Apollo 11's 1202 alarm was triggered by a rendezvous radar switch left on

As the Lunar Module Eagle descended toward the Moon on July 20, 1969, its guidance computer suddenly flashed a "1202" alarm — an executive overflow that meant the computer was being asked to do more than it could handle in real time. The cause was not a hardware failure but a mis-set switch: the rendezvous radar was left in a mode that fed extra data into the computer, overloading it during the critical landing phase. ^[1][2]

Why the computer overloaded

The Apollo Guidance Computer (AGC) was a marvel of its era, but it had limited processing power. During descent, it ran a program called P63 that guided the lander down. The rendezvous radar, designed to track the Command Module, was supposed to be off or in a standby mode. Instead, a switch was set to "LGC" (LM Guidance Computer), causing the radar to send a stream of interrupts that competed with the landing software. The AGC's priority display and executive system detected the overload and triggered the 1202 alarm. ^[1][3]

Mission Control had never seen this exact alarm in simulations, but a young flight controller, Jack Garman, had compiled a cheat sheet of computer alarm codes. He recognized 1202 as a non-critical overflow that the software could recover from — as long as the alarm didn't persist. Flight Director Gene Kranz and guidance officer Steve Bales made the split-second decision to continue the landing. ^[2][4]

The decision to land

The alarms recurred several times during the final descent, but each time the computer rebooted its executive and resumed the landing program. Because the AGC was designed with a priority-based task scheduler, it shed lower-priority jobs and kept the essential guidance running. The crew, Neil Armstrong and Buzz Aldrin, had only seconds of fuel remaining when they touched down in the Sea of Tranquility. ^[1][3]

“Give us a reading on the 1202 program alarm.”

— Neil Armstrong, during the Apollo 11 descent

Aftermath and lessons

• Post-flight analysis confirmed the rendezvous radar switch was left in the wrong position, a procedural error. ^[1]
• The 1202 alarm led to improved checklists and simulator training for later Apollo missions. ^[4]
• The AGC's robust design — prioritizing critical tasks — was vindicated, as it allowed the landing to succeed despite the overload. ^[3]

The 1202 alarm became a legendary example of calm decision-making under pressure. It also highlighted the importance of understanding software behavior in real time, a principle that still guides spaceflight and other high-stakes computing today. ^[2][4]