The multiple systems must naturally meet strict requirements for stability and security. All systems therefore undergo comprehensive testing before they are put into operation. Overall the total air traffic management system undergoes the following four test phases: A verification test in which the many systems are tested together, Validation test combining personnel, systems and procedures, A shadow-operation period, in which the new system eavesdrops on the old, and A semi-operational period, in which the new system takes over for the first time.