A NETWORK OF A DIFFERENT STRIPE

Zebras go wireless on the Kenyan plains.

IT WAS PRETTY PATHETIC," recalls electrical engineering student Pei Zhang, describing the scene. In the middle of the Kenyan plains, three Princeton University professors and four Ph.D. students-Zhang among them-all hovered over a metal box, waiting for a green light to flash.

But this was more than an ordinary LED. It was the culmination of three years spent developing and testing a wireless network like none attempted before: a network of computers carried in collars worn by wild zebras. Backed by a $1.3 million grant from the National Science Foundation, Princeton's ZebraNet project sought breakthroughs in two rather disparate fields: zebra behavior and wireless sensor-network computing.

ZebraNet wouldn't be able to advance either field unless the little green light glowed, showing the first communication between the base station in their hands and the computer that had been wrapped around a zebra's neck that morning.

With the zebra in sight, the seven academics anxiously watched the metal box. The light remained dim. Then, finally, a beam of green light appeared, and the group erupted in cheers. The startled zebra took off at a gallop.

Since that first glimmer of success in January 2004, ZebraNet has accumulated an impressive list of accomplishments. It has overturned long-held ideas about zebra behavior. It has met the ambitious goal of creating a wireless network of computers that can process position data and communicate with GPS satellites, a base station, and each other-all powered by a small bank of solar cells rated at 0.4 watts per computer. It has proven the potential to wirelessly update software on a network using peerto-peer communication. And perhaps most importantly, it has challenged a group of graduate students and their professors to take their research out of the laboratory and into one of the most demanding field environments imaginable. "Engineers spend a lot of time inside in a lab," says project leader Margaret Martonosi. "This was a once-in-a-lifetime opportunity."

The project grew out of electrical engineer Martonosi's work with portable, low-power computers. In one senior thesis project, her students developed a system for GPS-enabled Palm computers to give an automated tour of the Princeton campus, with information provided according to the user's location.

Princeton zoologist Dan Rubenstein, an international authority on zebras, learned about the automated tour and immediately saw possibilities for his own research. Before ZebraNet, Rubenstein had monitored zebra movements by learning the stripe patterns of individual animals and recording sightings. Other biologists use VHF collars that emit a "ping" signal to track large animals. A researcher takes several readings with an antenna and triangulates those readings on a map to home in on an animal. "It's slow, it's not particularly accurate, and it's very labor intensive," Rubenstein says.

Rubenstein and Martinosi wanted ZebraNet collars to collect GPS readings several times an hour and to store the readings in flash memory. Even more amazingly, they wanted two computers to be able to swap data whenever one collared zebra came within a kilometer or so of another one. This would mean that if a researcher could find just one collared zebra, he could wirelessly upload GPS data from several zebras. In the same manner, the researcher could perform a software upgrade to the entire system simply by getting within range of one zebra. This capability is essential, says Martonosi, "because it's extremely difficult to reboot a zebra."

SLEEPLESS NIGHTS

MOVING FROM these dreams to ZebraNet's reality would require dozens of technological advances and many sleepless nights, however. Martonosi put several electrical engineering undergraduates to work on small aspects of the project, while relying on four core graduate students who could commit to years of focus on ZebraNet. "We started from scratch," says one of those grad students, Chris Sadler. "We built everything."

The group's first priority had to be power efficiency. Sensor networks like ZebraNet are usually made up of remote nodes that must run on their own power for months or years. A sensor network, in the words of Sadler, is "the poster child for power supply difficulties." ZebraNet's computers are built around a microcontroller that can switch between two different clock speeds. The faster, 8-megahertz clock is only used for brief bursts of computing power, such as when receiving a fix from GPS satellites. The slower, 32-kilohertz clock handles routine functions at half the power consumption. ZebraNet's software turns on the battery-gobbling radio and GPS chip only when required.

As the group refined their system, they tested collars on bicycles, cars and horses. "Someone suggested we put the collars on grad students, but Margaret shot it down," recalls Zhang. "She said she won't be known as the professor who used GPS to track her grad students."