CarPlay GPS

Several months ago I pulled a trigger on a 2004 2005 Subaru Impreza Outback. Yes, such car exists, despite the confusing name. It was more popular in North America, with under 1000 made for the European market. It was always a head turner for me - as compact as the Impreza, with Outback-style higher clearance and bodywork. It is in a good shape and financially costed less than a pair of brake discs for a BMW (that is what the mechanic in a BMW service shop told me). And it is a dream to drive. 

It was missing a radio and I have gone full-in, soundproofing the cabin and installing a 2-DIN head unit (Alpine iLX-705D) combined with the Audiotec Fisher Match UP 6DSP amplifier. The DSP makes the real difference, the clarity of the music scene is unbelievable. Especially when a high resolution audio is played - the Alpine unit supports up to 24-bit/192 kHz FLAC files.

The head unit also has several modern convenience features, wireless CarPlay being one of them.

I have wireless CarPlay in my other Subaru (the Forester STI) and despite it being wireless, I have always been careful about placing the phone such that it could have a decent reception of GPS signals. It turns out this was entirely unnecessary.

Diving into the CarPlay protocol I was surprised to see that it mandates (the wireless variant) that the head unit feeds GPS location data to the phone (the phone is running the mapping software such as Google Maps). And there is even more. The radio has input for the speed signal. It is the pulse signal generated by a gearbox shaft, that the speedometer takes as input. Tapping into that signal may require some hackery - that depends on the car model, but here is a good guide. The Alpine unit recognizes that signal and also performs automatic calibration (no setup required). That is the assisting signal that the iOS CoreLocation Subsystem ingests. Did you know that an iPhone can use the car's gearbox as a sensor???

Apple CarPlay specifications require compatible head units to forward auxiliary sensor streams, including dead-reckoning status, heading, and wheel speed data, to the connected iPhone over the CarPlay communications link.

When CarPlay is active, iOS dynamically shifts primary location tracking from the iPhone’s internal hardware to the car’s sensor pipeline. The iOS CoreLocation framework runs sensor fusion: if satellite visibility drops, it seamlessly uses the head unit's speed pulse and inertial measurements to dead-reckon coordinate updates.

Then Google Maps simply requests high-accuracy location and heading updates from standard iOS CoreLocation APIs. Because iOS synthesizes the car’s speed pulses into that continuous stream, Google Maps continues tracking along the route smoothly through tunnels, parking structures, and dense urban corridors rather than freezing or drifting.

So the bottom line is - you can keep the phone in a pocket and it will still navigate well - using the car / head unit sensor fusion. Additionally, in tunnels the navigation puck will continue to advance at the car’s actual speed rather than assuming a static speed or losing lock entirely. If you stop in a tunnel or garage, the map will stop instead of continuing to coast forward along the route vector. It also eliminates the initial lag and GPS "snap" when pulling out of an underground garage or tight corner.