A CANCELLATION METHOD WITH RHOMBIC MESH AND ITS APPLICATION TO FALSE TARGET ELIMINATION IN ATCRBS
1984 Volume 1984 Issue 47 Pages 1-14
Details
1984 Volume 1984 Issue 47 Pages 1-14
The problem of false targets due to reflection by buildings, towers, hangers etc., have long been hindering the normal function of ATCRBS (Air Traffic Control Rader Beacon System), which imposes more workload on air traffic controllers and sometimes generates confusing situations.
Several preventive remedies, e.g., ISLS (Improved Side Lobe Suppression equipment) and the open array antenna, have been developed to cope with the problem.
However, false targets still remain due to the technical and economical limitations to the preventive remedies.
The purpose of this paper is to provide a new technique for these situations, which is dealt with as a part of studies on architectural control. A cancellation technique with rhombic mesh is deviced to cancell the reflected wave and proves to be effective by experiments reported here. Therefore, this technique has been applied to elminating false targets of ATCRBS. Its effectiveness has also been shown by evaluation tests including the flight test at the Sendai Airport.
Firstly, this paper briefly describes producing-mechanism of false targets in ATCRBS and surveys present remedies, which are only partially successful because of practical limitation. In the case of the problem at the Sendai Airport with considerably serious trouble, those remedies turned out to be ineffective.
A method of identification and estimation of the reflecting surface causing false targets is also shown as follows.
Pr=(ρEr/E0)2×P0………………(1)
where Pr, ρ, E, Er and P0 are the power reflected by a obstruction, the reflection coefficient of the surface, the electric field of the direct wave, that of the reflected wave via obstruction and the power of the conventional rader epuation of ATCRBS respectively.
Secondly, the fundamentals of a cancellation technique and the characteristics of rhombic mesh are investigated. Main components consist of a transmissible grating and a reflective screen, which are parallel to each other. The transmissivity of the grating is about 0.67 and the separation of these two is
d=(λ-4δ)/4 sin θ
where d, λ, δ and θ are the separation, the wave length, the phase delay of transmissivity through the grating and the grazing angle respectively. A grating with rhombic mesh is easy to control the transmissivity precisely by rotating the rhombic mesh to the direction of polarization despite of changing the size of rhombic mesh, and hence is employed here. The cancellation technique with rhombic mesh is devised and studied experimentally to obtain the parameters of a new cancellation panel. Attenuation over 18 dB is obtained in the direction of specular reflection.
Thirdly, the application of this technique to eliminating false targets in ATCRBS is studied experimentally. A test pannel, 2m×2m, for the Sendai Airport is developped and its parameters are determined from the above results as d=0.43λ, ϕ=18.5° at an incident angle of 20° when the size of rhombic mesh is Lw=0.73λ, Sw=0.36λ, where ϕ, Lw and Sw are the angle between the long dimension of mesh and the direction of polarzation, the long dimension of mesh and the short one.
The performance of the pannel is studied experimentally in the annechoic chamber and on the antenna testing tower.
Attenuation over 10dB is obtained for all angles, with a value of 18dB in the direction of specular reflection.
The pannel is expected to eliminate false targets at the Sendai Airport, since the control cower is identified as the reflecting source and their estimated levels requires attenuation over 10dB with the equation (1).
And hence a practical model, 4.5m×6.6m, is manufactured with alminium wire as a transmissible grating and with punching metal as a reflecting screen, of which the parameters are the same mentioned above.
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