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US DoD Poll On Leap Seconds

Posted by kdawson on from the great-leap-forward dept.

@10u8 writes "For time scales to leap, or not to leap, has been the question here before. The ITU-R will be considering leap seconds again in a few weeks. This week the USNO posted a survey about leap seconds by the US DoD. The issue has civil implications as well as technical ones, and there is a demonstrated way to respect the history, remove leaps from navigation and POSIX time, yet keep the sun overhead at noon."

4 of 314 comments (clear)

  1. Leap seconds fix a diferent problem by EmbeddedJanitor · · Score: 5, Informative
    Leap days correct our orbit around the sun to keep December/January in the middle of winter for the Northern Hemisphere.

    Leap seconds correct for the rotation of the earth to keep the sun above at noon.

    If we dispense with leap seconds then this relationship will slowly change and noon will eventually be dark.

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    1. Re:Leap seconds fix a diferent problem by Dannkape · · Score: 5, Informative

      According to wikipedia, there seems to have been 24 leap seconds in the last 36 years. For solar noon to move a single hour away would take over 5 millenia.

      Of course, they do give the news something harmless to report on every once in a while...

  2. Re:Automated and consistent leap seconds by techno-vampire · · Score: 5, Informative
    More to the point, it appears to average out, so we could be inserting them just to have to remove them a decade later.

    No they don't. If you'll look at the chart in the Wikipedia article, you'll see that since they started using them in 1972, they've never had to subtract a second. Either no change, or +1 second.

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  3. Re:Automated and consistent leap seconds by __aajfby9338 · · Score: 5, Informative

    Definitely your GPS. It cares about nanoseconds.

    But so long as all the satellites are in sync with their atomic clocks showing the same time, does it matter??? Even without them being in sync, doesn't the GPS use time and rough location to locate the satellites (unitil it's logged on) and then isn't it the round trip time taken by signal that's being measured? Is there any dependancy on leap seconds?

    GPS doesn't use UTC for its measurements; it uses its own system of GPS time for its measurements, and then calculates UTC using a correction value transmitted by the satellites in order to be able to display UTC (or any other UTC-derived time) for the user.

    Also, it doesn't "log in" in any usual sense, as the communication is purely one-way, from the satellite broadcasts to the receiver. Thus, it also doesn't measure round trip time, because there is no round trip. What it does is to receive the signals from multiple satellites, each of which essentially transmits a signal saying "I'm satellite number A, my location is B, and the time is C", and then solve a system of equations to figure out what time it was when it received the signals from each satellite, and thus how long each one-way trip took. Then it can do the geometry to figure out where it must be. The actual mechanism of accomplishing this is a whole lot more complicated, but on a very simple level, that's what's being done.

    The reason it takes at least four visible satellites to produce a 3D fix is because it needs to solve a system of at least four equations with four unknowns: X, Y and Z spatial coordinates, and time. More than four satellites are normally needed for good accuracy, since the each measurement is usually a lot more noisy and less precise than is desired. Additional measurements let the receiver do more math to try and filter out the noise.