A hybrid time calibration method for EAS ground-based timing arrays

In a ground-based Cherenkov EAS timing array, the reconstruction of the shower direction is based on the measured photon arrival times at the sensor stations. Relative time offset calibration down to sub-nsec precision is needed for optimal pointing precision. For large arrays, a direct time calibration by artificial light sources introduces a large effort. We discuss a new, hybrid method, that combines information obtained from extended air-showers and a few, dedicated individual station calibrations. We discuss the effect of using exclusively EAS information. The proposed method is particularly useful for large, several squared-km arrays.