We describe the Gamma Portable Radar Interferometer (GPRI), an instrument designed to perform ground-based radar interferometry to measure surface deformation with an accuracy < 0.2 mm and generate digital elevation models (DEMs). Ground-based interferometric observations are complementary to spaceborne or airborne SAR based measurements because they can be acquired continuously in order to track rapid deformation and mitigate the effects of temporal decorrelation and atmospheric phase variability. Results from test cases are shown including the Aletsch glacier, Aletschwald, and Kornhaus bridge in Bern.