A Hierarchical Tracking Strategy for Vision-Based Applications On-Board UAVs

In this paper, we apply a hierarchical
tracking strategy of planar objects (or that can
be assumed to be planar) that is based on direct
methods for vision-based applications on-board
UAVs. The use of this tracking strategy allows
to achieve the tasks at real-time frame rates and
to overcome problems posed by the challenging
conditions of the tasks: e.g. constant vibrations,
fast 3D changes, or limited capacity on-board. The
vast majority of approaches make use of featurebased
methods to track objects. Nonetheless, in
this paper we show that although some of these
feature-based solutions are faster, direct methods
can be more robust under fast 3D motions (fast
changes in position), some changes in appearance,
constant vibrations (without requiring any specific
hardware or software for video stabilization), and
situations in which part of the object to track is
outside of the field of view of the camera. The
performance of the proposed tracking strategy onboard
UAVs is evaluated with images from realflight
tests using manually-generated ground truthinformation, accurate position estimation using a
Vicon system, and also with simulated data from a
simulation environment. Results show that the hierarchical
tracking strategy performs better than
well-known feature-based algorithms and wellknown
configurations of direct methods, and that
its performance is robust enough for vision-in-theloop
tasks, e.g. for vision-based landing tasks.