TCAS behavior in New England airspace is being monitored and analyzed, making use of an omni-directional 1030/1090 MHz receiver. The receiver system, located in Lexington, MA, and operated by M.I.T. Lincoln Laboratory, is used to record Resolution Advisories (RAs). Omni-directional receptions make it possible to examine the air-to-air messages exchanged between aircraft for coordination of RAs. Omni-directional reception rates are also being studied. THe results indicated the percentage of aircraft that are TCAS equipped and the percentage of received signals that originate from TCAS and other systems. A third aspect of the program evaluates the availablity of 1090 MHz Extended Squitter data for use in collision avoidance systems. Data is recorded continuously, and the busiest periods are selected for focused attention.