  Ultra-wideband (UWB) systems are becoming increasingly popular as a means of
inter-robot ranging and communication. A major constraint associated with UWB
is that only one pair of UWB transceivers can range at a time to avoid
interference, hence hindering the scalability of UWB-based localization. In
this paper, a ranging protocol is proposed that allows all robots to passively
listen on neighbouring communicating robots without any hierarchical
restrictions on the role of the robots. This is utilized to allow each robot to
obtain more range measurements and to broadcast preintegrated inertial
measurement unit (IMU) measurements for relative extended pose state estimation
directly on SE_2(3). Consequently, a simultaneous clock-synchronization and
relative-pose estimator (CSRPE) is formulated using an on-manifold extended
Kalman filter (EKF) and is evaluated in simulation using Monte-Carlo runs for
up to 7 robots. The ranging protocol is implemented in C on custom-made UWB
boards fitted to 3 quadcopters, and the proposed filter is evaluated over
multiple experimental trials, yielding up to 56% improvement in localization
accuracy.
