  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 broad use. We address this
challenge by conducting a measurement study with a custom testbed to show the
benefits and limitations of Bluetooth Low Energy (BLE) in comparison to
distance estimation by ultra-wideband (UWB). Our results confirm that BLE-based
distance estimation is not sufficient in real scenarios where smartphones are
shielded heavily by the users' bodies. Yet, multi-path signal propagation
reduces the effect of body shielding. Finally, we demonstrate that UWB is more
robust to the environment than BLE.
