Dragon Endeavour performs port relocation to clear way for Starliner - NASASpaceFlight.com - NASASpaceflight.com

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— Chris Bergin – NSF (@NASASpaceflight) July 21, 2021

This is not the first time Crew Dragon has performed a port relocation – Resilience also performed the same maneuver on April 5 to clear the PMA-2 port for the arrival of Endeavour

Relocations of US crew vehicles is becoming commonplace on ISS as Crew Dragon, Cargo Dragon, and Starliner spacecraft share the only two available docking ports to American docking vehicles

Today’s relocation was performed in order to clear the PMA-2 port for the arrival of Boeing’s Starliner vehicle on the uncrewed OFT-2 mission, set to launch on July 30 for a docking to the ISS on July 31

Render of Starliner approaching the forward port while Crew Dragon sits docked to the Zenith port – via Mack Crawford for NSF/L2

One issue with vehicles docking to the PMA-3/IDA-3 port on the Zenith side of Harmony is that the station’s Ku-band Space to Ground Antenna (SGANT), located on top of the Z1 Truss, can get blocked by approaching vehicles

However, controllers on the ground are unable to see live footage from the docking cameras due to the lack of a Ku link, and for this reason PMA-2 can be the preferred port for some dockings – especially test flights – as, being located on the Forward side of Harmony, there are no SGANT masking issues with vehicles using this port

This is why the first, uncrewed docking of Starliner to the ISS will use the PMA-2 port, just like SpaceX’s Crew Dragon vehicle did on the uncrewed Demo-1 mission

After the departure of CRS-23 Cargo Dragon, the next crew rotation will arrive on a Crew Dragon on the SpaceX Crew-3 mission, launching from the Kennedy Space Center in Florida no earlier than 31 October