Calling occupants....

[bknight]

Why didn't the LMP transmit directly to the LM? Because the original spec called for only one astronaut to be on the surface at a time. When this was changed to two-man EVAs, it was easier to build a relay into one of the PLSSes than to add another receiver to the LM.

Seems odd that NASA planned on a two manned lander with only one on the surface at a time.  The planners did a poor job of mission specs when they came up with that one.

Not really. Remember that all nine Gemini EVAs had one person outside with one remaining in the spacecraft.

Yes, but that was a very different circumstance.

In what way?

1. Zero Gravity, somebody to drag the partner back into the hatch.
2. No PLSS, but connected to the spacecraft, someone needs to monitor the environment.
3. Job tasks related to one, whereas two were required for setup of different experiments.
Please understand, I'm not arguing with you just stating that to land two men on the moon and have one inside the landing craft seems poor planning.

[Dalhousie]

Why didn't the LMP transmit directly to the LM? Because the original spec called for only one astronaut to be on the surface at a time. When this was changed to two-man EVAs, it was easier to build a relay into one of the PLSSes than to add another receiver to the LM.

Seems odd that NASA planned on a two manned lander with only one on the surface at a time.  The planners did a poor job of mission specs when they came up with that one.

Not really. Remember that all nine Gemini EVAs had one person outside with one remaining in the spacecraft.

Yes, but that was a very different circumstance.

In what way?

1. Zero Gravity, somebody to drag the partner back into the hatch.
2. No PLSS, but connected to the spacecraft, someone needs to monitor the environment.
3. Job tasks related to one, whereas two were required for setup of different experiments.
Please understand, I'm not arguing with you just stating that to land two men on the moon and have one inside the landing craft seems poor planning.

If it was poor planning (or better, not ideal), that may have been one reason why it was changed.

However, keep in mind it it depends on how risks are assessed.  For example if the lunar surface is deemed hazardous then exposing two people to that hazard is twice the risk of exposing one if the second person offers no meaningful safety margin.

I learned this lesson while planning some underwater operations under hazardous conditions with a commercial diving firm.  It was deemed safer to have one diver on the seabed in constant communications and a second one standby on the surface for any rescue than it was to have both in the water at the same time.  There are so many factors, some quite subtle at play here.

At first impressions it would have been very difficult to get an incapacitated astronaut back into the LM, even with help.  A second person could help in sharing life support in the event of a failure or leak, or if an astronaut had difficulty getting getting up after a fall. Maybe there were still lurking fears that astronauts would disappear into deep pits of Gold dust.  Wasn't there a thread a while back discussing how Armstrong was initially secured to the LM by a safety line against such an occurrence?

What documentation is there for the change from one person to two person EVAs from the LM?  How did this related to the hatch opening direction decision, forcing the commander to be the one to go out first, or do a solitary EVA?

[ka9q]

Was this the aim for all APollo EVAs initially, or just Apollo 11? (I find it hard to imagine an H or J mission with just one person outside)

I don't think they were thinking that far ahead.

You do have to consider that communications between the two astronauts was more important than comm between either and the earth. That ruled out a LM relay between the two astronauts; they had to talk direct in case they were too far from the LM. By having the CDR relay the LMP to the LM, direct communications between the two could be maintained with just one VHF receiver on the LM.

Nowadays you'd use a mesh network where every station (LM, LRV, CDR, LMP) transmits and receives on a single channel and they automatically relay high speed digital packets in all directions as conditions permit. Or everyone would have two mesh radios on separate channels for redundancy.

When did the change from one person to two people on the lunar surface happen?

I know I've seen that discussed (that's how I know the reason for the relay) but I can't remember where. 

Did post Apollo 11 EVAs also relay through the commander's PLSS?

Yes, all of them. The only change came with Apollo 15, when they had the relay package on the rover in addition to the LM. The two were almost functionally identical, including the ability of the rover relay to transmit either PM (for voice) or FM (for voice/video) S-band to earth. PM had much better link margin, just as it did from the LM, so it was used through a low gain antenna during drives. FM required the LRV to be parked and the high gain antenna manually pointed at the earth.

