Satellite system may give ecologists a bird's-eye view

July 23, 2007 2:58 p.m.
Satellite Dish

Below left: From left, Kasdin and class of 2007 members Frank Mycroft and Stephen Savin hope to use the radio dish antenna located on the roof of Jadwin Hall in future simulations of the Hermes system.
Photos: Denise Applewhite

Aerospace engineering professor Jeremy Kasdin usually designs space systems to search for distant planets, but his latest endeavor is on the lookout for creatures close to Earth.

Kasdin and Martin Wikelski, professor of ecology and evolutionary biology, have collaborated with faculty colleagues and students to develop an innovative satellite system to track the migratory patterns of small birds. The researchers are now seeking support to launch the project.

The interdisciplinary effort began when Kasdin was approached by Wikelski, who was dismayed by the limitations of current tracking techniques and proposed the idea of a space-based system.

"As quantitative ecologists, we still have no clue why some populations fluctuate and others don't," Wikelski said. "We cannot be quantitative about population cycles. Where are the animals going? How long do they live? The ability to track them from space could be a major breakthrough."

Beyond providing critical information for conserving endangered species, the plan could reveal life-saving data about avian flu transmission patterns and help prevent billions of dollars of damage each year caused by bird strikes on airplanes.

But it also seemed nearly impossible: Current animal-tracking satellites, such as the joint U.S.-France Argos system, are unable to track animals weighing under 300 grams -- 10.5 ounces, or roughly the size of a small duck -- because the transmitters are too heavy. Wikelski wanted to detect messages sent from a transmitter small enough to be carried by birds that weigh about seven times less, such as the Swainson's thrush songbirds that face declining populations.

Some space scientists were skeptical, but Kasdin was undaunted. As the principal investigator of the Princeton Terrestrial Planet Finder project, an interdepartmental group designing a potential NASA mission to locate earthlike planets orbiting "nearby" stars, he's accustomed to the technical and political hurdles inherent to any proposed space mission. To him, Wikelski's challenge was the perfect opportunity to engage his students in a real-world project.

"This whole project is an excellent synergy between engineering and biology," he said.

He presented the challenge to the 30 students enrolled in his spacecraft design course in the fall of 2005 and by the end of the semester the students had completed initial plans for a satellite, which they dubbed Hermes.

Students with satellite dish

Two of the students, Frank Mycroft and Stephen Savin, focused on the communications link between the transmitters and the Hermes satellite for their senior thesis this past year. Their project earned them the Donald Janssen Dike Award for Excellence in Undergraduate Research, which is mechanical and aerospace engineering's top research prize. Classmate Paul Rosa, advised by associate professor Craig Arnold, developed longer-lasting energy storage devices, which would be critical for powering the tiny transmitters.

In May, the three undergraduates and their professors presented their work to an international group of researchers at a National Science Foundation-funded workshop at Princeton on animal tracking and physiological monitoring. Among the workshop attendees were officials from NASA and the European Space Agency.

"The Hermes idea is very good … much more detailed than what I've seen before," said Jan Dettman, a project manager within the European Space Agency's Human Spaceflight, Microgravity and Exploration Directorate. While his agency is not currently interested in building a satellite solely for bird-tracking, Dettman said a Hermes-like system using existing space platforms could one day become part of the agency's plans.

The Hermes satellite

The satellite transmitters currently available are relatively heavy, so less than 20 percent of bird species and 30 percent of mammal species are strong enough to carry them. But ground-based tracking systems already use commercially available transmitters that are much lighter; they can be attached to animals weighing as little as 40 grams, or 1.5 ounces. Wikelski himself is quite familiar with these gadgets, having spent many a long night following tagged birds in a specially rigged truck. But this technique is difficult, potentially dangerous and provides information about only one bird at a time. The goal of the Hermes satellite would be to pick out the subtle signals sent by many of these tiny transmitters at once, all from an orbit 300 miles overhead.

"The biggest challenge is the fact that the transmitters need to be so small that they emit really small amounts of power," said Mycroft. "How do you detect them from space?" Mycroft and Savin had to develop ways to enable the satellite to detect the signals against a very "noisy" background of random radio signals. Further complicating the task, the satellite must distinguish individual signals sent from birds and animals traveling together.

Their proposed solution is multi-dimensional, making use of four techniques designed to increase the likelihood of signal detection. First, they chose transmitters that emit 1.35-second-long pulses every 10 seconds. Since the pulses from different transmitters are not synchronized, they would often be separated in time, reducing the likelihood of overlap.

For those transmitters that do happen to be pulsing in rhythm, Mycroft and Savin took advantage of the fact that their transmissions arrive at the satellite in a broad range of frequencies. A receiver programmed to search simultaneously for signals in different areas of the spectrum would be capable of finding multiple signals at once.

The engineers still had to be prepared for the possibility that certain transmitters could send signals at the same time and frequency, so they used techniques like the ones used in cellular networks to ensure that the signals emitted by each transmitter were as different from one another as possible. Lastly, their satellite design incorporated a helical antenna, which helps detect weak and strong signals in the satellite’s path equally.

"Finding the signals is like looking for a needle in a haystack -- from space," said Savin, who enjoyed the experience despite long hours and late nights. "Working on something as an undergraduate that might get sent up into space was incredible."

Looking toward the future

The Hermes satellite is currently one of two proposed solutions being put forth by the International Cooperation for Animal Research Using Space Initiative, which was founded in 2002 and counts Kasdin and Wikelski among its lead investigators. Now that the system's feasibility has been demonstrated theoretically, the researchers face their next challenge: finding the necessary support to launch the project off paper and into the sky, whether from NASA and the European Space Agency, other funding agencies or partnerships with private industry.

Wikelski, for his part, is thrilled with the momentum. "The Hermes project gained credibility when engineers joined in," he said. "Without their strong backing, it wouldn't have gone anywhere. It has advanced much quicker than expected and is all falling into place."

He is optimistic that a Hermes-like project will be able to attract ongoing financial support, given its value to ecologists, public health officials and aviation experts. Already, multiple air forces have expressed interest in the idea since a better understanding of bird migration habits could help reduce the number of dangerous and expensive collisions between birds and airplanes. The same data also could map out the potential spread of the bird flu virus and inform readiness plans to combat a pandemic.

For the time being, Kasdin and Wikelski will continue to work with Arnold to make further modifications to the design of the transmitters and the satellite. Regardless of the satellite's future, the researchers have valued the opportunity to work with each other and their students on an important project.

"The Hermes project is a great example of what Princeton is all about -- great science, great engineering and great students," said Arnold. "For me, the opportunity to work with people on the other side of campus and get students excited about research was very rewarding."

Kasdin echoed his sentiments.

"I liked doing it for the sake of science and teaching," he said. "And, if we can make a contribution to conservation biology? What a fantastic use of technology."