diff options
| -rw-r--r-- | share/man/man4/udcf.4 | 18 |
1 files changed, 17 insertions, 1 deletions
diff --git a/share/man/man4/udcf.4 b/share/man/man4/udcf.4 index 8ff7d320a6f..7bbd7dba685 100644 --- a/share/man/man4/udcf.4 +++ b/share/man/man4/udcf.4 @@ -1,4 +1,4 @@ -.\" $OpenBSD: udcf.4,v 1.13 2006/10/27 11:29:06 mbalmer Exp $ +.\" $OpenBSD: udcf.4,v 1.14 2006/12/08 21:19:34 deraadt Exp $ .\" .\" Copyright (c) 2006 Marc Balmer <mbalmer@openbsd.org> .\" @@ -69,6 +69,22 @@ to see if it is still functional. The timedelta will eventually degrade to a lie as all computer internal clocks have a drift. .El +.Sh NOTES +DCF uses a 77KHz long wave radio signal transmitted from near Frankfurt, Germany. +Up to about 900km, the radio signal can travel directly to the receiver, +providing a linearly increasing time offset based on distance. +Due to the curvature of the Earth, beyond this distance the signal must +bounce off the lower ionosphere (residing at approximately 70km elevation +during the day, and 90km at night), thus causing a non-linearly increasing +time offset which can only be roughly calculated using trigonometry. +Since the distance and transmission geometry is not known, +the clock receivers and +.Nm +driver currently make no effort to calculate this offset. +We simply assume that the offset is small. +.Pp +In Germany, the train system uses DCF clocks. +As the distance from Frankfurt increases, trains can be expected to run late. .Sh SEE ALSO .Xr intro 4 , .Xr uhub 4 , |