In this paper we address the issue of autonomous navigation, that is, the ability for a navigation system to provide information about the states of a vehicle without the need for a priori infrastructures such as GPS, beacons, or preloaded maps of the area of interest. The algorithm applied is known as Simultaneous Localisation and Mapping (SLAM). It is a terrain aided navigation system which has the capability for online map building, while simultaneously utilising the generated map to bound the errors in the navigation solution. As no a priori terrain information nor initial knowledge of the vehicle location is required, this algorithm presents a powerful navigation augmentation system. More importantly, it can be implemented as an independent navigation system. This paper also describes a decentralised SLAM algorithm which allows multiple vehicles to acquire a joint 3D map via a decentralised information fusion network. The key idea behind this decentralised SLAM is to represent the map in information form (negative log-likelihood) for communication. Experimental results are provided using computer simulation to demonstrate the single-vehicle and multi-vehicles SLAM without the use of GPS and preloaded maps.