  The calculation of transport profiles from experimental measurements belongs
in the category of inverse problems which are known to come with issues of
ill-conditioning or singularity. A reformulation of the calculation, the
matricial approach, is proposed for periodically modulated experiments, within
the context of the standard advection-diffusion model where these issues are
related to the vanishing of the determinant of a 2x2 matrix. This sheds light
on the accuracy of calculations with transport codes, and provides a path for a
more precise assessment of the profiles and of the related uncertainty.
