  We present results from observation, correlation and analysis of
interferometric measurements between the three geodetic very long baseline
interferometry (VLBI) stations at the Onsala Space Observatory. In total 25
sessions were observed in 2019 and 2020, most of them 24 hours long, all using
X-band only. These involved the legacy VLBI station ONSALA60 and the Onsala
twin telescopes, ONSA13NE and ONSA13SW, two broadband stations for the next
generation geodetic VLBI global observing system (VGOS). We used two analysis
packages: nuSolve to pre-process the data and solve ambiguities, and ASCOT to
solve for station positions, including modelling gravitational deformation of
the radio telescopes and other significant effects. We obtained weighted root
mean square postfit residuals for each session on the order of 10-15 ps using
group delays and 2-5 ps using phase delays. The best performance was achieved
on the (rather short) baseline between the VGOS stations. As the main result of
this work we determined the coordinates of the Onsala twin telescopes in
VTRF2020b with sub-millimeter precision. This new set of coordinates should be
used from now on for scheduling, correlation, as a~priori for data analyses,
and for comparison with classical local-tie techniques. Finally, we find that
positions estimated from phase-delays are offset $\sim+3$ mm in the
Up-component with respect to group-delays. Additional modelling of
(elevation-dependent) effects may contribute to future understanding of this
offset.
