Accelerometer, gyroscope, and magnetometer (compass) sensor data.

No permission required.

Updates arrive at handle_info at the requested interval:

handle_info({:motion, %{
  accel:     {ax, ay, az},          # m/s² (gravity included)
  gyro:      {gx, gy, gz},          # rad/s
  mag:       {mx, my, mz} | nil,    # µT (microtesla), calibrated — key present only with :magnetometer
  heading:   float | nil,           # degrees [0, 360) from MAGNETIC north — key present only with :magnetometer
  timestamp: unix_ms
}}, socket)

The :magnetometer contract

The mag and heading keys are present exactly when you requested :magnetometer — on both platforms. When you did, they are always in the map, and each is nil when there's no reading yet: the device has no magnetometer at all, or the heading hasn't been fused. So match with nil, and don't assume a value is present:

case motion do
  %{heading: deg} when is_number(deg) -> rotate_needle(deg)
  %{heading: nil} -> show_calibration_hint()   # no magnetometer, or not yet fused
end

When you did not request :magnetometer, the map has neither key (the plain accel/gyro stream) — so a consumer that never asked for the compass keeps getting the exact same 3-key map, and pays no extra sensor/battery cost.

It's magnetic north, not true north — true north needs location + declination (out of scope; layer it with Mob.Location). Magnetometers drift until calibrated, so prompt the user to wave the phone in a figure-8, and note that many budget devices ship without one at all (there, heading/mag stay nil).

iOS: CMMotionManager — device motion with the XMagneticNorthZVertical reference frame when the magnetometer is requested and available (a calibrated field + a fused heading on one stream); nil/nil when requested on a device without one. Android: SensorManager — magnetometer + rotation-vector, registered only when :magnetometer is requested.