A dual linear polarization fringe fitting method for VLBI observation

In this paper, we propose a dual linear polarization fringe fitting method which can be used in the next generation VLBI system (VLBI global observing system, VGOS). The legacy geodetic VLBI observation mode is dedicated to the right circular polarization (RCP), which differs from the dual linear polarization adopted in the VGOS system. The algorithm presented in this work consists of the calibration part and combined fringe fitting part. In the calibration part, we derive the intermediate frequency (IF) delay and phase calibration information from each polarization product by carrying out fringe fitting on a strong reference source. Then this information is applied to the target sources. In the combined fringe fitting part, we combine 4 polarization products into the Pseudo-Stokes Intensity and in the meanwhile search for the appropriate differential parallactic angle, so as to maximize the amplitude of the intensity and therefore derive the delay as observable. Compared with single polarization, the combined product yields higher signal-to-noise ratio (SNR) and smaller fringe phase scatter.