Improving the accuracy of experimental prediction of the detectability of nonlinear objects by radar

Abstract

The possibility of calibration of nonlinear radar measuring systems using a physical standard scatterer is investigated. Suggestions on the shape of such a scatterer at super-high frequencies are formulated.Experimental prediction of radar detectability of objects with nonlinear electromagnetic properties includes measurement of their effective scattering cross section (ESCS) at harmonic frequencies of the incident wave [1]. Nonlinear scatterers include metal constructions containing butt, welded, or screwed joints, and semiconductor components of electronic devices [2].According to [1], the ESCS of an object at the nth harmonic frequency is defined as the ratio of the power Pn of the secondary field at input of the receiver of a radar measuring system (RMS) to the energy flux density rl o of the incident wave at the scatterer kx:ation. A distinctive t~ature of nonlinear scatterers is the dependence of the energy flux density of the secondary field l-! n on I-I 0, which is given by the power function Pn = ccFlg (where cz is constant tbr a given distance from the scatterer) [1][2][3]. Consequently, the ESCS of an object at the nth harmonic frequency is measured with a fixed energy flux density of the incident field. The incident wave power, needed to ensure the maximum signal-to-noise ratio at the RMS receiver input, is found from the relation rl n = F(rlo).The block diagram of a typical RMS for measuring the ESCS of objects at harmonic frequencies is shown in Fig. 1.The energy flux density l'I n (n = 2, 3) of the secondary field of the object at the second and third harmonic frequencies does not exceed -(40-50) dB relative to energy flux density of the field scattered at the radiation frequency [ 1, 2]. Higher harmonic frequencies are of no practical interest for estimation of radar detectability since the energy flux density of secondary radiation decreases according to 1/r 2(n+l) with the distance r to the object [4], and according to 1/n 2 with the harmonic frequency number (for n > 3) [5].To measure the ESCS of scatterers at harmonic frequencies, RMSs must be calibrated. Calibration consists in establishing a correspondence between the wave power Po radiated by the transmitting antenna of the system and the power of the secondary field at the receiving antenna input [5, 6]. Because of the lack of certified "'nonlinear" scatterers, RMSs are now calibrated by the method of indirect standards [5, 7]. The method is based on separate calibration of the transmitting and receiving channels of RMSs with the aid of standard antennas [5, 9] and consists of two steps: determination of the thctor of correspondence K l between the power Po and the enemy flux density of the incident wave 1-I o on the scatterer,determination of the factor of correstxmdence K 2 between the energy flux density FI n of the nth harmonic (n = 2, 3) secondary field and the power P, at the RMS receiver input, K 2 = l-ln/P n.