VSOP polarization observing at 1.6 GHz and 5 GHz

We present and discuss the results of 1.6 GHz and 5 GHz test polarization observations obtained with HALCA, the VLBA, and phased VLA, specifically to evaluate the feasibility of calibrating and imaging HALCA polarization data. Our analysis provided the first demonstration that centimeter-wavelength polarization imaging using HALCA is technically viable for sources with sufficiently high correlated polarized flux densities, despite a number of factors which conspire to complicate the calibration, including relatively low sensitivity, inability to observe calibrators, lack of varying parallactic angle, and absence of redundant measurements on space baselines. The test data described here are limited and cannot rule out instrumental variability, but indicate that the HALCA instrumental polarization leakage is ∼ 2-5% at 1.6 GHz and ∼ 9-11% at 5 GHz. The prospects for effective polarization calibration on future orbiting VLBI missions are encouraging, especially since improvements in sensitivity are expected.