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We present a pipeline that allows recovering reliable information for allfour Stokes parameters with high accuracy. Its novelty relies on the treatmentof the instrumental effects already prior to the computation of the Stokesparameters contrary to conventional methods, such as the M\"uller matrix one.The instrumental linear polarization is corrected across the whole telescopebeam and significant Stokes $Q$ and $U$ can be recovered even when the recordedsignals are severely corrupted. The accuracy we reach in terms of polarizationdegree is of the order of 0.1-0.2 %. The polarization angles are determinedwith an accuracy of almost 1$^{\circ}$. The presented methodology was appliedto recover the linear and circular polarization of around 150 Active GalacticNuclei. The sources were monitored from July 2010 to April 2016 with theEffelsberg 100-m telescope at 4.85 GHz and 8.35 GHz with a cadence of around1.2 months. The polarized emission of the Moon was used to calibrate thepolarization angle. Our analysis showed a small system-induced rotation ofabout 1$^{\circ}$ at both observing frequencies. Finally, we identify fivesources with significant and stable linear polarization; three sources remainconstantly linearly unpolarized over the period we examined; a total of 11sources have stable circular polarization degree $m_\mathrm{c}$ and four ofthem with non-zero $m_\mathrm{c}$. We also identify eight sources that maintaina stable polarization angle over the examined period. All this is provided tothe community for polarization observations reference. We finally show that ouranalysis method is conceptually different from the traditionally used ones andperforms better than the M\"uller matrix method. Although it was developed fora system equipped with circularly polarized feeds it can easily be modified forsystems with linearly polarized feeds as well.

Full-Stokes polarimetry with circularly polarized feeds - Sources with stable linear and circular polarization in the GHz regime