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uk.rec.gps (UK Sat Nav) (uk.rec.gps) for the discussion of all aspects of the UK use of Global Positioning Systems and any other satellite positioning/navigation systems which may be developed. Also any improvements, or extensions to the above and radio navigation systems.

GPS receivers and possible OHLE interference



 
 
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  #1  
Old September 8th 16, 10:30 PM posted to uk.railway,uk.rec.gps
NY
external usenet poster
 
Posts: 10
Default GPS receivers and possible OHLE interference

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 https://s15.postimg.org/d2qin2m3v/GPS.png - 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

  #2  
Old September 8th 16, 11:24 PM posted to uk.railway,uk.rec.gps
BevanPrice
external usenet poster
 
Posts: 1
Default GPS receivers and possible OHLE interference

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 https://s15.postimg.org/d2qin2m3v/GPS.png - 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.)



  #3  
Old September 8th 16, 11:54 PM posted to uk.railway,uk.rec.gps
Recliner
external usenet poster
 
Posts: 2
Default GPS receivers and possible OHLE interference

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 https://s15.postimg.org/d2qin2m3v/GPS.png - 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.

  #4  
Old September 9th 16, 04:09 AM posted to uk.railway,uk.rec.gps
Charles Ellson
external usenet poster
 
Posts: 1
Default GPS receivers and possible OHLE interference

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

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 https://s15.postimg.org/d2qin2m3v/GPS.png - 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.
  #5  
Old September 9th 16, 07:28 AM posted to uk.railway,uk.rec.gps
Jeremy Double
external usenet poster
 
Posts: 1
Default GPS receivers and possible OHLE interference

Å´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 https://s15.postimg.org/d2qin2m3v/GPS.png - 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.)


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.

--
Jeremy Double
  #6  
Old September 9th 16, 08:46 AM posted to uk.railway,uk.rec.gps
NY
external usenet poster
 
Posts: 10
Default 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
fix.

  #7  
Old September 9th 16, 09:37 AM posted to uk.railway,uk.rec.gps
Recliner
external usenet poster
 
Posts: 2
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.

  #8  
Old September 9th 16, 10:44 AM posted to uk.railway,uk.rec.gps
NY
external usenet poster
 
Posts: 10
Default GPS receivers and possible OHLE interference

"Recliner" wrote in message
...
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.


Good point. Maybe I get better results by getting a lock-on before getting
on the train because of the ability to read the "almanac" of currently
available satellites and their locations while there's still good reception.
I've got a little app called GPS Status which can download A-GPS info over
the internet which speeds up time to first lock considerably. Because of the
power consumption of leaving my phone's GPS receiver turned on all the time,
I tend to turn it on only when I need it, so it always needs to get an
up-to-date almanac. If I left it going all the time I'd probably get better
results, but my battery would only last a couple of hours. Making it last
from 8 AM to 6 PM (ie times when I can't be tethered to the charger), even
with GPS, wifi and mobile data normally turned off, is a problem out here in
the country because the phone uses so much power winding up the gain on the
mobile phone receiver to listen for incoming calls - oh to be in an area
that has good mobile reception.


GPS comes into the category of PFM (pure f***ing magic): the maths involved
in calculating your position based on time delays from satellites which
themselves are moving round the earth (*) makes my brain hurt! The fact that
it works at all, even with little quirks like going off course when you get
reflections or can only see a few satellites, is a miracle.


(*) For a long time, I thought that GPS satellites were geostationary, like
Sky satellites. Then I found out that they're not...

  #9  
Old September 9th 16, 12:04 PM posted to uk.railway,uk.rec.gps
Peter Crosland
external usenet poster
 
Posts: 17
Default GPS receivers and possible OHLE interference

On 09/09/2016 10:44, NY wrote:
"Recliner" wrote in message
...

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.


Good point. Maybe I get better results by getting a lock-on before
getting on the train because of the ability to read the "almanac" of
currently available satellites and their locations while there's still
good reception. I've got a little app called GPS Status which can
download A-GPS info over the internet which speeds up time to first lock
considerably. Because of the power consumption of leaving my phone's GPS
receiver turned on all the time, I tend to turn it on only when I need
it, so it always needs to get an up-to-date almanac. If I left it going
all the time I'd probably get better results, but my battery would only
last a couple of hours. Making it last from 8 AM to 6 PM (ie times when
I can't be tethered to the charger), even with GPS, wifi and mobile data
normally turned off, is a problem out here in the country because the
phone uses so much power winding up the gain on the mobile phone
receiver to listen for incoming calls - oh to be in an area that has
good mobile reception.


GPS comes into the category of PFM (pure f***ing magic): the maths
involved in calculating your position based on time delays from
satellites which themselves are moving round the earth (*) makes my
brain hurt! The fact that it works at all, even with little quirks like
going off course when you get reflections or can only see a few
satellites, is a miracle.


(*) For a long time, I thought that GPS satellites were geostationary,
like Sky satellites. Then I found out that they're not...


It also depends a lot on the software and hardware for the particular
GPS. The better ones have accelerometers built in that keep track during
signal interruptions. Others also use them to assume that you will
continue on the road you are on. I have never seen such weird results as
shown in the map you listed.
--
Peter Crosland

Reply address is valid
  #10  
Old September 9th 16, 12:13 PM posted to uk.railway,uk.rec.gps
Sam Wilson
external usenet poster
 
Posts: 1
Default GPS receivers and possible OHLE interference

In article ,
Peter Crosland wrote:

... The better [GPSs] have accelerometers built in that keep track during
signal interruptions. Others also use them to assume that you will
continue on the road you are on. I have never seen such weird results as
shown in the map you listed.


Have you ever seen a GPS receiver with the "I must be on a road" setting
trying to work out where it is when it's on a train? I've only seen it
once a long time ago but it was quite entertaining.

Sam

--
The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.
 




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