wild guess - electrons on the path mess with the signal in some way, with an effect proportional to their total number along the path (if these are meant to be

wild guess - electrons on the path mess with the signal in some way, with an effect proportional to their total number along the path (if these are meant to be very short pulses, maybe it smears them out in time, for example, or perhaps it changes their polarization). so if you know the distance you can get the average density along that path. then you assume that the universe is uniform (but evolving) and do the math (whether you call that exact or not...) oh, and presumably electron density is useful because it gives you (approx) proton density (assuming net charge of universe is zero!) which gives you ionized hydrogen density, which sounds like it would be a useful thing to know (you can see un-ionized hydrogen via absorption spectra, so if you add them together you get total hydrogen density, which is related to total normal matter density...) it's not clear to me how you do this without knowing how far away the sources are (see my earlier post).