A second key barrier to adopting probability based rules is that calculating the actual probability of collision is a difficult problem that requires a huge ‘Monte Carlo’ analysis that would tax the capabilities of standard computing systems.
This is a further advantage of the scalar factor method for calculating separation factor. It can translate easily into a close approximation of the probability of intersection (actual collision) using the following process:
Warning:
This formula only gives a good estimate of collision risk in the range from SF = 0 to SF = 3 at 2 sigma. Collisions are very rare indeed beyond this (6 standard deviations apart). Do not use this formula outside this range or collision risk will be over-estimated.
It should be noted that this calculation is the probability of the centre points of the ellipses colliding. It does not represent the complex probabilities of two curved well paths colliding in 3D space but serves as a useful guide.
The table on the following page gives an idea of what you could expect to see:
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