The observations were performed in three superposed
campaigns in order to obtain three sets of data with different temporal
resolutions: 
minutes, 1 day and 1 month. All observations were
accomplished with a 30m-single dish telescope of the IAR during
1995-1996. This telescope has a coverage from 
to 
in
declination and from 
to 
in zenith angle (four
hours). The two channel total power receiver with a corrugated horn feed was
operated at 1.42 GHz with a bandwidth of 20 MHz. At this frequency the HPBW
of the antenna
is 30 arcminutes. The temperature of the system was 90 K, and the sensitivity
in a single record 
. The
digitized signal and the telescope position were simultaneously acquired
by an IBM computer.
The observing procedure for point sources consisted of repeated orthogonal cross scans in declination and right ascension. The two sets of scans observed for a given source were averaged and Gaussian fitted to determine the peak flux density (which was calibrated using the scale of Wills 1975). A set of standard, steep-spectrum sources was observed for calibration and control purposes: PKS 0915-11, PKS 1308-22, PKS 1610-60, PKS 1814-63, PKS 1932-46 and PKS 2152-69 (see Table 1 (click here)). Sources under study are listed in Table 2 (click here) together with their coordinates, galactic latitude, redshift and identification.
G: Radio galaxy.
Q: QSO, G: Radio galaxy.
Cen A is the only extended source in our sample. A different observational
technique was used for this object: the
observations consisted of scans perpendicular to the line defined by the
outer radio galaxy and centered at the nucleus. The length of the scans
was 12 HPBW in order to allow a proper determination of the baseline (8
HPBW were sufficient for the other sources). Due to the low angular resolution
of the telescope we have observed with this procedure not just the nucleus but
also the jet and the Inner Lobes of Cen A. Variability, however, must come from
the innermost region if the timescales are of the order of a few months. The
emission form the outer jet and the Inner Lobes can be considered as a
quiescent component 
in such a way that the total flux density
observed is 
, where 
is the variable component.
The errors quoted with the variability data presented in this paper are rms fluctuations of the flux densities obtained from each set of scans. These errors are contributed by (i) absolute errors (i.e. errors with no dependence on the flux density of the observed sources), and (ii) intensity-proportional errors. Errors of type (i) are due to the noise and short-term instabilities of the receiver, confusion, and low-level interference. Errors of type (ii) are mainly originated in antenna pointing errors and variations of gain with zenith angle. In case of daytime observations there are additional contributions to the total errors from solar activity and differential heating and cooling of the antenna.
In order to determine the error budget we have studied the scatter of the peak flux densities associated to scans obtained within individual source observations. The rms fluctuations have been plotted against the flux densities of a large number of sources of different intensities. Errors of type (ii) are dominant for large flux densities whereas errors of type (i) are important for relatively weak sources. In both domains the errors can be fitted by linear functions, allowing a numerical determination. The total rms error for a flux density value S is then given by:
where 
and
. For daytime observations the absolute errors
are larger: 
. The confusion
contribution to 
is 0.08 Jy. In the case of the weakest
sources of our sample (PKS 1610-771) the total errors are about 4% of the
flux density.
In addition to these errors for individual observations, variations on the gain and sensitivity of the entire observing system can introduce a spurious low-level variability in the observed light curves. This effect can be determined by observations of calibration sources as we shall describe in the next section.
