Principles of Modern Radar - Volume 2 1891121537

17.5 2D-CCF Sidelobe Control 779higher than 240 m), the temporal ACF sidelobes at multiples of 1 μs should be reduced toavoid masking effects on targets appearing at those delays. A proper strategy to counteractthis effect is presented in the following.First, whereas the positions of these additional peaks in the ACF are a priori known,their value is data dependent and has to be estimated from the data themselves. To thispurpose the reference signal can be used to evaluate the values of the actual signal ACFat delays t k = kT sym (ˆα(t k )); these estimates can be then exploited to design a simple andeffective sidelobe reduction filter. Specifically, we start from the original CCF between thesurveillance and the reference signals for the m-th pulse, χ (m) (τ). The required sidelobesreduction can be obtained by subtracting from χ (m) (τ) a number 2K s of its replicas,appropriately scaled and delayed:χ (m)ASRF(τ) = χ (m) (τ) −K S∑k =−K Sk ≠ 0γ (m)k χ (m) (τ − kT sym ) (17.27)where K S is the number of range sidelobes to be removed on both sides of the ACFmain peak, and γ (m)k is the scaling factor for the k-th sidelobe at the m-th pulse that isobtained asγ (m)k = ˆα(m) (kT s )ˆα (m) (0)(17.28)The number K S of sidelobes to be reduced can be arbitrarily selected aiming atremoving periodical sidelobes contribution over a given range extent corresponding to thesurveillance area. The ASRF can be applied after the BWN by properly modifying thescaling factors, namely, the samples of the BWN-filtered reference signal ACF should beused.The ACF resulting from the cascade of the two different sidelobe reduction filters isshown in Figure 17-17 when the ASRF operates with K S = 3. The joint application of thetwo considered approaches in the range dimension provides a significant improvement ofthe PSLR and the effective removal of the isolated peaks, with only a very limited loss inSNR that is mainly due to the BWN.To show the benefits of the proposed techniques with reference to the target detectionissue, Figure 17-18 reports the results obtained for the same data set of Figures 17-16a–bafter applying the range sidelobes control networks (BWN + ASRF).Figure 17-18a shows the 2D-CCF evaluated without disturbance cancellation butusing a mismatched reference signal to achieve the desired sidelobe control in both therange and Doppler dimensions. In our specific case, it is not strictly necessary to apply theASRF since the maximum range searched for detection is limited to the parking area wherethe tests were performed. The 2D-CCF in the Doppler dimension shows severe sidelobesstructure that cannot be totally removed by using a conventional weighting network. In fact,these sidelobes represent Doppler ambiguities due to the nonregular beacon transmittingfrequency. Notice that, even configuring the AP to transmit the beacon signal at 1 msintervals, we experienced a variable beacon repetition period due to the capability ofthe WiFi system to “sniff” the shared medium before starting transmission. In contrast,the range sidelobes related to the Barker code due to the direct signal and other multipathcontributions have been significantly reduced with respect to Figure 17-16a. Consequently,isolated peaks now appear at zero velocity at ranges of about 70 m, 100 m, and 135 m,