Online software time calibration for a continuous air shower array

Time calibration is a crucial item for a shower array performance as it uses the time of flight method to reconstruct the arrival direction of the primary particle. This paper presents a software time calibration algorithm exploiting the continuous detector feature based on the assumption of locally flat shower front. On the small portions of the detector (tens of m²) a simple time-position fit of the arriving particles provides the time calibration constants of that part of detector. Then, a second step is needed to measure the time offset among the different portions obtaining the complete detector calibration. This algorithm is applied to ARGO-YBJ detector with a high trigger rate (several kHz) by means of an online processing and its performance is discussed.