Satellite interferometry has been used extensively for ground-motion monitoring with good success. In the case of landslides, for e xample, space-borne SAR interferometry has a good potential to get an overview on the slop e stability. The role of a space-borne INSAR as an element in a landslide or rock fall war ning system is constrained by the specific space-borne SAR imaging geometry, the typical multiple-week repeat-interval, and uncertainties in the data availability. Most of the se limitations can be overcome with an in situ radar imaging system. GAMMA has developed a portable radar interferometer that utilizes real-aperture antennas to obtain high azimuth resol ution. Images are acquired line by line while rotating the transmitting and receiving anten nas about a vertical axis. Phase differences between successive images acquired from the same location are used to determine line-of- sight displacements. The instrument operates at 17. 2 GHz and has measurement sensitivity better than 1 mm. The instrument uses two receiving antennas with a short baseline to form an interferometer. Phase differences between simulta neous acquisitions by these antennas are used to calculate the precise look angle relative t o the baseline, permitting derivation of the surface topography. Expected statistical noise in t he height measurements is on the order of 1 meter. In this contribution the design, measurement principles and characteristics of GAMMA's Portable Radar Interferometer are presented.