2021ApJS..254...28A

The Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) traces ionized gas in the Galactic midplane by measuring 4-8 GHz radio recombination line (RRL) emission. The nominal survey zone is 32.°3 > l > - 5°, |b| < 0.°5, but coverage extends above and below the plane in select fields and additionally includes the areas around W47 (l ≃ 37.°5) and W49 (l ≃ 43°). GDIGS simultaneously observes 22 Hnα (15 usable), 25 Hnβ (18 usable), and 8 Hnγ RRLs (all usable), as well as multiple molecular line transitions (including those of H₂¹³CO, H₂CO, and CH₃OH). Here, we describe the GDIGS survey parameters and characterize the RRL data, focusing primarily on the Hnα data. We produce sensitive data cubes by averaging the usable RRLs, after first smoothing to a common spectral resolution of 0.5 km s^–1 and a spatial resolution of 2.'65 for Hnα, 2.'62 for Hnβ, and 2.'09 for Hnγ. The average spectral noise per spaxel in the Hnα data cubes is ∼10 mK (∼5 mJy beam^–1). This sensitivity allows GDIGS to detect RRLs from plasma throughout the inner Galaxy. The GDIGS Hnα data are sensitive to emission measures EM >= 1100 cm^–6 pc, which corresponds to a mean electron density <n_e> >= 30 cm^–3 for a 1 pc path length or <n_e> >= 1 cm^–3 for a 1 kpc path length.