  Localization of low-cost unmanned aerial vehicles(UAVs) often relies on
Global Navigation Satellite Systems (GNSS). GNSS are susceptible to both
natural disruptions to radio signal and intentional jamming and spoofing by an
adversary. A typical way to provide georeferenced localization without GNSS for
small UAVs is to have a downward-facing camera and match camera images to a
map. The downward-facing camera adds cost, size, and weight to the UAV platform
and the orientation limits its usability for other purposes. In this work, we
propose a Monte-Carlo localization method for georeferenced localization of an
UAV requiring no infrastructure using only inertial measurements, a camera
facing an arbitrary direction, and an orthoimage map. We perform
orthorectification of the UAV image, relying on a local planarity assumption of
the environment, relaxing the requirement of downward-pointing camera. We
propose a measure of goodness for the matching score of an orthorectified UAV
image and a map. We demonstrate that the system is able to localize globally an
UAV with modest requirements for initialization and map resolution.
