From jray at bipm.org Tue Nov 4 00:20:41 2003 From: jray at bipm.org (Jim Ray) Date: Tue, 4 Nov 2003 09:20:41 +0100 Subject: [IGSMAIL-4679]: upcoming UTC anomaly Message-ID: <004801c3a2ac$89460d50$e57f68c1@TAI20> ****************************************************************************** IGS Electronic Mail 04 Nov 00:20:13 PST 2003 Message Number 4679 ****************************************************************************** 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.