  A linear response, local model for the DBS amplitude applied to gyrokinetic
simulations shows that radial correlation Doppler reflectometry measurements
(RCDR, Schirmer et al., Plasma Phys. Control. Fusion 49 1019 (2007)) are not
sensitive to the average turbulence radial correlation length, but to a
correlation length that depends on the binormal wavenumber $k_\perp$ selected
by the Doppler backscattering (DBS) signal. Nonlinear gyrokinetic simulations
show that the turbulence naturally exhibits a non-separable power law spectrum
in wavenumber space, leading to a power law dependence of the radial
correlation length with binormal wavenumber $l_r \sim C k_\perp^{-\alpha}
(\alpha \approx 1)$ which agrees with the inverse proportionality relationship
between the measured $l_r$ and $k_\perp $ in experiments (Fernandez-Marina et
al., Nucl. Fusion 54 072001 (2014)). This offers the possibility of
characterizing the eddy aspect ratio in the perpendicular plane to the magnetic
field and motivates future use of a non-separable turbulent spectrum to
quantitatively interpret RCDR and potentially other turbulence diagnostics. The
radial correlation length is only measurable when the radial resolution at the
cutoff location $W_n$ satisfies $W_n \ll l_r$, while the measurement becomes
dominated by $W_n$ for $W_n \gg l_r$. This suggests that $l_r$ is likely
inaccessible for electron-scale DBS measurements ($k_\perp\rho_s > 1$). The
effect of $W_n$ on ion-scale radial correlation lengths could be
non-negligible.
