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Old September 9th 16, 08:37 AM posted to uk.railway,uk.rec.gps
Recliner
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Default GPS receivers and possible OHLE interference

NY wrote:
"Jeremy Double" wrote in message
...
If you look at the map that the o.p. provided, there are some tall
buildings in that area, which might be either shielding or reflecting the
signal from one or other GPS satellite. That would explain why the track
goes astray before entering the tunnel.


Ah, so you think it's just the tall buildings causing reflections? You may
be right. My experience with GPS problems is that you either fail to get a
lock at all (too few usable satellites in range) or else you get "random
walk" tracks which show a lot of lateral jitter about the track that you are
supposed to be taking, or else there is a horizontal offset - the line
follows the route that you are taking but is displaced sideways from it.

I've never seen a case where the recorded track actually turns round and
heads in the opposite direction, which is why I suspected some interference
from the OHLE.


I'm very familiar with the problem of getting GPS reception inside a train
with heat-reflecting windows. I've found that the best thing is to start
recording before you get on the train, while there is still good reception
and plenty of visible satellites, and then the track will continue (maybe
with a bit more jitter) when you get on the train. If I leave it until I'm
on the train before turning on my phone's GPS receiver, it never gets a GPS
fix.


Even if you have a fix with six it more satellites before getting on the
train, you'll periodically lose it during the journey when the train is in
tunnels, deep cuttings and covered stations. The device will then have to
attempt to get a new fix, which will fail if the windows are shielded or
you don't hold it near a window.

Conversely, in something like the Mk 1 carriage I was travelling in
yesterday, even in an aisle seat you get a good fix. Perhaps surprisingly,
you also get a good fix in Shinkansen trains, on the rare occasions they're
not in tunnels.