By pinpointing locations on Earth from space, GPS systems have long
shown drivers the shortest route home and guided airline pilots across
oceans. Now, by figuring out how messed up GPS satellite signals get
when bouncing around in a storm, researchers have found a way to do
something completely different with GPS: measure and map the wind speeds
Improved wind speed measurements could help meteorologists better
predict the severity of storms and where they might be headed, said
Stephen Katzberg, a Distinguished Research Associate at the NASA Langley
Research Center in Hampton, Va., and a leader in the development of the
new GPS technique. On a global scale, experts hope to use the new
measurement method to better understand how storms form and what guides
The new technique could inexpensively provide a much more extensive view
of a storm's wind speeds than currently possible, its developers say.
Test flights on storm-hunting airplanes of the National Oceanic and
Atmospheric Administration (NOAA) - nicknamed Hurricane Hunters
-demonstrate that the system provides valuable information at little
additional cost, according to Katzberg and his colleagues.
An article describing the scientists' methods and findings has been
accepted for publication in Radio Science, a journal of the American
Hovering thousands of miles above Earth, GPS satellites constantly beam
radio waves toward the ground carrying information about both the
position of the satellite and the time the message was sent out. These
radio waves can reflect off a surface similar to the way visible light
reflects off a mirror.
When a radio wave from a GPS satellite strikes the surface of a body of
water, such as the ocean, about 60 percent of the signal reflects back
toward the sky, Katzberg said. Unlike a mirror, however, the surface of
the ocean is rarely calm and flat. Wind blowing over a body of water
generates heaving waves.
"Imagine you blow on a hot bowl of soup," he explained. "The harder you
blow, the bigger the 'waves' are in the bowl." When a GPS signal strikes
a wave, the rough surface distorts the reflection by scattering the
signals in various directions.
"The radio wave bounces off the waves," said Katzberg. "As the surface
gets rougher, the reflections get more disturbed and that's what we
The new method of calculating wind speeds is the fruit of years of
fine-tuning by scientists from NASA and NOAA, Katzberg added. In
operation, the measurements are taken by GPS receiver chips, similar to
those found in smartphones, located inside the aircraft.
A computer compares signals coming directly from satellites above with
the reflections from the sea below and calculates an approximate wind
speed with better than 5 meters per second (about 11 miles per hour)
accuracy. The wind speed of a mid-range, Category 3 hurricane, for
comparison, is about 55 meters per second (123 miles per hour).
Drops in the ocean
In order to measure hurricane wind speeds using the standard method,
scientists drop a 16-inch-long tube packed with scientific instruments
called a dropsonde. These dropsondes are attached to small parachutes
and jettisoned from airplanes, gathering information during their
descent. Each device measures pressure, humidity and temperature in
addition to wind speed. A typical Hurricane Hunter mission uses about 20
single-use dropsondes, each costing around $750.
Dropsondes provide 10 times more precise wind speed measurements than
the new GPS method can, so far. Their accuracy is about 0.5 meters per
second (1.1 miles per hour).
But, since the dropsondes are so expensive, their releases are spread
out around and in storms. This distance means meteorologists need to use
some guesswork to fill in the gaps. According to Katzberg, the
reflected GPS signal system can essentially run non-stop, constantly
gathering information about the wind below. The ultimate goal isn't to
replace dropsondes, but rather to add a much broader view of wind speeds
to the data the dropsondes provide.
"You were already going to have these GPS systems onboard, so why not
get additional information about the environment around you," said
Since the method requires large bodies of water to work, the system
can't be used over land. Also, in cases where the ocean's surface is
choppy without any wind, such as the eye of a storm, Katzberg says other
tools would need to be used instead to get an accurate measurement.
Although the new measurement technique is being tested on planes, it may
get implemented on satellites, according to Katzberg. In 2016, NASA
plans to launch a system of small satellites, called the Cyclone Global
Navigation Satellite System (CYGNSS), to measure reflected GPS satellite
signals from low orbit to monitor storm wind speeds from space.
And, looking further into the future, reflections of powerful satellite
broadcasts from DirecTV and Sirius XM Radio could be used in addition to
"Those signals are extremely powerful and easy to detect," said
Katzberg. "These satellites cost hundreds of millions or even billions
of dollars, but our system only costs a few hundred. We're taking
advantage of the expensive infrastructure that's already there."
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