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Ocean Surface Topography from Space
Keeping Current with Ocean Currents
July 30, 2004

Article courtesy of JPL

Imagine a place where the roads change constantly and last month's map may be completely out of date. That's the ocean.

Knowing where the currents were a week ago won't help a ship captain chart the best course or a clean-up crew anticipate where an oil spill is heading nearly as much as knowing where the currents are today.

Take Captain Karl Greig, for example, who skippers a large anchor-handling towing supply boat in the
Gulf of Mexico. He works for Edison Chouest Offshore moving oil rigs from one spot to another as they
search for oil and natural gas. Each rig comes with eight to 12 anchors along with heavy chain and
steel cables. Top towing speed is two to four knots, about as fast as strong ocean currents.

Greig needs up-to-the minute information on ocean currents and now he can get it, thanks to near
real-time ocean altimetry. Greig now regularly taps into the Web site of the University of Colorado's
Center for Astrodynamics Research, where the latest satellite measurements of sea surface height are
used to create maps showing the location, direction and speed of currents in the Gulf of Mexico.

Greig calculated that this information helped him shorten his towing time on one 724-kilometer
(425-mile) tow trip last year by more than 50 hours. "I was able to adjust my course to avoid
towing into the Loop Currents and to get one astern of us and increase our tow speed by almost
two knots," says Greig. "That saved the company chartering the equipment a considerable amount
of money."

Currents follow the ocean's ever-changing landscape. The best way to map global ocean currents
is from space with altimetry, measurements of sea surface topography. NASA satellites have been
making continuous measurements of ocean altimetry since 1992 beginning with the launch of
TOPEX/Poseiden followed by Jason. Now four different international satellites orbit Earth making
continuous measurements of sea surface height. The data are available to the public free of charge.

Scientists were quick to use altimetry data for studies ranging from long-term climate change
to predicting the next El Ni?o. For most researchers, the month it took for satellite data to
be downloaded, processed and delivered wasn't a problem.

However, with the advent of near real-time altimetry data just a few years ago and Web sites
that offer help in how to use it, a whole new group of people, such as Captain Greig, are taking
advantage of this new information both for work and for pleasure. Near real-time altimetry, as
global positioning has already done, is working its way from the military and scientific spheres
into everyday life.

"I couldn't do my job without it," says oceanographer Jenifer Clark, "I use it all over the world
for yacht races." Clark, who half jokingly calls herself "the AAA of the ocean," runs her own
company preparing real-time ocean charts for sailboat racing, ocean cruising, fisheries and other
uses. She uses a variety of sources, including buoys and satellite sea surface temperature
measurements and altimetry, to create her charts.

Near real-time altimetry data also helped Spanish authorities deal with the massive oil spill
caused when the oil tanker Prestige broke apart and sank in November 2002. Within hours of
the ship's sinking, the U.S. Navy's research laboratory began providing current information
for those in charge of clean up. The laboratory now produces a daily 72-hour forecast of
ocean currents for the area based on computer models that assimilate different types of satellite
data, including sea-surface temperature measurements and ocean altimetry.

"It is critical to have near real-time observations," says Greg Jacobs, head of the Ocean
Dynamics and Prediction Branch of the Naval Research Laboratory Stennis Space Center. "You
have the same problems predicting ocean currents as predicting the weather. It's the 'butterfly effect'
-- over time, a small change in one place can grow through time to completely alter the flow in another
area," explains Jacobs. "If we didn't have constant observations, we wouldn't have any idea about
what the currents or the weather would be like a week from now." The Naval Research Laboratory's
public Web site has a wide range of users, including fishermen, sailors and oil companies, according
to Jacobs.

Sources of near real-time ocean altimetry are growing along with the number of users. However,
being able to use satellite data does require skill. "The learning curve is pretty steep," says
Robert Leben, associate research professor at the University of Colorado's Center for Astrodynamics
Research. "The ocean is a complicated fluid system." Many altimetry data users, especially
ndustry, turn to consultants to help them translate the data into the specific information they
need, he says.

But, he adds, learning to read satellite maps of ocean currents is not unlike learning to
understand a weather map. "If you work at it, you can figure out how to get the information
you're looking for," Leben says, "and there are a lot of resources on the Web to help."

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