No cold load is available, so one has to rely on TREC for at least one receiver. can be measured with the interferometer (3 antennas required) using a strong continuum point source (OBSGAIN command).
This "single-dish" amplitude calibration is only a first step for interferometry. Additional degradation may come from phase noise. This degradation can be estimated from the amplitude of the phase calibrator (a quasar, usually). However, most quasars are variable sources, with typical time scales of months. Hence, flux referencing to a source of known flux is also necessary. Some quasars are regularly monitored to get the flux by comparison with planets and/or compact H regions. This secondary amplitude calibration allows the derivation of the true antenna efficiency (Jy/K conversion factor) at the time of observations.