  Ground penetrating radar mounted on micro aerial vehicle (MAV) is a promising
tool to assist humanitarian landmine clearance. However, the quality of
synthetic aperture radar images depends on accurate and precise motion
estimation of the radar antennas as well as generating informative viewpoints
with the MAV. This paper presents a complete and automatic airborne
ground-penetrating synthetic aperture radar (GPSAR) system. The system consists
of a spatially calibrated and temporally synchronized industrial grade sensor
suite that enables navigation above ground level, radar imaging, and optical
imaging. A custom mission planning framework allows generation and automatic
execution of stripmap and circular (GPSAR) trajectories controlled above ground
level as well as aerial imaging survey flights. A factor graph based state
estimator fuses measurements from dual receiver real-time kinematic (RTK)
global navigation satellite system (GNSS) and inertial measurement unit (IMU)
to obtain precise, high rate platform positions and orientations. Ground truth
experiments showed sensor timing as accurate as 0.8 us and as precise as 0.1 us
with localization rates of 1 kHz. The dual position factor formulation improves
online localization accuracy up to 40% and batch localization accuracy up to
59% compared to a single position factor with uncertain heading initialization.
Our field trials validated a localization accuracy and precision that enables
coherent radar measurement addition and detection of radar targets buried in
sand. This validates the potential as an aerial landmine detection system.
