Comment on ELI5: how do mobile devices know your movements?
FuglyDuck@lemmy.world 3 months ago
So, you are supposed to calibrate the pace length, it can guess that but it’s not as accurate.
It uses accelerometers for “fine” movements like bouncing in your pocket or being flipped over (or for augmented reality, etc,) but gps for very coarse positioning. (GPS knows you’re in a building, or maybe one of a few small buildings. WiFi location knows what room you’re in.)
They use everything together to create a picture of your movements.
skulblaka@sh.itjust.works 3 months ago
GPS is good enough now to know what room you’re in, I’ve tested it at my house. It can tell the difference between the 30 feet or so between my bedroom and front door.
FuglyDuck@lemmy.world 3 months ago
A radius of 5 meters- 15 feet or so. Which of you consider the scale of what’s going on (satellites in orbit, pinging your location to a relative pin prick, accounting for relativity,) it’s fucking impressive.
But you might be anywhere along a thirty foot line.
And that’s at its most optimal conditions. Inside a building (particularly commercial or industrial, and particularly larger buildings,) that becomes much, much more problematic.
Which is why they use combinations of things like WiFi location (“improved location services”) to increase accuracy as well as dead reckoning using your accelerometer.
But ultimately, that’s all just a guess, and they’ll frequently cheat. For example, if you pull under an overpass and stop, pretty quickly they’ll start wigging out bouncing you between the road above you or the one you’re actually on. Or when you decide to ignore navigation instructions on the highway- which is why until it updates the map it’ll still show you on the offered route. (Which might be 200 ft down the road, and 50ft away.)
All of that to say that they’re using a lot of tricks to convince you that it’s a lot more precise than it really is.
froh42@lemmy.world 3 months ago
There’s another trick - checking against street maps. When you walk/drive/ride around a corner the phone can use it’s inaccurate GPS reading and update it according to where the corner is. Of course it doesn’t use ONE corner. to fully update your position, it just nudges the position a bit closer.
In case you are navigating a lot of street corners, after a time an inaccurate GPS position will be corrected.
There’s even more input to the algorithm (location of known WIFI Hotspots for example etc) and they are all combined with a “Kalman filter”.
en.m.wikipedia.org/wiki/Kalman_filter
Sethayy@sh.itjust.works 3 months ago
As well as the us military intentionally keeps GPS readings inaccurate, as conspiracy theorist as that sounds lol
SkyezOpen@lemmy.world 3 months ago
This hasn’t been true for 24 years.
FuglyDuck@lemmy.world 3 months ago
Well, the civilian signals are, yes. Their hardware is likely much more accurate.
How else they gunna get a baddie with a stabby-slashy ninja missile dropped by some dude in a poorly air conditioned shipping container somewhere in like, Arizona.