  This paper introduces UVIO, a multi-sensor framework that leverages Ultra
Wide Band (UWB) technology and Visual-Inertial Odometry (VIO) to provide robust
and low-drift localization. In order to include range measurements in state
estimation, the position of the UWB anchors must be known. This study proposes
a multi-step initialization procedure to map multiple unknown anchors by an
Unmanned Aerial Vehicle (UAV), in a fully autonomous fashion. To address the
limitations of initializing UWB anchors via a random trajectory, this paper
uses the Geometric Dilution of Precision (GDOP) as a measure of optimality in
anchor position estimation, to compute a set of optimal waypoints and
synthesize a trajectory that minimizes the mapping uncertainty. After the
initialization is complete, the range measurements from multiple anchors,
including measurement biases, are tightly integrated into the VIO system. While
in range of the initialized anchors, the VIO drift in position and heading is
eliminated. The effectiveness of UVIO and our initialization procedure has been
validated through a series of simulations and real-world experiments.
