A new survey of radio sources at 151 MHz, which has not been described previously, is in progress at Cambridge. There are several of us working on it including Warner, Kenderdine, Waggett, Masson and Mayer. The results of the first observations are at present in a preliminary state but we hope that in time they will form the first part of the 6C survey. The purpose of the survey is not to reach the faintest sources detected so far in aperture synthesis observations but to study moderately faint sources at a low observing frequency and to cover a large part of the northern sky rapidly. The deepest survey made so far at a low frequency is that of Ryle and Neville (1962) at 178 MHz over a region of 50 square degrees near the north celestial pole. The faintest sources detected had flux densities of 0.25 Jy, corresponding to a source density of 104 sr−1. It is already 15 years since that survey, which was the first trial of aperture synthesis using the earth's rotation, and much more is now technically possible. One of the most interesting features of a low frequency survey is its ability to detect preferentially sources with steep radio spectra and to be sensitive to sources of very low surface brightness. We know that in many cases these two properties go together and are associated with old radio sources, or at least with those parts of sources which are old. Many of the weak radio galaxies in nearby clusters are obvious examples of this type of source while the final, and so far unidentified, stages of the development of the most powerful double sources may be exciting candidates for discovery.

To determine the frequency spectra of radio sources the flux densities of these sources should be measured at the widest possible frequency range. The use of different telescope types, methods of measurements and calibrations led in a number of cases to the considerable difference in the data of various observatories. At the present time for frequencies above 400 MHz the scales of flux densities - S for - S ≥ 1 Jy = 10−26w m−2Hz−1 presented by different authors coincide accurate to 5%. Up to now for frequencies below 200 MHz a single scale of fluxes recognized by all radioastronomers is absent. We attempted to determine such a scale for frequencies 180-10 MHz. We used both analysis of the published data and the results of the new measurements obtained with the UTR-2 Radiotelescope in the declination strip 10° – 20° at frequencies 10.0; 12.6; 14.7; 16.7; 20, and 25 MHz. The method of these measurements and the obtained results are described in detail in [1]. The UTR-2 Radiotelescope has 5 beams in declination with half power beam width in a zenith direction 20′ × 20′ at 25 MHz [2]. The absolute values of the flux densities of all observed discrete radio sources are defined during the experiments. Minimum flux density measured at frequency 25 MHz – 15-20 Jy, with a signal-to-noise ratio equal to 3-4. During measurements about 300 radio sources were found. Comparing these data with our earlier measurements obtained with the UTR-1 Telescope [3] as well as with corrected data of the both Pentincton [4] at 10.02 and 22.25 MHz and the Clark-Lake Observatories at 26.3 MHz [5] and the results obtained in Cambridge at 38 MHz [6] and 178 MHz [7, 8] we have derived some correcting factors allowing to bring the data of different catalogues to a single scale. The method for determining such factors is described in [9]. It was shown that below 200 MHz where the influence of ionosphere is great, it is necessary to take into account two corrections : regular scale shift of the one catalogue to another z12 = 1/z21 (1/z21 is the average value of the relation between flux densities of the first catalogue and the second one) and a scale shift caused by the multiplicative scatter. A value is greater than unity and it can be determined comparing with each other three different catalogues. The comparison results of a number of catalogues with data obtained with UTR-2 are shown in table I.

The University of Texas Radio Astronomy Observatory (UTRAO) is engaged in a survey of the entire sky north of −35° declination at various frequencies in the range 335-380 MHz. Primary goals are (i) determination of accurate (∼1″) positions for about 50,000 sources, followed by (ii) optical identification of the sources on the basis of exact radio-optical position coincidence; (iii) provision of rough structure models for all listed sources; and (iv) monitor the sky for variable sources on the time scale of 1 to 2 years. The survey is not expected to be a reliable source of absolute flux density information except for those sources known to be unresolved from other work.

The Bologna survey (Colla et al. 1970, 1972, 1973) does not represent a particularly new achievement since the first records refer to 1968. We will summarize here the main properties for a statistical use of the survey, the differential counts and the isotropy characteristic of this flux level, the latter being completely new.

One of the basic programs undertaken with the 1 mile Cross-type radio telescope at the Molonglo Radio Observatory has been a systematic survey of the sky south of +18° declination. Commencing in late 1967, the survey has been continuing at a low priority until it is now complete to a declination of −85°, except for some small areas affected by interference or instrumental problems which are being re-observed. A program of ‘deep surveys’ is also being undertaken; this involves multiple observations within narrow strips at selected declinations. The signal-to-noise ratio is some three to five times better than in normal surveying and the confusion limit is approached.

The Westerbork Synthesis Radio Telescope (WSRT) has now been used to make source surveys at frequencies of 610 and 1415 MHz. This paper summarizes the results concerning source counts and anisotropies in the distribution of sources from those surveys not concerned with clusters of galaxies.

The present 1400 MHz complete pencil-beam survey, made with the NRAO 300-foot telescope, consists of four parts which cover four different regions of the sky, as shown in Fig. 1.

The Parkes 2700-MHz survey for extragalactic sources now extends over virtually the whole sky available to the 64-m telescope at galactic latitudes |b| > 10°. Eleven parts of the catalogue have been published as Astrophysical Supplements of the Aust. J. Phys. (Numbers 19, 21, 26, 30, 32, 34, and 39); part 11 and subsequent parts consist of re-surveys of some areas to fainter flux density limits. The current version of the composite catalogue for the 7.36 sr covered (Fig. 1) contains about 7500 sources, together with the results of identification programmes and flux density measurements at other frequencies.

