New method for the time calibration of an interferometric radio antenna array

摘要

Digital radio antenna arrays, like LOPES (LOFAR PrototypE Station), detecthigh-energy cosmic rays via the radio emission from atmospheric extensive airshowers. LOPES is an array of dipole antennas placed within and triggered bythe KASCADE-Grande experiment on site of the Karlsruhe Institute of Technology,Germany. The antennas are digitally combined to build a radio interferometer byforming a beam into the air shower arrival direction which allows measurementseven at low signal-to-noise ratios in individual antennas. This techniquerequires a precise time calibration. A combination of several calibration stepsis used to achieve the necessary timing accuracy of about 1 ns. The groupdelays of the setup are measured, the frequency dependence of these delays(dispersion) is corrected in the subsequent data analysis, and variations ofthe delays with time are monitored. We use a transmitting reference antenna, abeacon, which continuously emits sine waves at known frequencies. Variations ofthe relative delays between the antennas can be detected and corrected for ateach recorded event by measuring the phases at the beacon frequencies.