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GPS interferometric reflectometry: applications to surface soil moisture, snow depth, and vegetation water content in the western United States

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GPS interferometric reflectometry is a new environmental sensing technique that can be used to measure near‐surface soil moisture, snow depth, and vegetation water content variations. The spatial scale of this technique, ~1000 m2, is intermediate to that of other in situ sensors (<1 m2) and satellites (>100 km2). Soil moisture and snow depth retrievals have accuracies of 0.04 m3/m3 and 0.04 m, respectively. These accuracies are sufficient for many hydrologic applications. Fortuitously, GPS interferometric reflectometry can be used with consumer‐grade off the shelf GPS instruments that are operated by the geodetic, geophysical, and surveying communities. This means that GPS data from thousands of sites are potentially available for environmental scientists seeking new in situ data for soil moisture, snow depth, and vegetation water content. The technique can be applied to data from existing archives or for new sites. Although the accuracy of the technique has only been evaluated for the GPS constellation, the technique can also be used for other navigation constellations such as GLONASS, Galileo, and Beidou. WIREs Water 2016, 3:775–787. doi: 10.1002/wat2.1167 This article is categorized under: Science of Water > Hydrological Processes Science of Water > Methods
Multipath geometry for a horizontal planar reflector. Satellite elevation angle is designated by the variable e. A GPS antenna measures the interference between the direct (blue) and reflected (red) signals. Examples of this interference are shown in the inset.
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Peak GPS‐IR vegetation index for the years 2007–2015 for PBO sites in California. In each year, the peak is compared to the average for 2008–2012 and reported as a percentage as defined in the color bar.
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Comparison between GPS‐IR vegetation index and NDVI. The green‐up period for the California site (blue symbols) is shown by the dashed line and the nondashed line is the senescent period. Results for a GPS site from eastern Wyoming (P042) are shown in green. At this site, there is a much stronger correlation between the GPS and NDVI data, and thus there is no depiction of separate green up and senescent periods.
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(a) GPS vegetation index for Plate Boundary Observatory site P208. (b) Cumulative North American Land Data Assimilation System (NLDAS) precipitation. (c) Normalized Difference Vegetation Index.
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Comparison between in situ soil moisture probes and GPS‐IR. Error bars represent one standard deviation. Root mean square error is 0.039 mm3/mm3.
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(a) Volumetric soil moisture (VSM) estimated using GPS‐IR for Plate Boundary Observatory site P038, located in eastern New Mexico. Soil moisture is estimated twice per day at this site. (b) Daily precipitation is measured with a Vaisala WXT520 sensor.
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GPS‐IR measurements of snow depth at Barrow, AK (Plate Boundary Observatory site SG27). To improve clarity of the snow depth estimate in blue, one standard deviation error bars are shown in gray.
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Utah State Daniel Experimental Forest comparison between GPS‐IR and in situ hand measurements measured in the GPS‐IR footprint.
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(a) Photo of the Niwot Ridge GPS antenna. Snow depth is also measured at the black and white pole by the Niwot Ridge Long‐Term Ecological Research group at 2‐week intervals during the snow season. (b) Snow depth measured by GPS‐IR (blue) and at the pole shown in panel (a) photograph (red).
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All Plate Boundary Observatory (PBO) sites (a: western United States; b: Alaska) are shown in gray. Those with potential for GPS‐IR have been highlighted in blue.
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(a, b) Signal‐to‐noise ratio (SNR) data are shown for the GPS L1 (gray) and L2 (black) frequency signals. The direct signal is represented by the low‐order polynomial fit (magenta). Elevation angles corresponding to this time period are shown in blue. (c) Reflection data for L1 and L2 data after direct signal effect has been removed. The L1 and L2 data have been offset from zero for clarity. (d) Periodogram of the reflection data from panel (c) are shown. The x‐axis is defined as the reflector height in meters, which is the vertical distance between the GPS antenna and the reflecting surface.
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(a) The first Fresnel zone for a single satellite track is depicted for satellite elevation angles of 7, 10, 15, 20, and 25 degrees and an antenna height of 1.8 m. (b) Map view for the GPS‐IR footprint in the western United States for all transmitting GPS satellites. The location of the GPS antenna is shown as the black square.
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