US Patent 9264151 Method and system for presence detection

The transmission system transmits an encrypted identifier ultrasonically in an open-air environment using an ultrasonic transmitter. The ultrasonic identifier is associated with a location, which may be a store or a department within a store, and may be received by a microphone on a mobile phone. The identifier may be used to infer presence of the mobile phone user at the location associated with the identifier. The transmitter may include an ultrasonic transducer. The system may further provide a reward, responsive to inferring presence at the location.