Just as sunlight glints off the ocean’s surface, so do radio signals from the
constellation of global positioning system (GPS) navigation satellites orbiting
Earth. Now, researchers from NASA’s Jet Propulsion Laboratory, Pasadena,
Calif., have shown that although these reflected signals are very weak, they can be
detected by airborne instruments and used to map ocean eddies.

Eddies, which affect shoreline weather and the fishing industry, represent
“one of the largest unknowns in Earth’s climate models,” said JPL physicist Dr.
Stephen Lowe. Lowe led two aircraft experiments to test how well GPS could be
used for altimetry, or measuring sea-surface height.

Eddies are currents that run in a circular path against the main flow of
current. Warm eddies have a higher surface height than the surrounding water,
while cool eddies are lower. Ranging from 10 to 100 kilometers (6 to 62 miles) in
size, many ocean eddies are either too small or don’t last long enough to be
spotted by the current generation of satellite ocean altimeters, whose
measurements of sea-surface height provide a picture of global circulation. Lowe
and his colleagues’ goal is to determine whether, in the future, reflected GPS
signals could be used to map small ocean circulation features such as eddies from

“Eddies are small features with a big impact,” said co-author Dr. Yi Chao,
a JPL oceanographer. “They’re where a lot of ocean physics happens and are an
integral part of our climate system. But we don’t have enough information about
them to include in our models. Coastal eddies also have a major role in regulating
the weather near the shore, and they are important for fisheries because they’re
where fish go to feed. In the open ocean, eddies bring nutrient-rich cold water up
to the surface and are an important part of the global carbon cycle.”

In the first experiment, designed to collect reflected GPS signals from a
variety of terrain, the scientists demonstrated that these signals could be detected
and used to calculate ocean height. In the second experiment, planned specifically
for ocean altimetry, they showed their technique has the potential to provide
ocean-height measurements precise enough to map ocean eddies. The results of
the latest experiment appear in the May issue of Geophysical Research Letters.

Today’s satellite ocean altimeters, including the U.S.-French
Topex/Poseiden and Jason 1 spacecraft, measure sea-surface height by sending a
radar pulse to the ocean’s surface and timing its return. While they measure ocean
surface topography very accurately, to within 2 centimeters (1 inch), they see only
the swath of ocean directly beneath them and take 10 days to make a complete
map of the global ocean. Since an ocean eddy lasts only a week or two, they may
only catch a portion of an eddy’s lifespan.

In contrast, an orbiting GPS altimeter would have no radar, making it
relatively inexpensive. The receiver would obtain position and timing information
from the GPS constellation of satellites and would measure ocean height using the
arrival time of GPS signals reflected from the surface. At any single time, it
would be able to produce about 10 simultaneous measurements across an area
thousands of kilometers wide. A constellation of about 10 such instruments,
capable of making up to 100 simultaneous ocean-height measurements, could
map ocean eddies globally.

The Global Positioning System is a Department of Defense-controlled
navigation system comprised of 28 Earth-orbiting satellites and a network of
tracking stations. By measuring the time it takes for signals to travel directly
between satellites and receivers, the positions of the satellites and receivers can be

In coming experiments, the JPL researchers will fly their equipment on
aircraft at different altitudes and speeds. They’ll be making ocean-height
measurements and comparing their results with those from other instruments.
They also have plans to improve their onboard receiver so that the instrument can
be flown on spacecraft.

“Our plan is not to replicate the very precise measurements that
Topex/Poseidon and Jason 1 make,” said Chao, “but rather to help fill in some of
the gaps in time and in coverage by looking between the satellites’ ground tracks
and close to the shore. We would like to provide a new data set to push the next
generation of climate models.”