Measurements of vegetation state are required both for modeling and satellite validation. Reflected GPS signals recorded by the Plate Boundary Observatory network provide a source of new information about vegetation state in the western United States and Alaska. The GPS ground stations were installed between 2005 and 2008 to measure plate boundary deformation. They operate continuously and transmit their data to a public facility at least once per day. However, they also act as bi-static radars by recording the interference between a direct GPS signal (transmitted at 1.5 GHz) and a reflected GPS signal. The frequency of this interference pattern primarily depends on the vertical distance between the antenna and the ground reflector. As an L-band sensor, the amplitude of the interference pattern depends on vegetation water content. A daily vegetation metric that depends on reflection amplitudes, Normalized Microwave Reflection Index (NMRI), is defined. A method for removing outliers caused by snow and rain is described. The footprint of NMRI depends on the antenna height and local terrain. The minimum footprint is 1000 m2. A database of more than 300 station NMRI time series has been compiled; these data span the period from 2007 to 2012. Comparisons between NMRI and in situ sampling of vegetation state are the subject of a companion paper.