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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 100-200 MHz band, looking for a 21-cm line signal from neutral atomic hydrogen at or before the epoch of reionization (EOR), and effects on that signal due to intervening matter. The project includes a test/development array at Green Bank, WV, and the primary array at the South African Square Kilometre Array (SKA) site. It began operation in 2009.
This type of observation (21-cm experiments) is experimental and as of 2016, PAPER has yet to identify a 21-cm signal from that era. It is considered inadequately sensitive and HERA is a follow-on project.