explain why german trains run late; help from jmc

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 ,