By Sid Perkins
Measurements of satellite signals bouncing off a calm mountain lake may pave the way for a fleet of spaceborne sensors or groups of seaside towers that can quickly and inexpensively monitor local or global changes in sea level.
Central Oregon’s scenic Crater Lake, one of America’s deepest bodies of water, sits within the caldera left behind by an ancient volcanic eruption. From an observation point on the edge of the ancient crater and 480 meters above the lake, scientists recorded the signals coming straight from Earth-orbiting Global Positioning System (GPS) satellites, as well as the signals bouncing off the water’s surface.
By measuring the slight delay between the direct and the reflected signals, the researchers could calculate the height of their observation point above the smooth lake to within 2 centimeters, says Robert N. Treuhaft, a physicist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Treuhaft and his colleagues report their field experiments in the Dec. 1, 2001 Geophysical Research Letters.
If scientists can develop similar methods of using GPS signals to accurately measure distances above rough ocean water, then inexpensive satellites carrying only GPS receivers could detect small changes in sea level. The satellites now used to monitor the rise and fall of the ocean’s surface are costly because they carry their own radar altimeters, Treuhaft notes. Furthermore, existing craft are limited because they can scan any particular spot on Earth only once every 10 days.
More frequent satellite coverage would enable researchers to track local changes in sea level associated with ocean eddies, says Treuhaft. These large, whirlpool-like swirls often provide nutrient-rich environments for plants and fish, and they can affect the dispersal of pollution.
Current ocean-monitoring satellites have trouble accurately measuring changes in sea level near coasts because stray radar reflections from land corrupt signals bouncing back from the water. However, GPS receivers looking out to sea from atop a 200-m tower or seaside cliff could continually monitor sea level changes up to 20 kilometers from shore.
