Using GPS Reflections to Estimate Terrestrial Water Cycle Variables

Eric Small

Abstract

GPS satellites transmit L-band microwave signals, which are strongly influenced by water at 
the surface of the Earth.  GPS signals take two different paths: (1) the “direct” signal 
travels from the satellite to the antenna; (2) the “reflected” signal interacts with the 
Earth’s surface before travelling to the antenna.  Standard geodetic-quality GPS systems 
measure a combination of the direct and reflected signals.  We have developed a new remote 
sensing technique to retrieve terrestrial water cycle variables from GPS data.  The sensing 
footprint is intermediate in scale between in situ observations and standard remote sensing 
measurements.  Snow depth, soil moisture, and an index of vegetation water content are 
estimated from data collected at operational GPS sites that make up NSF’s Plate Boundary 
Observatory Network.  The products are updated daily and are available online.  Validation 
studies show that retrieved products are of sufficient quality to be used in a variety of 
applications.  GPS systems designed for geodetic applications are ill-suited for monitoring 
the water cycle because the antennas used suppress reflections.  We are developing a new 
sensor designed to measure reflected GPS signals.  This will yield a more sensitive 
instrument that costs an order of magnitude less than existing geodetic-quality systems.  
Such a technology would have broad applications in both research and agricultural settings.