## 2 Detection Techniques
Shower arrays are made by a large number of detectors (scintillators, Resistive Plate Chambers (RPCs) or water Cherenkov tanks, for example) distributed in a regular grid over very large areas, of an order of 10\({}^{4}\)-10\({}^{5}\) m\({}^{2}\) (see Figure 2). The shower _\"size\"_, the total number of charged particles, and the shower arrival direction are the two key parameters reconstructed by all arrays. The majority of EAS arrays do not distinguish between the charged particles. From the measurement of the particle densities on the fired detectors of the array it is possible to determine the shower core position, that is, the point where the shower axis intersects the detection plane, and, via a Lateral Density Function (LDF), reconstruct the size of the shower. The LDF is of phenomenological nature, determined via Monte Carlo simulations for the particular experimental set-up [20]. The direction of the incoming primary particle is reconstructed with a _'time of flight'_ method making use of the relative times at which the individual detection units are fired by the shower front [20]. On general grounds, the instrumented area A determines the rate of high energy events recorded, that is, the maximum energy via limited statistics. The grid distance \(d\) determines the low energy threshold (small energy showers are lost in the gap between detectors) and the quality of the shower sampling. The particular kind of detector (scintillator, RPC, water tank) determines the detail of measurement (efficiency, resolution, energy threshold, quality) and impact on the cost per detector \(C_{d}\). In principle, best physics requires large area \(A\), small distance \(d\) and high quality of the sampling. However, the cost of an array increases with \(C_{d}\cdot A/d^{2}\), therefore a compromise is always needed. This is one of the reason why the typical total sensitive area of a classical array is less than 1% of the total enclosed area. This results in a high degree of uncertainty in the reconstruction due to sampling fluctuations which add to the shower fluctuations. The experiments devoted to study the PeV energy range have been operated at different altitudes, ranging from the extreme altitude (5200 m asl) of BASJE-MAS [21] to the sea level of KASCADE [22; 23; 24]. In Tables 1 and 2, the characteristics of air shower arrays operated in the last two decades to study Galactic CR physics from ground are summarized. The atmospheric depths of the arrays, the main detectors used, the energy range investigated, the sensitive areas of e.m. and muon detectors, the instrumented areas and the coverage (i.e., the ratio between sensitive and instrumented areas) are reported. The depth in atmosphere is crucial to fix the energy threshold, the energy resolution, the impact of shower-to-shower fluctuations, then the sensitivity to elemental composition. Figure 2: Example of a typical air shower array (Tibet AS\({}_{7}\) experiment located at the YangBaJing Cosmic Ray Observatory in Tibet (P.R. China) 4300 m asl). Generally speaking, near the depth of the maximum of the shower development, the number of secondary charged particles is almost independent of the mass of the primary particle, and the shower fluctuations are at minimum. For the knee energy region, this depth corresponds to \(\approx\)5000 m asl. Therefore, these extreme altitudes are suitable to have good energy resolution, to reconstruct the primary energy in a mass-independent way and to study the shower core region in great detail, where the hadronic component feeds the e.m. one deep in the atmosphere. As demonstrated by the ARGO-YBJ [30] and Tibet AS\(\gamma\)[31] experiments, observables related to the shower core properties are almost independent on the details of hadronic interaction models. At high altitudes, due the low energy threshold (\(\approx\)TeV), it is possible to cross-check the fluxes with direct measurements on a wide energy range (ARGO-YBJ) in the 5-250 TeV range).