GNSS-IMU-based navigation has been introduced in the mid-90s and still a key component of UAV navigation. However, it has a critical drawback to operate in downtown core areas where high-rise buildings often interfere with signals from GNSS. One emerging solution is to position UAV using Ultra-Wideband (UWB) due to its robustness to multipath and cm-level ranging error. This thesis is a study of GNSS-denied positioning based on UWB ranging.