  Determining when two individuals are within close distance is key to contain
a pandemic, e.g., to alert individuals in real-time and trace their social
contacts. Common approaches rely on either Bluetooth Low Energy (BLE) or
ultra-wideband (UWB) radios, that nonetheless strike opposite tradeoffs for
energy efficiency vs. accuracy of distance estimates.
  Janus reconciles these dimensions with a dual-radio protocol enabling
efficient and accurate social contact detection. Measurements show that Janus
achieves weeks to months of autonomous operation, depending on the
configuration. Several large-scale campaigns in real-world contexts confirm its
reliability and practical usefulness in enabling insightful analysis of contact
data.
