Mike Jacoubowsky

Nobody told me about the issues with GPS-enabled phones in China
displaying a pretty hefty offset to your location. Couldn't figure out
why I was getting so confused in Shanghai... the phone said I should be
"here" but it clearly wasn't the case. Once I understood that I might be
as much as a kilometer from the displayed position I could figure things
out from the map.

Never seen this anywhere else before, and a quick google turns up a
zillion people agreeing that this happens, but no clear consensus for
why. Guess this is one of those "TIC" things a customer who spends time
in China told me about. When you can't figure out something that should
be simple, the answer is always "TIC." This is China. :-)

--Mike Jacoubowsky
Chain Reaction Bicycles

Alan Browne

On 2011-11-21 19:32 , Mike Jacoubowsky wrote:
> Nobody told me about the issues with GPS-enabled phones in China
> displaying a pretty hefty offset to your location. Couldn't figure out
> why I was getting so confused in Shanghai... the phone said I should be
> "here" but it clearly wasn't the case. Once I understood that I might be
> as much as a kilometer from the displayed position I could figure things
> out from the map.
>
> Never seen this anywhere else before, and a quick google turns up a
> zillion people agreeing that this happens, but no clear consensus for
> why. Guess this is one of those "TIC" things a customer who spends time
> in China told me about. When you can't figure out something that should
> be simple, the answer is always "TIC." This is China. :-)

Between possible poor i) registration of the map data and ii) poor
location data on the cell towers that provide 'aiding' to the iPhone's
GPS and iii) the urban canyon's of Shanghai, it's not all that surprising.

"TIC" may introduce other gremlins of greater influence as well.

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nospam

In article <F96dnem-qJo1cFfTnZ2dnUVZ_sednZ2d@earthlink.com>, Mike
Jacoubowsky <MikeJ@ChainReaction.com> wrote:

> Nobody told me about the issues with GPS-enabled phones in China
> displaying a pretty hefty offset to your location. Couldn't figure out
> why I was getting so confused in Shanghai... the phone said I should be
> "here" but it clearly wasn't the case. Once I understood that I might be
> as much as a kilometer from the displayed position I could figure things
> out from the map.
>
> Never seen this anywhere else before, and a quick google turns up a
> zillion people agreeing that this happens, but no clear consensus for
> why. Guess this is one of those "TIC" things a customer who spends time
> in China told me about. When you can't figure out something that should
> be simple, the answer is always "TIC." This is China. :-)

china intentionally blurs the gps data unless you are approved for
accurate data.

Wes Groleau

On 11-21-2011 22:43, nospam wrote:
> china intentionally blurs the gps data unless you are approved for
> accurate data.

I'm no GPS expert, but AFAIK, "blurring" can only be done
by the controller of the GPS satellites.

--
Wes Groleau

Be spontaneous … today
http://Ideas.Lang-Learn.us/BlindDog?itemid=3984

JF Mezei

Quite possible that mobile tower location data is inaccurate.

While China could jam GPS frequencies on its territory, they couldn't
introduce an offset.

Urban canyons won't give you an error of 1km and it would be random in
nature as you move through the downtown core.

Since the iPhone doesn't tell you how it had gotten your current
location (wi-fi, mobile tower or real GPS), you can't know for sure. If
you had a real GPS with you, you could then compare a GPS-only source of
position with that of the iPhone.

Wes Groleau

On 11-21-2011 23:33, JF Mezei wrote:
> Quite possible that mobile tower location data is inaccurate.

And very likely that the WiFi DB in China is
not as well-developed as that in USA and Europe.

