The Vikram lander made an unsuccessful effort to soft-land on the lunar surface

BENGALURU: The changes the Indian Space Research Organisation (Isro) is making in the proposed Chandrayaan-3 mission are indicative of the things that may have gone wrong with Chandrayaan-2 that failed to soft-land a probe on Moon in September 2019.

From a new guidance algorithm to improved communication systems on the lander, the overview committee that met last Tuesday (November 12) has pointed to multiple recommendations made by the failure analysis committee (FAC) looking into Chandrayaan-2 failure .

As part of the final discussions at the meeting the committee accepted the “Propulsion+Landing Module” configuration for Chandrayaan-3, and said: “Guidance algorithm to be finalised considering all recommendations of FAC and also after detailed simulations.”

The TOI was the first to report that Chandrayaan-3 mission is likely to have this — lander & rover (landing module) and propulsion module— configuration on November 14.

Software & Algorithm Glitch?

Sources indicated that one of the main reasons that the FAC found for Chandrayaan-2’s failure in soft-landing Vikram (the lander) was a glitch in the software and the algorithm that saw the lander lose orientation just metres away from the lunar surface. They said that the velocity of the lander was higher than expected and that it may have had attitude errors.

The overview committee, last week noted: “...Also, after detailed simulations LDV (laser doppler velocity) sensor for the direct measurement of velocities (all three axes) must be incorporated. “The LDV sensor was developed even for Chandrayaan-2, but since it did not perform well in ground tests, it was not included,” a source said.

LDV sensors will be useful as they will help directly measure the velocity from the height of about 20km, sources added and said Bengaluru-based LEOS lab under Isro has developed the sensor.

Direct Velocity Measurement

Among other things, Isro is looking to improve the data transfer capability on the lander for the new mission, which will help transfer lunar surface images from the lander imager camera right from the beginning of the powered descent — likely to be from about 30km from the lunar surface.

“The camera was taking images even on Chandrayaan-2, but we didn’t have the capability to transfer real time, this time, the committee feels that we need this to get the right orientation,” another source said.

The committee has noted: “...Data rate, telemetry and orientation to be appropriately arrived at based on the descent trajectory.”

Power & Communication

Also, Chandrayaan-3 is expected to have solar cells on four sides of the lander— its predecessor had it only on three sides— to improve performance.

“Feasibility of populating solar cells on the fourth side vertical panel where Rover is accommodated to be studied to avoid power issues if landing happens with large attitude error resulting in absence of Sun in the plane,” the committee has said.

The panel, which will meet again later this year, has also tasked sub-committees to look at establishing margins for different touchdown conditions. “Strengthening of lander legs to be considered… power and communication between lander and ground to be ensured post landing irrespective of lander orientation,” the panel has noted.

Sources added that this may mean that the space agency will add more antennas on the lander to keep communication alive even if the lander does not land on the legs.

