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A pilot project has been proceeded to map 1 deg$^2$ on the Galactic plane forradio recombination lines (RRLs) using the Five hundred meter ApertureSpherical Telescope (FAST). The motivation is to verify the techniques andreliabilities for a large-scale Galactic plane RRL survey with FAST aiming toinvestigate the ionized environment in the Galaxy. The data shows that thebandpass of the FAST 19 beam L-band is severely affected by radio frequencyinterferences (RFIs) and standing wave ripples, which can hardly be correctedby traditional low order polynomials. In this paper, we investigate a series ofpenalized least square (PLS) based baseline correction methods for radioastronomical spectra that usually contain weak signals with high level ofnoise. Three promising penalized least squares based methods, AsLS, arPLS, andasPLS are evaluated. Adopting their advantages, a modified method named rrlPLSis developed to optimize the baseline fitting to our RRL spectra. To checktheir effectiveness, the four methods are tested by simulations and furtherverified using observed data sets. It turns out that the rrlPLS method, withoptimized parameter $\lambda = 2 \times 10^8$ , reveals the most sensitive andreliable emission features in the RRL map. By injecting artificial lineprofiles into the real data cube, a further evaluation of profile distortion isconducted for rrlPLS. Comparing to simulated signals, the processed lines withlow signal-to-noise ratio are less affected, of which the uncertainties aremainly caused by the rms noise. The rrlPLS method will be applied for baselinecorrection in future data processing pipeline of FAST RRL survey. Configuredwith proper parameters, the rrlPLS technique verified in this work may also beused for other spectroscopy projects.

Baseline correction for FAST radio recombination lines: a modified penalized least squares smoothing technique