Re: Why is that accuracy necessary?
"So long as the signal delay (and also change in signal delay per unit time) is known accurately"
That's the point - the signal delay isn't known accurately unless you've agreed clocks at the start of the journey.
"I sent this message at X"
"I received this message at Y"
The delay is only Y minus X *if* you can be sure that neither clocks have drifted.