GPS receivers and possible OHLE interference
"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