  Throughout the course of the COVID-19 pandemic, several countries have
developed and released contact tracing and exposure notification smartphone
applications (apps) to help slow the spread of the disease. To support such
apps, Apple and Google have released Exposure Notification Application
Programming Interfaces (APIs) to infer device (user) proximity using Bluetooth
Low Energy (BLE) beacons. The Private Automated Contact Tracing (PACT) team has
shown that accurately estimating the distance between devices using only BLE
radio signals is challenging. This paper describes the design and
implementation of the SonicPACT protocol to use near-ultrasonic signals on
commodity iOS and Android smartphones to estimate distances using
time-of-flight measurements. The protocol allows Android and iOS devices to
interoperate, augmenting and improving the current exposure notification APIs.
Our initial experimental results are promising, suggesting that SonicPACT
should be considered for implementation by Apple and Google.
