Distorted GPS signals help measure hurricane wind speed

Distorted GPS signals help measure hurricane wind speed

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Washington: Distorted GPS satellite signals could help researchers to measure and map the wind speeds of hurricanes. Test flights on storm-hunting airplanes of the National Oceanic and Atmospheric Administration (NOAA) – nicknamed Hurricane Hunters –demonstrate that the system provides valuable information.

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, about 60 percent of the signal reflects toward the sky, said Stephen Katzberg, a Distinguished Research Associate at the NASA Langley Research Centre. Since the surface of the ocean is rarely calm and flat, wind blowing over a body of water generates huge waves. 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 measure.” 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 metres per second (about 11 miles per hour) accuracy. The wind speed of a mid-range, Category 3 hurricane, for comparison, is about 55 metres per second (123 miles per hour). The reflected GPS signal system can constantly gather information about the wind below. However 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.

Source: phys.org