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 (OBS
GAIN
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.