The GPS/Compass button switches on a two tabbed window displaying the skyplot - placement of monitored GPS satellites and more useful information like current altitude, speed, GPS accuracy, current coordinates.

Tapping the skyplot switches on/off the GPS as well as the button in the bottom. The other tab displays the compass, azimuth, distance of your navigation target and tilt of the phone.

Tapping the tilting indicators opens a menu offering sensor calibration.