During the Apollo 11 lunar landing mission on July 20, 1969, the Lunar Module’s Apollo Guidance Computer (AGC) experienced multiple program alarms, specifically 1202, during the critical powered descent phase. These alarms indicated that the computer was overloaded and could not complete its tasks within the allotted time, leading to software restarts.

The root cause of the computer overload was an uncorrected design flaw in the rendezvous radar interface. The radar was left on and continuously fed data to the computer, consuming approximately 13% of the computer’s duty cycle, even when its data was not required for the descent. This excessive processing demand pushed the AGC beyond its capacity.

The immediate customer impact was the appearance of “1202 program alarm” messages on the DSKY (Display and Keyboard Unit), causing significant concern and requiring rapid assessment by the crew and Mission Control. Despite the alarms, the landing proceeded successfully, but under immense pressure.

Remediation involved a pre-flight assessment by Mission Control personnel, including Steve Bales, Jack Garman, and Russ Larson, who had determined that a 1202 alarm was a “go” condition unless it occurred too frequently or the trajectory deviated. Their real-time decision to proceed allowed the mission to continue to a successful lunar landing, despite the ongoing computer overload.