  Development of the hardware, data analysis, and simulation techniques for
large compact radio arrays dedicated to mapping the 21 cm line of neutral
hydrogen gas has proven to be more difficult than imagined twenty years ago
when such telescopes were first proposed. Despite tremendous technical and
methodological advances, there are several outstanding questions on how to
optimally calibrate and analyze such data. On the positive side, it has become
clear that the outstanding issues are purely technical in nature and can be
solved with sufficient development activity. Such activity will enable science
across redshifts, from early galaxy evolution in the pre-reionization era to
dark energy evolution at low redshift.

Instrumentation and Methods for Astrophysics