Influence of an extended source on goniopolarimetry (or direction finding) with Cassini and Solar Terrestrial Relations Observatory radio receivers

In a recent paper, Cecconi and Zarka (2005) provided analytical goniopolarimetric inversions, which allow us to retrieve the direction of arrival of an incoming electromagnetic wave, its flux, and its polarization state, also referred as direction finding inversions, to be used with measurements acquired with a system of electric dipole antennas on a three-axis stabilized spacecraft such as the Cassini/Radio and Plasma Wave Science/High Frequency Receiver or the Solar Terrestrial Relations Observatory/Waves receivers. In the present study, we establish the expressions of the measurements (autocorrelations and cross correlations) in the case of an extended source. We also analyze the effect of an extended source on the outputs of the analytical inversions presented in the former paper, which are supposing a point radio source. We show that for a source with an angular half width smaller than 5°, the induced biases are not significant.