How do car "GPS" locators work without GPS or line of sight antenna? Cellular? - Page 1
Forum section: electronics / general

author: Beamin
author_rank: Super Contributor
timestamp: September 24, 2021, 11:24:41 pm
content: When I owned a car dealership we would finance some car to high risk buyers with repos/bad credit/etc and the loan company would give them great rates like 24.99% daily APR (should be illegal, we didnt see that money but it got some into cars that couldnt otherwise.We had to install this tracker/immobilizer thing under the dash. This came with an IR remote where they had to enter a code each month to make their car start, like a prepaid phone. The first gen was awful, you had to locate the GPS antenna under plastic with a view of the sky and an IR led for the remote. Smarter customers realized you could follow the IR wire back to the box unplg it and put a paper clip in to start your car.Then the second gen came out RF remote and just a single box with five wires coming out. You could bury the thing deep in the dash in a "GPS farraday" cage with no regard to line of sight to GPS sats. Paper clip still worked if you could find the box.How did they do that? The company wouldnt tell us. I suspected it used cell towers to track because it worked in a metal garage or parking garage, then it would send out a SMS with ID long and lat every so often. Could it just have a list of every cell tower and figure out position based on that? Then the device would just need an IMEI to send texts to a cell carrier the loan company worked with. Or would it need to have to be dedicated like to only verizon towers then the box was treated like a cell phone to the tower? I know my iphone struggles when I'm around really tall buildings downtown with the GPS position so thats why I dont think they are GPS.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 24, 2021, 11:50:47 pm
content: I don't know how your trackers worked, but I can tell you a couple of ways that have been tried with some success.1. Triangulation from cell towers based on signal strength. The towers ID themselves so you know their location from mapping data, and in principle there are rings of constant signal strength around the towers. If you can hear three towers you can get an unambiguous solution for the location. Obviously antenna patterns aren't perfect and buildings and trees mess with signal, but generally these systems claimed accuracy to a few blocks or better. Probably more than good enough for this application, where you are just looking for folks skipping town. The five wire harness could have been carefully designed to provide cell phone reception.2. Tying into the cars signal traffic for speed and possibly direction information, augmenting this with modest quality rate sensors and possibly accelerometers and then doing dead reckoning. There are a number of tricks to making it work. The cars info on zero speed is pretty accurate and those periods can be used to measure and cancel rate sensor drift and accelerometer offset. Built in maps can be used to test/correct the position estimate with the assumption that most motion is on roads. These systems claim to delivery accuracy measure in modest numbers of meters.

author: james_s
author_rank: Super Contributor
timestamp: September 25, 2021, 12:16:08 am
content: It may have just been GPS still, GPS receiver technology has come a long way. I have an ancient handheld Garmin GPS that normally won't even lock indoors unless I set it right on the windowsill and even then it is unreliable and takes a while. I have some modern GPS modules that will get a lock within seconds even downstairs in a windowless room. It's truly remarkable when you consider how minute the signal is to begin with and how heavily it is attenuated by many building materials. I think GPS in general is one of the modern wonders of the world, if you proposed such a system I'd tell you it was impossible and would never work if not for the fact that it exists already and it does work.

author: Halcyon
author_rank: Unknown
timestamp: September 25, 2021, 12:27:13 am
content: Quote from: james_s on September 25, 2021, 12:16:08 amMobile phones are a great example of this. I'm able to get GPS fix with a high level of accuracy even when surrounded by concrete walls or under a metal roof.

author: Beamin
author_rank: Super Contributor
timestamp: October 09, 2021, 11:10:49 pm
content: Quote from: Halcyon on September 25, 2021, 12:27:13 amSo its using gps and not cell triangulation?

author: ivaylo
author_rank: Frequent Contributor
timestamp: October 10, 2021, 05:10:12 am
content: Quote from: Beamin on October 09, 2021, 11:10:49 pmOr Google’s car recording nearby visible wifi-s…

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 10, 2021, 09:51:41 am
content: I have a GPS tracker built into an ODB connector, it has a GPS receiver based on a u-blox AMY 6M chip, a GSM module and a LiPo battery. Plugged into the ODB connector below the steering wheel the GPS reception is fine and accurate.https://www.u-blox.com/sites/default/files/products/documents/AMY-6_ProductSummary_%28GPS.G6-HW-10039%29.pdf

author: Halcyon
author_rank: Unknown
timestamp: October 10, 2021, 10:56:10 pm
content: Quote from: Beamin on October 09, 2021, 11:10:49 pmCorrect. Purely GPS data. No extra "Googley" type data aggregation either. You can download applications to your phone that will decode the raw serial data from the GPS module. Cell triangulation wouldn't work particularly well in my area anyway since we're only serviced by a single cell.