The REFSMMAT: an explanation

REFSMMAT (Reference to a Stable Member Matrix) option. This extraordinary acronym refers to the simple idea of a reference orientation which can be well defined and used by the crew in their platform alignments. The initial 5½ hours of the mission uses the precise orientation of the launch site at Kennedy Space Center at the time of launch as the reference to which the platform is aligned. As the flight progresses, other REFSMMAT alignments will be brought into play, eight in total, which include one based on the plane of the ecliptic, another based on the alignment of the landing site at the time of lunar landing, and others based on the computed alignment for major engine burns.

The realignment of the platform was generally done by sighting on stars, and computer carried a catalogue of the positions of 37 prominent stars distributed across the sky.

[edit] Apollo 15

Scott, from 1998 correspondence - "But the crew had to know how to locate all 37 stars within the celestial sphere - one of the more interesting aspects of training. [We trained in] planetariums and [studied] the night sky while flying cross-country. The sky and its constellations became very familiar. Even though we did have a small diagram in the checklist, it was very important to be able to locate and positively identify each of the 37 stars. If the platform was too far from its desired orientation, the computer would not be able to point to the proper star."

Star maps - pages 6-8, 6-9

Star maps - page 6-10

The star maps from the G&C checklist are pages 6-8, 6-9 and 6-10.

The star reference numbers were given by their octal (base 8) number. The two stars used for Al Worden's first realignment were Antares (number 33) in the constellation Scorpius, and Dabih (number 41) in Capricornus. After the star sightings, the computer found that Al's measured angle between the stars, as given by Noun 05, differed from the angle the computer knows is between them, by only 0.01°. The difference between the intended platform orientation and its actual state was 0.019° in x, 0.021° in y, and 0.061° in z. These values were displayed through the Noun 93 display on the DSKY and were used to bring the platform back into correct alignment at 50 minutes into the flight.

The full list of stars, along with their reference numbers is as follows:-

Note that four of these are not stars as such but allow the crew member to refer to other celestial objects to the computer. (ap15fj)