## 2 Detection Techniques
This cross-calibration is important due to the conflicting results obtained not only by ground-based detectors, but also by direct experiments. In addition, the absolute energy scale can be calibrated at a level of 10%, exploiting the so-called _\"Moon Shadow\"_ technique [20]. On the other hand, experiments located deep in the atmosphere enhance the differences in the longitudinal development of EAS of different primary masses, as the shower is sampled well beyond its maximum. Therefore, the ratio N\({}_{e}\)/N\({}_{\mu}\) is, in principle, more suitable for elemental composition studies. However, shower fluctuations are much larger, making it difficult to interpret the data. In addition, the reconstruction of the energy is typically strongly model-dependent because 'a priori' assumptions on the primary composition are needed, and the calibration of the absolute energy scale is one of the major open issues. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Experiment** & **g/cm2** & **Detector** & **AE (eV)** & **e.m. Sens. Area (m2)** & **Instr. Area (m2)** & **Coverage** \\ \hline ARGO-YBJ [20] & 606 & RPC/hybrid with wide-FoV C Tel. & \(3\times 10^{11}\)–\(10^{16}\) & 6700 & 11,000 & 0.93 \\  & & & & & (carpet) \\ \hline BASJE-MAS [21] & 550 & scint./muon & \(6\times 10^{12}\)–\(3.5\times 10^{16}\) & \(10^{4}\) & \\ \hline TIBIET AS\(\gamma\)[25] & 606 & scint./burst det. & \(5\times 10^{13}\)–\(10^{17}\) & 380 & \(3.7\times 10^{4}\) & \(10^{-2}\) \\ \hline CASA-MIA [26] & 860 & scint./muon & \(10^{14}\)–\(3.5\times 10^{16}\) & \(1.6\times 10^{3}\) & \(2.3\times 10^{5}\) & \(7\times 10^{-3}\) \\ \hline KASCADE [22] & 1020 & scint./mu/had & \(2\times 10^{15}\)–\(10^{17}\) & \(5\times 10^{2}\) & \(4\times 10^{4}\) & \(1.2\times 10^{-2}\) \\ \hline KASCADE-Grande [27] & 1020 & scint./mu/had & \(10^{16}\)–\(10^{18}\) & 370 & \(5\times 10^{5}\) & \(7\times 10^{-4}\) \\ \hline Tunka [28] & 900 & open C det. & \(3\times 10^{15}\)–\(3\times 10^{18}\) & — & \(10^{6}\) & — \\ \hline IceTop [29] & 680 & ice C det. & \(10^{16}\)–\(10^{18}\) & \(4.2\times 10^{2}\) & \(10^{6}\) & \(4\times 10^{-4}\) \\ \hline LHAASO [11] & 600 & Water Č scint./muon/had wide-FoV Č Tel. & \(10^{12}\)–\(10^{17}\) & \(5.2\times 10^{3}\) & \(1.3\times 10^{6}\) & \(4\times 10^{-3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of different air shower arrays. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Experiment** & **Altitude** & **\(\mu\) Sensitive Area** & **Instrumented Area** & **Coverage** \\  & **(m)** & **(m2)** & **(m2)** & **Coverage** \\ \hline LHAASO & 4410 & \(4.2\times 10^{4}\) & \(10^{6}\) & \(4.4\times 10^{-2}\) \\ TIBIET AS\(\gamma\) & 4300 & \(4.5\times 10^{3}\) & \(3.7\times 10^{4}\) & \(1.2\times 10^{-1}\) \\ KASCADE & 110 & \(6\times 10^{2}\) & \(4\times 10^{4}\) & \(1.5\times 10^{-2}\) \\ CASA-MIA & 1450 & \(2.5\times 10^{3}\) & \(2.3\times 10^{5}\) & \(1.1\times 10^{-2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristics of different muon detectors operated in some shower arrays. The great variety of layouts, observables, and reconstruction procedures to infer the elemental composition is at the origin, in part, of the conflicting results reported by different ground-based experiments. Arrays focused on the investigation of the knee region operated so far are also characterized by a limited size of the instrumented area. They collected limited statistics above \(10^{16}\) eV, and were, therefore, unable to give a conclusive answer to the origin of the knee. The poor sensitivity to elemental composition, due to the small statistics, prevents discrimination against different mass groups, and only general trends can be investigated in terms of the evolution of \(\langle\ln A\rangle\) or of _\"light\"_ and _\"heavy\"_ components with energy.