The Calibration of the Geometry and Antenna delay in Askaryan Radio Array Station 4 and 5

The Askaryan Radio Array (ARA) experiment at the South Pole is designed to detect the radio signals produced by ultra high energy cosmic neutrino interactions in the ice. There are 5 independent ARA stations, one of which (A5) includes a low-threshold phased array trigger string. Each ARA station is designed to work as an autonomous detector. The Data Acquisition System in all ARA stations is equipped with the Ice Ray Sampler second-generation (IRS2) chip, a custom-made, application-specific integrated circuit (ASIC) for high-speed sampling and digitization. In this contribution, we describe the methodology used to calibrate the IRS2 digitizer chip and the station geometry, namely the relative timing between each pair of ARA antennas, deployed at 200 m below the Antarctic ice surface, and their geometrical positions in the ice, for ARA stations 4 and 5. Our calibration allows for proper timing correlations between incoming signals, which is crucial for radio vertex reconstruction and thus detection of ultra high energy neutrinos through the Askaryan effect. We achieve a signal timing precision on a sub-nanosecond level and an antenna position precision within 10 cm.