How ISRO plans to land the Chandrayaan 3 even if all the sensors fail to work. 

ISRO chairman S Somanath has said that the lander of India’s third lunar mission Chandrayaan-3, Vikram, will be able to make a soft-landing on the Moon's surface on August 23 even if all the sensors and two of its engines do not work.

Yet, embarking upon the lunar surface stands as a formidable endeavour, replete with intricacies that defy facile solutions. The act of descending to the moon's embrace necessitates a balletic fusion of precision navigation and masterful control. This culminating stage, christened the powered landing, orchestrates the symphony of deceleration. A kinetic ballet wherein thrusters ignite, coercing the lander's velocity from a brisk 1.6 kilometres per second to nought, all while sidestepping celestial impediments and courting a secure touchdown site.

In September of 2019, Chandrayaan 2, the previous lunar mission undertaken by ISRO, unfortunately, failed to land on the moon. The reason for this failure was attributed to a misaligned IMU, which caused the trajectory to deviate from its intended path. To prevent a similar situation from occurring in the upcoming Chandrayaan 3 mission, ISRO has developed a backup plan that relies on redundancy and autonomy. This plan will ensure a safe landing even if the sensors fail to operate properly.

Navigating the Lunar Terrain: Autonomous Precision

Chandrayaan 3's autonomous navigation and guidance system are poised to revolutionize lunar exploration. This advanced system boasts the ability to detect and proficiently navigate around obstacles present on the lunar surface. Anchored by a camera and a lidar (light detection and ranging) sensor, this system's prowess lies in its capacity to perceive and analyze the environment.

Smart Site Selection: Merging Predefined Criteria

A pivotal feature of Chandrayaan 3's autonomous system is its capability to identify an optimal landing site autonomously. By processing a set of predefined criteria—such as slope, roughness, and illumination—the system can discern the most suitable location for landing. This not only showcases the system's discerning intelligence but also enhances the overall mission's success rate.

The Resilient Backup Mode: Ensuring Safe Descent

In the event of sensor failure during the critical powered landing phase, Chandrayaan 3's onboard computer seamlessly transitions to a resilient backup mode. Here, the reliance is placed upon the thrusters and a precisely calibrated timer to regulate the descent process. The computer employs a preloaded trajectory that strikes a balance between minimizing fuel consumption and maximizing safety, firing the thrusters at predefined intervals and durations.

Keeping a Watchful Eye on Fuel Levels: Preventing Potential Shortages

During the landing process, the onboard computer of Chandrayaan 3 carefully observes the fuel levels. If the fuel levels drop below a set limit, the computer takes immediate action by initiating an abort sequence. This demonstrates the system's commitment to ensuring the safety of the mission, even in unexpected situations.

The margin of Error: Acceptable Precision

While the backup mode lacks the intricacies and flexibility of the primary sensor-driven system, it still possesses the capability to ensure a soft landing within an acceptable margin of error—approximately 100 meters. Considering the vast landing site spanning approximately 500 square kilometres, this level of precision is deemed commendable.

“The challenge is also to make sure that the fuel consumed is lesser, the distance

calculations are correct and all the algorithms work properly," the ISRO chief said.

He explained that the ISRO team this time has made arrangements to ensure that Vikram land properly even if there are some variations in the calculations.