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The application of time domain transformations to increase the dynamic range in the measurements of backscattering diagrams of the objects

DOI 10.18127/j00338486-201904-10


S.V. Elizarov – Ph.D.(Eng.), Head of Sector, PJSC «Radiofizika» (Moscow); Lecturer, Moscow Institute of Physics and Technology (State University)
S.A. Kiselev – Engineer of the 3rd category, PJSC «Radiofizika» (Moscow) 
E.V. Korotetsky – Engineer, PJSC «Radiofizika» (Moscow)
A.V. Tikhonova – Student, Moscow Institute of Physics and Technology (State University); Engineer, PJSC «Radiofizika» (Moscow)

The method of measuring backscattering diagrams of the object by the method of time domain transformations is studied in this paper. Such measurements can be performed in the anechoic chambers with poor anechoic conditions. The algorithm for reducing the residual background of the anechoic chamber is improved using software with the procedure of lowering unwanted signals in the time domain (gating). The practical significance of this method lies in the technology of choosing a temporary window for gating, depending on the presence or absence of cavities in the investigated object.
Basically noise is formed with the re-reflection of the signal from the elements of the environment (floor and walls of the anechoic chamber) during the measurements of backscattering characteristics reducing the dynamic range of the measurements.
It is possible to reduce the influence of interference signals by closing the reflection area with effective radio-absorbing material. In order to minimize parasitic reflections of electromagnetic field caused by environment we can also use mathematical processing to filter the received signal.
The process with a complex signal transmission coefficient can be carried out both in the frequency domain and in the time domain. Usually it is better to work with the frequency domain. It is necessary to filter interference signals for the measurements under poor anechoic conditions. In the time domain, unwanted harmonics in the spectrum of the signal are usually eliminated.
The conversion of the signal from the frequency domain to the time domain and back with the removal of unwanted interference signals is considered in this paper. In this case, we do not work with the analytical representation of the signal, but with a fragment measured in a certain frequency range.
The problem of calculating the optimal «window» functions is actually reduced to the mathematical problem of finding time-limited functions. To solve this problem Kaiser proposed a relatively simple approximation of these functions (the Kaiser window) as the best function.
The mechanism of getting rid of unwanted harmonics is the same conformal window, but in the time domain (gating). Thus, we reduce the level of the residual background of the anechoic chamber in the measurements of backscattering diagrams by post-processing of the initial data, increasing their reliability at low radar cross section values of the studied objects.
This paper presents two examples demonstrating the possibility to measure the objects with low backscattering levels with interference signals in the anechoic chamber of PJSC «Radiofizika». The algorithm of processing the experimental backscattering diagrams of the objects (401 frequency point in X range with rotation angle increments of 0,036°) using the «Time Domain» function was worked out on the metal reference bodies (cylinder and cylinder with a cavity at one end). Measurements and verification of backscattering diagrams of reference objects in X range in the specified sectors of the location angles are also carried out. All measurements are made for the case of monostatic location. The shape of the backscattering diagram and the median value of the radar cross section of the object were chosen as criteria for comparing the results of the accurate calculation and the experiment.
The analysis of the obtained results showed that the calculated and experimental backscattering diagrams of the reference bodies measured in an anechoic chamber with a low level of residual background coincided with the accuracy acceptable for such measure-ments.

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