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.