Our charges are made to the owners and operators of aircraft. In the case of airlines, our charges are to them. The charges to the airlines are weight- and distance-related. The larger the weight of the aircraft and the farther it flies in the airspace we're looking after, then the greater the charge.
With respect to privately owned aircraft, general aviation aircraft, it's generally just a flat annual fee, much like you pay for a licence on your car. But it's not a big factor.
So in terms of formula, it's weight- and distance-related charges to the airline itself. Probably over 90% of the countries in the world use that same formula.