Editing Apollo PGNCS (section)

==Navigational information==
Despite the word "primary" in its name, PGNCS data was not the main source of navigation information. Tracking data from NASA's [[Deep Space Network]] was processed by computers at Mission Control, using [[least squares]] algorithms. The position and velocity estimates that resulted were more accurate than those produced by PGNCS. As a result, the astronauts were periodically given state vector updates to enter into the AGC, based on ground data. PGNCS was still essential to maintain spacecraft orientation, to control rockets during maneuvering burns, including lunar landing and take off, and as the prime source of navigation data during planned and unexpected communications outages. PGNCS also provided a check on ground data.

The lunar module had a third means of navigation, the [[Apollo Abort Guidance System|abort guidance system]] (AGS), built by [[TRW Inc.|TRW]]. This was to be used in the event of failure of PGNCS. The AGS could be used to take off from the Moon, and to rendezvous with the Command Module, but not for landing.  During [[Apollo 13]], after the most critical burn near the Moon, the AGS was used in place of PGNCS because it required less electrical power and cooling water.