About the only difference is that the LRV PM transponder was not coherent, as it listened to the same uplink from earth as the LM but transmitted on its own downlink frequency. (The uplink and downlink frequencies in a coherent transponder have to be in a specified ratio of 221/240 for S-band). The ground could therefore select a return (astronaut->Houston) link by just switching earth receivers.

Selecting the forward link was a little more complex. Uplink voice was sent to the LM (and CSM) as NBFM on a 30 kHz subcarrier. A separate subcarrier of 124 kHz was used for the LRV, so the ground could select a relay by just modulating the appropriate uplink subcarrier. The LM and LRV transmitted to the astronauts on the same frequency, but voice-activated switching (VOX) keyed each transmitter only when the associated receiver was active; this avoided interference on the moon between the two VHF transmitters.

Commands were sent to both the LM and LRV on a 70 kHz subcarrier, but they could be addressed digitally.

[Dalhousie]

thanks!

[ka9q]

Sure.

That independent comm relay on the LRV really saved their bacon on Apollo 16 when the LM high-gain antenna failed. They used backup voice mode (direct PM on the main S-band carrier) and omni antennas during the landing and ascent but it was weak and often hard to understand. Video would have been impossible.

[bknight]

Sure.

That independent comm relay on the LRV really saved their bacon on Apollo 16 when the LM high-gain antenna failed. They used backup voice mode (direct PM on the main S-band carrier) and omni antennas during the landing and ascent but it was weak and often hard to understand. Video would have been impossible.

I didn't know that happened, so they just relayed the signal to the LM and used the big antenna to transmit the voice/images back to the MDSN?

[ka9q]

The steerable high-gain antenna on the upper right side of the Apollo 16 did not deploy; one of the pins used to immobilize and protect it during launch failed to disengage when commanded. You can easily tell you're looking at the Apollo 16 LM in pictures: the high gain antenna is pointed directly back at the western horizon instead of up at the earth.

During landing and ascent, they still had to use the LM comm system so they used the omni antennas (small conical corkscrews, one on front, one on back) and often the "backup voice" mode. This sounds noticeably different from the normal voice mode because the backup mode is essentially AM. (It's actually direct phase modulation or PM of the voice on the S-band carrier, but the modulation level is so low that mathematically it is very similar to AM).

The normal voice mode is narrowband FM on a subcarrier on the main PM S-band carrier (FM when video is being transmitted). As an AM signal gets weaker, the noise stays constant and the voice sinks into it. As an FM signal gets weaker, you first hear "popcorn" noise, then it gets louder and eventually blots out the voice.

During Apollo 16 surface operations, the partly failed LM comm system was simply left unused (except to monitor LM telemetry, which had to be sent at a low data rate) and the independent comm system on the LRV was used instead to talk to the astronauts. It had its own high gain antenna that could be pointed manually at earth, so they still had full video coverage of the EVAs.

[bknight]

The steerable high-gain antenna on the upper right side of the Apollo 16 did not deploy; one of the pins used to immobilize and protect it during launch failed to disengage when commanded. You can easily tell you're looking at the Apollo 16 LM in pictures: the high gain antenna is pointed directly back at the western horizon instead of up at the earth.

During landing and ascent, they still had to use the LM comm system so they used the omni antennas (small conical corkscrews, one on front, one on back) and often the "backup voice" mode. This sounds noticeably different from the normal voice mode because the backup mode is essentially AM. (It's actually direct phase modulation or PM of the voice on the S-band carrier, but the modulation level is so low that mathematically it is very similar to AM).

The normal voice mode is narrowband FM on a subcarrier on the main PM S-band carrier (FM when video is being transmitted). As an AM signal gets weaker, the noise stays constant and the voice sinks into it. As an FM signal gets weaker, you first hear "popcorn" noise, then it gets louder and eventually blots out the voice.

During Apollo 16 surface operations, the partly failed LM comm system was simply left unused (except to monitor LM telemetry, which had to be sent at a low data rate) and the independent comm system on the LRV was used instead to talk to the astronauts. It had its own high gain antenna that could be pointed manually at earth, so they still had full video coverage of the EVAs.

After spending about 30 minutes at ALSJ, searching for images that have LM(A lot of them), I now "see" that the high gain antenna is not deployed beside the LM as in the other missions, probably because of the failure.  I never noticed the missing deployment before.  As I said earlier I'm learning facts that I didn't realize existed that were unknown to me.