AbstractTerrain navigation has been used extensively by underwater vehicles in the last decade. By comparing bathymetric measurements with a digital terrain model (DTM), one can estimate a global position of a vehicle underwater, where GPS signals are unavailable. With the increasing threat of GPS signal jammers and spoofers to marine vessels, GPS-independent techniques are becoming more interesting for surface vessels as well. This paper explores the idea and challenges of using terrain navigation to detect GPS spoofing and as a substitute position source during GPS jamming. A robust navigation system is suggested based on a GPS-aided INS augmented by terrain navigation. The INS and terrain navigation system of the HUGIN AUV was adapted to a surface vessel and tested in two experiments on the coast of Norway in scenarios simulating GPS jamming. The results from the experiments clearly indicate the feasibility of such a system, if a DTM of the area is available and the terrain is well suited for terrain navigation.