  Cross-correlated contrast source inversion (CC-CSI) is a non-linear iterative
inversion method that is proposed recently for solving the inverse scattering
problems. In CC-CSI, a cross-correlated error is constructed and introduced to
the cost functional, which improves the inversion ability when compared to the
classical design of the cost functional by exploiting the mismatch between the
data error and state error. In this paper, the multi-frequency inversion for
electromagnetic waves is considered and a multi-frequency version of CC-CSI is
proposed. Numerical and experimental inversion results of both transverse
magnetic (TM) and transverse electric (TE) polarization demonstrate that, when
multi-frequency data are available, CC-CSI still outperforms the
multiplicative-regularized CSI method (MR-CSI) in the inversion of more
complicated scatterers.
