[time-nuts] Timing performance of servers

Bob Camp lists at rtty.us
Fri Oct 26 14:09:52 EDT 2012


Hi

If we are talking about timing performance of normal servers, multipath
really isn't much of an issue. 

Unless you are running an OS with real time code (as in *not* Windows and
*not* standard Linux) the timing stability isn't good enough to be bothered
by the level of distortion that multipath injects.

Yes, I have a fleet of Soekris NTP's that get around all that stuff. I would
not call them servers. Yes, on an unloaded box you get pretty good numbers
almost regardless of what you run. A server doing no work also isn't
something I would call normal.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Dennis Ferguson
Sent: Friday, October 26, 2012 1:53 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Timing performance of servers


On 26 Oct, 2012, at 08:06 , shalimr9 at gmail.com wrote:

> If you cannot see the horizon because of obstructions (what else?), these
obstructions are likely to be a source of multipath. So while technically
you do not need to see the horizon, any obstruction above the horizon could
cause problems. Of course, distant trees or a hill are less likely to be a
problem than your neighbor's garden shed with a tin roof.

Though, as I understand it, typical low-end GPS antennas are quite
sensitive to multipath arriving from below the horizon as well.
I think getting a sharp antenna cutoff at the horizon is the reason
that high-end antennas have choke rings.

> Also, some antennas are better at rejecting low angle signals than others.
While the software can reject some undesired signals, it can only do so if
the software can identify them as separate. If the multipath signal
destructively interferes with the desired signal, there is not much the
software can do.

Given that the transmitted C/A bandwidth is greater than 1 MHz, however,
I'm not sure that it is possible for multipath signals to destructively
interfere across the entire bandwidth; I think the issue is distortion,
with some frequencies in the bandwidth suffering destructive interference
while others are constructively interfered with.  This can be compensated
for in software, though it is much better not to have to.

Dennis Ferguson
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