  The current generation of air shower radio arrays has demonstrated that the
atmospheric depth of the shower maximum Xmax can be reconstructed with high
accuracy. These experiments are now contributing to mass composition studies in
the energy range where a transition from galactic to extragalactic cosmic-ray
sources is expected. However, we are still far away from an unambiguous
interpretation of the data. Here we propose to use radio measurements to derive
a new type of constraint on the mass composition, by reconstructing the shower
length L. The low-frequency part of the Square Kilometer Array will have an
extremely high antenna density of roughly 60.000 antennas within one square
kilometer, and is the perfect site for high-resolution studies of air showers.
In this contribution, we discuss the impact of being able to reconstruct L, and
the unique contribution that SKA can make to cosmic-ray science.
