PAPER (Precision Array for Probing the Epoch of Reionization) is a radio interferometer aimed at mapping the high redshift (e.g., 500 million years, cosmic time) universe by mapping the 100-200MHz band, looking for affects on cosmic microwave background's 21cm line signal.
The project includes a test/development array at Green Bank, WV, and the primary array at the Square Kilometre Array (SKA) site, South Africa. It began operation in 2009.
This type of observation is experimental and as of 2016, PAPER has yet to identify a 21-cm signal from the CMB. It is considered inadequately sensitive and HERA is a follow-on project.