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The cross-correlation function and template matching techniques havedominated the world of precision radial velocities for many years. Recently, anew technique, named line-by-line, has been developed as an outlier resistantway to efficiently extract radial velocity content from high resolutionspectra. We apply this new method to archival HARPS and CARMENES datasets ofthe K2-18 system. After reprocessing the HARPS dataset with the line-by-lineframework, we are able to replicate the findings of previous studies.Furthermore, by splitting the full wavelength range into sub-domains, we wereable to identify a systematic chromatic correlation of the radial velocities inthe reprocessed CARMENES dataset. After post-processing the radial velocitiesto remove this correlation, as well as rejecting some outlier nights, werobustly uncover the signal of both K2-18b and K2-18c, with masses that agreewith those found from our analysis of the HARPS dataset. We then combine boththe HARPS and CARMENES velocities to refine the parameters of both planets,notably resulting in a revised mass and period for K2-18c of$6.99^{+0.96}_{-0.99}$M$_{Earth}$ and $9.2072\pm0.0065$d, respectively. Ourwork thoroughly demonstrates the power of the line-by-line technique for theextraction of precision radial velocity information.

Revisiting Radial Velocity Measurements of the K2-18 System with the Line-by-Line Framework