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

TL;DR

This paper presents a novel time calibration method for interferometric radio antenna arrays, crucial for accurate cosmic ray detection, using a beacon-based phase measurement technique to achieve about 1 ns timing precision.

Contribution

The paper introduces a new calibration approach employing a continuous sine wave beacon to monitor and correct antenna delay variations in real-time.

Findings

Achieved approximately 1 ns timing accuracy.

Demonstrated effective correction of delay variations.

Validated method with the LOPES array data.

Abstract

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