A system can include an unmanned aerial vehicle and an altimeter disposed on the unmanned aerial vehicle. The altimeter can include an ultra-wideband radar antenna disposed orthogonally to a plane of straight and level flight of the unmanned aerial vehicle and having an omnidirectional azimuthal beam pattern orthogonal to the plane of straight and level flight of the unmanned aerial vehicle. The altimeter can be configured to determine an altitude of the unmanned aerial vehicle above a target surface based on time of flight of radar pulses between the ultra-wideband radar antenna and the target surface.