But what is your economy, and what is the distance (and is it a fixed region or anywhere out in the field)?
The economy is so that the 1cm precision you can get for $10.000 extra need to be "10cm for $10". What I hoped was that, with precision, a to b could be done cheaply compared to the cost of "where is a".

a and b will be within, say, 100 m of each other, the moving 'b' around with minimum homing may take 3 hours. I do not need to worry about reflections, consider it an "open field application"

Erik