Accuracy of energy measurements at cosmic-ray observatories

Ultra-high-energy cosmic rays are measured indirectly via air showers which are huge particle cascades that develop in the atmosphere. Thereby the atmosphere acts as a giant calorimeter that is read out from the ground. The currently most accurate method is the measurement of fluorescence light where the systematic uncertainty in the energy measurement can be reduced to 14% requiring extensive monitoring efforts as the creation as well as the attenuation of fluorescence light depend on the actual conditions of the atmosphere. In this contribution, we present an alternative detection technique that is less dependent on atmospheric conditions and thus potentially reduces the systematic uncertainty in the cosmic-ray energy measurement significantly. Air showers emit short radio pulses in the MHz regime such that the cosmic-ray energy can be determined via the measurement of the energy radiated by the air shower in form of radio waves. Two attractive aspects are that the atmosphere is transparent to MHz radio waves and the radio emission can be calculated from first-principles using classical electrodynamics. We present the systematic uncertainties of the radio method and the efforts to apply the radio technique for an improved absolute energy calibration of the Pierre Auger Observatory.