The signal quality of the Belgocontrol equipment can be influenced by physical obstacles, whether these are in the immediate vicinity of the antenna systems or not.
A physical object can influence electromagnetic radiation in several ways. On the one hand, it can absorb part of the radiation, causing a shadow zone to be created behind the object in which the field strength of the signal is too weak.
On the other hand, the object can reflect a part of the radiation. This reflected signal can cause interference with the directly received signal, culminating in a signal quality loss and in erratic results. Moreover, the object can create diffraction of the electromagnetic radiation.
These disruptions can have a severe impact on safety. A few examples:
- Reflections caused by ill-considered erection of buildings and other constructions can be the reason for an erroneous positioning of detected aircraft on the radar screen.
- If a construction creates a shadow zone for radar signals, aircraft in this zone will remain undetected and become an unknown factor for air traffic control.
- If the ILS (Instrument landing System) signal is disrupted by reflections on an object, there can be abrupt and sudden modifications in the approach profile, with possibly dramatic consequences for aircraft and passengers.
The signal of Belgocontrol equipment can be disrupted in such a way that the minimum signal requirements are no longer met. In that case, the equipment involved needs to be degraded or even put out of service.
Moreover, Belgocontrol's navigation, radar and communications systems can be disrupted by interferences from other transmitting equipment.
Measures of protection
The reflection of electromagnetic waves is a very complex matter and depends on the type and the frequency of the installation, the shape of the object, the dimensions, the materials used, the position and orientation with regard to the antenna… It is simply impossible to draw up just one set of rules for erecting a (temporary) construction.
That is why ICAO EUR DOC 015 presents a combination of surfaces (Building Restricted Areas) for each type of system. Based on this document servitudes have been drawn up. Objects that exceed the surfaces have to be separately examined by the relevant technical services of Belgocontrol. Only the Belgocontrol Urbanism Service can establish whether an object penetrates the servitude surfaces or not.
The routes from and to the airports are protected by the PANSOPS surfaces, as imposed by ICAO DOC.8168.Each route is protected by specific surfaces that guarantee a safety margin between the aircraft on that route on the one hand and each obstacle on the ground on the other hand.
New constructions or the raise of existing constructions (e.g. placing antennae on top of a building) that penetrate the PANSOPS surfaces constitute an obstacle with a direct impact on air traffic safety. The Urbanism Service will issue a height restriction. In some cases only a a negative opinion is possible for the obstacle concerned in order to guarantee the safety of the air traffic.Aside from the PANS-OPS another set of limiting surfaces is imposed by ICAO in ANNEX14. These surfaces, situated around the airports, do not fall under Belgocontrol's sphere of competence and are under the supervision of the airport operators involved. You can obtain more information on this subject via the Belgian Civil Aviation Authority.