This document provides the Algorithm Design Description (ADD) for the Surveillance and Tracking Module (STM) and the Threat Resolution Module (TRM) of the next generation Airborne Collision Avoidance System (ACAS Xu). ACAS Xu is designed for platforms with a wide range of surveillance technologies and performance characteristics, such as Unmanned Aircraft Systems (UAS). This document has to be read together with RTCA DO-365 – “Minimum Operational Performance Standards (MOPS) for Detect and Avoid Systems”.