number, but the wireless capabilities of each pod means that the network can be updated and changed after the pods have been deployed. The goal is to provide a pervasive, continuous, embedded monitoring presence in a range of environments and over large spatial areas.

The pods communicate wirelessly using standard radio components taken from the cordless phone market. Operating at 900 MHz, each pod can broadcast for hundreds of meters to other pods within its range, thus distributing data throughout the network. How and when distribution occurs depends on the parameters of any given application; for example, a pod can be configured to collect and transmit data to every pod within range every five minutes. In addition, specific portal pods can provide end-user access points through an Internet connection, so that information on the Sensor Web can be monitored outside of the pods by pushing data through the Internet to a computer or engineer. A Sensor Web could also be controlled and changed through that same connection.

But the point of a Sensor Web is not merely to collect data and transmit it to a person who then acts on that information. Rather, the ultimate goal of a Sensor Web is to analyze the data collected by its sensors and then to respond accordingly. Intelligence embedded in the network operating system means it can be left to operate without the need to communicate with any end user or control system.

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