[time-nuts] Timing performance of servers

Dennis Ferguson dennis.c.ferguson at gmail.com
Fri Oct 26 13:53:02 EDT 2012


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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