Satellite Reflectors Out Standing in Delta Fields

In the shadow of Donnelly Dome near Delta Junction, something unusual catches the eye: an inverted metal pyramid sits amid miles of muskeg.

The shiny pyramid, open to the sky like the beak of a just-hatched robin, isn't the work of an avant-garde sculptor. It's called a corner reflector, and Parker Martyn recently toted me to Delta Junction while he aimed it and others at a satellite.

Martyn is a calibration engineer at the Alaska Synthetic Aperture Radar Facility (ASF), housed in the Geophysical Institute. ASF people operate the big satellite dish on top of the Elvey Building at the University of Alaska Fairbanks. They use the dish to receive data from passing satellites and convert it to images used by scientists all over the world.

Satellites with synthetic aperture radar aren't limited to gathering images during the daytime or on cloudless days. These satellites send microwave pulses to Earth that penetrate clouds and darkness much like a camera uses the reflected light of a flash to create photographs. This allows scientists to take a detailed look at Earth from a satellite's-eye view no matter what the earthly weather.

When Martyn and I traveled to Delta Junction, he was anticipating a particular orbit of Radarsat, a Canadian satellite that converts microwave signals into pictures. Radarsat, launched in 1995, now zips around Earth 14 times a day about 500 miles over our heads.

Martyn was in Delta to assure the reflectors were pointed directly at Radarsat during its orbit. The calibration team at ASF has installed 16 of those upside-down, aluminum pyramids in Delta Junction in the past few years. These corner reflectors sit with jarring symmetry near Donnelly Dome, in farmers' fields, and in other areas throughout Delta Junction.

The guys at the Geophysical Institute machine shop constructed the corner reflectors by welding three triangular sheets of aluminum together. The geometry of the corner reflectors serves a crucial function for SAR technicians--when properly aimed skyward, the corner reflectors bounce microwave signals straight back at the satellite from where the signals came.

Earth-observing satellites, such as Radarsat, create images of our planet by sending down bursts of microwaves. Objects on Earth's surface can either scatter microwave signals or reflect the signals in varying degrees back to the satellites. Calm lakes, for example, tend to skip the microwaves away so they aren't received by the satellite. An image processor at ASF assigns the color black to calm lakes. Trees and buildings appear several shades lighter because they reflect more of the microwave signal back to the satellite. Corner reflectors, designed to boomerang microwaves directly back at a satellite, appear as bright white dots on satellite images.

ASF's calibration team uses custom computer software to find out if the corner reflectors are where they should be in the images, and if the signal that returns from the corner reflector is of the proper intensity. The process is like placing a 50-pound weight on a scale and seeing if the needle points to 50. If the software detects an error, technicians then tweak their image-processing equipment to print out accurate portraits of our planet.

Satellite images of Earth allow us the sort of view you could once only get from the windows of an Apollo launch or the space shuttle. Scientists can use ASF images to see how extensively a forested area is being clearcut, to calculate how fast glaciers move, or to assist ship captains trying to navigate through a maze of arctic sea ice.

While I'm on the subject of satellites, I need to correct a mistake I made two weeks ago. I wrote the elevation of GPS satellites was 12,500 feet instead of the proper 12,500 miles. As Geophysical Institute Project Engineer Dan Osborne pointed out, satellites can't orbit that low; if they could, they wouldn't be able to hurdle Denali.