This section contains a number of technologies and processes fundamental to the guidance and navigation of the Apollo spacecraft during various parts of its flight to and from the Moon. Perhaps one of the most fundamental technologies associated with guidance and navigation for Apollo spacecraft was the Primary Guidance and Navigation Control System, or PGNCS (pronounced to sound like “pings”). Both the Command and Lunar Modules possessed PGNCS, though there were slight differences between them based on the craft’s distinct needs. PGNCS consisted of a number of different systems and subsystems, such as an inertial measurement unit—which detected altitude and velocity—and a guidance computer, which contained information vital to navigation. The guidance computer controlled propulsion systems necessary to position the spacecraft on proper trajectories, orbits, or landings.