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

[Tim Shoppa]

Cellphones in use in urban canyons or places with foliage will have
substantial GPS signal losses.

Mapping programs hide this by using algorithms that guess you are
continuing to move along the same street at the same speed. e.g. just
slightly smarter than dead reckoning. It will always try to snap you to a
road unless that becomes truly hopeless.

These same programs use some "common knowledge" about traffic patterns to
try to draw you on correct side of road as if the GPS has finer resolution
than it actually has. e.g. if you are traveling northbound it assumes you
are on the right side of the street and will draw you on the eastern side
of the street. But this is just a guess and breaks down if right-hand
travel is not applicable (e.g. you are walking your dog and not driving).

Now an optimization on the above I have seen recently, is that if the
health app is seeing walking activity going on, it turns off the
right-hand-side-of-road optimization.

These same mapping programs also use clues from earthside cellphone base
stations as to location especially when GPS is less than optimal.

Almost certainly your data is showing not "raw GPS" reads but the overall
synthesis of all these algorithms.

If you are using a raw GPS receiver without any of these smarts it will not
do any dead reckoning or finer-than-actually-available spatial resolution
tricks. This raw GPS data may be available using some API in your phone but
most apps use the overall synthesized position.

Tim N3QE

On Sun, Feb 2, 2020 at 12:36 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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