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According to the mascon entry on wikipedia:

At that time, one of NASA's highest priority "tiger team" projects was to explain why the Lunar Orbiter spacecraft being used to test the accuracy of Project Apollo navigation were experiencing errors in predicted position of ten times the mission specification (2 kilometers instead of 200 meters). This meant that the predicted landing areas were 100 times as large as those being carefully defined for reasons of safety. Lunar orbital effects principally resulting from the strong gravitational perturbations of the MASCONS were ultimately revealed as the cause. William Wollenhaupt and Emil Schiesser of the NASA Manned Spacecraft Center in Houston then worked out the "fix" that was first applied to Apollo 12 and permitted its landing within 163 meters of the target, the previously-landed Surveyor 3 spacecraft.

I've been unable to find any reliable information on the fix mentioned here. What was the fix, exactly? Is there a reliable reference for this information?

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The Tindallgrams have some notes on the matter. See pages 307 to 354 of http://www.collectspace.com/resources/tindallgrams/tindallgrams02.pdf

Briefly, improvements were made in a couple of areas:

  • The model of the moon's gravity field (based on tracking data from earlier Apollo and Lunar Orbiter flights)
  • Real-time estimation of the spacecraft's trajectory - a new "Lear Processor" was brought online at Misson Control's Real-Time Computer Complex, which was running a specialized Kalman Filter to provide optimal use of all the tracking information.

It was too late in the design cycle to incorporate any changes to the gravity model into the LM guidance computer program, so instead the RTCC computed offsets to the landing site position based on the latest information during powered descent. This offset was verbally relayed to the astronauts who entered it into the PGNCS, adjusting the target landing site that the autopilot was aiming for so that the actual landing site would be closer to Surveyor.

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If you are still interested in EXACTLY what Emil devised for us, I have a write up he did for me on this specific matter.

email removed for privacy

H. David Reed

FLIGHT DYNAMICS OFFICER Apollo11, Apollo 12, A-13, A-14.

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    $\begingroup$ I'm sure many people here would be very interested in that write-up, preferably posted here as an answer. Alternately, could you post a summary here and link to a complete version hosted somewhere else? $\endgroup$
    – Makyen
    Nov 29 '18 at 2:24
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    $\begingroup$ Unfortunately, as this answer currently is, it's not really an answer to the question. The format Stack Exchange uses is that answers should contain an actual answer to the question, not just something that says you're willing to supply the answer, if asked. Please don't take that negatively. It just means that yes we'd really like to see the write-up you have, preferably posted here, if possible. So, please consider it as we've definitely asked to see it. Posting it here would make for a great answer. Thanks. $\endgroup$
    – Makyen
    Nov 29 '18 at 2:24
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    $\begingroup$ Stack Exchange is wide open to the internet, putting anyone's personal e-mail address here in a post opens them up to potential problems. I would not recommend it. It is more common to put it in a profile, without the (at) sign at least. If it is possible to add some highlights or a summary of what is in the write-up, that would make this a more appropriate Stack Exchange answer post. In any event, welcome to Space! $\endgroup$
    – uhoh
    Nov 29 '18 at 3:33

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