The flight range of the AR.Drone is limited because the wifi range is limited to somewhere between 30 – 100 meters depending on where you fly and whether you have line of sight.
The flight range of the AR.Drone can be drastically extended to 1.5 – 2.5 kilometers by using RC equipment. The 2.4 GHZ RC hack instructions describe how the range can be extended in full detail.
The actual flight range will depend on the RC transmitter & receiver used. According to the tutorial it has been tested to work with Spektrum DX6i and Spektrum AR6200 DSM2 6 Ch Rx Ultralite. The Spektrum DX7 with the AR6200 (2.4Ghz), Futaba 9CH with Assan module and X8R7 (2.4Ghz) and the JR790UL spcm Rx (72 Mhz) have also been tested to work according to the tutorial. You will need a 5 volt BEC (Battery-Elimination Circuit) to power the Arduino and the RC-receiver since the AR.Drone’s battery is 11.1 volt. This will cost you around $10.
The wifi signal is passed from the RC-receiver to the AR.Drone through a small Arduino compatible device called YellowJacket ($55). The YellowJacket is based on the Arduino mini and comes with on-board wifi. You will need an USB breakout board ($14) as well to be able to program or transfer the Arduino sketches to the YellowJacket.
This is not a cheap hack. You could buy an extra AR.Drone for what you will have to shell out for this hack. And you have to feel comfortable with a soldering iron, programming an RC-controller and command line stuff. Like most hacks discussed here this is not for the faint-hearted.