Introduction

At various times during Apollo lunar surface operations, transmission from Earth would trigger one or both of the astronauts' Voice Operated Relay (Vox) and produce an echo of the transmission in the air-to-ground recordings made on Earth.  The delay between the original transmission and the arrival of the echo would be twice the light travel time between the tracking station then in use and the landing site plus twice the signal transit time between the tracking station and the location where the recording was made.  The fact that the digital versions now available were made from original tape recordings introduced uncertainties such as differences in the original recording speed and the playback speed used in the digitization process.

An example has been published by David Keeports, Mills College, California, who has done an analysis of an Apollo 16 CapCom transmission heard for a second time after being picked up by a crewmember's microphone and being re-transmitted to Earth.   In a recording made in Houston, the two-way time delay was 2.712 seconds.  The transmission occurred at a ground elapsed time of about 120:26, or 18:20 UTC on 21 April 1972.  The tracking station in use was Madrid; and the distance from there to the Apollo 16 site was 385075 km, giving a Madrid-Moon-Madrid travel time of 2.568 seconds.  The Houston-Madrid-Houston remainder is 0.144 seconds and would be dependant on the actual links used in the NASA Communications Network (NASCOM). ¹

1 Although geostationary satellites were available during the lunar missions, the minimum delay resulting from use as part of the path from Houston to a tracking station or vice-versa was 0.238 seconds. 

Apollo 11 Examples

Hi Roland and all,

Apologies for coming in late - the sound is, without doubt, President Nixon's voice coming back from the Moon.

Bill Wood (I'm not sure if Bill is still following this group...) wrote this when I asked him about the echoes. Goldstone was uplinking audio to the LM during the EVA -

I'm sure he won't mind me quoting him -

"I was standing right behind the A/G tech during the call from President Nixon and was sorely tempted to inhibit the downlink from the LM during his speech to limit the amount of feedback from the astronaut's headsets to their microphones. The PLSS radio VOX circuits were tripping off, sending Nixon's voice back down to the earth after 2-1/2 seconds."

I have uploaded the audio - as it was recorded off Net 1 (i.e. the Air/Ground circuit) and put it in this directory -

<http://honeysucklecreek.net/audio/A11_Net1_audio/nixon/>

(The Honeysuckle audio is, as far as I'm aware, the only version of the audio we have that wasn't released in real time by NASA Public Affairs. The "mystery sound" is not an artefact of bleed-through - it was certainly there on the day.)

It's simple to show it was an "echo" caused by either Neil or Buzz's VOX cutting in.

I have not checked to see if the tape needs any speed correction (you'd do that by seeing if the Quindar tone is spot on or not), but it doesn't matter for this anyway.

So, using the file I uploaded (Nixon_HSK.mp3), I opened it and looked at the waveform.

At 29.67 seconds into that clip Nixon says "telephone call". (source_1.mp3)

At 32.25 seconds you hear the "echo" of "telephone call". (echo_1.mp3)

So that's a round trip of 2.58 seconds.

Then go to the mystery sound, which I call "echo_2.mp3", at 2m 08.79 seconds.

Go back 2.58 seconds to 2m 6.21 seconds - and you find "source_2.mp3".

It's just a part of Nixon saying - "seeing you on" [the Hornet on Thursday] - which is just what you suspected. :-)

best wishes

Colin Mackellar

Sydney

2011/8/1 Mike Dinn <mdinn@pcug.org.au>

One possibility is that it could be Collins. At the time we (HSK) were two way
with the CSM (via our wing HSKX/DSS42) whilst Goldstone was two way with the LM,
and EVAers. I remember Ernie Randall ("Network") asking for confirmation of
"President uplinking?" to Columbia, and my affirmative. There was a pretty
horrendous comm config between HSK, GDS and NASCOM, with the single uplink
through both stations, and the downlink from both spacecraft connected to Net1.
Hence the horrible echoes. We hadn't practiced that config before the mission
(not very possible) and in some ways it's amazing it worked at all!

Mike Dinn -- Canberra


For some portions of Apollo 12, the available audio recordings are unusual in that, when the CapCom 'keyed' his microphone, transmissions arriving from the Moon were not recorded. When he switched his microphone off, recording of the lunar transmissions resumed.  Audio for other portions of Apollo 12 do include both the CapCom's transmission and any lunar transmissions received while the CapCom's mike was keyed.

