SubwayAPPS: Using Smartphone Barometers for Positioning in Underground Transportation Environments

Location information that is crucial for all location-based services is almost always available due to a number of different positioning techniques and technologies such as GPS and WiFi positioning. However, positioning technologies cannot provide sufficient position information when a user is underground, e.g. travelling with a car through a tunnel or on subways on an underground public transportation network. While there have been a number of attempts to utilize expensive infrastructure and smartphone sensors to address this situation, all of these techniques are either limited in scope, very expensive, or somewhat limited in accuracy. In this paper, we present a novel smartphone-based approach called SubwayAPPS (Subway Air Pressure Positioning System) that for the first time utilizes relative air pressure changes as detected by smartphone barometers to position a user. We first demonstrate the feasibility of this approach by comparing the depth characteristics of five major underground transportation networks across the globe and show that our novel approach is feasible for positioning users while they are underground in these networks. Second, we show with two user tests in Brussels and London that our lightweight approach works well as other more complex techniques, e.g. techniques that rely on pattern matching using the build-in accelerometers or gyroscopes, under realistic conditions.