Synthetic Aperture Radar (SAR)
The Fairbanks skyline sports a new feature this month--a 30-foot-diameter dish antenna mounted on a sturdy pedestal atop the Geophysical Institute on the West Ridge of the University of Alaska Fairbanks campus. The new device can be seen from many of the major highways around Fairbanks, and is certainly noticeable from aircraft of all sorts.
Thus, a good many people have had a chance to wonder just what that big dish is and what it does. Oddly, its first uses may be for the benefit of the people who can't see it---those who live near or travel upon Alaska's seas.
The dish is a receiving antenna, not so different from antennas that pick up satellite television signals. Like these smaller kindred, it will lock on and automatically track transmitting satellites, receiving the energy waves that they send.
Although the final products are also visible images, this antenna will track satellites that are sensitive data-gathering devices. They will scan the ground and sea with an exquisitely accurate kind of radar, then send the radar's findings to the Fairbanks receiver. The information will be filtered and processed by specialized computers. The result: a detailed image of a portion of the earth's surface, captured on a screen or on film.
Ordinary radar has been around so long that most people have forgotten the word is a compression of the system's full name, radio detection and ranging. Radar uses radio-frequency energy to locate objects, for example a ship at sea.
The time interval between the instant a pulse of radio energy is transmitted and the instant the reflected pulse is received enables the system to measure how far away the object is. Given a large enough transmitting antenna, a radar system can provide a great deal of information about the object its beam encounters---the placement of the deckhouse, masts, stacks, perhaps whether there are missiles aboard that ship at sea.
In the language of radar experts, every transmitting antenna has an aperture. The new Fairbanks antenna will receive what is known as synthetic aperture radar (SAR). It was originally devised to use airplanes as the sensing platforms. Moving along a very straight path, an airplane emitting continuous pulses in a very constant microwave frequency can produce microwave echoes. These are processed as if they were transmitted by a single antenna with an effective aperture as long as a kilometer. The effect is of an aperture that is synthesized, or put together, to be far longer than it physically could be. The result is an image of far greater detail than earlier radar devices provided.
Now, with satellites entering the picture, an even larger aperture can be synthesized, Geophysical Institute scientists expect a satellite hundreds of kilometers up to provide images of such high resolution that ice floes only ten meters in diameter can be identified. SAR can provide that level of detail at night or through a cloud cover.
SAR, combined with the computers that make up the less visible portion of the new receiving facility, will enable researchers to study sea ice along Alaska's Bering, Chukchi, and Beaufort sea coasts with a new level of certainty. That will have use for off-shore petroleum activities, transportation, and even fishing fleets. Fishermen are even more likely to appreciate SAR's capacity to identify internal ocean waves, fronts, and eddies. SAR's next applications may be on solid ground. Judging by what airborne imagers have provided so far, the technology works well for finding geological features useful in mineral resource assessment and earthquake hazard identification. Permafrost detection and vegetation studies are also on the roster of forthcoming uses.
Although most of the funding for the Alaska SAR facility came from the U.S. National Aeronautical and Space Agency, the first satellites providing data won't be flown by NASA. Beginning in 1990, Europe, Japan, and Canada will be launching the SAR satellites, It may be late in the 1990s before a U.S. satellite joins the skyborne pack, yet thanks to international cooperation and the Alaska facility, SAR-generated images will soon be available for U.S. scientists.
It's tempting to say the big dish represents the tip of the iceberg. Actually, it's more accurate to say it represents a technology in which the sky truly is the limit.
