  ALTO is a wide field-of-view air shower detector array for very-high-energy
(VHE) gamma-ray astronomy, proposed to be installed in the Southern Hemisphere
at an altitude of about 5.1 km above sea level. The array will use water
Cherenkov detectors, as in the HAWC observatory, but combined with scintillator
detectors, to detect air showers induced by VHE gamma rays in the atmosphere.
It is being designed to attain a lower energy threshold, better energy and
angular resolution, and improved sensitivity. The array will consist of about
1250 small-sized (3.6 m diameter) detector units distributed over a circular
area of about 160 m in diameter. Each detector unit will consist of a water
Cherenkov detector and a liquid scintillation detector underneath which will
preferentially identify muons, facilitating the background (cosmic ray)
rejection, thereby improving the sensitivity. The background rejection will be
further enhanced by the close-packed arrangement and the small size of the
detectors which will allow a fine sampling of air-shower footprints at the
ground. In this contribution, we present the Monte-Carlo simulation of the
experiment performed using CORSIKA and GEANT4 simulation packages. The expected
performance of the array in terms of reconstruction accuracies of the shower
core and arrival direction, as well as preliminary estimate of the trigger
energy threshold after preliminary selection cuts for a point-like gamma-ray
source are presented.
