P23 cislunar navigation
The space between the Earth and the Moon is known as cislunar space and it must be accurately navigated if a spacecraft is to successfully voyage across it. By taking sightings of the stars, along with fixes of the Earth and Moon, a voyager can determine if his craft will reach the far side of the Moon at precisely the right time and with the correct velocity necessary to make a safe arrival. These sightings, combined with the laws of orbital motion, provide an accurate fix of the spacecraft's position. Any observed errors in position or velocity can be corrected by thrusting the spacecraft in the appropriate direction.
Having been conceived during the Cold War, Apollo's navigation was originally to have been an entirely self-contained exercise carried out by the crew without communication with the ground for fear of interference by the Soviets. This thinking extended even to the landing on the Moon. The requirement was dropped as the perceived threat receded, and as flights by earlier unmanned probes refined tracking techniques by ground based radar. Meanwhile, with programmers running out of memory in the spacecraft's computer, onboard navigation was scaled back to the return-to-Earth scenario in which communications with Earth were lost. For this contingency, the CMP continued to practise the skills of navigation.
Al uses P23 for cislunar midcourse navigation. With this program's help, he can calculate the position and velocity vectors (collectively known as the state vector) of the CSM/LM stack, using the Earth or Moon as one reference, and a combination of stars (or stars and planets) for additional references. Ground controllers also calculate the state vector independently using radar and tracking data. If the ground result is considered to be more accurate than that in the spacecraft, Mission Control will uplink it to the onboard computer which they do when they ask the crew to select POO (Program 00) and the Accept mode.
Scott, from 1998 correspondence - "[Whether] ground['s state vector is] more accurate depends on when and where - e.g., the onboard state is more accurate immediately after a maneuver. Also, a certain amount of ground tracking is required for the vector to converge. Also, given enough onboard SXT [Sextant] sightings, especially at lunar distances, maybe the onboard state would be better?"
Should an abort situation include the spacecraft losing radio contact with Earth, the CMP would have to navigate himself. Therefore Al Worden makes a series of cislunar navigation sightings to keep him trained in the procedures and also to give him experience in using a horizon for a reference. Prior to experience gained during the Gemini program, sightings of the Earth were to be taken using known landmarks until it was shown that many were obscured by cloud. To get around this, and to simplify crew training, the Earth's horizon was chosen, though it is ill-defined due to the hazy atmosphere. Experiments found that, with practise, astronauts could learn to mark their sightings at a consistent level on the horizon, a level that varied from person to person. This is only a problem with marking on the Earth, as the Moon is airless and its horizon is stark and sharply defined. (ap15fj)