Comment by eqvinox
Almost, they use accelerometers and gyros, 'dead reckoning' is the keyword to look for. The wheels are a bit unreliable because the diameter changes slightly with pressure and temperature.
Almost, they use accelerometers and gyros, 'dead reckoning' is the keyword to look for. The wheels are a bit unreliable because the diameter changes slightly with pressure and temperature.
The legal requirements on that (in most places) are that the speedometer is in something like a -0%/+10% range, i.e. never shows lower than you're actually driving. Not only is that not helpful for navigation (but you could compensate that/shift the error window), but the precision is also pretty low (which you can't easily compensate).
(There are two precision problems here — tyre diameter changes slighly while you're driving, but also it's not precise to begin with before you even turn on the car, due to tyre wear.)
You'd need to do something like calibrating wheel speed data while you have good GNSS reception, then you could use it for dead reckoning. But accelerometers and gyros are cheap…
P.S.: I didn't say wheel speed data isn't used, just that it wouldn't be precise enough on its own.
Are there modern cheap IMUs that are able to hold position for some time available?
My understanding is that using plain accelerometers and gyros will drift quite soon due to noise, while some cars (e.g. Tesla) maintain their position quite well when doing to an underground parking structure and then coming back. So I actually do believe that they put a lot of weight to tracking wheels, which I believe would not accumulate that much error. (For more precise location they'd still need to use accelerometers to detect downhill/uphill, though.)
So I believe that the precision actually favors wheels, not IMUs.
In the underground parking case it doesn't actually matter too much if there's a certain % offset, because you're very likely driving a near-closed loop of some sort; and as you say, the wheel size ratio can be calibrated upon good GNSS data.
Wheels are still used for legal telemetry: speedometer and odometer.