July 15, 2013
Distorted GPS Signals Used To Measure Hurricane Winds
Brett Smith for redOrbit.com - Your Universe Online
Truck drivers and commuters rely on GPS technology to get themselves where they need to go in the shortest amount of time. During massive storms, these GPS signals can be buffeted by storm conditions. Now, scientists have figured out how to measure wind speeds based on these disruptions, according to a new study in the journal Radio Science.
GPS satellites continuously send out radio waves toward the Earth containing information about each satelliteâs position and the time of transmission. When a GPS signal strikes a body of water, about 60 percent of the signal reflects back toward the sky, the researchers said. When the surface of that water is disrupted by high velocity winds, the resulting waves scatter GPS signals in different directions.
"Imagine you blow on a hot bowl of soup," explained study co-author Stephen Katzberg, a research associate at the NASA Langley Research Center. "The harder you blow, the bigger the 'waves' are in the bowl."
"The radio wave bounces off the waves," Katzberg continued. "As the surface gets rougher, the reflections get more disturbed and that's what we measure."
Using GPS receiver chips like those found in smartphones, the scientists were able to collect data while on board storm hunting aircraft - referred to as Hurricane Hunters. On the plane, a computer compares signals coming from satellites above to the reflections from the sea below and calculates an estimated wind speed.
This method is much cheaper than the current method that measures hurricane wind speeds using 16-inch tube called a dropsonde, which is filled with instruments, attached to small parachutes and dropped from airplanes. Each non-reusable device records pressure, humidity, temperature and wind speed and costs around $750.
While dropsondes provide 10 times more precise wind speed measurements than the GPS method, they can be cost prohibitive at times - leading to limited use during each storm and forcing meteorologists to estimate wind speeds within the gaps.
According to Katzberg, the new GPS method can continuously gathering information on the storm. The NASA scientist said he hopes the new system can supplement the dropsonde system and give a more comprehensive view of a major storm.
"You were already going to have these GPS systems onboard, so why not get additional information about the environment around you," Katzberg said.
Another limiting factor on the GPS system is that is requires large bodies of water to work and also does not function well in the eye of a storm because of the choppy waters without any winds that are typically found there.
The study researchers said they expect a new NASA system, called the Cyclone Global Navigation Satellite System (CYGNSS), to measure GPS signals from low orbit, enabling the observation of storm wind speeds from space. They also speculated that powerful satellite broadcasts from DirecTV and Sirius XM Radio could also be used in a next-generation GPS system.
"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."