  An ultrasonic Positioning System (UPS) has outperformed RF-based systems in
terms of its accuracy for years. However, few of the developed solutions have
been deployed in practice to satisfy the localization demand of today's smart
devices, which lack ultrasonic sensors and were considered as being `deaf' to
ultrasound. A recent finding demonstrates that ultrasound may be audible to the
smart devices under certain conditions due to their microphone's nonlinearity.
Inspired by this insight, this work revisits the ultrasonic positioning
technique and builds a practical UPS, called UPS+ for ultrasound-incapable
smart devices. The core concept is to deploy two types of indoor beacon
devices, which will advertise ultrasonic beacons at two different ultrasonic
frequencies respectively. Their superimposed beacons are shifted to a
low-frequency by virtue of the nonlinearity effect at the receiver's
microphone. This underlying property functions as an implicit ultrasonic
downconverter without throwing harm to the hearing system of humans. We
demonstrate UPS+, a fully functional UPS prototype, with centimeter-level
localization accuracy using custom-made beacon hardware and well-designed
algorithms.
