Editing Apollo PGNCS (section)

==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|REFSMMAT]] (for "Reference to Stable Member Matrix"). There were two reference coordinate systems used, depending on the phase of the mission:  One centered on Earth and one centered on the Moon.