NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: What time is it?
From: UNK
Date: 2004 Nov 11, 09:57 -0500
From: UNK
Date: 2004 Nov 11, 09:57 -0500
On Wednesday, November 10, 2004 9:16 AM Trevor J. Kenchington asked: > Could you add one further detail: How does "GPS time" relate to UTC > and/or the various grades of UT? GPS time is the number of seconds since 06-Jan-1980 00:00:00.0 UTC, and is steered to an accuracy of better than 1 microsecond. It is encoded in the GPS NAV message as the number of weeks since 06-Jan-1980 and the number of seconds since the start of the week. As such, it does not include leap seconds, so a naive reading of GPS time would be fast of UTC today (13 or 14 seconds, I believe, for the number of leap seconds since 06-Jan-1980). However, elsewhere in the GPS messages it gives the correction, so receivers can display UTC or UTC-derived zone time. Interestingly, on 22-Aug-1999, the 10-bit week number field wrapped around from 1023 to 0, causing some early GPS receivers to lose track of the date. The US Naval Observatory's GPS page at http://tycho.usno.navy.mil/gps.html has some info. -- Peter
