Does the Pebble have an integrated GPS or does it rely on a phone with GPS?

I wonder if the form of exercise also has an impact. I'm averaging 12.5km walking/running per day or 7.5+ miles but my primary form of exercise is weight lifting which wouldn't be very taxing on the watch at all. On the other hand someone doing jumping jacks and various other flail-y cardio exercises might be triggering the screen to turn on an off and making the accelerometer work for its supper. Rapid changes in air-pressure might also be triggering the mic/DSP to listen for speech.