High-energy cosmic rays impinging on the atmosphere of the Earth induce cascades of secondary particles, the extensive air showers. Many particles in the showers are electrons and positrons, which due to interactions with the magnetic field of the Earth emit radiation with frequencies of several tens of MHz. In the last years, huge progress has been made in measuring the characteristics of extensive air showers through their radio signal at these frequencies.
The radio technique is now routinely applied to measure the properties of cosmic rays, such as their arrival direction, their energy, and their particle type/mass. Air showers with zenith angles above 60$^\circ$ have a large footprint of the radio emission on the ground which can be detected with sparse arrays with kilometer-scale spacing. With the Auger Engineering Radio Array (AERA) these "horizontal air showers" are measured, demonstrating the feasibility of the radio technique for highly inclined showers.
At present, the Auger Collaboration is upgrading its detectors. The upgrade includes the installation of radio antennas on each of the 1661 water-Cherenkov detectors of the array. The main objective of the radio upgrade (RD) is to measure horizontal air showers and to determine the properties of cosmic rays up to the highest energies. The combination of water-Cherenkov detectors and radio antennas will provide muon-electron separation for horizontal air showers at the highest energies.
Details of the technical implementation and the expected performance will be presented.