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

Looks like most of your route was under dense foliage, as well.  That could
cause GPS
signal dropouts.

Dana

On Sun, Feb 2, 2020 at 11:36 AM Chris Wilson chris@chriswilson.tv wrote:

On 2/2/20 9:35 AM, Chris Wilson wrote:

GPS (and time derived from GPS) on a phone is a funny thing.  They
obsess about energy consumption ( how many picojoules/fix kind of
things).  Therefore, they do fixes on a sort of "as needed" basis and
feed that to the API.  A couple things can screw this up:
The legal requirement driving GPS is the E-911 service, but they only
need good accuracy when you're making a 911 call.
Most phones make heavy use of Assisted-GPS - the cellsite gives them an
estimate of position and the approximate code phase and timing, so that
acquisition can be fast (and consume small power).
A mapping application (or the phone API, I don't know) could also do
some sort of "forward estimation" of position (i.e. you were heading 130
degrees at 5 m/s so after 10 seconds, your position is X meters north
and Y meters East.

Note also that the mapping applications target people driving so they
do interesting things like snapping to likely positions (i.e. you're not
likely to be in the middle of a river, so they snap to the road) and
they filter out small velocity variations.  If you're walking, the
application might shift to a different position filter (particularly in
urban areas, where multipath is a reality, but position is aided by
things like known WiFi access points, etc.)

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@chriswilson.tv wrote:

Hello everyone,

Sorry  fore  the  late reply, work took me away again. Many thanks for
the  detailed  replies, I now understand how  cell phone GPS might not
be as accurate as the one in a dedicated GPS tracking device, and that
certainly  is  the case in tests I have done. All very interesting and
much  appreciated,  both  the replies here and bu direct e-mail, thank
you again!

Monday, February 3, 2020

Best regards,
Chris        mailto:chris@chriswilson.tv

j> On 2/2/20 9:35 AM, Chris Wilson wrote:

j> GPS (and time derived from GPS) on a phone is a funny thing.  They
j> obsess about energy consumption ( how many picojoules/fix kind of
j> things).  Therefore, they do fixes on a sort of "as needed" basis and
j> feed that to the API.  A couple things can screw this up:
j> The legal requirement driving GPS is the E-911 service, but they only
j> need good accuracy when you're making a 911 call.
j> Most phones make heavy use of Assisted-GPS - the cellsite gives them an
j> estimate of position and the approximate code phase and timing, so that
j> acquisition can be fast (and consume small power).
j> A mapping application (or the phone API, I don't know) could also do
j> some sort of "forward estimation" of position (i.e. you were heading 130
j> degrees at 5 m/s so after 10 seconds, your position is X meters north
j> and Y meters East.

j> Note also that the mapping applications target people driving so they
j> do interesting things like snapping to likely positions (i.e. you're not
j> likely to be in the middle of a river, so they snap to the road) and
j> they filter out small velocity variations.  If you're walking, the
j> application might shift to a different position filter (particularly in
j> urban areas, where multipath is a reality, but position is aided by
j> things like known WiFi access points, etc.)

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