Radiometry performance of the VGOS receivers of the Onsala twin telescopes

Conference poster, 2023

With the introduction of the VLBI Global Observing System (VGOS) the parallel use of the VGOS receiver as
radiometer in order to estimate the wet propagation delay was recognised as a future possibility. That is when
observations 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 Water
Vapour Radiometer (WVR), is that the radiometer will observe the same atmospheric volume that is causing the
signal 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 than
the 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 to
increase the sensitivity for water vapour. The radiometric measurements shall be made within a couple of GHz
from 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 to
assess the radiometric stability of these receivers at the higher end of the frequency band. We have used one of
the Onsala Twin Telescopes for this purpose, which is able to observe both polarizations in the frequency band
15.36–15.58 GHz. The system temperature has been observed at different elevation angles in order to separate
the atmospheric sky brightness temperature and the receiver noise temperature. The observations are carried out
during different atmospheric conditions and the estimated sky temperatures are compared to the observations
done with one of our stand-alone WVRs. By using one-frequency algorithms we may also, during cloud-free
conditions, compare the wet propagation delays using 20.7 GHz observations from the stand-alone WVR and
15 GHz observations from the VGOS receiver.