simulation and established a modeling and simulation masterplan, which included the High Level Architecture. Two protocols for M&S interoperability already existed: Distributed Interactive Simulation (DIS), focusing on real-time platform level simulation with a fixed object model, and Aggregate Level Simulation Protocol (ALSP) focusing on simulation of aggregate with time management, ownership management and flexible object models, called confederation models. The purpose of HLA was to provide one unified standard that would meet the simulation interoperability requirements of all US DoD components. The development of HLA was based on four prototypical federations: the Platform Prototype Federation, the Joint Training Protofederation, the Analysis Protofederation and the Engineering Prototype Federation. The HLA specification was prototyped and refined, until HLA 1.3 was finally released. To facilitate usage outside of the defense community, HLA was then transitioned into an IEEE standard, maintained by Simulation Interoperability Standards Organization (SISO). To facilitate the migration for DIS users, a Federation Object Model corresponding to the fixed object model of DIS was also developed as the Real-time Platform Reference FOM (RPR FOM). The following HLA versions exist: HLA 1.3 HLA 1.3 was published in March 1998 by DMSO. It consists of: U.S. Department of Defense, Rules Version 1.3 U.S. Department of Defense, High Level Architecture Interface Specification Version 1.3 U.S. Department of Defense, High Level Architecture Object Model Template Version 1.3 The US DoD also published interpretations for HLA 1.3: U.S. Department of Defense, Interpretations of the High Level Architecture Interface Specification Version 1.3, Release 3 HLA 1516-2000 HLA IEEE 1516-2000 was published in 2000 by IEEE. It consists of: IEEE Std 1516–2000 – Standard for Modeling and Simulation High Level Architecture – Framework and Rules IEEE Std 1516.1–2000 – Standard for Modeling and Simulation High Level Architecture – Federate Interface Specification IEEE 1516.1–2000 Errata (2003-oct-16) IEEE 1516.2-2000 – Standard for Modeling and Simulation High Level Architecture – Object Model Template (OMT) Specification Major improvements in IEEE 1516-2000 included an XML-based FOM with detailed data type specifications, as well as an improved DDM design. The IEEE 1516-2000 standard was also complemented by a recommended development process as well as a recommended VV&A process: IEEE 1516.3-2003 – Recommended Practice for High Level Architecture Federation Development and Execution Process (FEDEP). This standard would later become IEEE Std 1730-2010 Distributed Simulation Engineering and Execution Process (DSEEP) IEEE 1516.4-2007 – Recommended Practice for Verification, Validation, and Accreditation of a Federation an Overlay to