There are eyes in the sky, looking down on the Yukon -- and they're providing reams of useful information.

Data gathered by remote sensing can be used for a variety of purposes, such as updating maps, searching for valuable mineral deposits, evaluating forest resources, or tracking the changes caused by human activity.

Remote sensing is any method of gathering information from a distance, says Jim Hawkings, a biologist with the Canadian Wildlife Service in Whitehorse.

"When people talk about remote sensing, they're usually talking about satellite imagery," he says. "However, it might mean digital images or conventional photographs taken from an aircraft. Conventional photographs have also been taken from the space shuttle."

Satellite images from the American Landsat series of satellites have been available to the scientific community for a quarter of a century. The satellite currently sending back information is Landsat 5, which is due to be replaced later this year when Landsat 7 is launched. Landsat 6 was lost in the ocean after a failed launch.

The Landsat satellites collect information in the visible range of light, using two different systems, Hawkings says.

The Multi-spectral Scanner (MSS) uses four parts of the visible light spectrum. Its images achieve a resolution of 90 metres at ground level.

"That was the first satellite data that was really widely used," says Hawkings. "That data goes back to the 1970s."

The newer Thematic Mapper (TM) uses seven parts of the spectrum and has a ground resolution of 30 metres. Its images are much more detailed than MSS images and provide far more information.

"That was the state of the art until the late 80s," says Hawkings. "Now there are much more advanced satellites up there. The French SPOT satellite has a ground resolution of 10 metres in black and white, and others soon to be operational will provide data in the 2 to 5 metre range."

Canada has recently launched a new satellite that gathers information in a different way. Radarsat carries sensors that work outside the visible light spectrum, in the range used by ground-based radar.

Because radar sensors can penetrate cloud cover, RadarSat was intended to keep track of the movements of sea ice. However, the radar images are proving very useful in monitoring changes in tropical rainforests, rice-growing regions, and other areas that are often obscured by clouds.

Although individual radar images contain less information than optical images, a series of radar images can give the same range of information as optical images, Hawkings says. Moreover, stereo radar images can be processed to provide a three-dimensional image of the earth.

Until the early 1990s, the information delivered by the satellites could only be processed efficiently by larger, expensive computers, mainly at universities and large research facilities. However, rapid changes in computer technology have made the images useful to far more people.

"Now you can deal with this information on a mainstream home computer," Hawkings says. "However, the data have become more expensive."

In order to recover more of the costs associated with the satellites, the Canadian government has turned the marketing of images over to a private agency, and prices have increased substantially. A full scene of Thematic Mapping data, using seven bands of visible light and covering an area 160 by 160 kilometres, now costs about $5000, says Hawkings.

"That's for a single image, one place on a particular date. If you want to buy another image of that place, on another date, it will cost you more."

Even at that price, the remote image often provides information more cheaply than other methods, such as conventional aerial photography or aerial surveys, he says.

For more information about remote sensing, contact Environment Canada, Whitehorse.