The first GPS satellite was launched in 1978, and was built to aid navigation by transmitting radio signals from space to Earth. In 2000, President Bill Clinton removed the system’s military encryption. Called “selective availability,” this encryption had prevented users from calculating position coordinates in real time. Today, there are 31 GPS satellites operated by the US, plus more from other countries, all producing freely-available data.

In recent years, radio scientists have found a way to use the GPS constellation for purposes far beyond its intended use. The bounce-backs of GPS signals can be used to measure soil moisture, sea surface height, sea ice, wind velocities, and snow depth. After bouncing off of the Earth, the weak return signals are received by other satellites and contain information about the surface they hit. This process is called the Global Navigation Satellite System Reflectometry (GNSS-R).

Now, researchers have developed a new method to measure melting of the Greenland ice sheet using the GNSS-R. 

Water reflects more microwave radiation than ice, causing stronger bounce-backs from melt regions. Researchers use this property to make maps of the reflectivity of the ice sheet during melt and non-melt seasons. Furthermore, the GPS signals can penetrate hundreds of feet into the ice, much deeper than existing measurements. As a result, the technique may be able to detect previously invisible subsurface melt, a new research area. 

Spaceborne sensors are notorious for unplanned failures. The ability to make melt measurements from military-backed GPS satellites will provide a useful and more securely funded point of comparison for conventional measurements, potentially at greater subsurface depths than before.