  Micro aerial vehicles (MAVs) hold the potential for performing autonomous and
contactless land surveys for the detection of landmines and explosive remnants
of war (ERW). Metal detectors are the standard detection tool but must be
operated close to and parallel to the terrain. A successful combination of MAVs
with metal detectors has not been presented yet, as it requires advanced flight
capabilities. To this end, we present an autonomous system to survey
challenging undulated terrain using a metal detector mounted on a 5 degrees of
freedom (DOF) MAV. Based on an online estimate of the terrain, our
receding-horizon planner efficiently covers the area, aligning the detector to
the surface while considering the kinematic and visibility constraints of the
platform. As the survey requires resilient and accurate localization in diverse
terrain, we also propose a factor graph-based online fusion of GNSS, IMU, and
LiDAR measurements. We validate the robustness of the solution to individual
sensor degeneracy by flying under the canopy of trees and over featureless
fields. A simulated ablation study shows that the proposed planner reduces
coverage duration and improves trajectory smoothness. Real-world flight
experiments showcase autonomous mapping of buried metallic objects in undulated
and obstructed terrain.
