[HN Gopher] The digital ranging system that measured the distanc... ___________________________________________________________________ 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: ___________________________________________________________________ (page generated 2022-04-23 23:00 UTC)