5 ms·

I'm curious as to why they're using atomic clocks in space for a high resolution time source, rather than an atomic clock on earth... Latency?

fennecfoxen 11y ago
At a guess, they need a very small number of clocks (for uniformity) and they use the existing clocks in space because that signal distribution was more effective than new terrestrial infrastructure.
jewel 11y ago
I think it's a matter of price. GPS receivers are cheap and widely supported by timekeeping software. The cheapest atomic clock I can find is $1,500, but it's a hard thing to search for since the term is used to refer to radio-synchronized clocks. If you did have an atomic clock in every datacenter, you'd still need to make sure that they didn't develop a problem that caused them to drift apart, and also you'd need to make sure that they were in sync with each other in the first place.
- ploxiln 11y ago
  it would be a real problem if they drifted at all ... that's kinda the point of atomic clocks, no?
  - detaro 11y ago
    Of course atomic clocks drift as well, and as soon as you run multiple you have to keep that in mind. GPS (or rather a GPS-controlled quartz) is probably better quality than cheap atomic clocks. GPS is a convenient way to get well-synced precise time nearly everywhere you can see the sky.
    - ploxiln 11y ago
      GPS satellites themselves use atomic clocks (and to find your position you use their also-broadcast orbital positions and math for the effects of relativity etc)
      - detaro 11y ago
        Yes, I'm aware. ?
        ploxiln 11y ago
        GPS satellites use atomic clocks which don't drift aka don't become 0.1% slower or faster than they should be. OK, technically they drift, but not enough to be noticeable by anyone. They're the standard to which other clocks are compared. According to https://en.wikipedia.org/wiki/Atomic_clock#Evaluated_accuracy https://en.wikipedia.org/wiki/Atomic_clock#Evaluated_accurac... existing atomic clocks can drift by 1 second in 100 million or 150 million years. You won't notice that in your high-speed datacenter applications over the course of a few decades. (unless perhaps if one is plain broken, and it's noticed by comparing to the others and taken out of service)
      - KMag 11y ago
        I think you missed the word "cheap" in the GP.
  - mentat 11y ago
    Within their frame of reference. Many many frames exist.
    - ploxiln 11y ago
      That's true, but if it's sitting in your datacenter which is stably attached to some point on the surface of the earth, that's a well defined frame of reference.
- madaxe_again 11y ago
  $1500 is nothing if you're a telco. If cost were the reason they wouldn't blink at having five caesium fountains in each location - but as another observed it's likely about synchronicity - relativity is a problem if you're looking to have two clocks on opposite sides of the planet in perfect sync.
varjag 11y ago
GPS reference clocks are nanosecond resolution. Synchronizing from Earth would be challenging due to varying transmission distances and relativistic effects.
chiph 11y ago
A ground-based transmitter could be destroyed in an attack, so having the clocks on orbit meant they were protected and would still be operational for a retaliatory ICBM launch. Edit: So far as latency - the on-board clocks get resynced today every now and again against a ground-based reference time as they pass overhead of a USAF transmitter (primary + backup). If they used only moderately-accurate clocks as a cost-saving measure (or were at the limit of late 1970's technology) drift would be more severe and the tail-end of each orbit would be less correct than at the start of the orbit. So additional ground stations would be required in parts of the planet not always so friendly to the US, and they would have to coordinate their time sources, allowing for the light-speed delay travelling around the earth. Much more expensive/harder solution from an operational aspect. Edit Edit: Don't forget that the GPS system was designed around military requirements. All the satellites (even today) have nuclear weapon detonation detectors on board and are a primary input to NORAD. > The 2nd SOPS has three missions: global navigation, time transfer and nuclear detection. http://www.schriever.af.mil/GPS/ http://www.schriever.af.mil/GPS/
- madaxe_again 11y ago
  Totally get it in the military context, and why it's useful in the context of a missile lobbing holiday, but was rather more curious as to why it'd be used as a time source for synchronising packets in commercial applications as the article describes. Based on what others have said and my own inkling I think it's about synchronisation at a fine timescale without having to trouble yourself with bothersome shit like relativity, and leaving that particular pain in the ass to the US military. That's what I meant by latency, but phone keyboard, walking, lazy!
  - chiph 11y ago
    What it is is outsourcing your time source to the USAF. Aka introducing an external dependency. If these people are upset over 13 microseconds, wait until the DoD turn selective availability back on during a war or start adjusting satellite positions to avoid incoming ASAT missiles. Interesting fact: GPS time doesn't take leap seconds, etc. into account. As of last summer, it's ahead of UTC by 17 seconds. http://tycho.usno.navy.mil/leapsec.html http://tycho.usno.navy.mil/leapsec.html
elfchief 11y ago
The problem with atomic clocks in general is that they have very good short term stability, but not-so-great long-term stability. So they are very good at measuring the amount of time that has passed between two events, but not as great for telling you exactly what time an event happened. The other problem is that there's not really such thing as a single, true time. Precision timekeeping is one of those places where relativity matters -- differences in altitude and the geology of the earth under your feet at a particular point change how fast any particular clock 'ticks'. So to accommodate these differences, if you want events to be synchronized across multiple locations over a long period, you both need a single agreed-upon "current time", and some way to get information about that time information to the sites that need it. You could surely do this with directly connected wires that were very carefully measured, but that gets expensive very quickly. The GPS system already requires super-precise time just to function, so it makes for a great distribution mechanism of "what time is it" information, where the people running the GPS system are bothered with all the details of figuring out and standardizing a single time, and you 'just' have to listen. The GPS folks also keep track of things like how fast the GPS standard clock is drifting from the UTC standard clock (since no two clocks will agree over the long term, period). And this is actually where the problem happened -- Two of the parameters broadcast (A0 and A1) were incorrect, and it's these parameters that are used to specify the current time difference between the GPS master clock and the UTC master clock (the GPS master clock is disciplined to be as close to UTC as possible, but is always off by a minute amount, and that's what A0/A1 represent). Pretty much it boils down to, much as it is in distributed systems, "there is no now". Even with maximum care and the highest quality clocks, there's really not a definition of 'now' that's consistent across more than a single location. GPS is currently the closest thing we have to a globally-distributed definition of 'now'.
- eru 11y ago
  Google is also working on a globally distributed `now': http://research.google.com/archive/spanner.html http://research.google.com/archive/spanner.html Of course, they make use of GPS, but also add their own atomic clocks.
  - elfchief 11y ago

