Some Examples and Results

Aircraft on airports (Fig. 2) are potentially dangerous for the landing systems which are serving their guidance for the safe landing even under worst condition in case. The aircraft are taxiing after the roll-off or before starting in the radiation field of the landing systems. Aircraft today can be very large, considering the 747-type or even in future the Airbus A3xx-type. The scattering effects of the aircraft are almost impossible to estimate but has to be calculated numerically by the adequate methods having in mind that the aircraft are taxiing close to the ground in the radiation field of the horizontally polarized antenna of the ILS-Localizer. It has been decided to apply an approximate improved Physical Optics method (IPO; see above) which application has to be justified by a more rigorous method, namely the Method of Moments MoM. Fig. 4 shows a numerical calculation of the MoM surface-currents on a 747-type aircraft. The large rim currents as well as the increased currents on the higher tail fin can be nicely seen. It should be pointed out that the ground is included in this calculation in contrast to the standard RCS-treatment using the plane wave approach.

Cranes on airports are a particular problem, because they are used during the construction of hangars and terminals and have a large height and large horizontal dimensions co-polarized to the ILS-Localizers. Usually arrays of such tower cranes (Fig 3) are used. The horizontal jibs are turned according the constructional needs and also due to the wind conditions which may create dangerous worst case conditions. The jibs have been modeled for the analysis by the Method of Moments MoM. Fig. 5 shows the horizontal scattering patterns for 3 different jib positions. It can be clearly seen that scattering maxima are varying depending on the jib angle. However, the forward scattering beam is formed in any case.

Humped runways on airportsare a particular problem for the landing systems, for the ILS and in particular for the MLS. The specific problem of the humped runway is that that the aircraft are landing in the "shadow" of the antennas of the landing systems. Distorting scatterers are located in many cases in the region of the hump and its scattering pattern is affecting much less than "wanted direct signal". By that the electrical distortions are amplified virtually. The basic task is to calculate numerically the exact direct signal in the shadow of the hump. Fig. 6 shows the adequately optimized application of the so-called parabolic equation PE for the calculation of the fieldstrength in the required height of 4m. The comparison shows an excellent agreement between the measurement and the calculation.
 

Conclusion