  This paper investigates an infra-structure free global localization of a
group of communicating mobile agents (e.g., first responders or exploring
robots) via an ultra-wideband (UWB) inter-agent ranging aided dead-reckoning.
We propose a loosely coupled cooperative localization algorithm that acts as an
augmentation atop the local dead-reckoning system of each mobile agent. This
augmentation becomes active only when an agent wants to process a relative
measurement it has taken. The main contribution of this paper is addressing the
challenges in the proper processing of the UWB range measurements in the
framework of a loosely coupled cooperative localization. Even though UWB offers
a decimeter level accuracy in line-of-sight (LoS) ranging, its accuracy
degrades significantly in non-line-of-sight (NLoS) due to the significant
unknown positive bias in the measurements. Thus, the measurement models for the
UWB LoS and NLoS ranging conditions are different, and proper processing of
NLoS measurements requires a bias compensation measure. We also show that, in
practice, the measurement modal discriminators determine the type of UWB range
measurements should be probabilistic. To take into account the probabilistic
nature of the NLoS identifiers when processing UWB inter-agent ranging
feedback, we employ an interacting multiple model (IMM) estimator in our
localization filter. We also propose a bias compensation method for NLoS UWB
measurements. The effectiveness of our cooperative localization is demonstrated
via an experiment for a group of pedestrians who use UWB relative range
measurements among themselves to improve their shoe-mounted INS geolocation.
