[time-nuts] GPS location inaccuracies from a cell phone

[Bob kb8tq]

Hi

Cell phone GPS is very much a “that depends” sort of thing. 

In some cases, the raw data comes to the phone, but the location 
processing takes place “in the cloud” somewhere. Loose cell coverage 
with this sort of device and you also loose location. This sort of processing
does allow “tower time of arrival” sort of location to be blended in. That
may or may not be a good thing.

With a true GPS in the phone, you next get into antenna location. There
isn’t much space in a phone so where ever it goes will be a compromise. 
The main goal of the phone is the cell connection. I’d bet the GPS antenna
very much gets the not quite so good location for it’s antenna.

Cell phones “wake up” on a fairly regular basis. They spew RF for a bit and
then go back to sleep. Depending on how long this goes on for the GPS may
be blocked out (RF overload) for a short bit or for quite a while. 

Even with an “ideal” GPS, you have multi-path issues. If not caught and 
suppressed they will make the location “jump”. How well this or that chip 
set or approach deals with them is variable. 

GPS units do have firmware in them. That firmware can and does have
bugs in it. Some are quite reproducible others not so much. Hop on this
road in Montana going that way with my Garmin and you can very
repeatably watch the track wander off by a couple miles. The map data
is correct … the GPS, not so much.

Those are just the first few that come to mind. There are certainly other 
issues as well. 

Lots of fun

Bob

> On Feb 2, 2020, at 12:35 PM, Chris Wilson <chris at chriswilson.tv> wrote:
> 
> 
> 
>  02/02/2020 17:28
> 
> Hopefully not too off topic a question, but GPS experts abound here...
> 
> I am running a tracking device server on one of my PC's and an option
> is to use an app on a cell phone and the phone acts as a tracking
> device. But it shows seemingly random anomalies in position. For
> example I walked the dogs around the wood earlier. 99% of the tracking
> of the phone is correct, but I see occasional abnormalities where the
> track veers off into the distance to a "dead end" where I have
> certainly not taken the phone.
> 
> Could it be because I am not keeping the phone in a constant
> orientation? I do not see such anomalies with a "proper" tracking
> device, say in a vehicle? Where the trace veers off I may have been
> bending down burning some rubbish. Te phone would have been in the top
> pocket of my overalls. Any idea why these anomalies occur please? The
> track can be seen at http://www.chriswilson.tv/phone.jpg
> 
> Thanks!
> 
> 
> 
> -- 
>       Best Regards,
>                   Chris Wilson.
> mailto: chris at chriswilson.tv
> 
> 
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