Only a handful of specialist aircraft have been available as well as UAV's (Unmanned Aerial Vehicles).
Most of these aircraft and Helicopters although they do the job well, are very expensive to acquire and maintain and requiring very specialist crews and engineers to support them. Limited numbers due to high purchase costs are available within a government's fleet and thus a less than adequate patrol schedule is met. Other limiting factors for these aircraft are range and capability in order to perform their missions perfectly and economically. UAV's for example are great patrol tools, however, they lack payload and are unable to carry crews, making them perfect for detection only. Their inability to be operated off site is also a hindrance the as the aircraft always have to return back to the base of departure. Expensive to purchase and very limited capability.
Helicopters are probably the most popular form of surveillance and patrol aircraft globally, however, limited range, extremely high acquisition costs and equally high operating costs do not make them the most cost affective choice, especially if you need to cover vast areas. Of course they have hover capability but that's not always essential when patrolling and detection is your biggest requirement.