I have a pretty tricky (well for me at least) problem.  I how it can
be solved mathematically

I have a system with two points, A1 and A2.  There is a transmitter at
point A1 and a receiver at A2.

A1 sends out a pulse P1 which arrives at point A2 at time T1
A1 sends out another Pulse P2 which arrives at point A2 at time T2
A1 sends out another Pulse P3 which arrives at point A2 at time T3
This cycle continues to P(n) and T(n)
The time between sending out the pulses P1, P2, P3 etc is reasonably
accurate, in the millisecond range.

How could I predict a very accurate time of travel between points A1
and A2 (nanoseconds)?
What are the minimum number of output pulses?
How much better does the accuracy get with more pulses?
How could I implement this is code?

thanks