By figuring out how messed up GPS satellite signals get when bouncing around in a storm, NASA Langley researchers have found a way to do something completely different with GPS: measure and map the wind speeds of hurricanes.
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.
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. Unlike a mirror, however, the surface of the ocean is rarely calm and flat. Wind blowing over a body of water generates heaving waves.
The radio wave bounces off the waves, and as the surface gets rougher, the reflections get more disturbed, which is what the researchers 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 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).
Although the new measurement technique is being tested on planes, it may get implemented on satellites. 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.