Fluctuating properties of the atmosphere, and in particular its water vapour content, give rise to phase fluctuations of astronomical signals which, if uncorrected, lead to rapid deterioration of performance of (sub)-mm interferometers on long baselines. The Atacama Large Millimetre/submillimeter Array (ALMA) uses a 183 GHz water vapour radiometer (WVR) system to help correct these fluctuations and provide much improved performance on long baselines and at high frequencies. Here we describe the design of the overall ALMA WVR system, the choice of design parameters and the data processing strategy. We also present results of initial tests that demonstrate both the large improvement in phase stability that can be achieved and the very low contribution to phase noise from the WVRs. Finally, we describe briefly the main limiting factors to the accuracy of phase correction seen in these initial tests; namely, the degrading influence of cloud and the residual phase fluctuations that are most likely to be due to variations in the density of the dry component of the air.

We have used multifrequency follow‐up observations of a sample of extragalactic sources from the 9C survey at 15 GHz to make deductions about the expected source population at higher radio frequencies, such as those in the lower frequency bands of the Planck Surveyor satellite. In particular, we have made empirical estimates of the source counts at 22, 30, 43 and 70 GHz and compared these with both known data and current theoretical predictions. We have also made an estimate of the count at the Atacama Large Millimeter Array (ALMA) frequency of 90 GHz, with a view to assessing the possible population of point sources available for the phase calibration of that instrument.

We have carried out extensive radio and optical follow-up of 176 sources from the 15-GHz 9th Cambridge survey. Optical identifications have been found for 155 of the radio sources; optical images are given with radio maps overlaid. The continuum radio spectrum of each source spanning the frequency range 1.4-43 GHz is also given.Two flux-limited samples are defined, one containing 124 sources complete to 25 mJy and one of 70 sources complete to 60 mJy. Between one-fifth and one-quarter of sources from these flux-limited samples display convex radio spectra, rising between 1.4 and 4.8 GHz. These rising-spectrum sources make up a much larger fraction of the radio source population at this high selection frequency than in lower frequency surveys.We find that by using non-simultaneous survey flux density measurements at 1.4 and 15 GHz to remove steep-spectrum objects, the efficiency of selecting objects with spectra rising between 1.4 and 4.8 GHz (as seen in simultaneous measurements) can be raised to 49 per cent without compromising the completeness of the rising-spectrum sample.

We present results from a 3‐yr study of the 15‐GHz variability of 51 9C sources. 48 of these sources make up a subsample of a larger one complete to 25 mJy in 9C, and as the sources are selected pseudo‐randomly the results should be representative of the complete sample. 29 per cent of this subsample are found to be variable above the flux calibration uncertainties of ∼6 per cent. 50 per cent of the flat‐spectrum objects are variable whilst none of the steep‐spectrum objects or the objects with convex spectra peaking below 5 GHz are variable. Nine of the objects studied have convex spectra and peak frequencies above 5 GHz; eight of these were found to vary at 15 GHz, suggesting that the high‐frequency peaking class in this sample is largely populated by objects with jets aligned close to the line of sight whose emission is dominated by beamed components.

Martin Krause, Paul Alexander, Rosie Bolton, Jorn Geisbusch, David A. Green, and Julia Riley, 'Measurements of the cosmological evolution of magnetic fields with the Square Kilometre Array', Monthly Notices of the Royal Astronomical Society, Vol. 40 (2): 646-656, first published online 12 November 2009. The version of record is available online at doi: 10.1111/j.1365-2966.2009.15489.x Published by Oxford University Press on behalf of the Royal Astronomical Society. ?? 2009 The Authors. Journal compilation ?? 2009 RAS.We investigate the potential of the Square Kilometre Array (SKA) for measuring the magnetic fields in clusters of galaxies via Faraday rotation of background polarized sources. The populations of clusters and radio sources are derived from an analytical cosmological model, combined with an extrapolation of current observational constraints. We adopt an empirical model for the Faraday screen in individual clusters, gauged to observations of nearby clusters and extrapolate the polarization properties for the radio source population from the National Radio Astronomy Observatory Very Large Array Sky Survey. We find that about 10 per cent of the sky is covered by a significant extragalactic Faraday screen. Most of it has rotation measures between 10 and 100 rad m???2. We argue that the cluster centres should have up to about 5000 rad m???2. We show that the proposed mid frequency aperture array of the SKA as well as the lowest band of the SKA dish array are well suited to make measurements for most of these rotation measure values, typically requiring a signal-to-noise ratio of 10. We calculate the spacing of sources forming a grid for the purpose of measuring foreground rotation measures: it reaches a spacing of 36 arcsec for a 100 h SKA observation per field. We also calculate the statistics for background rotation measure (RM) measurements in clusters of galaxies. We find that a first phase of the SKA would allow us to take stacking experiments out to high redshifts (>1), and provide improved magnetic field structure measurements for individual nearby clusters. The full SKA aperture array would be able to make very detailed magnetic field structure measurements of clusters with more than 100 background sources per cluster up to a redshift of 0.5 and more than 1000 background sources per cluster for nearby clusters, and could for reasonable assumptions about future measurements of electron densities in high-redshift clusters constrain the power-law index for the magnetic field evolution to better than m = 0.4, if the magnetic field in clusters should follow B ??? (1 + z) m