See SEP.

The radius of a sphere within which there is a 50 percent probability of locating a point or being located. SEP is the three-dimensional analogue of CEP.

A statistical measure of the 3-D positioning precision. The SEP value is defined as a sphere's radius, when centred at the true position, encloses 50% of the data points in a 3-D scatter plot. Thus, half the data points are within a 3-D SEP sphere and half are outside the sphere.