The Earth's not slowing down fast enough to suit Motorola

[Dave Farber <dave () farber net>]

Date: Tue, 07 Oct 2003 23:33:45 -0700
From: Paul Eggert <eggert () CS UCLA EDU>
Subject: The Earth's not slowing down fast enough to suit Motorola

Motorola reports that several GPS receivers in its Oncore line will
misdisplay the date on 28 Nov 2003 at midnight UTC.  For a one-second window
the receivers will mistakenly report the date as 29 Nov instead of 28 Nov.

Here's why.  Every couple of years or so for the past three decades, the
International Earth Rotation Service has announced a leap-second because the
Earth is rotating slightly more slowly than an 86400-second day would
suggest.  But since 1 Jan 1999, we've had an unusually long dry spell
without any leap seconds.  The GPS week number in the UTC correction
parameter is 8 bits long, which allows for 256 weeks of unambiguous time
calculation.  Until now this parameter has never rolled over, but because of
the dry spell 28 Nov will be exactly 256 weeks after the most recent leap
second, and the rollover will contribute to the bug.
  <http://www.motorola.com/ies/GPS/docs_pdf/notification_oncore.pdf>

Steve Allen writes in <http://www.ucolick.org/~sla/leapsecs/onlinebib.html>
that some JDAM smart bombs and other munitions are rumored to contain these
receivers.  Anyone intending to use those weapons around the magic window
might want to reschedule their bombing runs for some other time. ...

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