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We present the Generalized Polarization CALibration pipeline (GPCAL), anautomated pipeline for instrumental polarization calibration of very longbaseline interferometry (VLBI) data. The pipeline is designed to achieve a highcalibration accuracy by means of fitting the instrumental polarization model,including the second-order terms, to multiple calibrators data simultaneously.It also allows using more accurate linear polarization models of calibratorsfor D-term estimation compared to the conventional way that assumes similarlinear polarization and total intensity structures. This assumption has widelybeen used in the existing packages for instrumental polarization calibrationbut could be a source of significant uncertainties when there is no suitablecalibrator satisfying the assumption. We demonstrate the capabilities of GPCALby using simulated data, archival Very Long Baseline Array (VLBA) data of manyactive galactic nuclei (AGN) jets at 15 and 43 GHz, and our Korean VLBI Network(KVN) observations of many AGN jets at 86, 95, 130, and 142 GHz. The pipelinecould reproduce the complex linear polarization structures of several sourcesshown in the previous studies using the same VLBA data. GPCAL also reveals acomplex linear polarization structure in the flat-spectrum radio quasar 3C 273from the KVN data at all four frequencies. These results demonstrate that GPCALcan achieve a high calibration accuracy for various VLBI arrays.

GPCAL: a generalized calibration pipeline for instrumental polarization in VLBI data