Depend on how the data was encoded.
If it was an old-style am signal, and it was just audio (say, the pilot reporting her position and time) then you'd just tune your radio receiver into whatever gets a strong signal and you'll hear her voice a bit on the deep side talking a bit slow. How big-a difference is 0.5c going to make?
If it was a modern wireless broadband video stream, I don't know the standard. I know it has to be able to handle streaming between different speed networks. I imagine you could work something out with the engineers so the receiver does not make assumptions about the incoming data's clock speed. That's just an engineering problem.
Of course, for sufficiently high speeds you may need a different kind of receiver completely.
In this example, the transmission was by prior arrangement - how you pick out a signal when there was no prior arrangement is a SETI problem innit :) Normally you'd know the relative speeds anyway.