Introduction A dedicated 4-stations 24 hours experiment is proposed. A set of RR and RL scans will be produced. It was found that RL fringes were found at all baselines, and the fringe amplitude for RL scans was at the level 0.08-0.20 of the fringe amplitude for RR scans. It means that the effect of polarization impurity in normal RR scans is at the level 0.01-0.04, roughly 10 times smaller than in RL scans. Instruments polarization is depends on time relatively weak, but may change after any manipulation with appliances.
Instrumental polarization shows strong frequency dependence at many stations.
Motivation Residual phase scattering at X-band after wideband synthesis for RL scans is about 1 radian. If the factor which caused this scattering is related to instrumental polarization we can expect that it affects normal RR scans with the magnitude 10 times less what correspond to 0.1 radian. Excessive residual phase scattering was noticed in early 80-s, was picturesquely described by T. Herring in his Ph.D. theses dated in 1983 but was not explained up to now. To measure polarization impurity on stations participating in the experiment. To investigate dependence of instrumental polarization on time frequency parallactic angle To investigate how residual phases depend on time frequency parallactic angle To determine relative jumps in group and phase delays for RR and RL scans.