  Mobile contact tracing apps are -- in principle -- a perfect aid to condemn
the human-to-human spread of an infectious disease such as COVID-19 due to the
wide use of smartphones worldwide. Yet, the unknown accuracy of contact
estimation by wireless technologies hinders the broader use. We address this
challenge by conducting a measurement study with a custom testbed to show the
capabilities and limitations of Bluetooth Low Energy (BLE) in different
scenarios. Distance estimation is based on interpreting the signal pathloss
with a basic linear and a logarithmic model. Further, we compare our results
with accurate ultra-wideband (UWB) distance measurements. While the results
indicate that distance estimation by BLE is not accurate enough, a contact
detector can detect contacts below 2.5 m with a true positive rate of 0.65 for
the logarithmic and of 0.54 for the linear model. Further, the measurements
reveal that multi-path signal propagation reduces the effect of body shielding
and thus increases detection accuracy in indoor scenarios.
