FOI conducts research on cost effective integrated navigation systems based on satellite navigation and MEMS-IMU(Micro Electro Mechanical Systems). Both civilian and military applications often require sufficiently accurate navigation in indoor and urban environments. Ultra tightly integrated navigation systems is a new and effective integration method, which requires accurate modelling of MEMS-IMU errors. MEMS-IMU sensorshave larger errors and are more difficult to model than traditional IMU-sensors. An automated process for determining the dominant stochastic. IMU-errors from measurements have been developed. The effect of different stochastic IMU-errors on the navigation performance have been analysed by using Monte Carlo-simulations. Three different trajectories have been investigated: stationary, low dynamics and a circular path. The purpose of this report is to analyse the effect of the stochastic errors on the navigation solution. The results indicate good conformity between the generated IMU-signals and measurements. For IMU-error parameters corresponding to a Crista MEMS-IMU, the navigation performance using stochastic errors was significantly degraded for the circular path compared to the stationary and low dynamics path.