In this work the design, implementation and real-time tests of a navigation system for the autonomous surface vehicle MACÁBOT is presented. This vehiclerepresents a versatile platform to perform several tasks inthe marine environment, such as; ports maintenance,marine productive ecosystems studies and bathymetries.The navigation system is responsible for accuratelydetermining the position, velocity and attitude of the vehicle.It represents a fundamental component to autonomouslycarry out any of the aforementioned tasks. In this work, thenavigation system is developed based on a GPS aided strapdown inertial navigation system using an extended Kalmanfilter sensor fusion algorithm. In order to provide an adaptive approach to the sensor fusion algorithm tuning afuzzy inference system is used. The navigation system wasimplemented as a package for the Robot Operating System,benefiting from the advantages of heterogeneity, integrationand hardware abstraction. Real time tests of theMACÁBOT on a local creek were carried out, showingsatisfactory performance of the navigation system in bothposition and velocity estimates. In addition to these tests,simulations of GPS outages were carried out with theregistered data to evaluate the performance of thenavigation system in such cases.