  Ultra-high-energy cosmic rays (UHECR), of energy >10 EeV, arrive at the Earth
regularly, but their sources, acceleration mechanisms, details of propagation
through the universe, and particle composition remain mysteries. In addition,
their interactions with the atmosphere show an unexpectedly high muon flux
compared to simulations. To address these issues, the Pierre Auger Observatory,
a hybrid 3000 square km ground based cosmic ray detector, is being upgraded,
notably adding a completely new detection layer to measure the radio frequency
emission of extensive air showers. This Radio Detector extends the vertical
shower techniques developed in earlier radio arrays, such as the Auger
Engineering Radio Array, to horizontal showers, with a precision that is
expected to be similar to existing ground array techniques. It will provide a
novel measurement for inclined showers, complementary to the other techniques.
Details of the detection technique, the design and production of the full 1660
station Radio Detector and the expected reach in addressing the open questions
in UHECR astroparticle physics are presented.
