Through Kansas, Missouri, Iowa and Wisconsin, across the Keweenah Peninsula of Michigan and into Canada, Alan Jenkins followed as far as he could.
"That’s where I lost the signal,” he said.
Jenkins, assistant director of the George M. Sutton Avian Research Center near Bartlesville, was tracking a bald eagle. A tiny radio transmitter on the bird’s back emitted a signal that Jenkins followed using a directional antenna on his truck or, sometimes when he would lose contact, from an airplane.
"It was an adventure,” he said of that 1989 project.
For three decades, radio frequency (RF) devices have been vital tools for biologists to understand behavior of many species. They have tracked movements of countless animals, from insects and rodents up to the largest sea creatures. However, Global Positioning Satellite (GPS) technology is taking over.
GPS equipment provides much better and much more data, such as exact location, temperature, altitude, speed and direction. This equipment, which communicates with satellites, is significantly more expensive — $2,000 to $3,000 for a single device. Radio transmitters, receivers and antennae can be bought for a few hundred dollars. Also, GPS tracking requires a satellite fee of $30 to $200 a month, depending on how much data you want and how frequently you want it.
"There’s a lot more cost up front,” said Don Wolfe, biologist for the Sutton center.
Still, in many cases, GPS can be cheaper since researchers don’t have to drive hundreds of miles, camp out and trudge through backcountry for weeks to stay within the limited range of radio frequency transmitters.
RF devices have a range of one-half to two miles on the ground, depending on terrain and strength of the signal, or up to 10 miles if monitoring from aircraft.
RF signals can be tricky to track. "The signals can bounce and bend around hills,” Wolfe said.
Since they have only to send a basic signal to a nearby receiver, radio transmitters can be smaller and lighter than GPS devices, which must have a signal powerful enough to reach satellites orbiting hundreds of miles overhead.