SonicPACT: An Ultrasonic Ranging Method for the Private Automated Contact Tracing (PACT) Protocol. - John Meklenburg; Michael A. Specter; Michael Wentz; Hari Balakrishnan; Anantha P. Chandrakasan; John Cohn; Gary Hatke; Louise Ivers; Ronald L. Rivest; Gerald Jay Sussman; Daniel J. Weitzner (CoRR, 2020)