Visual Line of Sight

The maximum height that any UAV can fly at without special permission from the CAA is 400 feet (122 m) above ground level. This is normally high enough for most applications. But most importantly, the aircraft must stay in sight (VLOS).

You fly the aircraft using radio control from the ground. The signal is encrypted similar to a mobile phone signal but much safer. If there is a breakdown in the signal then safety protocols take over. The pilot has to be able to see the aircraft at all times. This is called Visual Line of Sight or VLOS.