Advanced Imaging

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Advanced Imaging Magazine

Updated: January 12th, 2011 10:01 AM CDT

Eye in the Sky

Latest Canadian SAR satellite delivers higher resolution, more flexibility
RADARSAT-2
© Canadian Space Agency and MacDonald, Dettwiler and Associates
RADARSAT-2 is a second-generation Synthetic Aperture Radar (SAR) satellite with a number of advancements, including three-meter ultra-fine beam for high-resolution imaging and increased data storage.
A RADARSAT-2 image of Greenland.
© Canadian Space Agency and MacDonald, Dettwiler and Associates
A RADARSAT-2 image of Greenland.
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By Barry Hochfelder

"You send out the signal and sample the return pulses over time," says Doug Rae, Manager Business Development—Integrated Information Solutions at MDA. "Then, with some signal processing, it's turned into imagery. There are advantages. It works in clouds and at night and all weather conditions. With a camera, you need a flash. You can't put a flash on a spacecraft. Optical [imaging] is really geared to morning because there are fewer clouds. With SAR, it doesn't matter."

MDA takes an order from the Canadian government or a private customer of the area to be monitored and programs the satellite for that job. "All the data is processed on the ground," Rae explains. "Information is stored in the memory on the satellite or can be down-linked later. On the ground, they decode the data and digitally convert it from raw signals into an image. It used to be done with ground lenses, but that was incredibly complicated. Now we use Fourier and other methods like that to resolve differences in time and velocities."

Unlike optical cameras, brightness is not an indication of light reflection, but rather how well the radar signal is sent back. With water, for example—especially if it's calm—the pulse reflects away, so the water appears dark, Rae says. "With mountains, they're rocky and tend to reflect the signal back. They reflect the radar pulse back better. Things coming from behind the satellite come back slower; from the front, they come back a little faster so you can tell where it's coming from. Range is perpendicular to where the satellite is traveling.

"If you send a signal straight down," he adds, "the signal will come back at the same time. Between a 20-50-degree angle, the time between the nearest data and farthest gives time to differentiate different parts of the data."

The spacecraft communicates with the ground using a combination S-band (uplink and downlink) and X-band (downlink only). The X-band provides a signal that can be received by ground station antennas with diameters as small as three meters. Data is down-linked to network stations around the world. As soon as the satellite enters the network stations' "mask" data can be down-linked in real time.



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