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Author
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Topic: Lunar Module: Changing landing location
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Obviousman
Member Posts: 438
From: NSW, Australia
Registered: May 2005
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posted 03-18-2009 02:56 AM
   
I'm trying to find a reference for something I read. It was how lines of code were saved in the PNGS when the pilot wanted to change the landing position.Instead of telling PNGS that the landing point had changed, they told PNGS that where the LM had changed the appropriate amount to make the previously calculated landing point now equate to the revised landing point. Can someone give me a reference?
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garymilgrom
Member Posts: 1966
From: Atlanta, GA
Registered: Feb 2007
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posted 03-18-2009 09:41 AM
I read this story in How Apollo Flew to The Moon, a really great book by David Woods. This book is a treasure trove of detailed Apollo technical information, yet written in a compelling manner so you can't put it down! One of the best books I've ever read on any aspect of the space program.It's available in softcover or Kindle editions from Amazon.com |
Philip
Member Posts: 6002
From: Brussels, Belgium
Registered: Jan 2001
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posted 03-18-2009 10:17 AM
For those who wonder what PNGS means: Primary Navigation & Guidance SystemMore info: The Star Charts of Apollo  |
ilbasso
Member Posts: 1522
From: Greensboro, NC USA
Registered: Feb 2006
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posted 03-18-2009 11:01 AM
Actually, it's correctly referred to as the PGNCS (Primary Guidance, Navigation, and Control System) or PGNS. The astronauts called it "pings" even though the middle letters were reversed to pronounce it that way. |
Obviousman
Member Posts: 438
From: NSW, Australia
Registered: May 2005
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posted 03-19-2009 04:01 AM
Thanks ibasso.It was NOUN 69, but I am trying to remember (and get a reference for) why it was decided to make the LM think was in a different position, rather than tell it to land in a different position. The former saved lines of code... but why? |
xlsteve
Member Posts: 392
From: Holbrook MA, USA
Registered: Jul 2008
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posted 03-19-2009 07:34 AM
quote:
Originally posted by ilbasso:
The astronauts called it "pings" even though the middle letters were reversed to pronounce it that way.
And it certainly sounds better than "pigness" if you pronounced it phonetically. |
ilbasso
Member Posts: 1522
From: Greensboro, NC USA
Registered: Feb 2006
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posted 03-19-2009 08:28 AM
I may be oversimplifying things but I think the process was to tell the lunar module that they were redesignating the landing point to be 400 feet farther away rather than updating the gyro platform with the lunar module's actual position. Out in space, a 400-foot error in position was not a big deal. When you consider the equipment they were working with, a 400-foot error at the distance of the Moon was pretty darned impressive! However, once you are getting ready to land on a particular spot on the ground, obviously you want to be more accurate. As you are coming in, you see that the landing point the guidance computer is designating is off by 400 feet from what your eyeballs and map are telling you is the correct spot. What do you do? One solution is to update the guidance system with the correct position of the LM.
Updating the REFSMMAT ('reference to a stable member matrix') data in the guidance computer was a tedious process involving sighting three reference stars through the sextant and having the computer lock them in. It's not something they would want to do in real-time in the middle of a landing! The other solution is just telling the PGNCS to land 400 feet farther downrange than the computer was targeting. You see the computer is bringing you in short. You know where you're actually heading and you are tracking it with your eyes. Redesignating where the computer thinks it was heading was by far the easiest thing to do in the moment. The computer will think you have landed 400 feet away from the 'correct' spot, but you know better. You can update the REFSMMAT after you have landed, and everyone is in agreement again. The ability to change the targeting point was added to the Apollo 12 mission and subsequent ones. Noun 69 was the targeting correction display. Verb 21 allowed them to change just the downrange targeting position. There's a good discussion of this in the Apollo 12 Lunar Surface Journal starting at time stamp 110:21:49. |
wdw
Member Posts: 58
From: Scotland
Registered: May 2007
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posted 05-07-2009 07:02 AM
First, thanks to garymilgrom for the kind words on my book.An excellent, if technical reference on the ability to update the landing site position is to be found in one of the excellent Apollo Experience Reports, TN D-6846: Mission Planning for Lunar Module Descent and Ascent. A huge range of these reports are available. The original poster isn't clear in what is being asked. I'm unsure whether he is asking about the LPD - the Landing Point Designator, used during final approach to let the commander know where the computer is taking them and give him the opportunity to change that point; or whether he is talking about the Noun 69 retargeting of the computer, used from Apollo 12 onwards. This latter technique had radio tracking from Earth determine the LM's trajectory error. Then they would compensate for any error by altering the computer's idea of where the landing site was, kidding it into taking them where they wanted to go. This occurred early in the descent to give the computer time to alter the trajectory. One other thing. The mention of the REFSMMAT is a red-herring. It had nothing to do with landing site targeting. The REFSMMAT was no more than a definition of an orientation in space. The guidance platform was then orientated to match (using two stars). There was no position or velocity information involved in the REFSMMAT. However, having been aligned to a REFSMMAT, the guidance system could meaningfully measure the spacecraft's position and velocity (known as its state vector). I guess that a possible technique would have been to modify the state vector to make the spacecraft land where they wanted. They chose to modify the position on the Moon that was being aimed at. |
Obviousman
Member Posts: 438
From: NSW, Australia
Registered: May 2005
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posted 05-07-2009 11:07 PM
Thanks David.I think I mixed some things up. I'm pretty sure it's the NOUN 69 used on 12 and later, but I had thought it was change that had been made earlier and applied to Apollo 11 as well. |
p.j. mcgill
New Member Posts:
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posted 05-08-2009 06:28 AM
There is a great discussion on this topic in Apollo by Murray and Cox. The chapter covers the issues the crew encountered when training for Apollo 12 and achieving a pin point landing. I believe (haven't got the book here) that Neil Armstrong credits Emil Schiesser with the idea of telling the computer "the landing site has changed". I think the main advantage was that the LMP required fewer keystrokes into the DSKY than any other technique. |
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