Coherent radio signals in the MHz range are emitted from extensive air showers initiated by high-energy cosmic rays. Observing this emission enables precise measurement of the energy of the primary particle. Compared with those made with the well-established fluorescence technique, radio measurements are less dependent on atmospheric conditions, and thus offer the potential of energy determination with reduced systematic uncertainties. Combining these accurate measurements with absolute predictions of the radio signal by first-principle calculations based on classical electrodynamics then allows a determination of the cosmic-ray energy scale.
This approach is discussed in the context of the Auger Engineering Radio Array (AERA), sited at the Pierre Auger Observatory. More than 150 autonomous radio stations, covering about $17\,\rm{km}^{2}$, are used to record radio emission in the $30$ to $80\,\rm{MHz}$ band from showers produced by primary particles with energies of about $10^{18}\,\rm{eV}$.
The systematic uncertainties of the measurement using AERA are presented. The calibration of the antenna is identified as the dominant uncertainty. The antenna response was measured in a recent calibration campaign using a flying drone. To this end, an octocopter was used to place a calibrated source at any position above the array. The campaign results in an antenna calibration accuracy of about $10\,\rm{\%}$.