The algorithms presented in this book were designed to achieve an acceptable trade-off between contradictive requirements to the software of small UAV navigation systems: sufficient accuracy and reliability in order to perform required flight missions on the one hand, and acceptable cost and simplicity of this software on the other hand. The core of modern navigation systems is integrated Strapdown Inertial Navigation System (SINS) and GPS, so in this book, the SINS algorithms and the algorithms of sensor fusion are described primarily. Inertial sensors (rate gyros and accelerometers) used in SINS are manufactured on the basis of the MEMS-technology. That is why they possess poor accuracy and need to be corrected with other sensors (GPS, magnetometers, and barometric altimeters). It is necessary to take into account that flight missions of small UAVs are characterized by small flight distances, small flight times, small flight speeds, etc. These properties of small UAV flight missions and properties of MEMS-sensors create a practical background for simplification of the SINS algorithms, simultaneously preserving their accuracy at acceptable levels. The navigation algorithms for gyro-free SINS are also considered. Increasing reliability of the UAV navigation systems requires a solution of the problems of the detection of the faulty sensors. These algorithms are described. Some practical aspects of the operation of navigation systems such as initial alignment, sensors calibration, and laboratory, ground, and flight testing of integrated SINS for small UAVs are also presented.This book will be useful for a wide circle of researchers, engineers, and graduate students involved in modern UAV design and manufacturing.