GPS Reflections

 
 

Reflected GPS signals were first proposed as a remote sensing tool by Martin-Neira in 1993.  Most of the early studies of GPS reflections were developed for aircraft or space platforms.  In those experiments, the receiver/antenna systems were optimized to receive the reflected signals.

 

Changes in the observed GPS interference patterns allow us to measure changes in  surface soil moisture, snow cover, and vegetation water content.  For examples, changes in frequency in the above interference patterns are used to retrieve the height h of the antenna above the reflecting surface.  These changes in h are directly related to snow depth.  Changes in phase offset are used to retrieve soil moisture.  Changes in amplitude are related to vegetation growth, i.e. large amplitudes are observed at bare soil sites, and amplitudes decrease during the vegetation growing cycle. We have focused our initial research on taking advantage of existing commercially-available GPS receiver/antennas. This allows us to take advantage of thousands of GPS sites operating around the world.

Our research takes advantage of specular reflections observed at geodetic GPS sites. Such a GPS site is shown to the left.  The antenna is protected by an acrylic dome.  The antenna has been carefully designed so that it can be drilled into bedrock (note the drill-braced monument). The antenna’s phase center is typically about two meters above the ground.  Because users want to measure position, this GPS instrument is designed to suppress reflected signals.

The primary observable of a GPS receiver is the distance between the satellite and the antenna. However, depending on the surface, a GPS receiver also senses a reflected signal; the direct signal is much stronger than the reflected signal.  The beating of the direct and reflected signals turns a GPS receiver into an interferometer.

To first order, the observed frequency of the interference depends on the height of the antenna above the reflector (h) and the GPS transmit frequency (1.5 or 1.2 GHz).

h

GPS satellite

Direct Signal

Reflected Signal

Interference pattern for an antenna 2 meters above the reflecting surface.

Interference pattern for an antenna 1.2 meters above the reflecting surface.

GPS antenna