Jimanator:
I did the math and as long as the receiver is more than 0.26 meters from the transmitter it won't be quicker than a nanosecond, so I don't need to worry about picoseconds.
Surely it's the difference in distance of the two receivers that counts not the distance from the transmitter. If the difference is always considerably greater than 0.3m then you don't need better than nanosecond resolution otherwise...
Steve