Date: Tue, 07 Oct 2003 23:33:45 -0700 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. ...

for all you NTP/GPS junkies out there... :) --bill