Circular error probable

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In the military science of ballistics, circular error probability or circular error probable (CEP) is a simple measure of a weapon system's precision. It is defined as the radius of a circle into which a missile, bomb, or projectile will land at least half the time. Examples include:

A Trident II warhead has a CEP of 90 meters (with GPS guidance); thus, there is a 50% probability that each warhead will impact within 90 meters (assumning GPS is used).
For LGM-30 Minuteman III warheads, the CEP is 275 meters for the three 170 kt W62 warheads contained in General Electric (GE) Mk 12 RVs, and 220 meters for the three 335 kt W78 warheads contained in GE Mk 12A RVs.
In its most accurate mode, a Joint Direct Attack Munition provides a CEP of 13 meters or less when GPS data is available.
Russian newest quasi-ballistic Iskander missile has CEP of 10 meters or less, when all guidance modes are used, making it one of the most accurate ballistic missiles ever.

The impact of munitions near the target tends to be bivariate normally distributed around the aim point, with most reasonably close, progressively fewer and fewer further away, and very few indeed at long distance. One component of the bivariate normal will represent range errors and the other azimuth errors. Unless the munition is arriving exactly vertically downwards the standard deviation of range errors is usually larger than the standard deviation of azimuth errors, and the resulting confidence region is elliptical. Generally, the munition will not be exactly on target, i.e. the mean vector will not be (0,0). This is referred to as bias. The mean error squared (MSE) will be the sum of the variance of the range error plus the variance of the azimuth error plus the covariance of the range error with the azimuth error plus the square of the bias. Thus the MSE results from pooling all these sources of error. The square root of the MSE is the circular error probable, commonly abbreviated to CEP. Geometrically, it corresponds to radius of a circle within which 50 % of rounds will land.

It should be noted that the concept of CEP is only strictly meaningful if misses are roughly normally distributed. This is generally not true for precision-guided munitions. Generally, if CEP is n meters, 50 % of rounds land within n meters of the target, 43% between n and 2n, and 7 % between 2n and 3n meters. If misses were exactly normally distributed as in this theory, then the proportion of rounds that land farther than three times the CEP from the target is less than 0.2%. With precision-guided munitions, the number of 'close misses' is higher.