cheap GPS

Jerry Scharf scharf at vix.com
Fri Aug 20 16:42:30 UTC 1999



On Fri, 20 Aug 1999, Alex P. Rudnev wrote:

> Through I did not see anything to worry about - your GPC may be show you 
> the wrong date, but why it can affect the accuracy at all (except some 
> short time around the very moment itself).
> 

This is not true. There is things called the catelog and ephemerus, which
are time based and give detailed corrections of the orbital position of
the satellites that GPS derives locaton from. If the receiver does not
handle rollover correctly, it will not correctly return time or position.
Most receivers built since the mid 90s have handles this, and even more of
the precision time sources have handled this, but nothing is perfect. It's
really easy for the manufacturer to test this, but almost impossible for a
user to (you need a GPS simulator.)

The good thing about modern NTP systems is that they don't accept times
that are way off (there was a bad incident of a wacko clock many years
ago) so if the GPS reports a 1980 date, the software would not believe it.
That would mean losing synch with the GPS, but that should not be the end
of time for a reasonalbly configured system.

I'll be watching all my clocks, but a lot of people won't be.

jerry





More information about the NANOG mailing list