[time-nuts] GPS shielding by power lines?

David Ackrill dave.g0dja at tiscali.co.uk
Sun Aug 10 03:30:11 EDT 2008


Alan Melia wrote:
> Hi all,  in the process of setting up a GPS time standard for a Radio
> Astronomy facility (amateur) we installed a GPS receiver in a small cabin
> with a translucent roof, thinking that would not impede the GPS signal.
> After a lot of head scratching as to why we were not getting the performane
> we got at another site, we realised that the "convenient position" for the
> cabin was directly below a three phase 11kV power distribution line ( common
> UK rural electricity distribution system). 

Since the satellite signals are in the high UHF range, arround 1575MHz, 
the AM receiver test is not going to tell you alot about the noise at 
the frequency that the GPS receiver is using.  So, even if you had 
detected any noise from the overhead lines, it might not have been proof 
that this was the cause of the problems.

I find that the positioning of the antenna for a GPS receiver can be 
very touchy.  You really need a good view to the horizon and, despite 
what you might see on simple presentations of the satellite positions, 
they do tend to be mainly in the southern sky when viewed from the UK.

See 
http://www.guralp.com/articles/20060405-howto-gps-troubleshooting/print 
for details.

The other effect that you may notice is that your 'good' position for 
the antenna isn't so good all of the time as the satellites appear to 
move round the sky and the signal strengths from each alters.

Dave (G0DJA)



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