Question [NASSP 8] [Apollo 8] How do I do P23 Cislunar Navigation?

Photon

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Hey, first post here, and still kind of new to NASSP, so please keep that in mind.

I was doing Apollo 8 and I had just finished TLI and went on to continue the Flightplan checklist, which included "Perform P23 Cislunar Navigation (STAR 14 ENH)" or the first P23.

When following the checklist, I come to a point where it wants me to load a Star Code, a Landmark option, and a Horizon option. I not only have no clue how to acquire this info necessary to continue P23 (AKA The Landmark and Horizon options mostly), but I also cannot find out which star it wants me to mark and its relative Landmark and Horizon options. It would be nice to get any help possible or a link to any threads that may help. Thanks in advance.
 

Thespacer

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Hi and welcome.

Have a look at this thread, for a first try:
Thread 'NASSP: P23 Program Explanation'
https://www.orbiter-forum.com/threads/nassp-p23-program-explanation.36210/

Most of us have struggled with P23 when starting out, but threads like the above should answer your immediate questions. One thing to keep in mind is the limitations of orbiter mean that achieving consistently “good” results from P23 is hard, if not impossible. So don’t feel too bad if it feels like it doesn’t work as smoothly as, say, P52. And so you know, an accurate P23 is not necessary to accomplish the mission because it is intended to update your state vector, and MCC should be providing that to you anyway. It really is just a contingency procedure (loss of comms) that the crew exercised.

In your case, you need to mark Star no. 14 (use F9 to turn on the star markers), and use the Earth’s near horizon (“ENH”). The thread linked above should help fill in the gaps.
 

Soumya-8974

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:welcome: to Orbiter Forum. If you follow the instructions linked above, then you will have fewer problems with P23.
 

STS

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You can see a "demo" of how P23, Earth Landmark is performed on Apollo 8 on the following video of my attempt to do the mission on realtime.

I used NASSP 7, but well, the things you have to enter on the AGC should be the same.

This is based on the information contained on the thread that @Thespacer quoted, in wich I contributed also.

 

MrFickles

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After having more experience with the sim, I realized that the explanation I gave a few years back is not 100% accurate.

You should not place the star with any part of the Earth/Moon's horizon. Instead, the highest point or lowest point of the body when looking at it through the sextant is the substellar point of the body with the selected star. Align your star with either of those 2 points when performing your P23s depending on whether you're using the near or far horizon.

If you're not sure which point corresponds to which horizon, one of them is usually lit and the other is in shadow, the lit horizon is usually the correct one.
 
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