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Post by ka9q on Feb 19, 2011 17:10:54 GMT -4
Mike Collins makes a similar comment in "Carrying the Fire" and then goes on to explain what caused it. Did he? What page? I read it a while ago so I must have forgotten that he mentioned it. The transcripts were prepared from the LM and CSM tape recordings, so the actual recordings must exist somewhere. Since they were talking to each other over a pair of full duplex radio links, the tape must have recorded it. The "music" probably appears on both radio links since the LM radios were designed to send back whatever tones they received from the CSM. Of course, the artifacts might be different.
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Post by ka9q on Feb 19, 2011 17:42:47 GMT -4
I found a complete description of the Apollo VHF ranging system in a Bellcomm memo on the NASA Technical Reports Server (that thing is a goldmine). It's NASA-CR-95421, file name 19790073126.pdf. I read and understood it. There's no question in my mind that this system would create all sorts of audible artifacts in operation, especially at its range limits where it is frequently reacquiring lock on the LM.
The CSM transmits one of three audio ranging tones through the same VHF transmitter the CMP uses to talk to the LM. The tone frequencies are 247 Hz, 3.95 kHz and 31.6 kHz. They can be transmitted individually, or the 247 and 3.95 tones can be XORed with each other. They are transmitted at 100% modulation mixed with clipped voice. All this would produce a wide range of audible intermodulation products and harmonics.
The LM receiver sends back all three audio tones on the same VHF transmitter that the LM crew uses to talk to the CMP so that the CSM can measure the round trip delay. The two lower frequency tones are simply fed back to the audio input of the LM VHF transmitter but the 31.6 tone is outside the normal voice bandwidth. Because it's used for continuous tracking, achieving a good signal-to-noise ratio is important. So the LM separately regenerates it before feeding it back to the VHF transmitter.
But there's an ambiguity problem with such a high frequency tone: the CSM can measure the phase difference between the transmitted and received 31.6 tones but it doesn't know how many complete cycles there are in addition. So it first uses the two lower frequency tones in a predetermined sequence to acquire the LM, determine the approximate range, and resolve this ambiguity.
During normal tracking, there wouldn't be too much interference with the voice from the 31.6 tone. Acquisition is a different story. Voice is momentarily interrupted while the lower frequency tones are sent, and if the crews can hear them I can easily see how they'd describe them as "strange space music" -- especially if they had no idea how the ranging system operated (and they probably didn't, since they didn't need to know how it worked just to use it.)
Since Apollo 10's Snoopy and Charlie Brown were at the extreme range limits of the VHF tracking system, it would have been breaking and reacquiring lock quite often, easily explaining the weird sounds the crew heard.
This memo also gives the acquisition sequence. With this information, I could easily write a program to simulate the same sounds the crew would have heard.
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Post by supermeerkat on Feb 19, 2011 20:26:21 GMT -4
Mike Collins makes a similar comment in "Carrying the Fire" and then goes on to explain what caused it. Did he? What page? I read it a while ago so I must have forgotten that he mentioned. From Chapter 13 of Carrying The Fire by Michael Collins: "It was interference between the LM's and Command Module's VHF radios. We heard it yesterday when we turned our VHF radios on after separating the two vehicles, and Neil said that it 'sounds like wind whipping around the trees.' It stopped as soon as the LM got on the ground, and started up again just a short time ago. A strange noise in a strange place." Page 420 (in my edition of the book, anyway).
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Post by ka9q on Feb 20, 2011 2:28:13 GMT -4
Hmm. That sounds like something different. Did they have the VHF ranging system on? I wouldn't see any need for it during the separation and descent.
I threw together a simple program to generate the ranging tones and I wouldn't describe it as "wind whipping around the trees". They sound more like the initial handshake of a dialup modem. Of course, in the late 1960s most people weren't familiar with the sound of dialup modems...
A windy, whistling sound sounds more like the "spurs" that many radios generate on certain frequencies.
