Distorted GPS Signals Can Accurately Measure Hurricane Wind Speed

Severe weather has often been the bane of GPS navigation systems, causing countless and unwanted communications disruptions en route. But scientists have now learned how to at least get some benefit from the inconvenience by figuring out how to use the distorted GPS signals to measure wind speed, a feat that reseachers say will enable more accurate measurements of hurricane winds.

GPS navigation systems function by orbiting satellites continuously, sending communication bursts to Earth via radio waves. Each signal contains information about the satellites' position and time of transmission.

But when the satellite waves hit a body of water, about 60 percent of the signal reflects towards the sky. And when the water surface is disrupted by high speed winds, the GPS signal gets scattered in different directions.

"Imagine you blow on a hot bowl of soup," Stephen Katzberg, a research associate at the NASA Langley Research Center, said in a news release. "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."

Knowing that the signal disruption occurs and how to measure it enabled researchers with a novel and inexpensive method to calculate wind speeds. Using the new calculations, GPS receivers onboard airplanes can measure wind speed with better than five meters per second (11 mph) accuracy.

The new system will not only allow for more accurate wind speed measurements, but will save money. The current standard for measuring wind speed involves jettisoning small instruments called dropsondes from airplanes as they fly near hurricanes. A typical hurricane data collection mission will use about 20 dropsondes, each costing about $750.

Dropsondes are very accurate devices, but the high cost of deploying them means their release is spread out in and around storms, which leaves scientists stuck filling in the gaps with guesswork.

Katzberg said the new reflected GPS signal system can operate continuously, gathering non-stop information about the wind below. When used in tandem with the dropsonde system, the spectrum of data collected during hurricanes will be broader and more accurate than ever before.

"You were already going to have these GPS systems onboard, so why not get additional information about the environment around you," said Katzberg.