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We compare the observational properties of the circular polarization in Galactic jet sources with that observed in AGN, and outline the constraints they place on the mechanism responsible for the circular polarization. We also discuss the implications of the time scale of polarization variations on the mechanism responsible for the circular polarization.

We have measured annual cycles in the time scales of the rapid, intra-day variability of the total and circularly polarized flux density, of the unusual BL Lac source PKS 1519-273 at 4.8 and 8.6 GHz. The data were collected at the ATCA over the last seven years, and establish unequivocally interstellar scintillation as the principal mechanism responsible for this cm-wavelength intra-day variability.

We report strong variability in the circular and linear polarization of the intraday variable source PKS 1519-273. The circular polarization varies on a timescale of hours to days at frequencies between 1.4 and 8.6 GHz, and is strongly correlated with variations in the total intensity at 4.8 and 8.6 GHz. We argue that the variability is due to interstellar scintillation of a highly compact (15-35 \mu as) component of the source with -3.8\pm 0.4% circular polarization at 4.8 GHz. We find that no simple model for the circular polarization can account for both the high magnitude and the frequency dependence in PKS 1519-273 at centimeter wavelengths.

Intra-day variability (IDV) of active galactic nuclei (AGN) has been detected from gamma-rayenergies to radio wavelengths. At high energies, such variability appears to be intrinsic to thesources themselves. However, at radio wavelengths, brightness temperatures as high as 1018 to1021 K are encountered if the mv is intrinsic to the source. We discuss here the accumulating evidenceshowing that, at radio wavelengths where the highest brightness temperatures are encountered,interstellar scintillation (ISS) is the principal mechanism causing IDV. While ISS reduces the impliedbrightness temperatures, they still remain uncomfortably high.

An overview is presented of a number of astrometry and astrophysics programs based on radio sources from the Parkes 2.7 GHz catalogues. The programs cover the optical identification and spectroscopy of flat-spectrum Parkes sources and the determination of their milliarc-second radio structures and positions. Work is also in progress to tie together the radio and Hipparcos positional reference frames. A parallel program of radio and optical astrometry of southern radio stars is also under way.

An observational pilot program is described which provides a first step towards the construction of an inertial extragalactic reference frame from optical and radio measurements. Candidate objects will be selected from extragalactic compact radio sources which display optical counterparts. Galactic radio stars will be secondary reference objects. The reference frame will be defined by the radio positions of the extragalactic objects. Based on a coordinated effort in both hemispheres, a global homogeneous net of about 400 quasars and 100 radio stars is expected to result from a five year campaign.