The doctoral project will apply recently developed methods within nonlinear control theory, and also develop new methods for nonlinear analysis and observer design, in order to develop nonlinear observers for tight GPS/INS integration. The theoretical res ults will be verified by experiments at the GPS/INS laboratory at the Norwegian University og Science and Technology which is built as a part of the Department of Engineering Cybernetics and Telecommunications' joint commitment to research on nonlinear ob server design for GPS/INS integration. This will enable concurrent development and testing of methods and algorithms, and will promote high quality, effective observer algorithms. The project is a continuation of previous research works at the Department of Engineering Cybernetics, and is a part of the collaboration with the Department of Telecommunications on GPS/INS integration. The project will develop navigation systems with higher integrity and reliability, provding high-accuracy measurements in 6 d egrees of freedom, and this will amongst other allow for more accurate, reliable and safe marine operations.