4 ms·

Author here if anyone has questions. I encourage you to try out the interactive page that shows how the ranging system worked: https://righto.com/apollo/ranging

by kens 5y ago

Author here if anyone has questions. I encourage you to try out the interactive page that shows how the ranging system worked: https://righto.com/apollo/ranging.html https://righto.com/apollo/ranging.html

dtgriscom 5y ago
> the spacecraft couldn't simply receive and retransmit the signal, because the Doppler shift from the upwards journey would be lost. How is that true? Isn't "simply receiving and retransmitting" the signal just the same as a reflective surface, which works just fine for Doppler relative speed measurements? (I'm ignoring the retransmission increasing the signal strength over a simple reflection.) My guess would be that the frequency difference makes it easier to detect the faint return signal over the strong transmitted signal.
- dtgriscom 5y ago
  Ah: I read further. I believe you meant that it couldn't just send the signal back at a fixed frequency; the frequency had to be based on the incoming frequency.
- kens 5y ago
  It depends which signal they retransmit. If they amplified and retransmitted the raw 2 GHz signal, you are correct that it would be the same as a reflection, but stronger. Unfortunately, if the spacecraft is transmitting and receiving on the same frequency at the same time, the outgoing signal will overwhelm the signal you're trying to receive and you'll get feedback on the spacecraft. Kind of like getting feedback from a microphone and speaker. And it's also going to be very hard to detect the signal on the ground, as you mention. Alternatively, they could extract the pseudo-random signal from the carrier and retransmit it at a different, fixed frequency. In this case, the problem is that the ground station doesn't have the spacecraft's transmission frequency for comparison, so it's hard to extract the Doppler shift. You could put an extremely precise frequency reference on the spacecraft and a matching frequency reference on the ground, but that's a lot harder than shifting the frequency in the transponder. Also, this approach only gives half the Doppler shift of the transponder approach, since you lose the Doppler shift in the upward direction.
gist 5y ago
For a future blog post, as a non engineer I'd be curious how an engineering team designs a system (like those you detail) and others in that mission when there is no way to test them fully in advance of the mission. I know that's a very basic general question but essentially how to you know a hypothesis you can't test works (even with various degrees of pre-testing and scientific knowledge. I am not talking about that 'we know that sound travels' or things that are known but things you are pretty sure but not absolutely sure until you have actually done them. (I do know things are done in stages with various testing and small steps but I guess I am unclear on how once lives are at stake (meaning liftoff from the moon) you really know 'it will all work as planned' meaning to a super high degree of likelihood enough to take the risk.
- simne 5y ago
  Soviet (Russians) don't fully test systems in few programs, including first Moon automatic flights and R-7 rocket. BUT! Because of this, R-7 made first successful flight from 7 or 8 try, and the same was with first soft landing - 6 ships crashed before success. Other programs, like Buran or Lunokhod, was tested on small analogues or on bench. Examples: for Lunoknod created web wheels for Russian all-road truck, and it made thousands off-road kilometers; for Buran was created few ships analogue, program named "Bor"; for rockets, usually created prototype from parts of previous generations rockets, looking Frankenstein. When create analogues was impossible, created very simple designs, just like programmers create micro-kernel systems. And in nearly all Soviet human programs, except "Voskhod" (sunrise, their success I think just luck), used rule, that ship should make 3 fully automatic flights without strict remarks, before carry human. And yes, Soviet space program was more automated then American. - Soviet ships was capable to make all things without human on board. And they even made automatic flight around Moon without humans, but all flights had big problems. But Leonov said, if human was accepted, he would be fight for survival and made success (he was really brave).
- kens 5y ago
  Testing the systems for Apollo was a huge task, more than I can describe here. Everything from component testing to putting a Lunar Module in a giant vacuum chamber on Earth. Each flight pushed things further so they could test in Earth orbit (Apollo 7), then Lunar orbit (Apollo 8). For the ranging system specifically, they could do most of the testing on Earth. As long as they could predict the antenna behavior at long distances, everything could be tested pretty accurately. In addition, the Apollo Guidance Computer provided the same ranging data in a completely redundant way. So even if the ranging system completely failed, they still had a backup.
IMSAI8080 5y ago
Hi Ken. Just wanted to say I've always found your blog superb. The quality of your articles is excellent and the subjects always fascinating.

