Independent navigation

On all return flights from the Moon, the CMP was kept busier than his crewmates, because of his role as the navigator. He regularly realigned the guidance platform to keep it oriented according to whatever REFSMMAT was in force at that point of the mission. He was also responsible for maintaining an autonomous ability to navigate the spacecraft to an accurate splashdown in the event of a loss of communications with Earth. All the way out to the Moon, he had kept his techniques and skills of cislunar navigation up to date with a series of practise sessions whereby his measurements of the angle between the Moon or Earth and a star could be processed by Program 23 to yield the spacecraft's state vector and provide a basis for calculating their trajectory. Readers are directed to Chapter 6 for a more detailed description.

From Apollo 14 onwards, NASA decided to simulate this 'no-communications' scenario by having the CMP maintain an entirely independent state vector. There were two areas of the computer's memory where the numbers relating to the state vector were kept: the CSM slot and the LM slot. During the coast home, mission control left the state vector in the CSM slot alone for the CMP to refine during his navigation exercises. The only time they touched it was to take account of any engine burns. They would then download it to Earth, bias it with the effect of the burn, and reinstate it for further use by the CMP. Of course, the crew were never expected to rely solely on their state vector. As a precaution, the ground's version was always kept in the LM slots, since there was no longer a LM attached.

As Apollo 15 hurtled home, its CMP Al Worden was continuing this practice. From the Capcom console, Bob Parker reported the results of Worden's P23, navigation sightings: ''15, Houston. Looks like a good set of P23s again, Al. And your gamma, right now, on your vector, is 6.5.'' In the clipped, economical parlance of NASA's astronauts, what Parker was saying was that Worden's state vector, if calculated forward to re-entry at Earth, would result in the spacecraft meeting Earth's atmosphere at an angle of 6.5 degrees. This was the ideal value - a point not lost on Worden.

''It sounds like, after a while, we might get along without you, huh, Bob?'' After all, this was the object of the exercise.

"No comment," returned Parker.

Worden continued to rub it in. "As a matter of fact, if you guys keep working on your ground [calculated] vectors, they might even converge to the onboard vectors pretty soon."

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