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Post by George Tirebiter on Feb 20, 2011 3:33:16 GMT -4
I think I heard about that mission back in the early 1980s. Isn't that the one that was attacked by General Zod? I thought they died from a solar flare—or was that Apollo 19?
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Post by Count Zero on Feb 20, 2011 21:50:18 GMT -4
I wouldn't mind being attacked by Sara Douglas.
Actually, I'm not into horror movies, but I like the looks of this one. Although some things have to pass with a wink (secret Saturn V launch, in-LM video, etc.), it looks like they did a good bit of research on the actual records to get the "look" right. Most people don't know that LK even existed, so they win some points right there.
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Post by supermeerkat on Feb 21, 2011 8:26:21 GMT -4
Most people don't know that LK even existed, so they win some points right there. For me, the LK, alongside seeing Apollo hardware on the big screen, is a major reason just to see that film
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Post by lukepemberton on Feb 21, 2011 19:47:30 GMT -4
For me, the LK, alongside seeing Apollo hardware on the big screen, is a major reason just to see that film Actually, that had me thinking supermeerkat. Wouldn't it have been great if the USSR had managed to land men on the moon too? That would have been incredible. As an aside to the last comment: I will probably go and see the film, although the trailer does give the impression that it will be a lunar version of Blair Witch. It is likely to annoy the hell out of me in that case.
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Post by banjomd on Feb 22, 2011 12:51:26 GMT -4
. . . Wouldn't it have been great if the USSR had managed to land men on the moon too? . . . I feel that, if the USSR had landed on the Moon, there's a chance that Apollo would have continued, possibly culminating in a lunar base.
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Post by supermeerkat on Feb 22, 2011 13:29:27 GMT -4
I feel that, if the USSR had landed on the Moon, there's a chance that Apollo would have continued, possibly culminating in a lunar base. Had the USSR beat the US, it almost certainly would have triggered a Mars race, but otherwise I agree the space race would have continued.
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Post by banjomd on Feb 22, 2011 13:35:03 GMT -4
supermeerkat, I didn't mean if the USSR had landed first; I meant that if they had not quit and continued their lunar program to success, Apollo might have gotten funding to continue.
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Post by echnaton on Feb 22, 2011 13:40:50 GMT -4
I'm not so sure. The technological and financial commitment needed to build a lunar base dwarfs that of the commitment for Apollo. Apollo was started during a period of fantastic economic growth, while the landings occurred while growth was slowing and about to stop while at the same time the demand for government funding of earthly projects was picking up. For the cold war, détente was replacing full fledged competition.
I just don't see the resources being available to sustain Apollo and expand scope of the moon exploration program to put a base on the moon.
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Post by ka9q on Feb 22, 2011 14:27:41 GMT -4
Apollo was so expensive in part precisely because it was a crash program. Maybe that was the only way it could have happened given the political tone of the day, but it inevitably collapsed once it had succeeded in beating the Russians to the moon by the end of the decade.
Any chance for a sustainable lunar base depends on a careful, methodical approach that emphasizes affordability over speed. Apollo made any number of design decisions that would probably have been radically different had sustainability been the goal.
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Post by PeterB on Feb 23, 2011 8:06:12 GMT -4
I'm reminded of the service notice at a cafeteria across the road from work, somewhat tongue-in-cheek. Adapted for NASA, it would be:
I think Apollo was an example of case 3, and the Shuttle was an example of case 2. I have a feeling that a couple of Soviet programs (Voskhod and early Soyuz) were examples of case 1.
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Post by randombloke on Feb 23, 2011 8:13:28 GMT -4
The "cheap-fast-good" triumvirate (for variable values of good) has nearly universal jurisdiction I think. About the only way you can get all three is if you have a good team and a complete, invariant, spec to meet. But all that really means is that you've off-set the fast with other people's time, even if you've never met them...
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