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
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.
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.
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.
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.
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.
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).
> 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.
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.
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.
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.
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.
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?
For complete details on how the Apollo ground stations stayed synchronized, see "Apollo Precision Frequency Source and Time Standard" [1]. Stations stayed synchronized to one part in 10^10. Each had two rubidium frequency standards and two crystal frequency standards. Each frequency standard had its own power supply and battery pack for redundancy. The sites were synchronized to the WWV time radio station and VLF signals.
The specification for Apollo was to maintain the frequency within 5x10^-11 for a one year period. The VLF system had 1 microsecond resolution. Jitter in the WWV pulse was +/- 0.5 milliseconds. They calculated and measured the propagation time from the transmitting station so they could account for that, within about 1 millisecond.
It's amazing how much redundancy and complexity there was for everything in Apollo. Even something like ground station time synchronization that nobody thinks about (except for supernova87a), had teams of people, a bunch of research, and racks of equipment.
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.
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.
A modern phone contains around 50-70 individual ranging receivers, all capable of operating simultaneously (each with its own spread spectrum sequence, Doppler shift etc).
The GNSS chip typically has 30-60. The 3G modem (if enabled) has ~10 independent receivers (called "fingers" in CDMA parlance). WiFi has 1 (legacy 802.11b).
As comparison, a modern phone contains around 50-70 individual ranging receivers similar to the Apollo ranging receiver, all capable of operating simultaneously (each with its own spread spectrum sequence, Doppler shift etc).
The GNSS chip typically has 30-60. The 3G modem (if enabled) has ~10 independent receivers (called "fingers" in CDMA parlance). WiFi has 1 (legacy 802.11b).
“ 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.
The specific case of SR-71 to F-117 seems incredibly unlikely - at least based on my memory of Ben Rich's (was an engineer at Lockheed Skunkworks who worked on the A-12/SR-71 program and then was Director/President for F-117 program) memoirs.
What did happen was that some of the program managers who initiated the stealth fighter program were not aware initially aware of the stealth characteristics of SR-71. This was rectified.
But in this case, the two were being worked on literally at the same time. NRL flew the experimental precursors to GPS in the late 1960s and early 1970s. NTS-4, the first operation GPS satellite, launched in 1974.
In NRL's case, the challenge would have been that the engineers were in different divisions. Radar ranging of the moon was done by Space Sciences, and Transit and Timation were done by Spacecraft Engineering.
An excellent book about the guidance computer on the Lunar Module, is Sunburst and Luminary.
https://www.sunburstandluminary.com/SLhome.html
It describes in a very entertaining style about building testing and editing the software and hardware of the Apollo program. It includes a lot of commentary and anecdotes about the era and life style of many of the program's engineers, designers, pilots, technicians, etc.
Very recommend.
36 comments
[ 5.4 ms ] story [ 95.2 ms ] threadFor 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.
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.
https://adsabs.harvard.edu/full/1996ESASP.377...83F
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).
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.
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.
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.
> 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...
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?
For ranging specifically, the codes were transmitted and received at the same site so time synchronization was not an issue.
The specification for Apollo was to maintain the frequency within 5x10^-11 for a one year period. The VLF system had 1 microsecond resolution. Jitter in the WWV pulse was +/- 0.5 milliseconds. They calculated and measured the propagation time from the transmitting station so they could account for that, within about 1 millisecond.
It's amazing how much redundancy and complexity there was for everything in Apollo. Even something like ground station time synchronization that nobody thinks about (except for supernova87a), had teams of people, a bunch of research, and racks of equipment.
[1] PDF page 135 in https://ntrs.nasa.gov/api/citations/19650025875/downloads/19...
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.
The GNSS chip typically has 30-60. The 3G modem (if enabled) has ~10 independent receivers (called "fingers" in CDMA parlance). WiFi has 1 (legacy 802.11b).
The GNSS chip typically has 30-60. The 3G modem (if enabled) has ~10 independent receivers (called "fingers" in CDMA parlance). WiFi has 1 (legacy 802.11b).
You can see the equipment from this post being tested and used!
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.
Yes, it is possible, but for military programs very typical, program running in 1980s forgot knowledge of 1960s.
Example, Lockheed SR-71 used stealth technologies (even U-2 used), but insiders said, all these achievements was lost, when worked on F-117.
What did happen was that some of the program managers who initiated the stealth fighter program were not aware initially aware of the stealth characteristics of SR-71. This was rectified.
In NRL's case, the challenge would have been that the engineers were in different divisions. Radar ranging of the moon was done by Space Sciences, and Transit and Timation were done by Spacecraft Engineering.