Reflected GPS Signals

Most scientists use GPS to measure position. However, there is a growing community of GPS scientists that are using noise on the GPS signal to do their science. For these applications, reflected GPS signals (also known as multipath) are used.

Reflected GPS signals were first proposed as a remote sensing tool by Martin-Neira in 1993. Most of these 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. Research pioneered by the PBO H₂O group takes advantage of reflections observed at geodetic-quality GPS sites (Figure 1). The antenna is inside the hemispherical acrylic dome. The antenna is typically about two meters above the ground. This particular GPS antenna was designed to suppress reflected signals (specular reflections). However, reflections can still be measured at many existing sites, allowing us to take advantage of thousands of GPS sites operating around the world.

For most people that use GPS receivers, the observed quantity is the distance between the satellite and the antenna. However, depending on how smooth the surface is, a GPS receiver can also sense a reflected signal. The beating of the direct and reflected signals turns a GPS receiver into an interferometer (Figure 2). To a first order, the observed frequency depends on the height of the antenna above the reflector (H) and the GPS wavelengths (19 cm for L1 and 24.4 cm for L2). These effects can be seen in all data. We focus on the signal to noise ratio (SNR) data.

Figure 1.Top: PBO GPS receiver site P041, located at Marshall, Colorado. Specular reflections, where the incident ray and the reflected rays make the same angle with respect to the surface.

Top: PBO GPS receiver site P041, located at Marshall, Colorado. Specular reflections, where the incident ray and the reflected rays make the same angle with respect to the surface.

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Figure 2.Cartoon of the GPS reflection method. The angle of the satellite signal with respect to the planar surface is e, the elevation angle. The direct signal is shown in blue. The reflected signal travels a further distance (shown in red); the GPS receiver measures the interference between the direct and reflected signals. Examples of this interference are given in the middle of the figure. e is the angle of the satellite with respect to the horizon. (Credit: Kristine Larson.)

Cartoon of the GPS reflection method. The angle of the satellite signal with respect to the planar surface is e, the elevation angle. The direct signal is shown in blue. The reflected signal travels a further distance (shown in red); the GPS receiver measures the interference between the direct and reflected signals. Examples of this interference are given in the middle of the figure. e is the angle of the satellite with respect to the horizon. (Credit: Kristine Larson.)

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Below we summarize the kinds of way you can use the reflected signals in SNR data to study the water cycle. At top we start with the "bare soil" reflection. This sine wave has a certain frequency, phase, and amplitude. If instead of soil, the signal bounces off the snow layer, the frequency changes (dashed line). If the reflected surface has vegetation on it (particularly vegetation with water in it), the reflection is suppressed. So the frequency doesn't change much, but the amplitude can change a lot. Finally, the hardest signal to see is soil moisture changes when it rains. This causes a very small phase shift in the SNR data.

(Credit: Kristine Larson.)

(Credit: Kristine Larson.)

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