diff options
author | Eric S. Raymond <esr@thyrsus.com> | 2013-09-27 16:24:34 -0400 |
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committer | Eric S. Raymond <esr@thyrsus.com> | 2013-09-27 16:24:34 -0400 |
commit | 0fde30987d1893fcca4e1834c3a0694627757b47 (patch) | |
tree | f1cfb489f0fdb51d8abd6b142a7bd09f06ddd5c3 /timebase.c | |
parent | f7b91c2037fa7b31c8929ff8e4815987a27d121a (diff) | |
download | gpsd-0fde30987d1893fcca4e1834c3a0694627757b47.tar.gz |
Caveat about PPS accuracy.
Diffstat (limited to 'timebase.c')
-rw-r--r-- | timebase.c | 16 |
1 files changed, 9 insertions, 7 deletions
@@ -73,13 +73,15 @@ we don't even know what century it is! Therefore, we must assume the system clock is reliable to within a second. -However, none of these caveats affect the usefulness of PPS, which -tells us top of second to 50ns accuracy and can be made to condition a -local NTP instance that does *not* rely on the system clock. The -combination of PPS with NTP time should be reliable regardless of -what the local system clock gets up to. That is, unless NTP clock -skew goes over 1 second, but this is unlikely to ever happen - and -if it does the reasons will have nothing to do with GPS idiosyncracies. +However, none of these caveats affect the usefulness of PPS, which +tells us top of second to theoretical 50ns accuracy (actually about 1 +microsecond over RS232 and roughly one poll interval over USB) and can +be made to condition a local NTP instance that does *not* rely on the +system clock. The combination of PPS with NTP time should be reliable +regardless of what the local system clock gets up to. That is, unless +NTP clock skew goes over 1 second, but this is unlikely to ever happen +- and if it does the reasons will have nothing to do with GPS +idiosyncracies. This file is Copyright (c) 2010 by the GPSD project BSD terms apply: see the file COPYING in the distribution root for details. |