[HN Gopher] The digital ranging system that measured the distanc...
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The digital ranging system that measured the distance to the Apollo
spacecraft
Author : picture
Score : 73 points
Date : 2022-04-23 17:29 UTC (5 hours ago)
(HTM) web link (www.righto.com)
(TXT) w3m dump (www.righto.com)
| kens wrote:
| 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
| IMSAI8080 wrote:
| 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.
| gist wrote:
| 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.
| someguydave wrote:
| you can artificially create RF channels with long delays for
| bench testing a system like this
| kens wrote:
| 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.
| TedDoesntTalk wrote:
| How did the AGC have the same ranging data? I thought it
| was a completely different system.
| kens wrote:
| The AGC computed the position (including range) using
| inertial guidance and star sightings by the astronauts.
| So yes, it was a completely independent system, but the
| results should be the same.
| pwr-electronics wrote:
| As the article below explains, it's a combination of
| structured qualitative analysis and a review process. That
| process builds on top of all the other application-specific
| or discipline-specific processes, like a hierarchy. The
| higher up you go, the more generic it gets. The lower down
| you go, the more you critique the exact math or test or
| whatever.
|
| https://adsabs.harvard.edu/full/1996ESASP.377...83F
| ghaff wrote:
| For the Saturn V rocket/Apollo as a whole, The Saturn V Story
| documentary is quite good. It's on Amazon but I'm sure can be
| found elsewhere.
|
| The short answer is that, as you say, things were done in
| stages, problems were found and fixed, people did die (the
| Apollo 1 fire), and the Apollo 11 moon landing was a pretty
| close thing.
| tails4e wrote:
| Thanks. Really nice writeup, as usual. 1 meter accuracy is also
| very impressive. I wonder what is achievable today?
| ericbarrett wrote:
| Layman speculation--Using exactly this system (it's quite
| brilliant!) I bet we could use much higher frequencies due to
| modern electronics, antenna design, and so forth, giving
| additional precision, and maybe the ability to measure craft
| attitude. I could also see additional systems (a laser
| rangefinder...? or whatever) augmenting it without the mass &
| energy penalty they would have imposed 50 years ago.
| tyldum wrote:
| As someone who does modern ranging, you are pretty much spot
| on.
|
| Fewer and fewer do ranging these days, and the few that does
| only uses it for contingency situations or to validate GPS
| receivers during launch and early phase.
| crimsonday wrote:
| Today, about 1mm, which is x1000 better accuracy:
|
| > The distance to the Moon can be measured with millimeter
| precision.
|
| > The Moon is spiraling away from Earth at a rate of 3.8
| cm/year
|
| From:
| https://en.m.wikipedia.org/wiki/Lunar_Laser_Ranging_experime...
| nikanj wrote:
| The laws of physics haven't really changed, so I'd guess
| something similar. It doesn't seem like their limiting factor
| was the computers
| GlenTheMachine wrote:
| " As far as I can tell, there isn't any direct connection between
| the Apollo ranging system and GPS. GPS grew out of the Transit
| (Naval Navigation Satellite System), the Timation satellite
| program, and USAF Project 621B (history)."
|
| I can't 100% verify this, but Transit and Timation were both US
| Naval Research Lab (NRL) programs. NRL has a thirty foot radio
| antenna on the roof of the administration building that
| contributed to determining the range to the moon itself prior to
| Apollo. IDK for certain whether that antenna was used for ranging
| the Apollo spacecraft, but it's possible; certainly the NASA and
| NRL personnel knew each other and contributed to each others'
| programs.
| CaliforniaKarl wrote:
| As a companion piece to this post, I suggest watching
| CuriousMarc's still-in-progress series "Apollo S-Band
| Communications" at
| https://www.youtube.com/playlist?list=PL-_93BVApb58SXL-BCv4r...
|
| You can see the equipment from this post being tested and used!
| supernova87a wrote:
| Great article!
|
| One thing I wasn't able to tell from the description, did having
| multiple sending/receiving locations around the world require
| very tight time synchronization? (like being able to match up
| signals sent from one location to another received across the
| globe?) How was the time accuracy in those days?
| kens wrote:
| Yes, the ground stations were kept in time sync with rubidium
| atomic clocks and other complex systems.
|
| For ranging specifically, the codes were transmitted and
| received at the same site so time synchronization was not an
| issue.
| CrimsonCape wrote:
| Maybe a good follow-up post would be to compare the Apollo
| hardware to modern hardware. It would be nice to see the
| differences in size, complexity, etc.
| tyldum wrote:
| It's starting to transition into software defined these days.
| The antennas themselves are similar size, depending on
| frequency but the backend systems are now down to a standard
| server (at least up to 250-ish megasymbols and increasing).
|
| Things like viterbi are designed to be done efficiently in
| hardware. Doing it in software is limited to CPU core speed.
| Multi threaded solutions are hard to engineer as the signal is
| a continuous stream and cannot trivially be split up for
| parallel processing.
|
| I do grounds stations for a living. Currently building edge
| clouds to do this at scale where the dishes are placed around
| the globe.
| crimsonday wrote:
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(page generated 2022-04-23 23:00 UTC)