How can my iPhone know my location whilst in airplane mode?
If you have a device with iOS 8.2 or earlier, Airplane Mode will also
  turn off GPS.


Thus with iOS 8.3 and above the GPS stays on while in Airplane Mode.


https://support.apple.com/en-us/HT204234


GPS works independently of phone or WiFi signals and is "receive only", so keeping it enabled even in Airplane mode doesn't influence other electronical devices nearby.