On the measurement of the proton-air cross section using longitudinal   shower profiles
  In this paper, we will discuss the prospects of deducing the proton-air cross section from fluorescence telescope measurements of extensive air showers. As it is not possible to observe the point of first interaction $X_{\rm 1}$ directly, other observables closely linked to $X_{\rm 1}$ must be inferred from the longitudinal profiles. This introduces a dependence on the models used to describe the shower development. The most straightforward candidate for a good correlation to $X_{\rm 1}$ is the depth of shower maximum $X_{\rm max}$. We will discuss the sensitivity of an $X_{\rm max}$-based analysis on $\sigma_{\rm p-air}$ and quantify the systematic uncertainties arising from the model dependence, parameters of the reconstruction method itself and a possible non-proton contamination of the selected shower sample. 
Ralf Ulrich, Johannes Bl\"umer, Ralph Engel, Fabian Sch\"ussler,   Michael Unger