At ground level, surface detector arrays sample the secondary particles from extensive air showers induced by primary cosmic rays with ultra-high energy interacting with the Earth's atmosphere. The recorded events are used to study the energy spectrum, arrival directions, and mass composition of the cosmic rays.
The surface detector of the Pierre Auger Observatory comprises 1660 water-Cherenkov stations deployed on a tessellated triangular grid. Each station is filled with 12 tons of ultra-purified water viewed by three 9-inch photomultipliers, which record the Cherenkov light produced mainly by electrons and muons entering the detector. Physics events are selected by exploring the temporal and spatial combination of the individual detector information in real time. The reconstruction algorithm determines the arrival direction and the energy of the primary particle, and will be described for selected events with zenith angles less than $60^\circ$ (a dataset referred to as vertical). The presented reconstruction procedure has been successfully applied to measured and simulated showers.