  The Auger Engineering Radio Array (AERA) is an extension of the Pierre Auger
Cosmic-Ray Observatory. It is used to detect radio emission from extensive air
showers with energies beyond $10^{17}~$eV in the $30 - 80~$MHz frequency band.
After three phases of deployment, AERA now consists of more than 150 autonomous
radio stations with different spacings, covering an area of about $17~$km$^2$.
It is located at the same site as other Auger low-energy detector extensions
enabling combinations with various other measurement techniques. The radio
array allows different technical schemes to be explored as well as
cross-calibration of our measurements with the established baseline detectors
of the Auger Observatory. We report on the most recent technological
developments and give an overview of the experimental results obtained with
AERA. In particular, we will present the measurement of the radiation energy,
i.e., the amount of energy that is emitted by the air shower in the form of
radio emission, and its dependence on the cosmic-ray energy by comparing with
the measurement of the the well-calibrated Auger surface detector. Furthermore,
we outline the relevance of this result for the absolute calibration of the
energy scale of cosmic-ray observatories.
