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
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.)
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