  A robust and accurate positioning solution is required to increase the safety
in GPS-denied environments. Although there is a lot of available research in
this area, little has been done for confined environments such as tunnels.
Therefore, we organized a measurement campaign in a basement tunnel of
Link\"{o}ping university, in which we obtained ultra-wideband (UWB) complex
impulse responses for line-of-sight (LOS), and three non-LOS (NLOS) scenarios.
This paper is focused on time-of-arrival (TOA) ranging since this technique can
provide the most accurate range estimates, which are required for range-based
positioning. We describe the measurement setup and procedure, select the
threshold for TOA estimation, analyze the channel propagation parameters
obtained from the power delay profile (PDP), and provide statistical model for
ranging. According to our results, the rise-time should be used for NLOS
identification, and the maximum excess delay should be used for NLOS error
mitigation. However, the NLOS condition cannot be perfectly determined, so the
distance likelihood has to be represented in a Gaussian mixture form. We also
compared these results with measurements from a mine tunnel, and found a
similar behavior.
