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 and based on the assumption of locally flat shower front; accordingly, taken a small portion 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. In a second step, the time offsets among the different portions are measured and the complete detector calibration is obtained. The implementation of this algorithm in an online environment like the ARGO-YBJ data acquisition system has been studied. The results of this investigation are reported and discussed.