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 \arcdeg \leq l \leq 75 \arcdeg and -2 \arcdeg \leq b \leq 2 \arcdeg; 175 \arcdeg \leq l \leq 207 \arcdeg and -2 \arcdeg \leq b \leq 1 \arcdeg ) observable with the telescope in radio recombination lines ( RRLs ) .
Processed data sets are being produced in the form of data cubes of 2 \arcdeg ( along l ) \times 4 \arcdeg ( along b ) \times 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 , respectively .
RRL maps with 3.4 \arcmin resolution and line flux density sensitivity of \sim 0.5 mJy will enable us to identify new HII regions , measure their electron temperatures , study the physics of photodissociation regions ( PDRs ) with carbon RRLs , and investigate the origin of the extended low density medium ( ELDM ) .
Twelve Hn \alpha lines fall within the 300 MHz bandpass of ALFA ; they are resampled to a common velocity resolution to improve the signal-to-noise ratio ( SN ) by a factor of 3 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 HII region complex S255/S257 has detected Hn \alpha and Cn \alpha lines with SN > 10 .