Conventional mission-critical systems cannot prevent mission failure in dynamic battlefield environments in which the execution situations or missions change abruptly. To solve this problem, self-adaptive systems have been proposed in the literature. However, the previous studies do not offer specifics on how to identify changes in a system situation or to transform situation information into the actions the systems must take in dynamic environments. This paper proposes a situation-awareness based self-adaptive system architecture (SASA) to support more efficient adaptation and, hence, achieve more accurate and successful missions, even in dynamic execution environments. A case study for air defense systems (ADS) using tests in a HLA/TRI-based real-time distributed simulation environment was implemented.