I always carry the AirPort Express, doubles as a USB charger.

Beware, the built-in GPS antenna is located next to the zoom buttons alone the top edge. For the fastest initial lock, hold the N810 vertically / perpendicular, the zoom button facing up towards sky, do not tilt the N810. Go to a clear area with most visibility of the sky and start you map application. The N810 should be able to lock down to your position in 15 sec on a clear day. Longer if it is cloudy. Once the N810 has your initial position, you can tilt the screen so you can read it.