by kens 5y ago

dtgriscom 5y ago

> the spacecraft couldn't simply receive and retransmit the signal, because the Doppler shift from the upwards journey would be lost. How is that true? Isn't "simply receiving and retransmitting" the signal just the same as a reflective surface, which works just fine for Doppler relative speed measurements? (I'm ignoring the retransmission increasing the signal strength over a simple reflection.) My guess would be that the frequency difference makes it easier to detect the faint return signal over the strong transmitted signal.

dtgriscom 5y ago
Ah: I read further. I believe you meant that it couldn't just send the signal back at a fixed frequency; the frequency had to be based on the incoming frequency.
kens 5y ago
It depends which signal they retransmit. If they amplified and retransmitted the raw 2 GHz signal, you are correct that it would be the same as a reflection, but stronger. Unfortunately, if the spacecraft is transmitting and receiving on the same frequency at the same time, the outgoing signal will overwhelm the signal you're trying to receive and you'll get feedback on the spacecraft. Kind of like getting feedback from a microphone and speaker. And it's also going to be very hard to detect the signal on the ground, as you mention. Alternatively, they could extract the pseudo-random signal from the carrier and retransmit it at a different, fixed frequency. In this case, the problem is that the ground station doesn't have the spacecraft's transmission frequency for comparison, so it's hard to extract the Doppler shift. You could put an extremely precise frequency reference on the spacecraft and a matching frequency reference on the ground, but that's a lot harder than shifting the frequency in the transponder. Also, this approach only gives half the Doppler shift of the transponder approach, since you lose the Doppler shift in the upward direction.

gist 5y ago

For a future blog post, as a non engineer I'd be curious how an engineering team designs a system (like those you detail) and others in that mission when there is no way to test them fully in advance of the mission. I know that's a very basic general question but essentially how to you know a hypothesis you can't test works (even with various degrees of pre-testing and scientific knowledge. I am not talking about that 'we know that sound travels' or things that are known but things you are pretty sure but not absolutely sure until you have actually done them. (I do know things are done in stages with various testing and small steps but I guess I am unclear on how once lives are at stake (meaning liftoff from the moon) you really know 'it will all work as planned' meaning to a super high degree of likelihood enough to take the risk.

simne 5y ago
Soviet (Russians) don't fully test systems in few programs, including first Moon automatic flights and R-7 rocket. BUT! Because of this, R-7 made first successful flight from 7 or 8 try, and the same was with first soft landing - 6 ships crashed before success. Other programs, like Buran or Lunokhod, was tested on small analogues or on bench. Examples: for Lunoknod created web wheels for Russian all-road truck, and it made thousands off-road kilometers; for Buran was created few ships analogue, program named "Bor"; for rockets, usually created prototype from parts of previous generations rockets, looking Frankenstein. When create analogues was impossible, created very simple designs, just like programmers create micro-kernel systems. And in nearly all Soviet human programs, except "Voskhod" (sunrise, their success I think just luck), used rule, that ship should make 3 fully automatic flights without strict remarks, before carry human. And yes, Soviet space program was more automated then American. - Soviet ships was capable to make all things without human on board. And they even made automatic flight around Moon without humans, but all flights had big problems. But Leonov said, if human was accepted, he would be fight for survival and made success (he was really brave).
kens 5y ago
Testing the systems for Apollo was a huge task, more than I can describe here. Everything from component testing to putting a Lunar Module in a giant vacuum chamber on Earth. Each flight pushed things further so they could test in Earth orbit (Apollo 7), then Lunar orbit (Apollo 8). For the ranging system specifically, they could do most of the testing on Earth. As long as they could predict the antenna behavior at long distances, everything could be tested pretty accurately. In addition, the Apollo Guidance Computer provided the same ranging data in a completely redundant way. So even if the ranging system completely failed, they still had a backup.

IMSAI8080 5y ago

Hi Ken. Just wanted to say I've always found your blog superb. The quality of your articles is excellent and the subjects always fascinating.