  We report on measurements of Bluetooth Low Energy (LE) received signal
strength taken on mobile handsets in a variety of common, real-world settings.
We note that a key difficulty is obtaining the ground truth as to when people
are in close proximity to one another. Knowledge of this ground truth is
important for accurately evaluating the accuracy with which contact events are
detected by Bluetooth LE. We approach this by adopting a scenario-based
approach. In summary, we find that the Bluetooth LE received signal strength
can vary substantially depending on the relative orientation of handsets, on
absorption by the human body, reflection/absorption of radio signals in
buildings and trains. Indeed we observe that the received signal strength need
not decrease with increasing distance. This suggests that the development of
accurate methods for proximity detection based on Bluetooth LE received signal
strength is likely to be challenging. Our measurements also suggest that
combining use of Bluetooth LE contact tracing apps with adoption of new social
protocols may yield benefits but this requires further investigation. For
example, placing phones on the table during meetings is likely to simplify
proximity detection using received signal strength. Similarly, carrying
handbags with phones placed close to the outside surface. In locations where
the complexity of signal propagation makes proximity detection using received
signal strength problematic entry/exit from the location might instead be
logged in an app by e.g. scanning a time-varying QR code or the like.