Clearly, there may be examples of Apollo 12 astronaut transmission that weren't recorded on the CapCom loop when he had his microphone keyed.  Hermann Dür calls attention the one example found in an Onboard Transcript (10 Mb; PDF) made soon after the mission from LM onboard recordings.  In the following sequence from early in EVA-2, the transmissions shown in red don't appear in the the Technical Air-to-Ground (A/G) Transcript (27 Mb; PDF) or in the relevant audio clip (4 Mb; MP3), both made from the CapCom Loop. 

The times shown are those found in the A/G transcript; the text is from the ALSJ.


One possibility is that Pete switched from dual-mode comm to primary or secondary and used the Push-to-talk switch on the bottom of his RCU so that his comment would go no farther than the LM.  It is an open question if, having done so, Pete's remark would have appeared in the onboard recording.  It is also reasonable to ask if Pete would have taken the time for such self-censorship.  He was being careful to avoid swearing but it is not clear that he would have considered his "Indians" comment  worth the trouble of self-censorship.  If he had, he probably wouldn't have said what he did.  It seem much more likely that the two transmissions weren't recorded in Houston simply because Ed had his microphone keyed.

The analysis presented below demonstrates that the two lunar transmissions shown in red arrived at Houston during CapCom Ed Gibson's subsequent transmission, shown in blue.  Key elements in the analysis are: (1) the time delay introduced by radiowave travel between the landing site and Houston; and (2) evaluation of the times given in the A/G and LM transcripts.

Light (and Radio) Travel Time

In this analysis we deal with times at which transmissions were recorded in Houston, which we will call Houston Recorded Time (HRT), and times in the recording made onboard the LM, which we will call Lunar Recorded Time (LRT).  The difference is the radio travel time.

The transmissions listed above left the Moon between about 04:56 and 04:57 UTC on 20 Novermber 1969. The tracking station facing the Moon at that time was Goldstone.   JPL's Horizons System gives a range from Goldstone (35.4 N, 116.9 W) to the Apollo 12 landing site (3.0 S, 23.4 W) of 385005 km or 1.284 light-seconds.



The Apollo 12 one-way time delay is certainly not less than 1.284 seconds and could be as long as 1.356 seconds.  In the following, we use 1.3 seconds.  In converting HRT to LRT or vice versa, transmissions that leave the Moon at, say, 132:00:00 LRT arrive in Houston at 132:00:01.3 HRT.  Similarly, a transmission leaving Houston at 132:00:00 HRT arrives at the Moon at 132:00:01.3 LRT.

Times of recorded transmissions

Comparisons between various Apollo Technical Air-to-Ground (A/G) transcripts and the corresponding digitized recordings show that time intervals in the raw A/G transcripts are usually good to one or two seconds  over intervals of a minute or two.  For the purpose of this analysis we need time intervals that are more accurate than the one-way delay of approximately 1.3 seconds.

To make the following table, we used the recording made from the CapCom's loop, determined transmission start times relative to the start of the selected clip (column 2).  An attempt has been made to identify transmission that start at mid-second.  Clearly, precision at the level of 0.1 seconds would require graphical represetnations of the audio wave forms.  Next, we determined start times relative to the start of the selected sequence (column 3); and then added the numbers in column 3 to the Houston Recorded Time of the first transmission to get inferred HRTs (column 4). The average difference between the the inferred HRTs and those given in the raw A/G transcript (column 5) proved to be about one second, so we reduced each of the inferred HRTs by that amount.  Finally, for each of the inferred HRTs, we added 1.3 seconds for Ed Gibson's transmissions and subtracted 1.3 seconds for crew transmissions to get inferred onboard Lunar Recorded Times (LRTs).




Although Ed Gibson's transmission shown in blue lasted about four seconds, a change in the quality of the background indicates that he keyed his microphone about one second before he started speaking and that he turned his microphone off nearly a second after he finished speaking.  The total time his microphone was keyed is between 4 and 6 seconds, with the likely value toward the upper end of the range.

From the flow of the dialog prior to Ed's transmission, it is clear that Al Bean's "Okay" immediately followed Pete's transmission that started at 131:59:36.5 HRT and ended at 131:59:49 HRT.  If the delay between the end of Pete's transmission and the start of Al's "Okay" was a half second, Al's "Okay" would have been recorded unless Ed keyed his microphone no later than 131:59:49.5 HRT - a half second before Ed started speaking.

