As for the cause of the altitude estimation issue, ispace points to a probable source of confusion during the mission. While en route to the landing site, Hakuto-R flew over a substantial cliff that turned out to be the rim of a crater. At this point, the spacecraft’s onboard sensor recorded an altitude reading of 3 kilometers, exceeding the predetermined estimated value. The software erroneously interpreted this as an abnormal reading and subsequently disregarded all subsequent altitude measurements.
ispace had implemented the capability to reject abnormal altitude measurements into the lander’s software as a safety measure in case of sensor hardware malfunctions. However, this safeguard backfired during Mission 1 because the simulations of the landing sequence did not account for the lunar environment along the spacecraft’s trajectory. Additionally, the decision to change the landing site occurred after the critical design review had already been completed in 2021.
