Countermeasures and strategies

The effects of a building (object) on a radar-system system can be influenced by several physical principles

• absorption of the illuminating wave by transforming the energy into heat. A narrow band absorption can be accomplished by thin sheets and resonant absorbers, e.g. arborbers acc. the "Salisbury screen" or "Jaumann"-principles /1/, while broadband absorption requires a lot of space or volume which is unrealistic for this application. In any case the efficiency of the absorption is dependant on the angle of incidence. In case of a small grazing incidence an effective absorption is not possible. These principles are realized elsewhere in anechoic chambers and for camouflaging tasks. This method is questionable on an airport despite the frequency range of the radar and nav-systems (Table 1).

 
• destructive interference, i.e. cancellation of a certain (reflected/diffracted) signal in a certain direction by the superposition of an antiphase signal of the same amplitude and reversed phase (Fig. 6,8). By its nature this method is narrow band and spatially selective. To broaden the operating band, cascading of individual measures for adjacent bands are proposed, but are not feasible in general for the airport environment. This is the classical method for improving the matching of antenna reflectors, radomes etc. However, it is not an absorption process. In general the wave is directed in to other directions, e.g. into the interior of the building where it might be reflected again.

 
• diffus reflection and diffraction , i.e. reducing the scattering directivity (gain). This principle is realized e.g. by the serrations for the reflectors of nearfield measurement systems. Another application is to subdivide an effectively reflecting plane surface into randomly arranged sub-surfaces, random in terms of shape/volume and/or material. This method may be combined additionally with a target to suppress the reflection/diffraction in a certain direction. Depending on the realization the bandwidth can be wider. However, for the low navaids frequencies this method is unfeasible.

 
• redirection of the incoming wave into spatial directions which do not harm the radar-system. In principle this method works only for a certain direction or , in a wider sense, for a limited small spatial angular range (Fig. 6,7,8). It should be pointed out explicitly that the redirected wave is not absorbed. From an application point of view the transformation of (partially) reflected waves into the transmitted waves is also a redirection and equivalent. The real methods may be very much different for existant buildings or for buildings in planning or under construction. The required frequency range, the polarisation of the waves and the spatial directions/coverage are important factors in selecting the adequate method. In many cases there will be no reasonable answer to the problem.

This principle is tried to be realized by (additional) reflecting screens, by special multilayer glas-structures or by a suitable orientation of the building if possible and by constructing the building in an appropriate way. For the lower navaids frequencies only the shape of the building is a feasible parameter. However in any case the building or the surfaces must be large compared to the wavelength. Otherwise the surface is not reflecting in an optical sense and the effect is not broadband. E.g., reflecting screens do not work in the intended sense if they are not large compared to the wavelength.

All the additional methods are very costly in parts and put a lot of constraints on the construction of the building. Due to that, a thorough analysis and a well founded reasoning must be performed before a method is realized to protect a particular radar system. These methods are justified only when

• it is proven that this building/object is distorting the radar system in an inacceptable way. The decisive system parameter(s) and the operational system coverage on the particular airport have to be treated This proof has to be executed before a measure is realized. Otherwise a tremendous unneeded investment may be exercised.

 
• the cost/effectiveness is justified.

 
• the other systems on the airport are not affected by the realized method to protect the radar system. It is inacceptable that the modified building/object might work for a particular radar system (e.g. MSSR) in a certain direction but is catastrophic for the operationally much more important landing system ILS. There are examples on airports where buildings designed for radar operation have been terribly distorting the ILS-system.