There are a lot of cool concepts in those books, but the sophon faster-than-light communication that props up a lot of the story was a bit too much for me.
The theory of quantum entanglement is sound though, and it took a while for a single sophon to arrive before the FTL communication. Layman here, but I thought it was relatively had science/speculative fiction.
Give distances to pulsars with an arbitrary scale. The receiver can solve for the scale once they figure out which pulsars you are referring to. Identifying pulsars could be as easy as a pulse modulated with the frequency of the pulsar...
1) The one relative to the receiver of the coordinates
2) You start by explaining the encoding first, before sending coordinates, maybe by establishing X and Y axes as well as compass direction based on “landmarks” that should be visible from the receiver.
That’s the first thing that came to my mind at least.