Sensing vegetation growth with reflected GPS signals

Estimates of vegetation state are required for hydrometeorological modeling and validation of satellite estimates of land surface conditions. A linkage is described between vegetation growth and ground reflected multipath at GPS stations. Reflections are sensitive to conditions over a ∼1000m² area, larger than typical in situ observations but smaller than space-based products. At two agricultural test sites, vegetation height and water content are inversely correlated with the magnitude of ground reflected multipath measured by geodetic-quality GPS stations. This relationship was tested further at Plate Boundary Observatory (PBO) network GPS sites, using Normalized Difference Vegetative Index (NDVI) to gauge vegetation status. NDVI is inversely correlated with the magnitude of multipath at nine sites located in grassland, shrubland and cropland. Multipath variations lag NDVI by approximately three weeks. Multipath statistics from existing sites are calculated daily and could be used to estimate biophysical properties in non-forested regions, which represent ∼80% of land area.