fennecfoxen 11y ago

At a guess, they need a very small number of clocks (for uniformity) and they use the existing clocks in space because that signal distribution was more effective than new terrestrial infrastructure.

jewel 11y ago

I think it's a matter of price. GPS receivers are cheap and widely supported by timekeeping software. The cheapest atomic clock I can find is $1,500, but it's a hard thing to search for since the term is used to refer to radio-synchronized clocks. If you did have an atomic clock in every datacenter, you'd still need to make sure that they didn't develop a problem that caused them to drift apart, and also you'd need to make sure that they were in sync with each other in the first place.

ploxiln 11y ago
it would be a real problem if they drifted at all ... that's kinda the point of atomic clocks, no?
- detaro 11y ago
  Of course atomic clocks drift as well, and as soon as you run multiple you have to keep that in mind. GPS (or rather a GPS-controlled quartz) is probably better quality than cheap atomic clocks. GPS is a convenient way to get well-synced precise time nearly everywhere you can see the sky.
  - ploxiln 11y ago
    GPS satellites themselves use atomic clocks (and to find your position you use their also-broadcast orbital positions and math for the effects of relativity etc)
    - detaro 11y ago
      Yes, I'm aware. ?
      - ploxiln 11y ago
        GPS satellites use atomic clocks which don't drift aka don't become 0.1% slower or faster than they should be. OK, technically they drift, but not enough to be noticeable by anyone. They're the standard to which other clocks are compared. According to https://en.wikipedia.org/wiki/Atomic_clock#Evaluated_accuracy https://en.wikipedia.org/wiki/Atomic_clock#Evaluated_accurac... existing atomic clocks can drift by 1 second in 100 million or 150 million years. You won't notice that in your high-speed datacenter applications over the course of a few decades. (unless perhaps if one is plain broken, and it's noticed by comparing to the others and taken out of service)
    - KMag 11y ago
      I think you missed the word "cheap" in the GP.
- mentat 11y ago
  Within their frame of reference. Many many frames exist.
  - ploxiln 11y ago
    That's true, but if it's sitting in your datacenter which is stably attached to some point on the surface of the earth, that's a well defined frame of reference.
madaxe_again 11y ago
$1500 is nothing if you're a telco. If cost were the reason they wouldn't blink at having five caesium fountains in each location - but as another observed it's likely about synchronicity - relativity is a problem if you're looking to have two clocks on opposite sides of the planet in perfect sync.

