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.