Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why each GPS satellite holds four atomic clocks?

  1. Oct 9, 2008 #1
    Each of the 24 GPS satellites holds four atomic clocks! Why four and not one or two? They are not cheap!!!
    Thanks
     
  2. jcsd
  3. Oct 9, 2008 #2

    russ_watters

    User Avatar

    Staff: Mentor

    Redundancy.

    And compared to the cost of launching them into space, they probably are pretty cheap.
     
  4. Oct 9, 2008 #3
    Ok, redundancy, but ... why 4 and not 5 or 3 or ... XY?

    Thanks.
     
  5. Oct 9, 2008 #4

    Danger

    User Avatar
    Gold Member

    Perhaps for the same reason as the redundancy of space shuttle computers; they essentially 'vote' upon decisions. Three might not produce enough of a majority to be fail-safe, and five is overkill.
     
  6. Oct 9, 2008 #5

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    The block 1 satelites used rubidium clocks and had three of them as the normal backup - 3 can vote and still have a backup if one fails.
    Prototype ceasium clocks where developed and one clock was added alongside the existing 3 ( so the rubidium ones could be used if it didn't work).
    In later model satelites 2 rubidium and 2 ceasium were used, so each had a backup.
    Then when ceasium proved best they were switched to all ceasium, but since the design hardware and software was for four clocks it was as easy and safer to fit four of them.

    (Not sure of exactly what changes where when and on what block numbers)
     
  7. Oct 9, 2008 #6

    Danger

    User Avatar
    Gold Member

    Thanks for a great explanation, Mgb. :approve:
     
  8. Oct 10, 2008 #7

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2015 Award

    It's also worth pointing out that "not cheap" is relative statement. Nothing in orbit is ever cheap. So the differential cost isn't always driven by the cost of the instrument.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Why each GPS satellite holds four atomic clocks?
  1. Atomic clock ? (Replies: 6)

Loading...