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Comment by spdustin | Hacker News Reader

CarPlay is essentially a conditional pair of video inputs. Any system that supports on-screen rear-view camera and that has a wheel speed sensor can support CarPlay.

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Is wheel sppeed really the only car sensor exposed to the app? That is kind of sad.

What do you really need? Is the car moving forward or backward is the only one you can't figure out from GPS on the phone (this is possibly what they are getting from wheel speed - GPS speed is more accurate if you have a signal).

There are a lot of nice to haves of course. GPS does eat phone battery so better if the car can give you that. There is a lot of other car data that is interesting, why force plugging a OBDII dongle in to get DTCs, RPM, O2 sensor values, or whatever. However for car play to work at all it doesn't need anything more.

Nav apps on phones will use dead-reckoning if they don't have a GPS signal, so they don't really even need the wheel-speed sensor, but I'd guess they use it just to increase accuracy.. e.g. in a long tunnel.

Increasing accuracy is a wild understatement. Dead reckoning with mobile phone hardware won't give you a usable result for long. Maybe your experiences are from tunnels with dedicated beacons to tell the phone where you are.

Most vehicular tunnels aren't too terribly long, if you're stuck in one in traffic, yeah dead reckoning drifts quite a bit. But if you're driving through one for a minute or two, it's sufficient.

On a side note I've personally had bad experience with beacons in train tunnels telling me I'm miles away from where I'm actually at.

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