  We report on the results of a measurement study carried out on a commuter bus
in Dublin, Ireland using the Google/Apple Exposure Notification (GAEN) API.
This API is likely to be widely used by Covid-19 contact tracing apps.
Measurements were collected between 60 pairs of handset locations and are
publicly available. We find that the attenuation level reported by the GAEN API
need not increase with distance between handsets, consistent with there being a
complex radio environment inside a bus caused by the metal-rich environment.
Changing the people holding a pair of handsets, with the location of the
handsets otherwise remaining unchanged, can cause variations of +/-10dB in the
attenuation level reported by the GAEN API. Applying the rule used by the Swiss
Covid-19 contact tracing app to trigger an exposure notification to our bus
measurements we find that no exposure notifications would have been triggered
despite the fact that all pairs of handsets were within 2m of one another for
at least 15 mins. Applying an alternative threshold-based exposure notification
rule can somewhat improve performance to a detection rate of 5% when an
exposure duration threshold of 15 minutes is used, increasing to 8% when the
exposure duration threshold is reduced to 10 mins. Stratifying the data by
distance between pairs of handsets indicates that there is only a weak
dependence of detection rate on distance.
