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.
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