The Traffic Alert and Collision Avoidance System (TCAS II) is now operating in all commercial airline aircraft to reduce the risk of midair collisions. TCAS II determines the relative positions of nearby aircraft, called intruders, by interrogating their transponders and receiving their replies. An intruder deemed a potential threat will trigger a resolution advisory (RA) that consists of an audible alert and directive that instructs the pilot to execute a vertical avoidance maneuver. Lincoln Laboratory has investigated the possibility of increasing the capability of TCAS II by incorporating the horizontal maneuvering of aircraft. Horizontal RAs can be computed if the intruder horizontal miss distances at closest approach are known. Horizontal miss distances can be estimated with range and bearing measurements of intruders. With this method, however, large errors in estimating the bearing rates will result in large errors in calculating the horizontal miss distances. An improved method of determining the horizontal miss distances may be to use the Mode S data link to obtain state data (position, velocity, and acceleration) from intruder aircraft.