P52 platform realignment

Throughout the mission, one of the Command Module Pilot's jobs is to occasionally realign the guidance platform. There are some 45 such realignments, a task usually known by the name of the program used, program number 52, or simply "P52".

Guidance and navigation are crucial to any journey and even more so in the ballistic dance of getting to the Moon and back. The crew must be able point the spacecraft to precise, well known directions, so that their engines will send them where they want to go. The spacecraft carries a gyroscopically stabilised platform within the IMU (Inertial Measurement Unit) which remains fixed in attitude while the spacecraft rotates around it, connected to it by three orthogonal gimbals. The only way to ensure that the platform is properly aligned is to compare it to a fixed attitude reference - the stars are almost always used. Initially, the CSM (Command/Service Module)'s platform was aligned before launch. Realignment is achieved by using P52 on the CMC (Command Module Computer). With the spacecraft held in a steady attitude the sextant is pointed at a specified star, marked, then to a second star, where another mark is taken. The computer, knowing the attitude of the spacecraft (relative to its own idea of where the stars are), now has new values of where the stars are located. The amount of drift the platform has experienced since the previous realignment is calculated, and displayed as the amount of correction needed to move, or "torque" the gimbals to bring the platform back into accurate alignment. Known as "gyro torquing angles", they are displayed on the DSKY as "Noun 93". These angles are usually very small, and are expressed in thousandths of a degree.

See also: Optics cover jettison

[edit] Apollo 15

From the 1971 Technical Debrief (ap15fj):

Worden: (Technical Debriefing) The guidance in the CMC was just dead-on, like what we looked at in simulation. I could almost repeat the numbers verbatim, because we had seen them so many times in simulation. It was just absolutely perfect, dead-on. The Z-torquing angle that we got after we got insertion was about half, as I recall, and that's just about what the first P52 showed, right in that ball park. We have the numbers written there somewhere [in their Flight Plan], but the guidance was right-on, super. We had no problems at all with the alignment. In fact, that was generally true with all the alignments. The first alignment went very smoothly. Tracking it in Orb Rate [the rate at which the vehicle was rotating as it orbited the Earth] was no problem. The Z-torquing angle came up about the same as they had called up.