The Galactic Arecibo L-band Feed Array HI (GALFA-HI) Survey is an ongoing survey of the Galaxy in the 21 cm neutral hydrogen line conducted with the Arecibo Observatory 305-m telescope.
With the combination of sensitivity (60-100 mK), angular resolution (~4'), velocity resolution (0.18 km/sec), and spatial coverage (~13,000 deg^2), GALFA-HI provides the best window into the diffuse interstellar medium (ISM) in the pre-SKA era.
The survey started in 2005 and operates mainly commensally with other Arecibo surveys, pioneering a new mode of operation for radio telescopes.
The core GALFA-HI collaboration between Wisconsin, UC Berkeley and Columbia is undertaking a systematic exploration of the outstanding questions regarding the diffuse ISM, including the flow of atomic gas into galaxies and the conversion of diffuse ISM into molecular clouds. Science highlights, publications, and survey details can be found at: http://sites.google.com/site/galfahi/Home
The first GALFA-HI data release covers about 50% of the Arecibo sky and is publicly available at http://purcell.ssl.berkeley.edu/.