A multiple-sweep-frequencies scheme based on Eigen-decomposition to compensate ionospheric phase contamination

This paper presents an improved scheme to compensate nonlinear phase path contamination when the backscattered signal propagates through the ionosphere in high-frequency skywave radar systems. The ionospheric variation often causes the spread of the ocean clutter spectrum in the frequency domain. The energy of the first-order component of ocean clutter dominates in Doppler spectrum, and thus its spreading may submerge the neighboring low-velocity target easily. The instantaneous frequency (IF) estimating algorithm based-on eigen-decomposition has been introduced to estimate the frequency fluctuation due to ionospheric phase path variation and the compensation is carried out before the coherent integration. In the proposed multiple-sweep-frequencies scheme we construct a new "sweep-frequency" dimension by using different transmitting frequencies, and because of the different variation of Doppler frequencies for the target echo and ocean clutter first-order Bragg lines with the varying transmitting frequencies, that can be considered as different "sweep-frequency angles", the full-rank autocorrelation matrix used in eigen-decomposition can be formed. Better estimation accuracy is achieved and significant spectral sharpening can be observed in the resultant spectrum. To avoid the additional systemic complexity due to the multiple frequencies sweep operation, a segmenting range transform in an assistant channel is proposed to obtain the 'sweep-frequencies' dimension data and estimate the ionospheric contamination. Experiments show that the proposed scheme is effective and its performance is discussed.