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During the normal progress of design, fabrication, and integration of communications subsystems for spacecraft and for ground installations, every effort is made to assure that the equipment meets certain specifications relating to performance, environment, reliability, and interface capability. These specifications are based on the best available definition of requirements and interface characteristics of complementing subsystems. Frequently, in the field of manned spaceflight, the spacecraft subsystems, the launch vehicle subsystems, and the ground systems must be designed and constructed concurrently. This means that the operating and interface characteristics of one subsystem are not available for use by the engineers in the design of the other subsystems. Close technical liaison among the various engineering groups is essential in the accomplishment of overall systems' integrity. Component and subsystem testing has been developed to a high degree, but the results of these are necessarily limited. They cannot validate the overall systems' performance and compatibility. It is considered mandatory that the interfacing subsystems be mated to form a complete system in a controlled test environment as early as practicable in any program, especially in one involving communications systems as new and as complex as those for Apollo. This must be accomplished at such a phase in the program that corrective engineering details can be fed back to the cognizant design, fabrication, or integration groups involved in time for necessary modifications prior to the beginning of the flight phase.