CRADLE: Combined RF/Acoustic Detection and Localization of Passive Tags

We introduce CRADLE, or combined RF/acoustic detection and localization, for the remote parametric ranging of passive tags. In this approach, a tag consists of an RF antenna connected to an ultrasonic transducer via a passive electrical network. A reader looks for tags by transmitting both continuous-wave RF and pulsed ultrasound into a region of interest; when a tag is within that region, the ultrasound incident on the tag transducer varies the transducer capacitance, creating a time-varying load for the tag antenna and thus parametrically modulating the RF waves reradiated from the tag. The reader localizes the tag by demodulating the received signal to recover the acoustic time of flight. In this paper, we present a proof-of-concept CRADLE system, describing the design of a custom tag and reader. The results of our localization experiments show that we can achieve sub-decimeter accuracy at reader-tag distances of up to 6 m using the proposed system; future improvements in the design of the tag and reader can result in localization of centimeter-sized tags with sub-centimeter accuracy out to distances of several meters.