From: John De Armond
Subject: Re: GPS location on cell phones
Date: Thu, 14 Jun 2007 03:23:35 -0400
On Thu, 14 Jun 2007 01:37:35 GMT, "Tom J" <email@example.com> wrote:
>I can tell you for sure GPS on phones work. A man killed two real
>estate ladies about 3 miles form where we live just north of Atlanta.
>He made a cell phone call to his girl friend within minutes and told
>her he was leaving the state but would keep in touch. She called the
>police. The police monitored his transmissions as he traveled and
>drove right to his motel door 3 days later as he was making a call in
>Minnesota. This was over 2 years ago.
>Now whether they work for any application other than that, I don't
>know. My phone is not GPS equipped and if it ever dies, I not only
>will have to get another phone, but since my plan is not offered
>anymore, will have to change to a more expensive plan.
Let's clear up some misconceptions. Allegedly for 911 purposes, the FCC had decreed
that any cellphone user be locatable within something like 200 feet. That does not
require GPS and in fact, most phones lack GPS receivers.
The method being used by the cell phone companies is a sophisticated form of old
fashioned triangulation. The system looks at the relative time-of-flight of the
signal from the handset to three or more base stations and computes the location
using simple geometry. No additional circuitry or software is needed in the phone.
It's all done by the cell system. This is a fancy version of old fashioned RF
direction-finding as practiced by the military from back before WWII and by ham radio
operators everywhere in an activity called "fox hunting".
As a separate but related matter, many cellphone makers are now including GPS
receivers in their handsets. My old tri-mode phone had the then-brand-new SiRF chip
set. DFing in the analog world is more resource-intensive than in the digital world
so there was pressure to use the GPS chips. Digital DFing is easy so the pressure
for GPS chips has lessened but many phone makers are including them as features.
They serve two different roles. One is presenting a navigation screen to the user.
the other is location reporting. GPS chipsets are STILL power hogs compared to
digital phone circuits so the chipset is normally off unless the user is navigating.
The chipset can be remotely commanded on, as can your phone, but since it isn't
necessary for location reporting to the degree required by the FCC, they usually
aren't even during 911 calls.
Despite the many myths created in the movies and fiction, GPS is pretty lousy for
this kind of stuff. The chips require relatively a lot of power and given the space
constraints in a modern cellphone, don't work very well because of the limitations on
antenna size. The GPS in my old phone was essentially worthless unless I was
standing out in the open. In areas with large steel structures or high mountains,
low-end civilian GPS receivers are easily fooled by multipath reflections. Digital
radiolocation is simply more accurate and more reliable.
I haven't read the details (and am not particularly interested) in that crook chase
you mention but I can reliably assume that he was caught using radiolocation and not
GPS, media reports to the contrary.
One other note. Modern digital cellphones can be remotely converted into totally
different devices by simply pushing out new firmware over the air. You won't know it
and can't tell. The FBI did that in one of the recent mob cases. They pushed
firmware to his phone that turned it on, activated the mic and turned it into a
personal wiretap. This worked even though his phone was "off".
Anyone who's doing something that might get some 'crat's panties in a wad needs to
know this. Political dissidents, civil libertarians and so on. The ONLY way that a
phone is truly off is with the battery removed. The fibbies got a court order in the
mob case but in this post-911 kinder and gentler police state, I'd expect that to be
the exception instead of the rule. Especially since it's push-button easy for them