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The development of an unambiguous unit-distance code configuration for automatic altitude reporting via the ATC Radar Beacon System is described. This code provides for compatible altitude reporting in increments of 100, 250, and 500 feet within one common system. The code is developed by using Karnaugh mapping techniques. Some assumptions and requirements are set forth to provide guidance for the determination of the most desirable code. A brief presentation of a developmental code in use in the United Kingdom is also given. It is concluded that the flexibility of the unit-distance code affords a satisfactory compromise which can satisfy the objectives of the various users.