Procedural control

Procedural control is a method of providing air traffic control services without the use of radar. It is used in regions of the world, specifically sparsely populated land areas and oceans, where radar coverage is either prohibitively expensive or is simply not feasible. It also may be used at very low-traffic airports, or at other airports at night when the traffic levels may not justify staffing the radar control positions, or as a back-up system in the case of radar failure.

In air traffic control, the risk of aircraft colliding is managed by applying separation rules. These rules require aircraft to be separated by either a minimum vertical distance, or if vertical separation is not feasible, by a minimum horizontal distance defined by various means. One of the means of determining horizontal separation is by a controller observing the radar returns of the aircraft to be at least a minimum horizontal distance apart. This is the essence of radar control and is probably the form of air traffic control most familiar to lay people.

However, in times gone by radar was not very common, and in certain parts of the world today it still is not common, on grounds of cost or technical feasibility. Procedural control is a form of air traffic control that can be provided to aircraft in regions without radar, by providing horizontal separation based upon time, the geography of predetermined routes, or aircraft position reports based upon ground-based navigation aids, for those aircraft that are not vertically separated.

The central rule of procedural control is that each aircraft is cleared onto a predetermined route (airway), and no aircraft traveling on the same or intersecting routes at the same level shall come within 10 minutes' flying time of another (or sometimes 15 minutes depending on the accuracy of the available radio navigation beacons).

Using procedural control, the controller must maintain a mental picture of the location of aircraft based on each aircraft's flight progress strip, which contains its route, altitude and estimated times over reporting points. That information is compared against all other aircraft in the sector to determine if there are any conflicts. For aircraft that conflict, the controller issues an altitude, speed or routing change that both separates the conflicting aircraft from each other, while still remaining separated from all others. After all conflicts have been resolved in this way, the sector is considered "separated" and the controller only needs to check again for conflicts when new aircraft are added, or if an aircraft needs to change its altitude, or if the aircraft reaches a reporting point significantly earlier or later than previously estimated.

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Wikipedia