If you are hardware inclined and can use something else for GPS, some of the inexpensive fitness bands from Asia contain Nordic nRF51 / nRF52 microcontrollers a

If you are hardware inclined and can use something else for GPS, some of the inexpensive fitness bands from Asia contain Nordic nRF51 / nRF52 microcontrollers and are completely reprogrammable. Roger Clark's site has some good info including http://www.rogerclark.net/arduino-on-the-id100hr-fitness-tracker/ http://www.rogerclark.net/arduino-on-the-id100hr-fitness-tra... (I believe the watch pictured is actually an ID107HR, not an ID100HR) The ID107HR is nice because it opens with a few screws rather than cracking glue apart and it also exposes the programming pads. You can write Arduino code and interface with the heart rate sensor, accelerometer, OLED screen, etc.