Subject: [IGSMAIL-4679]: upcoming UTC anomaly
From: "Jim Ray" <[email-masked]>
To: <[email-masked]>
Cc: <[email-masked]>
Date: Tue, 4 Nov 2003 09:20:41 +0100

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IGS Electronic Mail      04 Nov 00:20:13 PST 2003      Message Number 4679
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Author: Jim Ray

FYI, here is my synthesis of reports from a variety of sources
floating around the Internet.

At the end of 27 Nov 2003, exactly 256 weeks will have elapsed since
the last UTC leap second, at the end of 31 Dec 1998.  Because the GPS
week number counter in the UTC subframe data message is only 8 bits
(maximum value of 256), it will reset.  Receivers that do not account
for the truncated GPS week values when determining UTC from GPS time
may report an invalid UTC date.

According to Motorola, their VP Oncore, UT Oncore, GT Oncore, and M12
Oncore receivers (which are widely used for timing applications) will
report the day as 29 Nov instead of 28 Nov for the first second after
the end of 27 Nov 2003.  Afterwards, the correct date will be reported.
See the Motorola announcement at:
  http://www.motorola.com/ies/GPS/docs_pdf/notification_oncore.pdf

It is not clear yet which other receivers might be affected.  JPO is
in the process of organizing tests.  My impression, though, is that
the core functions of geodetic receivers are unlikely to notice this
anomaly since they operate in GPS time, which will be unaffected.
Users who rely on the UTC output from any receiver should take heed,
however.

It is highly ironic that this event occurs in the midst of discussions
to change UTC on account of overly frequent leap seconds.