Autonomous navigation (or localisation) is the process of determining a platform's pose without the use of any a priori information external to the platform except for what the platform senses about the environment. That is, the determination of the platform's pose without the use of predefined maps or infrastructure developed for navigation purposes such as terrain aided navigation systems or Global Navigation Satellite System (GNSS). The objective of this book is to both develop and demonstrate autonomous localisation algorithms for airborne platforms. The emphasis is placed on the importance of the algorithms to function appropriately and accurately using low cost inertial sensors (where the rapid drift in navigation output requires an increasing reliance on frequent absolute sensing), within an environment where the highly dynamic nature of the platform motion provides unreliable and infrequent absolute sensing.