A number of surveys have been carried out at a frequency of 5 GHz at the National Radio Astronomy Observatory (NRAO) and at the Max-Planck-Institut für Radioastronomie (MPIfR), with the aim of determining the number-flux density relation for the sources detected and also of obtaining their radio spectra and optical identifications. The surveys fall into two categories: first, the strong source (S) surveys, which are intended in due course to cover the whole northern sky and to be complete above a flux density of about 0.6 Jy; second, surveys of limited areas of sky down to lower levels of the flux density.

One of the many uses to which a radio survey may be put is an analysis of the distribution of the radio sources on the celestial sphere to find out whether they are bunched into clusters or lie in preferred regions of space. There are many methods of testing for clustering in point processes and since they are not all equally good this contribution is presented as a brief guide to what seem to be the best of them. The radio sources certainly do not show very strong clustering and may well be entirely unclustered so if a statistical method is to be useful it must be both powerful and flexible. A statistic is powerful in this context if it can efficiently distinguish a weakly clustered distribution of sources from an unclustered one, and it is flexible if it can be applied in a way which avoids mistaking defects in the survey for true peculiarities in the distribution of sources.

Surveys of the sky between declinations +25° and −90° at 2700 MHz (11 cm) have been in progress for the past 10 years. Excluding some regions close to the galactic plane the whole sky south of +25° has been surveyed to a flux density limit of 0.6 Jy at 2700 MHz and within this area surveys to limits of 0.35, 0.25 or 0.1 Jy have been made covering 3.5 sr. Flux densities have been measured at 5000 MHz for all sources stronger than 0.35 Jy at 2700 MHz. The source positions have an average accuracy of 10″ arc in both coordinates and the positions have been examined for optical identifications on Palomar, ESO or SRC sky survey plates, which now cover 95% of the area. The first part of this paper concerns the relationships between the spectral indices α(2700 to 5000 MHz) and the identifications of the 2300 sources with galactic latitudes greater than 10°. It is a statistically significant sample, since the sources stronger than 0.35 Jy cover 3.5 sr. It is also a representative sample, since no selection was made on the basis of spectral index or identification. It cannot however be claimed as a complete sample, for two reasons. A substantial fraction of sources found in radio surveys at high frequencies are variable - variations of up to a factor of three can occur on a time scale of a year - thus the various sections of the survey are complete only for the relevant epoch. Many of their optical counterparts are also variables - variations of up to a factor of 100 can occur on a time scale of one year. It is hoped to make some assessment of the effect of these two factors in the next two years, when second-epoch Parkes surveys will begin and SRC Schmidt plates will overlap the Palomar Sky Survey.

Spectral index distributions can be indicated conveniently by gSv1(αv1,v2), i.e. the distribution of the two-point spectral index between frequencies v1 and v2 for a sample of radio sources complete to flux density S at the selection frequency v1 (flux densities will be expressed in Jy and frequencies in GHz). Such a detailed specification is necessary because g(α) has been found to depend on all three parameters. The effect of varying v1 is well-known: the fraction of flat-spectrum sources increases with selection frequency. Variations of g(α) with S have also been found: both the fraction of flat-spectrum sources and the mean spectral index of the steep-spectrum sources depend on S, at least at high frequencies. The magnitude of this dependence on S as well as g(α) itself appear to depend on the choice of v2, or rather on the relation between v2 and v1. This complex behaviour of g(α) is not unexpected if one considers that the redshift and luminosity distributions of a source sample vary with selection frequency and flux density limit. Intrinsic or induced correlations between spectral index and redshift or luminosity, or different redshift distributions of flat-and steep-spectrum sources may cause variations of g(α) with v and S. An understanding of the behaviour of g(α) requires observations over large ranges of frequency and flux density. Even then, it will be difficult to interpret these data without information about the composition of the various samples, which can be obtained only through optical work. Here we describe recent spectral index information for weak sources selected at 1.4 GHz, as well as work on spectral index/optical identification correlations in 5 GHz samples.

The main cosmological tests using radio sources as probes are summarized below.

The relation between angular size and flux density depends on the world model and more strongly on evolution in radio source properties with epoch. I shall consider here the simplest forms of evolution that explain the observed θ-S relations (Swarup 1975; Kapahi 1975a,b; Ekers, this symposium).

The median values of angular sizes of weak extragalactic radio sources, the flux densities of which lie in the range of about 0. 3 to 5 Jy at 327 MHz, have been determined for a new sample of 119 sources observed during 1973-74, and agree well with the value of about 10 arc sec determined earlier by Swarup (1975). For 8 different flux density ranges, the angular size distribution for the All-sky, 3CR and Ooty radio sources have been compared with theoretical predictions based on the evolutionary model by Kapahi (1975) and show a remarkable agreement with his model except that the best fit is found for a linear size evolution proportional to (l+z)−1.

Complete samples of quasars in the 3CR and 4C catalogues have been observed with a resolution of 2″ arc in RA and 2″ cosecδ in dec with the Cambridge 5-km telescope at 5 GHz (Riley & Pooley 1975; Jenkins et al 1977), from which it has been possible to determine the overall angular sizes of their radio structures (LAS). The angular diameter-redshift test for quasars has been reexamined using these data; details are described fully elsewhere (Hooley et al 1977).