[time-nuts] GPS shielding by power lines?

[Magnus Danielson]

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). We extended the cable and moved
> the antenna about 20 - 30 feet to the side of the line run, which was
> mounted on wooden poles at about 25 feet. In this position we immediately
> got a reliable fix. The fix and number of usable satellites degrades as we
> move nearer the lines.
> 
> The thought was that there as interference arcing or corona noise from the
> line insulators, and a receiver (AM) was deployed to listen for what was
> expected to be a substantial wide band noise signal....we didnt hear one! We
> are now confused about what the effect is. The signal could not be
> "screened" by the wires which are about 3 feet apart, but they definite
> provide a cone of interference directly under the run. The experiment was
> later repeated with two further different GPS receivers and produced the
> same result.
> 
> Has anyone seen this before? have you any idea of what level noise we should
> be looking for? I believe this is a wide signal so maybe an AM receiver is
> not the best choice The area is a rural, horticultural area (called market
> garden in the UK) We are obviouslt concerned to trace any noise sources in
> the vicinity of the Hydrogen line frequency at 1420MHz.

It can also be a very simple case of strong multipath causing 
cancelation reflections strong enought to obscure the signal. Be sure to 
plot signal strength and if possible delta distance from solution for 
each satellite along with elevation and azimuth. See if a clear pattern 
emerges such that certain angles is problematic.

Just to make you look in another direction.

Cheers,
Magnus