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

[Chris Wilson]

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 at chriswilson.tv


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

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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