1. دانشگاه صنعتی خواجه نصیرالدین طوسی دانشکده مهندسی هوافضا Faculty of Aerospace Engineering K.N. Toosi University of Technology http://aero.kntu.ac.ir The present study developed a new fault tolerant navigation system to improve the reliability of an Reusable Launch Vehicle (RLV) navigation system. The proposed system contains fault detection and isolation features and can reconfigure the system against occurring faults in the RLV navigation system. For this purpose, fault tolerant navigation system is constructed to detect, isolate and reconfigure predefined faults operating as a soft sensor based on the symmetrical dynamic behaviour of the yaw and pitch channels of the vehicle. In this regards, the dynamic model of the yaw channel of the vehicle is identified using the recursive least squares (RLS) method. Based on symmetrical behaviour of the RLV in the yaw and pitch channel, identified dynamic model of the yaw channel put in place of dynamic model of the pitch channel and accordingly pitch-rate gyroscope output is estimated to provide fault tolerant navigation solution. Abstract The survival of an Reusable Launch Vehicle (RLV) after navigation failure during payload injection is of critical importance for guidance and control systems. The high cost of research and development and the scientific components confirm the need for reliability and emphasize the need for more reliable systems. Fault Detection, Isolation and Recovery (FDIR) are essential for the reliability assessment of RLVs. Detection is the process of recognizing the existence of a fault. Isolation is the localization of a fault within the system by providing information. Recovery is the process of limiting fault propagation and enabling service to be restored to an acceptable system state. This paper proposes analytical fault-tolerant navigation system based on the symmetrical dynamic behaviour of the yaw and pitch channels. In this study, all navigation sensors are fixed separately in the flight channels (roll, pitch and yaw) and the symmetrical dynamic behaviour of the yaw and pitch channels are used to develop a soft backup sensor. When a fault occurs, the residual exceeds its corresponding threshold and transmits an alarm signal to the fault tolerant system to accommodate the fault. The faulty gyro output is then replaced by that developed by the soft sensor. Symmetrical dynamic behavior between the pitch and yaw channels of the RLV were used in the development of the proposed soft sensor. In spite of the complexity of detecting and isolating bias and drift faults, effective detection of these types of fault was accomplished using the proposed soft sensor. Reconfiguring a predefined fault is successfully designed and numerically simulated by the proposed method. Conclusion Introduction Figure 2: Comparison of pitch rate of fault at t = 60 s. Novel Fault Tolerant Navigation Sensor for a Reusable Launch Vehicle M. Hasani, A. M. Khoshnood, J. Roshanian Figure 1:Comparison of pitch rate of fault at t = 60 s. Procedures