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by benjam47 663 days ago
It is how Apollo navigated, although both the ground (via ground tracking) as well as the crew (via locating stars through a extant, and the Apollo computer having a database of the position of several dozen bright stars) could update their current position throughout the flight.
2 comments
embedded_hiker 663 days ago
Apollo used star sightings to check the accuracy of the gyros that measured which way the spacecraft was pointed. The stars could not be used to determine position like a ship at sea could do.

Besides inertial navigation, they had a transponder that would echo back a continuous pseudorandom bit stream, and the delay gave a precise measurement of distance.

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benjam47 663 days ago
Thank you for the correction, but are you sure that is accurate? I was definitely under the impression that although their position was normally updated by the ground (to the AGC, via their uplink capability), and the sextant was normally used to determine their orientation, the astronauts could use their optical equipment and calculations to determine their position as well as their orientation, albeit it with less precision. This NASA website (https://www.nasa.gov/history/afj/compessay.html#:~:text=Opti...) seems to say as much:

"Optical navigation subsystem sightings of celestial bodies and landmarks on the Moon and Earth are used by the computer subsystem to determine the spacecraft's position and velocity and to establish proper alignment of the stable platform."

And Wikipedia (https://en.m.wikipedia.org/wiki/Apollo_PGNCS):

"The CM optical unit had a precision sextant (SXT) fixed to the IMU frame that could measure angles between stars and Earth or Moon landmarks or the horizon. It had two lines of sight, 28× magnification and a 1.8° field of view. The optical unit also included a low-magnification wide field of view (60°) scanning telescope (SCT) for star sightings. The optical unit could be used to determine CM position and orientation in space."

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bigiain 663 days ago
The errors would be less of a problem than Apollo's, when your longest possible flight is only 45 minutes or so. And I'm not sure, but I guess the ballistic portion of the flight is uncontrolled (since the steering is from the rocket motors), so perhaps only the first few minutes are all it needs to maintain accuracy for?
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