I am sure this will be an easy one for the Apollo buffs.

The story of the the 1202 computer alarm going off at a very inconvenient time during Eagle's powered descent to Mare Tranquilitatis is well-known and often recounted in documentaries, as it ads a lot of drama to the story (as it did, undoubtedly, to the descent).

None of the documentaries I watched, however, offered a reason for the alarm (meaning data overload of the onboard computer, if I understand it correctly) - fair enough, I thought, computer glitches occured before the invention of Windows.

I was surprised, then, that in the beautiful documentary, "In the Shadow of the Moon", Buzz Aldrin offered a simple explanation: "Being Dr. Renezvous", and contrary to the flight manual, he had left the rendezvous radar on in case he needed it quickly, and the data flow resulting from both the landing and the rendezvous radar caused the overload (The film is on YouTube here, and the story begins at 6:45 into the segment).

My questions - Is this the accepted explanation for the alarm? Were there any consequences from it? I guess Aldrin's explanation for him leaving the rendezvous radar on is very reasonable. Was there a modification on subsequent flights to allow for the added data flow, or was the dual radar just avoided? I also got the impression Aldrin rolled his eyes a bit that nobody anticipated that he would do what he did. What came out of the debriefings regarding this error?

Thanks.

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Easy to understand, sort of. But not easy to explain in a few words!

The story is told in considerable detail in Murray and Cox's "Apollo - The Race to the Moon". It suggests Aldrin's answer is correct, but incomplete. The key was that the rendezvous radar was taking up a large portion of the computer's time, all the while performing a useless task.

The task related to the radar's mode switch, which could be in one of four settings. For the descent to the Moon, the radar was originally to be in one mode. Not long before launch, it was decided to use a different mode, which required the development of new software and new procedures. The new software was loaded, but then the engineers decided the new procedures were too much to introduce at such a late stage. So they came up with a method of disabling the software which they thought was failsafe. Unfortunately, all they did was make the LM computer undertake an impossible task (described in the book as trying to calculate an angle with a sine and cosine of 0). Doing that apparently used nearly one-fifth of the computer's time, and contributed to the computer indicating that it was overloaded.

It sounds like it wouldn't have been that hard to remedy for later missions. What puzzles me is why it was never picked up in any of the simulations - Aldrin presumably didn't do anything in the landing that he hadn't already done in the sims.

Anyway, I'm sure I haven't given the story full justice, so if anyone can tell it better than me, go right ahead.

I just happened across this thread. The best available account of the Apollo 11 alarms is in my paper "Tales from the Lunar Module Guidance Computer" which can be found at www.doneyles.com/LM.

In addition to the explanations given above, what I've heard and read (quoting from memory, sorry, no link), the LM's computer was designed in such a way that if peripheral software crashed, such as the area sorting out the radar data, the main core was not affected. Individual parts of the memory could be dumped and rebooted without any damage to main computer action.

The Apollo 11 flight journal (http://history.nasa.gov/alsj/a11/a11.html) shows this:

Apparently, in the pause before the first "we're go," the speaker glanced over to the software people at Mission Control, got their nod that the alarm was no problem, and gave the ok to the LM.

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Credis in alienos extraterrenos terram visitantes? Credis in celeritam super lucem?
Credis in viam inter tempores? O tu stulte, aquire vitam.

and remembering that all of this was just radio communication garbage to give the impression of a lunar landing... the area 51 and hanger 18 film crew added it just to add to the drama.... Sorry folks, you can not fool a fool.
Its great to see the answers are still so clear. ( not mine ).. as Kleindoofy said. I actually remember all this at the time... Those computers were very basic and, multi tasking was beyond them.

Donelyes, if you are indeed the same Don Eyles who wrote that paper, a big welcome to the BAUT forum. We don't hear much from Apollo originals these days.

For those who don't know the name, a Don Eyles is mentioned on page 358 of Andrew Chaikin's book A Man on the Moon, and is shown and spoken about in the NASA movie, Apollo 14: Mission to Fra Mauro some time after the lunar module Antares and the command and service module Kitty Hawk undocked in lunar orbit:
As we now know, Antares eventually landed at Fra Mauro and the mission was a success.

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Don't criticize what you can't understand. — Bob Dylan, “The Times They Are A-Changin'” (1963)
Some people think they are thinking when they are really rearranging their prejudices and superstitions. — Edward R. Murrow (1908–65)

Welcome to BAUT, Don. Many of us have already heard of you (and thanks, Kiwi, for filling in the rest). It's an honor to have you here. Thanks for posting the link. Great details!

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"Transport of the mails, transport of the human voice, transport of flickering pictures - in this century, as in others, our highest accomplishments still have the single aim of bringing men together." St. Exupery

Welcome aboard!

I see y'all had 152K of memory (less than the earliest floppy disk capacity) for your Colossus program to get us to the Moon. I can guess why the assembler language was useful.

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Lighten up! This is a stellar board!

Don´s link is broken [due to the period on the URL]. Here´s a good one:

http://www.doneyles.com/LM/Tales.html

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What brings us together is stronger than what pulls us apart

Hey Argos, thanks a lot. And indeed, a great link with some awesome stuff.

Yes, that's an amazing website.

You know, for a hoax, it is amazing the amount of detail that went into it.

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At night the stars put on a show for free (Carole King)

One Earth, One Sky - IYA 2009
All moderation in purple

Soon we'll know what a flight computer with some imparted kinetic energy can do to a frog! Yay for science.

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"Ignorance more frequently begets confidence than does knowledge" -- Charles Darwin | Meet the OOONG TOE.
"Your right to hold an opinion is not being contested. Your expectation that it be taken seriously is." -- Jason Thompson
'No, mad's when you froth at the mouf,' said Gaspode. 'He's insane. That's when you froth at the brain.'

You can hear Steve Bales on the recording of the Mission Control loop cutting off Gene Kranz to announce that they're go on the alarm:

http://history.nasa.gov/alsj/a11/A11LandingMOCR.mp3

He would have seen it even before the Capcom acknowleged it.

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Everything I need to know I learned through Googling.

@ToSeek

Why thank you!

I wasn't aware of that recording, or better said, I've never had (or taken) the time to delve deeper into the Apollo 11 surface journal. Very interesting, very educational.

On that recording (39':31"), at the 24':56" time mark, a 1202 (not 1201) alarm is reported which doesn't seem to cause very much concern, and the voices say:

"12...1202 alarm"
"Yeah, the same thing we had."
(interruption by retro-control)
"We're...We're go on that flight."
"We're go on that alarm?"
"It's, ... if it doesn't re-occur, we'll be go, he's taking in ...h now."
"Rog."

At 28':43" there's the 1201 alarm:

"1201 alarm"
"Same type, we're go, flight."*
"Ok, we're go."
(this "same type, we're go" is then repeated by the Capcom.)

*this is the interruption ToSeek referred to.

So, the "same type" is referring to the 1202 alarm that had occured 3':43" seconds earlier.

Interestingly enough, at the 36':16" time mark, a few minutes after the landing, the rendezvous radar is mentioned again:

"Has he got the rendezvous radar off yet?"
"Negative, it's still on."
"Ok, [he] should be getting that off pretty shortly."

So flight control seems to have been aware that the rendezvous radar was on, at least after the alarms.

Btw, the right channel (Catcom/Eagle) was mostly static. I'm assuming that was correct.

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Credis in alienos extraterrenos terram visitantes? Credis in celeritam super lucem?
Credis in viam inter tempores? O tu stulte, aquire vitam.

Charlie Duke was CapCom and spoke of this in his book, Moonwalker.

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Lighten up! This is a stellar board!