  High-accuracy absolute localization for a team of vehicles is essential when
accomplishing various kinds of tasks. As a promising approach, collaborative
localization fuses the individual motion measurements and the inter-vehicle
measurements to collaboratively estimate the states. In this paper, we focus on
the range-only collaborative localization, which specifies the inter-vehicle
measurements as inter-vehicle ranging measurements. We first investigate the
observability properties of the system and derive that to achieve bounded
localization errors, two vehicles are required to remain static like external
infrastructures. Under the guide of the observability analysis, we then propose
our range-only collaborative localization system which categorize the ground
vehicles into two static vehicles and dynamic vehicles. The vehicles are
connected utilizing a UWB network that is capable of both producing
inter-vehicle ranging measurements and communication. Simulation results
validate the observability analysis and demonstrate that collaborative
localization is capable of achieving higher accuracy when utilizing the
inter-vehicle measurements. Extensive experimental results are performed for a
team of 3 and 5 vehicles. The real-world results illustrate that our proposed
system enables accurate and real-time estimation of all vehicles' absolute
poses.
