When the LM's rendezvous radar (which tracked the still-orbiting Command/Service Module (CSM)) was powered up during Apollo 11's descent (a step designed to lessen the crew's workload in case of an abort), the radar's CDUs were energized and took stock of the state of the attached ATCA assembly. By chance, the power-up happened at a moment when the CDU's 800 Hz power supply happened to produce energy that wasn't phase aligned with the ATCA's power (a power-up a fraction of a second earlier or later would have resulted in aligned phases and no problems). The CDUs used their 800 Hz power as a reference signal for interpreting the ATCA's position and orientation, and because the signals from the ATCA were out of phase, this produced readings that were far out of range from what the CDUs expected.