[aprssig] GPS in cell phones

Jim Lux jimlux at earthlink.net
Sun Mar 6 12:27:50 CST 2005

Andrew Rich (VK4TEC) wrote:
Actually signal strength has nothing to do with it.
It works off the timing advance in GSM.
As you get further away from a base the TALIM changes
Get 3 and you can work out where you are

On Thu, 2005-03-03 at 22:39 -0500, A.J. Farmer (AJ3U) wrote:
> You mentioned in a previous e-mail that many newer cell phones are GPS
> enabled.  However, they do not actually have GPS recievers in them.  The
> location of the phone is determined based on signal strength and direction
> from cell towers and triangulation...  There is no direct reception of
> satellite GPS signals by the phone itself.  Just FYI.

Actually, the phones DO receive GPS signals (the timing and/or signal
strength (RSSI) approaches (both are used) don't provide sufficient
resolution for E-911 requirements).  However, often, they're not a full GPS

They only receive the codes and determine the "observables" (code phase and
rate), and they do this with ephemeris information supplied by the cell
site. Since the cell site is reasonably close to the cell phone, the
satellites in view are likely to be the same, the cell site can tell the
phone's receiver which satellites to look for. Even better, the range of
code phases over which they need to search can be restricted.  Once the
receiver has acquired the PN code and determined code (and potentially
carrier) phase and rates, it sends that back to the cell site, which
actually does the navigation solution.

This is all pretty well laid out in the latest Motorola spec sheets (try
http://www.synergy-gps.com/ which is a big distributor of Motorola modules).
Try this for the Oncore FS module:
http://www.synergy-gps.com/FS_Oncore_Brochure.pdf which actually has an
autonomous mode, as well as a pair of "assisted" modes.

They do all this for several reasons:
1) The biggie: lower power.  They rate these receivers in
"millijoules/fix".. you want to push all the processing off the phone, if
you can.
2) cost of implementation: simple receiver is much cheaper than a whole GPS.
Cellphones are a VERY price sensitive device, and this sort of division of
components is probably the lowest cost way to meet the E-911 mandates.
3) Revenue stream for location aided services/telematics:  They can sell you
your position, or, more likely, sell you navigation aids based on your
current position (What Chinese restaurants are near here and how do I get
there in my car?.. for $1, we'll tell you) (or, more cynically... She's
walking in front of a Prada store, let's send a customized video ad to her
phone for shoes right now)

Many of the phones have a "hidden" option to display the GPS information as
a diagnostic feature (try entering a number like ##GPS#). A bit of googling
will find the options for your phone in some cases.  My Treo650 doesn't
support this particular one (although it has GPS inside), but my wife had a
LG PM325 for a week, and it worked for that one. It was very cool, in a geek
sort of way, displayed the PRN sequences it was tracking, the SNR, and code
and carrier phase, etc.  But, of course, no nav solution.

There ARE other schemes out there for position finding for phones (not
everyone has a GPS phone, and there is understandable consumer resistance to
forcing people to buy new phones, when you have a perfectly good phone that
works). A lot of it is based on accurately determining angle of arrival
using interferometry between two antennas at a cell site, then combining
that information with signal strength and time delays to arrive at a
position solution.  There is a huge amount of "art" in combining angle of
arrival, signal strength, and trilateration data to come up with accurate
(enough) position estimates, but there's also a big financial incentive to
make it work. Phones (in the US, anyway) are heavily subsidized by the
cellular providers, so they really, really want to get the prices down.

Jim Lux, W6RMK

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