Al's "Okay" would have taken about a half second to say.  Pete's "If you see any Indians" can be said in four seconds although, knowing Pete's speech habits, it seems more likely that he would have paused for emphasis after "indians", making his transmission 4.5 seconds long. With a half second gap after Al's "Okay", the two tranmissions combined would have started at 131:59:49.5 HRT and ended 5.5 seconds later at 131:59:55 HRT, which is one second after Ed finished speaking.

One final point is that, although most of the inferred LRT times agree within two seconds with the LRT times given in the onboard transcript, for the three transmissions of interest, the inferred times are greater than the onboard LRTs by 5, 5, and 4 seconds.

On the audio tape made from the CapCom loop, Pete's transmission that preceeds Al's"Okay" starts at 131:59:36.5 HRT and ends 12.5 seconds later. Necessarily, the transmission would have ended at 131:59:48.7 LRT.  However, although Al's Okay is clearly a response to Pete, the on-board transcript gives a start time for Al's "Okay" as 131:59:43, nearly six seconds earlier.  The flow of the dialog supports the time inferred from the air-to-ground audio recording, rather than the onboard time the three transmissions of interest.

The onboard audio, once it is digitized, will allow a resolution of this issue.  One prediction from this analysis is that, in the onboard recording, the start of Ed's "Roger" transmission will come about 1.4 seconds before the end of Pete's "If you see".

Another Example

This second instance comes just before Al joins Pete on the surface early in EVA-1.  In this case, the available audio recorded in Houston does have both CapCom and crew transmission.   Although Pete's transmmissions shown in red are fainter than Ed's they are clearly audible in the recording; but do not appear in the A/G transcript.  That suggests the possibility that the transcript was prepared from a recording that did not include crew transmissions when Ed had his microphone keyed.

115:46:03 Conrad: All right. Let me know so I can photograph you.
115:46:06 Bean: Okay.
115:46:12 Conrad: Okay, contingency sample 8; f/8.  (Pause)  (F/)8 and five (foot focal distance).  (I've got to) step back. (Long Pause)
115:46:59 Conrad: We sampled in quite a few places, Houston, so
I'm taking a bunch of pictures.
115:47: Conrad: Four.
115:47:04 Gibson: Roger, Pete.
115:47:03 Conrad: Five pictures.
115:47:06 Gibson: Pete, for your information for those photos, your shadow lines right now is about 45 feet on a level plane.
115:47:16 Conrad: Okay, very good. Contingency sample area I got, deploy the color chart (laughter). Take your time, Al.

Transmission	A/G MP3 Clip		Tech. A/G Transcript			Onboard Transcript
	elapsed (mm:ss)	interval (mm:ss)	Inferred HRT (hh:mm:ss)	Tech. A/G HRT (hh:mm:ss)	Infrrd - Tech (s)	Inferred LRT (hh:mm:ss)	OB Trans LRT (hhh:mm:ss)	Infrrd - Tech (s)
Conrad: All right ...	31:17.5	0:00	115:46:03	115:46:03	0	115:46:01.7	115:46:01	0.7
Bean: Okay.	31:20	0:02.5	115:46:05.5	115:46:06	-0.5	115:46:04.2	115:46:04	0.2
Conrad: Okay, contingency ...	31:26	0:08.5	115:46:11.5	115:46:12	-0.5	115:46:10.2	115:46:10	0.2
Conrad: We sampled ...	32:13.5	0:56	115:46:59	115:46:59	0	115:46:57.7	115:46:57	0.7
Conrad: Four.	32:20	1:02.5	115:47:05.5	na	na	115:47:04.2	na	na
Gibson: Roger, Pete.	32:20	1:02.5	115:47:05.5	115:47:04	1.5	115:47:06.8	115:47:02	4.8
Gibson: Pete, for your ...	32:22	1:04.5	115:47:07.5	na	na	115:47:08.8	115:47:04	4.8
Conrad: Five pictures.	32:23.5	1:06	115:47:09	na	na	115:47:07.7	115:47:03	4.7
Conrad: Okay, very good ...	32:30	1:10.5	147:47:13.5	115:47:16	-2.5	147:47:12.2	115:47:14	-1.8

As with the first case, the inferred LRTs for the four overlapping transmissions are significantly later than the times given in the Onboard Transcript.