A Survey of Ionized Gas in the Galaxy, made with the Arecibo telescope (SIGGMA), uses the Arecibo L-band Feed Array (ALFA) to fully sample the Galactic plane (30° ≤ l ≤ 75° and –2° ≤ b ≤ 2°; 175° ≤ l ≤ 207° and –2° ≤ b ≤ 1°) observable with the telescope in radio recombination lines (RRLs). Processed data sets are being produced in the form of data cubes of 2° (along l) × 4° (along b) × 151 (number of channels), archived and made public. The 151 channels cover a velocity range of 600?km s–1 and the velocity resolution of the survey changes from 4.2?km s–1 to 5.1?km s–1 from the lowest frequency channel to the highest frequency channel. RRL maps with 34 resolution and a line flux density sensitivity of ~0.5 mJy will enable us to identify new H II regions, measure their electron temperatures, study the physics of photodissociation regions with carbon RRLs, and investigate the origin of the extended low-density medium. Twelve Hnα lines fall within the 300?MHz bandpass of ALFA; they are resampled to a common velocity resolution to improve the signal-to-noise ratio (S/N) by a factor of three or more and preserve the line width. SIGGMA will produce the most sensitive fully sampled RRL survey to date. Here, we discuss the observing and data reduction techniques in detail. A test observation toward the H II region complex S255/S257 has detected Hnα and Cnα lines with S/N 10.
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