Chandrayaan-3 was the third spacecraft of Lunar exploration series. The spacecraft included a Lander for soft landing and scientific exploration near Lunar south pole. The Chandrayaan-3 mission carries Propulsion Module (PM) and Lander Module (LM) as composite until reaching Moon Bound Orbit of 125 km x 160 km. Post separation from the PM, Lander remained in this orbit for 6 days to verify and validate the performance of all sub-systems followed by powered descent. The powered descent is initiated from an Orbit of 125 km x 30 km. During descent, control and guidance is based on inertial navigation solution aided with absolute sensors in different phases. Moreover, accuracy of derived altitude from altimeters for navigation aiding depends on accurate instantaneous attitude. However, there is no aiding for attitude during descent and is purely determined by the inertial gyro performance. Also, the navigation mode is purely inertial from descent start (~30 km) till 6.8 km altitude (for duration of ~800 sec) and can be extended till 1.3 km under contingency. Final bias update is done ~4 hours prior to descent start. Thus, highly accurate and stable inertial sensors are required to achieve precise and soft landing which is achieved with LIRAP system (LASER gyro based Inertial Reference Unit and Accelerometer Package). LIRAP is strap-down inertial navigation system consisting of four Ceramic Servo Accelerometer (CSA) and four ISRO Laser Gyro Digital (ILGD). These are 21 micro-g and 0.015 deg/hr class sensors respectively
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Philip et al. (2024) studied this question.