  Monocular visual navigation methods have seen significant advances in the
last decade, recently producing several real-time solutions for autonomously
navigating small unmanned aircraft systems without relying on GPS. This is
critical for military operations which may involve environments where GPS
signals are degraded or denied. However, testing and comparing visual
navigation algorithms remains a challenge since visual data is expensive to
gather. Conducting flight tests in a virtual environment is an attractive
solution prior to committing to outdoor testing.
  This work presents a virtual testbed for conducting simulated flight tests
over real-world terrain and analyzing the real-time performance of visual
navigation algorithms at 31 Hz. This tool was created to ultimately find a
visual odometry algorithm appropriate for further GPS-denied navigation
research on fixed-wing aircraft, even though all of the algorithms were
designed for other modalities. This testbed was used to evaluate three current
state-of-the-art, open-source monocular visual odometry algorithms on a
fixed-wing platform: Direct Sparse Odometry, Semi-Direct Visual Odometry, and
ORB-SLAM2 (with loop closures disabled).
