| Re: [GMCnet] GPS Accuracy (was: Tires) [message #65560] |
Tue, 24 November 2009 16:37  |
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ljdavick
Messages: 3548
Registered: March 2007
Location: Fremont, CA
Karma: -3
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Some cell phones have real GPS, but many do use towers to triangulate from. Some even use WiFi hotspots. Whatever gets you there.
My 1st gen iPhone has only the mock-gps for mapping and it's served me very well in a pinch.
Larry Davick
The Mystery Machine
----- Original Message -----
From: "Jay Rabe" <jayrabe@hotmail.com>
To: gmclist@temp.gmcnet.org
Sent: Tuesday, November 24, 2009 2:13:43 PM GMT -08:00 US/Canada Pacific
Subject: Re: [GMCnet] GPS Accuracy (was: Tires)
I'd heard that the GPS apps in cell phones are not "real" GPS. That is, they use triangulation from cell phone towers to determine your location, rather than receiving and triangulating from actual GPS satellite signals. As such, they are no good in the boondocks where there's no cell phone coverage.
Comments?
J
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Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, Ca
Howell EFI + EBL + Electronic Dizzy
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| Re: [GMCnet] GPS Accuracy (was: Tires) [message #65582 is a reply to message #65560] |
Tue, 24 November 2009 20:09  |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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Senior Member |
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Almost no current cellphones actually contain a true GPS system. If it was, it would not work most of the time - just like even the newest of my stand-lone units. One of the things that never seems to get talked about (if you were installing car phones before cell) is the innovation that makes the cell technology work at all.
The system is comprised of cells (Duh). These cells are the area served by a single tower.
When a handset is moving (part of the original intent), the system has to know where it is so it can pass it off to the cell it is moving into. It can not do triangulation when an area is served by very few towers. It does this with a radial and bearing system. That is why cell towers have three antennas. That gets you the bearing and I have forgotten how they get radial distance. It is some sort of convoluted time in transit puzzle.
The some of the GPS handsets have the means to receive location information, but more often it is used in the central signal handling. Even those that are advertised as GPS handsets do not actually have usable location data onboard. I accosted a friend that has been an engineer with Motorola and On-Star when I was trying to hack into my T730c to retrieve the location data, he finally admitted that I could get it out of there because it isn't in there. Many On-Star units do have real GPS.
As to speed and travel distance accuracy, it is now much better than it was even 5 years ago. I have one old unit that will report velocity with a 0.1 resolution every second. It still is not accurate for distance traveled because the log is relatively low density - corners get cut. The most recent actually stores travel distance so well that I was able to use it to establish the error in the coaches odometer and straight or twisty road makes no difference.
I'll go back and sit down now.
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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