P.V. Baybakov – Head of Sector,

JSC «VNIIRT» (Moscow)

E-mail: oscillatorium@gmail.com

V.N. Zhurakovsky – Ph.D. (Eng.), Associate Professor,

Bauman Moscow State Technical University

E-mail: vnzh521@yandex.ru

In radar detection complexes, there is the problem of measuring an aircraft elevation angle under the influence of passive interference in the form of reflection the target signal from the underlying surface. Such interference distorts the radar signal amplitudephase distribution in the receiver, which complicates the object elevation determination. 

Amplitude methods based on using the information about the antenna pattern do not allow to take into account the interference  influence on the received signal, since this effect is of a quasi-random nature and depends on many factors: distance to the target, actual elevation of the target, reflective properties of the underlying surface, angle of the surface, propagation medium properties. Thus, such methods, for all their simplicity and the results adequacy obtained outside the interference zone, do not provide satisfactory results for low-angle tracking purposes, because with multi-channel reception in the antenna system, the input data vector  corresponding to the channel ratio in the analytically obtained antenna pattern is not match. To analyze this effect and test various elevation measuring methods, a mathematical model was developed. It forms the input signal of the antenna system under the influence of the above factors. Also, this model allow us to create a statistical information data about input signals under deterministic conditions for the direct and reflected signals formation (a surface slope, a surface reflection coefficient, etc.). 

Based on these results, the possibility of applying the maximum likelihood method to estimate the elevation angle of an aircraft was considered. The objective function in this method is a function that takes into account signal amplitudes and phases similarity of the antenna system channels input vector with the vector from the statistical data. The amplitudes similarity estimates by a cosine measure and is taken into account with a certain weighting factor in the objective function. Due to the randomness of the signal phases in the receivers for the phase information analysis, phase differences use in the receiving channels, and do not depend on the signal  initial phase. The similarity of differences estimates by the angular deviation from the statistical information and applies with its own weighting factor in the objective function. The objective function result is the vector from the statistical data that is the most similar to the input data vector, which, among other things, has information about the true target angle. As a result of this approach, it is possible to significantly improve the accuracy of determining the target parameters in the passive interference influence zone, while not compromising the quality of parameter measurements outside this zone. 

This approach has some drawbacks, including: the model adequacy control, long time to calculate the statistical data, long time to search for the desired vector, strong attachment to the antenna system parameters taken into account when calculating the statistical data – receiving beams angles, the signal carrier frequency, etc. Insufficient search speed of the needed vector can be corrected by developing specialized algorithms based on machine learning methods, such as using decision trees, kd-trees and other wellknown models. A statistical data can be generated in advance in a multiprocessor system using multi-threaded mode, which increases the speed and, as a result, the accuracy of the data.

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