varjag 11y ago

GPS reference clocks are nanosecond resolution. Synchronizing from Earth would be challenging due to varying transmission distances and relativistic effects.

chiph 11y ago

A ground-based transmitter could be destroyed in an attack, so having the clocks on orbit meant they were protected and would still be operational for a retaliatory ICBM launch. Edit: So far as latency - the on-board clocks get resynced today every now and again against a ground-based reference time as they pass overhead of a USAF transmitter (primary + backup). If they used only moderately-accurate clocks as a cost-saving measure (or were at the limit of late 1970's technology) drift would be more severe and the tail-end of each orbit would be less correct than at the start of the orbit. So additional ground stations would be required in parts of the planet not always so friendly to the US, and they would have to coordinate their time sources, allowing for the light-speed delay travelling around the earth. Much more expensive/harder solution from an operational aspect. Edit Edit: Don't forget that the GPS system was designed around military requirements. All the satellites (even today) have nuclear weapon detonation detectors on board and are a primary input to NORAD. > The 2nd SOPS has three missions: global navigation, time transfer and nuclear detection. http://www.schriever.af.mil/GPS/ http://www.schriever.af.mil/GPS/

madaxe_again 11y ago
Totally get it in the military context, and why it's useful in the context of a missile lobbing holiday, but was rather more curious as to why it'd be used as a time source for synchronising packets in commercial applications as the article describes. Based on what others have said and my own inkling I think it's about synchronisation at a fine timescale without having to trouble yourself with bothersome shit like relativity, and leaving that particular pain in the ass to the US military. That's what I meant by latency, but phone keyboard, walking, lazy!
- chiph 11y ago
  What it is is outsourcing your time source to the USAF. Aka introducing an external dependency. If these people are upset over 13 microseconds, wait until the DoD turn selective availability back on during a war or start adjusting satellite positions to avoid incoming ASAT missiles. Interesting fact: GPS time doesn't take leap seconds, etc. into account. As of last summer, it's ahead of UTC by 17 seconds. http://tycho.usno.navy.mil/leapsec.html http://tycho.usno.navy.mil/leapsec.html

elfchief 11y ago

The problem with atomic clocks in general is that they have very good short term stability, but not-so-great long-term stability. So they are very good at measuring the amount of time that has passed between two events, but not as great for telling you exactly what time an event happened. The other problem is that there's not really such thing as a single, true time. Precision timekeeping is one of those places where relativity matters -- differences in altitude and the geology of the earth under your feet at a particular point change how fast any particular clock 'ticks'. So to accommodate these differences, if you want events to be synchronized across multiple locations over a long period, you both need a single agreed-upon "current time", and some way to get information about that time information to the sites that need it. You could surely do this with directly connected wires that were very carefully measured, but that gets expensive very quickly. The GPS system already requires super-precise time just to function, so it makes for a great distribution mechanism of "what time is it" information, where the people running the GPS system are bothered with all the details of figuring out and standardizing a single time, and you 'just' have to listen. The GPS folks also keep track of things like how fast the GPS standard clock is drifting from the UTC standard clock (since no two clocks will agree over the long term, period). And this is actually where the problem happened -- Two of the parameters broadcast (A0 and A1) were incorrect, and it's these parameters that are used to specify the current time difference between the GPS master clock and the UTC master clock (the GPS master clock is disciplined to be as close to UTC as possible, but is always off by a minute amount, and that's what A0/A1 represent). Pretty much it boils down to, much as it is in distributed systems, "there is no now". Even with maximum care and the highest quality clocks, there's really not a definition of 'now' that's consistent across more than a single location. GPS is currently the closest thing we have to a globally-distributed definition of 'now'.

eru 11y ago
Google is also working on a globally distributed `now': http://research.google.com/archive/spanner.html http://research.google.com/archive/spanner.html Of course, they make use of GPS, but also add their own atomic clocks.
- elfchief 11y ago