The objective of the Thermal Control System Automation Project (TCSAP) is to develop an advanced Fault Detection, Isolation, and Recovery (FDIR) capability for use on the Space Station Freedom (SSF) External Active Thermal Control System (EATCS). Real-time monitoring, control, and diagnosis of the EATCS will be performed with a Knowledge-Based System (KBS).The EATCS provides heating, cooling, and control necessary to maintain elements, systems, and components within their required temperature ranges. The EATCS design has evolved from the single-phase fluid system used in Apollo and Space Shuttle to a two-phase system (ammonia liquid and vapor mixture) on SSF. Both active and passive components of the EATCS can potentially fail or become blocked. As a result, a variety of failure modes exist. When this is combined with the continuous range of normal operating conditions, EATCS management becomes very complex.The TCSAP KBS is a combination of three distinct elements that interact with each other. The first is a quantitative model of the EATCS, providing step-wise steady state values for any EATCS configuration. The model is used in sensor validation and component diagnosis by comparing observed sensor readings with their computed values. Inconsistencies between observed and expected values imply either instrumentation failure or actual off-nominal behavior of the EATCS. The second element is a rule-based system containing safety critical and non-critical FDIR rules focused directly on the EATCS. The rules use both quantitative and qualitative values for reasoning and diagnosis. Quantitative sensor values are obtained from an external source and qualitative representations are derived from the history of the quantitative data. The third KBS element is the Human Interface (HI). The HI implements graphically oriented monitoring and control capabilities for the EATCS. The interface attempts to “intelligently” support the operator by supplying information of the type and quantity most likely needed in a given context. The HI also allows the user to specify configuration changes such as the closing or opening of a valve. These changes are transmitted to the EATCS hardware as well as affecting the internal KBS quantitative model.The KBS utilizes conventional software and a real-time expert system tool called G2. The tool eases development of reasoning techniques required for automating the EATCS monitoring, control, and FDIR tasks. The resulting KBS utilizes a combination of model-based sensor validation, rule-based fault descriptions, and model-based diagnosis of unanticipated faults.