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Old September 9th 16, 03:09 AM posted to uk.railway,uk.rec.gps
Charles Ellson
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Posts: 1
Default GPS receivers and possible OHLE interference

On Thu, 8 Sep 2016 22:54:00 -0000 (UTC), Recliner

BevanPrice wrote:
On 08/09/2016 22:30, NY wrote:
We've just got a dashboard camera for the car, and it also has a GPS
receiver. I was looking at the tracks of my wife's journey to work. At
one point she passes under a long bridge beneath many OHLE-electrified
(*) railway tracks close to a station. The GPS track seems to go
haywire, showing her going roughly opposite to the real direction, and
this begins just *before* she goes under the tracks and therefore while
she still theoretically has GPS reception.

Intriguingly, the "dilution of precision" figures (a measure of how much
error there may be in each reading) does not get worse when she
approaches the bridge and the tracks, even though as she starts to turn
north, the GPS track shows her turning south.

Does the radiation from OHLE lines (presumably 50 Hz with lots of
harmonics and noise due to arcing) corrupt GPS signals in such a way as
to still show a reliable GPS signal (low DOP) but with lat-long
coordinates varying in the wrong direction? I'd have expected (if
anything) a loss of signal (no GPS fix) or else points that were
distributed at random causing a very wiggly line.

This is the track - the green
arrows show what was actually recorded and I've drawn a red line that
shows where she actually went.

(*) 25 kV AC overhead electrification, for non-railway people in uk.rec.gps

More likely to be reduction of signal when passing under the bridge. You
get the same effect with hand-held GPS units in trains when they are in
cuttings (especially tree-surrounded), or inside tunnels. Except in some
types of train (e.g. Voyagers), these GPS units generally perform O.K.
on either electrified or non-electrified lines.

(Voyagers and some other trains have window coatings that largely block
the GPS signal.)

Yes, for example you get a good GPS signal in older TGVs (including 373s),
but not in new ones, which must have coated windows.

As this involves a dashboard camera the GPS receiver will already be
partly screened from the full range of available satellites by the car
body. There should be a noticeable difference between getting a fix of
position after switch-on inside the car and doing the same standing in
the open, better demonstrated if it has the mode (usually involving a
hidden menu) available showing satellite positions.