It is likely that the noise tube intensities have changed over the fairly lengthy time since they were calibrated in the lab. When the visibility data are calibrated with the SY table (§4.3.15), the resulting gains are usually fairly close to 1.0. The task CDCAL appeared in 31DEC26 to try to calibrate the noise tube values found in the CD table. You must first run TYAPL to bring the gain values close to 1.0 and then CALIB to refine the values. The resulting SN table should have gain amplitudes not too far from 1.0. CDCAL then copies CD file version one to version two as a backup. It then re-writes CD table one with the input CD table values times the corresponding SN gain amplitudes squared. If the SN table contains solutions for more than one time at each antenna, the amplitudes are scalar averaged to avoid having amplitudes degraded by real phase variations. If all worked well you should be able to return to the TYAPL step followed by CALIB to have an SN table filled with amplitudes of one.