5 ms·

The digital ranging system that measured the distance to the Apollo spacecraft

131 points by picture 5y ago

pomian 5y ago
An excellent book about the guidance computer on the Lunar Module, is Sunburst and Luminary. https://www.sunburstandluminary.com/SLhome.html 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.
- ncmncm 5y ago
  Ordered. Looks fascinating!
ncmncm 5y ago
This is easily the most fascinating article Ken has ever posted, topping even the lunar lander computer, which was a high bar.
tails4e 5y ago
Thanks. Really nice writeup, as usual. 1 meter accuracy is also very impressive. I wonder what is achievable today?
- nikanj 5y ago
  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
- ericbarrett 5y ago
  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 5y 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.
- crimsonday 5y ago
  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_experiment https://en.m.wikipedia.org/wiki/Lunar_Laser_Ranging_experime...
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.
supernova87a 5y ago
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 5y ago
  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.
  - kens 5y ago
    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. [1] PDF page 135 in https://ntrs.nasa.gov/api/citations/19650025875/downloads/19650025875.pdf https://ntrs.nasa.gov/api/citations/19650025875/downloads/19...
CrimsonCape 5y ago
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.
- crimsonday 5y ago
  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).
- tyldum 5y ago
  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 5y ago
  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).
CaliforniaKarl 5y ago
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-BCv4rVHL-8GuC2WGb https://www.youtube.com/playlist?list=PL-_93BVApb58SXL-BCv4r... You can see the equipment from this post being tested and used!
GlenTheMachine 5y ago
“ 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.
- simne 5y ago
  > 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.
  - icegreentea2 5y ago
    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.
    - simne 5y ago
      I read Ben Rich's memoirs. But fortunately, I seen very detailed photos of stealth modifications of U-2, and nothing about it was in memoirs.
  - GlenTheMachine 5y ago
    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.

131 points by picture 5y ago

An excellent book about the guidance computer on the Lunar Module, is Sunburst and Luminary. https://www.sunburstandluminary.com/SLhome.html 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.

ncmncm 5y ago
Ordered. Looks fascinating!

ncmncm 5y ago

This is easily the most fascinating article Ken has ever posted, topping even the lunar lander computer, which was a high bar.

tails4e 5y ago

Thanks. Really nice writeup, as usual. 1 meter accuracy is also very impressive. I wonder what is achievable today?

nikanj 5y ago
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
ericbarrett 5y ago
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 5y 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.
crimsonday 5y ago
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_experiment https://en.m.wikipedia.org/wiki/Lunar_Laser_Ranging_experime...

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.

supernova87a 5y ago

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 5y ago
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.
- kens 5y ago
  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. [1] PDF page 135 in https://ntrs.nasa.gov/api/citations/19650025875/downloads/19650025875.pdf https://ntrs.nasa.gov/api/citations/19650025875/downloads/19...

CrimsonCape 5y ago

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.

crimsonday 5y ago
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).
tyldum 5y ago
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 5y ago
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).

CaliforniaKarl 5y ago

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-BCv4rVHL-8GuC2WGb https://www.youtube.com/playlist?list=PL-_93BVApb58SXL-BCv4r... You can see the equipment from this post being tested and used!

GlenTheMachine 5y ago

“ 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.

simne 5y ago
> 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.
- icegreentea2 5y ago
  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.
  - simne 5y ago
    I read Ben Rich's memoirs. But fortunately, I seen very detailed photos of stealth modifications of U-2, and nothing about it was in memoirs.
- GlenTheMachine 5y ago
  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.