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'[OT] Cell Phone GPS? not exactly'
2006\05\17@231301 by Robert Rolf

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Bob Axtell wrote:

{Quote hidden}

Anyone doing this in the USA? Certainly no one in Canada.

There is now a GPS/Cellphone box sold in the UK that uses a full
GPS receiver tied to a cell phone (using SMS) to track your kids.
Conceivably it could also be used in North America, but the first
thing any kidnapper is going to do is toss the kids cell phone.

> Two cell towers can triangulate a phone very well; even though only one
> cell tower passes traffic
> (talk signals), a second one can through software be used to locate the
> phone. The GPS function
> normally offered is actually THIS location, offset by the
> very-well-known GPS location of the
> cell tower. The accuracy varies considerably, according to the angle
> between the two towers.

Not exactly...
I think you've confused the two different technologies used here.

http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=12235&&pageID=2

"The handset takes GPS measurements and sends them back to the PDE
for calculation. There, the PDE processes the data with knowledge of
the GPS satellite constellation as provided by a wide area reference
network (WARN), providing the differentially-corrected position to
the requesting entity. This could be the handset, but for E911 the
request originates from the network and is not sent back to the device."

Basic E911 uses the tower triangulation method. It does NOT require GPS
receivers in the handset, but works indoors and is not overly accurate.
It does NOT work well in the country because of a lack of triangulation
(or very acute look angles).

GPS augmented E911 uses a partial GPS receiver in the handset, that
basically forwards raw satellite timing info to the base station,
which then does the calculations to get a full fix (sub 5 meter accuracy
in optimal conditions).

The option in the handset turns on and off this 'forwarding' of GPS
signals.

> These services are sold to police agencies as additional income to the
> phone companies.
>
> Notice that these services are not available where there is not a second
> cell tower in range, so it only
> works well in cities.

GPS augmented E911 works just FINE with a single tower. Hence it's
desirability. The down side is that it doesn't work well indoors because
the GPS signal is usually too weak.

>>The options you describe sound like your phone gives you the option to
>>disable GPS tracking for all purposes except 911 traces.

E911 doesn't require cooperation from the phone. You can be (and
sometimes ARE if the NSA is interested) tracked as long as your
cell phone is powered.

> Normally, you will want to do this, because increased xmt power to meet
> the needs of the distance
> (second) cell tower means the battery will not last as long.

Not exactly. CDMA requires close power matching of the received signals
in order for the rake receiver to work reliably. The other tower(s) will
deal with whatever they see for power level.
A tracked phone may be polled more frequently by the network, leading
to higher power consumption, but not by much.

>>There are some newer phones (usually smart phones) that have a full GPS
>>implementation and are therefore of calculating the fix by themselves.

At a CONSIDERABLE cost in power, even with the newest low power chip sets.
It takes quite a bit of horsepower to lock on and track 4+
spread spectrum satellite signals. Usually the GPS receiver is only powered
up on request of the tower, with important tracking info passed to the
phone enable a near instant track lock.

> satellite phones offer full GPS all the time.

Really? I haven't seen this feature mentioned in any of the
Irridium receiver docs I looked at.
I suppose this is so the satellite network can point the
antenna beam (phased array) at the correct location to maximize signal.

There have been a regular series of articles on GPS and E911 at
http://www.GPSWORLD.com but I couldn't immediately find the ones that
described the GPS/E911 system in detail. Feel free to browse their site.
It is chock full of good info.

Robert

2006\05\17@232349 by Robert Rolf

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Detailed article on E911/GPS and LBS (location based services).

http://uc.gpsworld.com/gpsuc/data/articlestandard/gpsuc/092006/310844/article.pdf

Robert Rolf wrote:

{Quote hidden}

2006\05\18@000312 by Herbert Graf

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face
On Wed, 2006-05-17 at 21:12 -0600, Robert Rolf wrote:
> >>Some service providers offer a "locate" service that for a fee will
> >>locate your phone by GPS. Generally it's meant for parents keeping track
> >>of kids.
>
> Anyone doing this in the USA? Certainly no one in Canada.

Actually it does exist in Canada. Bell Mobility was piloting such a
product in the Toronto area about a year ago, they said it would be
available shortly after the news report I saw.
http://www.bce.ca/en/news/releases/bc/2005/08/09/72623.html

No idea on costs, TTYL

2006\05\18@001614 by Robert Rolf

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Herbert Graf wrote:

> On Wed, 2006-05-17 at 21:12 -0600, Robert Rolf wrote:
>
>>>>Some service providers offer a "locate" service that for a fee will
>>>>locate your phone by GPS. Generally it's meant for parents keeping track
>>>>of kids.
>>
>>Anyone doing this in the USA? Certainly no one in Canada.
>
>
> Actually it does exist in Canada. Bell Mobility was piloting such a
> product in the Toronto area about a year ago, they said it would be
> available shortly after the news report I saw.
> www.bce.ca/en/news/releases/bc/2005/08/09/72623.html
>
> No idea on costs, TTYL

At the above URL..


"The service can
be added to new and existing Bell Mobility accounts for a $5 monthly fee,
which includes 20 locates per month. Out of bundle locates are $0.25 per
locate. There is also a pay-per-use option available at $0.50 per locate.
    More information on Seek & Find is available online at
http://www.bell.ca/seekandfind


Service is available on select AGPS cell phones."

AGPS = Augmented GPS.

Bob was right about it being a profit center for the cell companies.
Thats the REAL reason they're deploying it (and location based
chargeable services)
E911 is secondary since all the companies keep pushing their
implementation dates for E911 back and back and back.

Robert

2006\05\18@015242 by Bob Axtell

face picon face
I am familiar with GPS ONE, a service that locates a cellphone using two
cell towers. It works
when GPS cannot work, i.e. in a building.

So perhaps I am mixing up the technologies... That was over a year ago,
and that's a LOT of
elapsed time these days.

Thanks for the links.

--Bob

Robert Rolf wrote:
{Quote hidden}

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