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iPhone gyroscopes, of all things, can uniquely ID handsets on anything earlier than iOS 12.2

ibmalone Silver badge

It doesn't matter much either way. If it's stationary then you need enough sensor noise to get the jitter out, test mode on my phone seems to confirm there should be enough on a normal office table, though you still need to wait long enough to get the different combinations that will allow you to estimate the full calibration matrix. Because they're looking at changes between samples moving about is fine too, and they seem to do a trick with looking at the differences between pairs of outputs of similar values. There's not much of the work that is actually concerned with gait. There is a little discussion about bias correction in the javascript API causing a problem, waiting for the phone to be stationary is a suggested strategy for that. However the 100 samples they need is less than 1 second of data.

To crudely summarise, say you've got an accelerometer and it outputs integer values from 0 to 255. You have some calibration that turns that into physical units, say m/s^2. You still only have 256 possible values. Maybe they cover -20 to 20m/s^2, so they're about 0.156m/s^2 apart (-20m/s^2, -19.844m/s^2...). Look at the values you've collected over some set of samples and take the difference between close ones, you'll see +/0.156, +/-0.312 etc. On a calibrated device that 0.312 will not be exact, say it's 0.312445, this value will be different between phones. With the gyroscope you've got a slightly more complex scenario with 6 (I think, guessing it's symmetric) independent calibration values that can be extracted in a similar way. Because the devices are accelerometers the method works equally well stationary (values zero with noise), in steady motion (values zero with noise again) and constant acceleration (constant value with noise), but a bit of changing acceleration, unless incredibly jerky, is not a problem.

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