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FAA's Aviation Research and Development Service (ARDS), has recently embarked on an active program of establishing an altitude reporting subsystem for the Air Traffic Control Radar Beacon System (ATCRBS), internationally known as Secondary Surveillance Radar (SSR). This action is in response to a long-standing requirement for the provision of altitude information to the Air Traffic Control System by a means other than flight plan and voice transmission of altitude information from the cockpit. The use of the ATCRBS for altitude reporting utilizes precisely the same techniques used for the identification function on Mode A. The major difference is that while codes are manually selected for identification, they would be automatically selected for altitude through an automatic encoding system from a source of barometric altitude. The essential characteristics of the experimental subsystem are detailed, and the Federal Aviation Agency's (FAA) progress to date is described.