  This paper proposes an approach for fusing direct radiometric data from a
thermal camera with inertial measurements to extend the robotic capabilities of
aerial robots for navigation in GPS-denied and visually degraded environments
in the conditions of darkness and in the presence of airborne obscurants such
as dust, fog and smoke. An optimization based approach is developed that
jointly minimizes the re-projection error of 3D landmarks and inertial
measurement errors. The developed solution is extensively verified against both
ground-truth in an indoor laboratory setting, as well as inside an underground
mine under severely visually degraded conditions.
