With the introduction of the VLBI Global Observing System (VGOS) the parallel use of the VGOS receiver asradiometer in order to estimate the wet propagation delay was recognised as a future possibility. That is whenobservations can be carried out at higher frequencies, closer to the water vapour emission line at 22.2 GHz.An advantage of having the radiometer in the VLBI telescope, compared to the use of a stand-alone WaterVapour Radiometer (WVR), is that the radiometer will observe the same atmospheric volume that is causing thesignal propagation delay.We have assessed this method using simulations and arrived at the following two important conclusions:(1) the receiver’s measurements of the sky brightness temperature is likely to be the main error source, rather thanthe algorithm error introduced when calculating the wet delay from the observed sky brightness temperatures;(2) the method requires an extension of the frequency range of the receiver well beyond 14 GHz in order toincrease the sensitivity for water vapour. The radiometric measurements shall be made within a couple of GHzfrom the emission line at 22.2 GHz.In spite of the fact that the present VGOS receivers observe at too low frequencies we find it meaningful toassess the radiometric stability of these receivers at the higher end of the frequency band. We have used one ofthe Onsala Twin Telescopes for this purpose, which is able to observe both polarizations in the frequency band15.36–15.58 GHz. The system temperature has been observed at different elevation angles in order to separatethe atmospheric sky brightness temperature and the receiver noise temperature. The observations are carried outduring different atmospheric conditions and the estimated sky temperatures are compared to the observationsdone with one of our stand-alone WVRs. By using one-frequency algorithms we may also, during cloud-freeconditions, compare the wet propagation delays using 20.7 GHz observations from the stand-alone WVR and15 GHz observations from the VGOS receiver
