Long Range RF communication
If you are able to see the devices, then we can only assume line of sight, 2km distance 433Mhz (70cm) should be fine with quite a low power solution. If you can't see them then that drastically reduces the transmit range at 70cm without increasing power consumption. As with all radio communication it can be power hungry. I have created similar projects with the arduino using a radiometrix NTX2 transmitter at 434.650Mhz. My solution to save power was to turn the transmitter on, send a location 'ping' and then turn off the transmitter again rather than constantly transmitting. Easily done with an arduino.