ED-256 consists of two Volumes plus supplementary material: Volume I: MOPS for ACAS Xa with Xo Functionality This document sets forth minimum operational performance standards for the Active (Xa) and special Operations (Xo) system variants of Airborne Collision Avoidance System X (ACAS X) equipment. ACAS X is intended to improve air safety by acting as a last-resort method of preventing mid-air collisions or near mid-air collisions (NMAC) between aircraft. By utilizing surveillance information from Secondary Surveillance Radar (SSR) and ADS-B technology, ACAS X equipment operates independently of ground-based aids and air traffic control (ATC). Aircraft equipped with ACAS X have the ability to interrogate airborne transponders and receive ADS-B Messages to determine the location of other aircraft in the vicinity and assess the risk of collision. Volume II: Algorithm Design Description (ADD) 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 X). The algorithms are described at a sufficiently high level to allow for implementation in a variety of software languages and hardware platforms, thereby providing maximum freedom to manufacturers while ensuring the intended output from the system. This document provides descriptions of the algorithm’s operations.