We present the full source catalogue from the Australia Telescope 20 GHz (AT20G) Survey. The AT20G is a blind radio survey carried out at 20 GHz with the Australia Telescope Compact Array (ATCA) from 2004 to 2008, and covers the whole sky south of declination 0 • . The AT20G source catalogue presented here is an order of magnitude larger than any previous catalogue of high-frequency radio sources, and includes 5890 sources above a 20 GHz flux-density limit of 40 mJy. All AT20G sources have total intensity and polarization measured at 20 GHz, and most sources south of declination −15 • also have near-simultaneous flux-density measurements at 5 and 8 GHz. A total of 1559 sources were detected in polarized total intensity at one or more of the three frequencies.The completeness of the AT20G source catalogue is 91 per cent above 100 mJy beam −1 and 79 per cent above 50 mJy beam −1 in regions south of declination −15 • . North of −15 • , some observations of sources between 14 and 20 h in right ascension were lost due to bad weather and could not be repeated, so the catalogue completeness is lower in this region. Each detected source was visually inspected as part of our quality control process, and so the reliability of the final catalogue is essentially 100 per cent.We detect a small but significant population of non-thermal sources that are either undetected or have only weak detections in low-frequency catalogues. We introduce the term Ultra-Inverted Spectrum to describe these radio sources, which have a spectral index α(5, 20) > +0.7 and which constitute roughly 1.2 per cent of the AT20G sample.

We present the results of a high angular resolution study of the BL Lac object Markarian 501 in the radio band. We consider data taken at 14 different epochs, ranging between 1.6 and 22 GHz in frequency, and including new Space VLBI observations obtained on 2001 March 5 and 6 at 1.6 and 5 GHz. We study the kinematics of the parsec-scale jet and estimate its bulk velocity and orientation with respect to the line of sight. Limb-brightened structure in the jet is clearly visible in our data, and we discuss its possible origin in terms of velocity gradients in the jet. Quasi-simultaneous, multiwavelength observations allow us to map the spectral index distribution and to compare it to the jet morphology. Finally, we estimate the physical parameters of the parsec-scale jet.

The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries. The SKA will be 50 times more sensitive than any existing radio facility. A majority of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from 300 MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is aimed squarely in this frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phase-array feed systems on parabolic reflectors. This large field-of-view makes ASKAP an unprecedented synoptic telescope poised J. Wall is the overall editor.