Everyone has heard of Leap Year, where we have an extra day in the calendar to keep time in sync. Did you know there are also leap seconds that are introduced at half year intervals? Leap seconds are introduced to correct time according to the earth's rotation, which is not exact from year to year, and has been on an ever so slightly slowing trend for a while.
Why is this important? GPS actually uses a very accurate time measurement that is embedded in the messages from the satelites to the receivers we use. This allows for many different things to happen, one of them being that you get a more accurate position because of the accurate time measurement from each satelite to the rover, and how long the message took to get from one to the other, therefore measuring distance.
The reason I bring this up now, is that the end of this month (December 2008) there will be a leap second added to the clock. In the past this has caused some difficulties with certain GPS units, as the time measurement didn't match what it was supposed to, and they required an update to correct the problem. I am not aware of any potential problems with GPS from this, as we just added a leap second in 2005 and that was when a few of these problems surfaced. Prior to 2005, there hadn't been a second added since 1998.
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