direct sun light or light scattered by near-by parts of the LM impinging on the outside prism. Adding the sun shade also allowed increasing the number of view positions from three to six. Software The onboard guidance software used a Kalman filter to merge new data with past position measurements to produce an optimal position estimate for the spacecraft. The key information was a coordinate transformation between the IMU stable member and the reference coordinate system. In the argot of the Apollo program this matrix was known as REFSMMAT (for "Reference to Stable Member Matrix"). There were two reference coordinate system used, depending on the phase of the mission, one centered on Earth and one centered on the Moon. 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 abort guidance system (AGS), built by 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. Apollo 11 During the Apollo 11 mission, two PGNCS alarms (1201 "No VAC areas available" and 1202 "Executive alarm, no core sets") were relayed to mission control as the first lunar landing was being attempted on July 20, 1969. The computer system overload was caused by the simultaneous capture of landing radar data and rendezvous radar data. Support staff at Mission control concluded that the alarms could be safely ignored and the landing succeeded. See also Saturn V instrument unit − guidance system used by the Apollo launch