--
Wes Groleau

Be spontaneous … today
http://Ideas.Lang-Learn.us/BlindDog?itemid=3984

Chris Blunt

On Mon, 21 Nov 2011 16:32:33 -0800, Mike Jacoubowsky
<MikeJ@ChainReaction.com> wrote:

>Nobody told me about the issues with GPS-enabled phones in China
>displaying a pretty hefty offset to your location. Couldn't figure out
>why I was getting so confused in Shanghai... the phone said I should be
>"here" but it clearly wasn't the case. Once I understood that I might be
>as much as a kilometer from the displayed position I could figure things
>out from the map.
>
>Never seen this anywhere else before, and a quick google turns up a
>zillion people agreeing that this happens, but no clear consensus for
>why. Guess this is one of those "TIC" things a customer who spends time
>in China told me about. When you can't figure out something that should
>be simple, the answer is always "TIC." This is China. :-)

I never heard of this phenomenon before, but reading this reminds me
that I noticed exactly that happening when I was in Tibet last year. I
was using my GPS to navigate around Lhasa but the streets I was on
never seemed to match up to the location on the GPS map. When I
analysed it more carefully it seems the map was offset from the actual
location by a few hundred metres. This was using Google Maps on a
Nokia phone.

I did use the same phone in Beijing but didn't have a problem there.
The actual location always appeared to be correctly shown.

I assumed at the time that is was simply due to Google not aligning
the maps for Tibet correctly, but perhaps there was more to it than
that and someone was deliberately introducing the error into the GPS
signal somehow.

Chris

JF Mezei

Chris Blunt wrote:

> never seemed to match up to the location on the GPS map. When I
> analysed it more carefully it seems the map was offset from the actual
> location by a few hundred metres. This was using Google Maps on a
> Nokia phone.


With the satellite display on a GPS you would see if you were getting
sufficient satellites to have accurate location. With himalayas hiding a
good portion of the south, and generally not many satellites in the
north, it is possible that you have bad geometry of SVs and thus
inaccurate location.

(I am not sure if the okia has a real GPS or some combo of
GPS/wi-fi/mobile tower triangulation.

The further apart the satellites are, the more accurate your fix will be.

Note that the USA does have the ability to enable "selective
availability" over a region of the globe (when a satellite passes over a
region, it activates SA and deactivates it as it leaves that region). So
it is possible that the original induced errors were activated for that
part of the globe. However, lack of line of sight to satellites beyond
the himalayas to the south is more likely the reason.

Chris Blunt

On Tue, 22 Nov 2011 00:37:01 -0500, JF Mezei
<jfmezei.spamnot@vaxination.ca> wrote:

>Chris Blunt wrote:
>
>> never seemed to match up to the location on the GPS map. When I
>> analysed it more carefully it seems the map was offset from the actual
>> location by a few hundred metres. This was using Google Maps on a
>> Nokia phone.
>
>
>With the satellite display on a GPS you would see if you were getting
>sufficient satellites to have accurate location. With himalayas hiding a
>good portion of the south, and generally not many satellites in the
>north, it is possible that you have bad geometry of SVs and thus
>inaccurate location.
>
>(I am not sure if the okia has a real GPS or some combo of
>GPS/wi-fi/mobile tower triangulation.
>
>The further apart the satellites are, the more accurate your fix will be.
>
>Note that the USA does have the ability to enable "selective
>availability" over a region of the globe (when a satellite passes over a
>region, it activates SA and deactivates it as it leaves that region). So
>it is possible that the original induced errors were activated for that
>part of the globe. However, lack of line of sight to satellites beyond
>the himalayas to the south is more likely the reason.

I do remember checking at the time that the GPS was receiving signals
from 4 or 5 satellites, so I know I was wasn't relying simply on
mobile tower triangulation. I don't think the problem occurred on
trips in other parts of Tibet away from the city of Lhasa itself
either.

Quite possibly it was due to poor satellite geometry due to the
proximity of nearby mountainous terrain.

Chris

Alan Browne

On 2011-11-21 23:13 , Wes Groleau wrote:
> On 11-21-2011 22:43, nospam wrote:
>> china intentionally blurs the gps data unless you are approved for
>> accurate data.
>
> I'm no GPS expert, but AFAIK, "blurring" can only be done
> by the controller of the GPS satellites.

He may mean the aiding data from the cell towers. You are right that
the GPS signal itself can only be 'blurred' by the operator.



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