  While it is often critical for indoor-location- and proximity-aware
applications to know whether a user is in a space or not (e.g., a specific room
or office), a key challenge is that the difference between standing on one side
or another of a doorway or wall is well within the error range of most RF-based
approaches. In this work, we address this challenge by augmenting RF-based
localization and proximity detection with active ultrasonic sensing, taking
advantage of the limited propagation of sound waves. This simple and
cost-effective approach can allow, for example, a Bluetooth smart-lock to
discern whether a user is inside or outside their home in order to lock or
unlock doors automatically. We describe a configurable architecture for our
solution and present experiments that validate this approach but also
demonstrate that different user behavior and application needs can impact
system configuration decisions. Finally, we describe applications that could
benefit from our solution and address privacy concerns.
