On Wed, 24 May 2017, Pete Biggs wrote: > The GPS time system is also notoriously very precisely wrong. The time > was set when the first satellite was sent up and has never been > corrected since - so hasn't taken account of leap seconds or > relativistic effects. All that matters for GPS is that the time on each > satellite transmission is identical, and to that end you can get a > precision of about 3ns (which is what you need to get metre GPS > accuracy) and which you then have to correct for all the various > artefacts since inception. Lower cost GPS receivers get about 50ns > accuracy, which is probably still OK for a system clock. The good thing > is that the corrections necessary are well known and updated > frequently. http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html https://www.aapt.org/doorway/TGRU/articles/Ashbyarticle.pdf http://www.timetoolsglobal.com/information/gps-ntp-server/ I'll accept that it doesn't include leap seconds, but it does provide the offset from GPS time to UTC, so that's not an issue. I understand the exact opposite as far as relativistic effects, with countering them being necessary for it to work as a positioning system. jh