Since most of Covid-19 infection has spread from person to person indoors, smartphones should be able to find out indoor contacts when making a risk evaluation of infection for users. There are two methods to detect whether people are inside a building or not.
1. The GPS signals are usually weaker indoors than outdoors because buildings are blocking the signals sent by GPS satellites. So, if smartphones find weak GPS signals, their users are likely in a building.
2. People are normally more physically active outdoors than indoors. We know smartphones can detect their own vibrations. So, if smartphones find few vibrations, their users are likely in a building.
A combined use of these two methods would be more accurate in detecting indoor contacts than one use of them.
Like indoor environment, public transportation is often crowded with poor ventilation. With the help of GPS function, a smartphone can tell which form of transport a user is travelling by calculating the user’s speed. For example, a walk has an average speed of 5 kph, a bicycle has 20, a bus has 30, a train has 200 and a plane has 600.
Contacts on public transportation and inside buildings are considered at higher risk than in the open air, and therefore should be given more risk points by digital contact tracing apps.
