The scattering matrix is a 2x2 square matrix containing four complex-valued polarization measurements (amplitude & phase) forming one full (= quad) polarimetric set of coherent observations. An often recorded set of polarizations is the combination: HH (horizontal receive - horizontal transmit), HV (horizontal recive - vertical transmit), VH (vertical receive - horizontal transmit) & VV (vertical receive - vertical transmit). The scattering matrix is fully suficient for describing scattering from coherent targets (dominating the resolution cell), but not for incoherent tragets (mix of scattering contributions in the resolution cell). For the latter, the coherency and the covariance matrices are the more appropriate descriptions of scattering from incoherent targets.