[PW] ? cell phone GPS

[jtg]

Ok: Unless your 'phone actually incorporates a GPS receiver it doesn't have
one, if you see what I mean <g>.

GPS (Global Positioning System) is passive: a GPS receiver is receiving more
or less line of sight (which is why they tend to be unreliable in cities
with tall buildings) very accurately timed transmissions from a
constellation of military satellites and it calculates a position on Earth
by triangulation in 3 dimensions. A refinement to give back the accuracy
available to military users to civilian users is to add in a reference
signal from a fixed geographical point and this is D (Differential) GPS.

Unless your GPS position is then transmitted to a base station, only the
receiver knows where it is (you may know as well, if you can find the
wretched thing). 

Cell phones themselves are continually transmitting a broadcast signal
basically saying "Can any cell-tower hear me?". Any tower that can receive
responds and with a bit of electromagickery the best signal path gets used
until another is better. If only one tower can receive the signal then the
phone system knows that your 'phone is somewhere within that tower's area of
coverage or `cell': if more than one tower is receiving then by evaluating
the relative signal strengths the area can be narrowed accordingly. All
sorts of things can affect the accuracy unless in an open area with multiple
towers.

In a dire emergency the 'phone system might obtain a faster location than
the GPS - it could take up to 10 minutes to get a decent lock on a useable
number of satellites.

So, 

4: see above.
1: Possibly - some 'phones actually go into passive mode when powered off
and still transmit their ID to be ready for a quick start-up. Most don't
bother, just holding the last contact in memory.
2: No power, no signal. A flat or removed battery and the 'phone is dead as
far as the system is concerned.
2a: Some 'phones will function in emergency without a SIM card - I'm not
sure how the system handles these phones and whether they can be located.
3: If it really is a GPS equipped 'phone, the mode can probably be toggled
on and off to conserve power: consult the manual!   

HTH

John Germain

Jersey
British Channel Islands

-----Original Message-----
From: project-wombat-bounces at lists.project-wombat.org
[mailto:project-wombat-bounces at lists.project-wombat.org] On Behalf Of Mary
Barna
Sent: Thursday, October 04, 2007 8:46 PM
To: Project Wombat
Subject: [PW] ? cell phone GPS

This is for me.  I am technically challenged. I know that in your cell
phone is a chip whereby you can be traced through GPS.  I have several
questions, 
1. Does this work even if the phone is shut off?
2. Does this work if your battery runs down?
3. Do you have to turn on the GPS in some way?
4. How do "they" find you?

******************************************************
Mary Barna, Director
Valley Community Library
739 River Street
Peckville, PA  18452
phone (570) 489-1765  fax (570) 383-9657
email -- mbarna at albright.org
webpage -- www.lackawannacountylibrarysystem.org/valley
 
******************************************************
 

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