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:[email protected] 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.) j> _______________________________________________ j> time-nuts mailing list -- [email protected] j> To unsubscribe, go to j> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com j> and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
