In order to provide accurate positioning in many different environments and for many different platforms, the output from different sensors must be combined to provide the navigation solution. Evaluating the performance for all the diverse operating conditions is difficult in the field and it is almost impossible to repeatedly generate the exact test conditions. Therefore, it is necessary to simulate the various environments and platforms and model the sensor behaviour to understand how the navigation solution will perform. We have extensive experience with modelling navigation sensors and platforms and developing navigation algorithms. Navigation algorithms for unmanned vehicles, underwater or in the air, have been developed and validated at TRT (UK).