[Lonesome in Massachussets]

Short answer: We measure phase history and amplitude of the two signals. Details:

https://physics.stackexchange.com/questions/235579/how-were-the-solar-masses-and-distance-of-the-gw150914-merger-event-calculated-f

[InterceptPoint]

Much thanks for your response.

While the explanation is a bit over my head it is refreshing to learn that the question I posed was an importantant one, that it had obviously been addressed by the LIGO folks and that the methods they developed are well documented in the technical literature.

Once again, thanks for your response.