Re: iPhone location

In article <9pne25du3fadcejvf8852l1cdsf15fg4tg@xxxxxxx>, BGN
<nickmooney@xxxxxxxxxxx> scribeth thus
On Wed, 3 Jun 2009 22:43:34 +0100, tony sayer <tony@xxxxxxxxxxxx>
wrote:

In article <spld25ttmubrraoou0vnpm1l4qcf1sk484@xxxxxxx>, BGN
<nickmooney@xxxxxxxxxxx> scribeth thus
On Tue, 2 Jun 2009 21:33:21 +0100, tony sayer <tony@xxxxxxxxxxxx>
wrote:

In article <210b255rkr55o1i39mun8tumhbuu5jod44@xxxxxxx>, BGN
<nickmooney@xxxxxxxxxxx> scribeth thus
On Tue, 02 Jun 2009 12:05:41 +0200, Fedi El Arbi <arbifedi@xxxxxx>
wrote:

Hi everybody,

in the context of my thesis, I am writing a location based software for
the iPhone. I have in the theoretical part of my thesis to explain the
positioning methods used by the iPhone.

According to the documentation of the CoreLocation Framework, the iPhone
uses GPS, WPS (WLAN positioning with Skyhook technology), CellID and
cell triangulation as location methods.

Does anyone know, what is meant with cell triangulation in this case?
How does cell triangulation work? Would the position be determinated by
the iPhone or by the network operator?

You need to look up aGPS.

<http://en.wikipedia.org/wiki/AGPS>

When using the network the phone finds out where it is based on the
mobile phone cell/mast. You'll have to hunt around for this - I don't
know if the mast says "this is my location" or if the mast says "I'm
mast number 12345" and then it checks with some database somewhere to
see where mast number 12345 is.

Aha, this appears to clarify it:

<http://en.wikipedia.org/wiki/Google_Maps#Google_Maps_for_Mobile>

"The "my location" feature works by utilizing the GPS location of the
mobile device, if it is available. This information is supplemented by
the software determining the nearest cell site. The software then
looks up the location of the cell site using a database of known cell
sites. The software plots the streets in blue that are available with
a yellow icon and a green circle around the estimated range of the
cell site based on the transmitter's rated power (among other
variables). The estimate is refined using the strength of the cell
phone signal to estimate how close to the cell site the mobile device
is."


I rather doubt that the actual signal strength as that is a -very-
variable variable;!.

That's pasted from the Wiki article, I suggest you alter the article
if you have facts and sources for them.

Well it would work to an extent but its very crude and will give
serious dilution of navigational precision as they say;..

But it's only going to be used for an initial idea of where the device
is and in low GPS signal conditions, so is better than nothing.

Well as you say better than nothing..

Tho if you were out in the country it could be out by several
kilometres.!..



What it will be looking for is timing, the times
the signal gets to the base stations by such means you can work it out.

So you think a mobile device is more likely to use some kind of ping
reply instead of signal strength?

Effectively yes this is how GPS works you time the signals or rather the
differences between them and knowing the times and therefore the
distances- you can compute the position relative to the base stations
which are at known points and do it that way.

How would a mobile phone know the distance from the mast if it had
only one cell tower to speak to?

Thats it .. it needs three to triangulate. You cant do it with one or
two three is needed ..


The mobile phone network is not configured with the same accuracy of
the GPS system as it's not designed for it.

No it never was designed for navigation, just that someone saw an
application there and made it workable:).

Course a gps receiver working off the satellites is just using the
mobile to display the positional info its receiving from the sats or in
some instance will send that info back to the office for tracking..



But in order to do it how you ought to do you need three base stations
in range to triangulate the position.

I understand how GPS works, and three+ base stations is NOT how it
works.

I think you might be at cross purposes GPS needs three or more
"satellites" to work with not base stations..

All aGPS needs is a reference point ('you are somewhere around
mast 12345') and then a few GPS signals. The 'you are around here'
from the mast is by far the most helpful cold signal an aGPS unit can
have to quickly work out where it is.

Well if your using the output of a GSM base station then you can be
anywhere within the area that base station is receivable over.. Course
that can be literally hundreds of metres in say Central London but miles
out in the country..

I think you need to be careful with phrases like "a few GPS signals" as
they are satellite originated;)..


The time the signals take is a
constant unless there is serious multipath in an urban "clutter"
environment to affect that, but the timing of the signals is far better
than relying on the signal strength which can vary whether or not your
indoors or outdoors or upper floor, lower floor, building composition
and surrounds etc etc......

It would be nice if that's how it worked, but it doesn't.

No it doesn't;!..well its not that accurate;!..

--
Tony Sayer




.