What the method to calculate rocket probable error?

Answer 1

For unguided rockets probable errors are usually calculated by the equation

rep = dep = (sr)(cep)/1000

where rep is range error probable, dep is deflection (cross range) error probable,
sr is slant range to target (usually measured in feet or meters), and
cep is circular error probable in mils (1 mil = 0.0573 degrees)

To find standard deviations for use in probability models use sigma-x = rep/1.1774
sigma-y = dep/1.1774, where sigma-x corresponds to range direction and sigma-y
corresponds to deflection direction.

The 1.1774 comes from equating the Circular Bivariate Normal distribution (otherwise known
as the Rayleigh distribution, or the polar form of the bivariate normal distribution) to 0.50
and solving for radius r.

1 - exp(-0.5*r^2) = 0.50 ===> r = 1.1774 (note -0.5*r^2 means minus .5 times r squared)

This is the radius of a circle, in standard deviation measure, which is expected to contain 50%
of rocket impact points.