For permanent monitoring of the remote location in this case, a minimum of five drones are required. And this assumes zero refuelling time! Add the refuelling time, and the fleet size grows to 6 or 7 drones. Upgrading the fuel to liquid hydrogen to increase the range significantly reduces the number of drones required. Using 1kg of liquid hydrogen, rather than 1 kg of aviation kerosene, would approximately triple the range of the drone, meaning it has 12 hours of flying time.