A “5 meter” precision bomb misses by more than 5 meters half the time

In short: Circular error probable, or CEP, is the radius that catches only half of the shots. The other half land outside it, and roughly one in twenty lands more than twice as far out. Read any accuracy figure as a median, never as a guarantee.

Defense brochures love a single number. A guided bomb “accurate to 5 meters” sounds like every round hits inside a 5 meter circle. That is not what the number says.

Read also: Hardening the satellite is wasted money if the ground station stays soft · One exquisite satellite loses to sixty cheap ones, and revisit rate is why · Jamming does not stop the drone that is not listening

What CEP actually measures

CEP is a statistic borrowed from ballistics. Fire many rounds at one aim point, draw the smallest circle around that point that contains 50% of the impacts, and its radius is the CEP. Half the impacts sit inside. Half sit outside.

So a weapon with a 10 meter CEP will put plenty of rounds at 12, 15 or 20 meters. The spec sheet has not lied. It has simply answered a narrower question than the reader thinks it answered.

The JDAM numbers, read properly

The US Joint Direct Attack Munition is the textbook case. According to published figures, it reaches a CEP of 5 meters or less with a GPS signal. If GPS is jammed or lost, the inertial system alone still holds about 30 meters for free flight times up to 100 seconds. During testing, more than 450 JDAMs were dropped with a published accuracy under 10 meters CEP.

That GPS versus inertial gap explains why jamming is a real threat to precision weapons, and why designers keep an inertial fallback in the loop.

If the impacts follow a normal circular spread, the radius rules of thumb look like this:

Why planners care about the outer ring

The median miss decides whether a target is destroyed. The outer ring decides who else gets hurt. Near civilians, that tail is the number that matters.

A second trap hides in the word “aim point”. CEP measures error around where the weapon was told to go. If the target coordinates are 20 meters off, a perfect bomb hits the wrong spot perfectly. That is why targeting now leans on sensor fusion and fresh imagery, often from a satellite constellation with a high revisit rate, as much as on the guidance kit itself.

Questions to ask about any accuracy claim

• Is the figure CEP, R95, or something else? The gap is roughly a factor of two.
• Was it measured with GPS available, denied, or degraded?
• How many test shots produced it? Ten drops and 450 drops are different evidence.
• Does it include target location error, or only the weapon’s own error?
• What happens to the spread at maximum range compared with mid range?

The formal definition and the math behind the multipliers are laid out in the Wikipedia entry on circular error probable.

A quick way to use this

Next time a vendor quotes a single accuracy number, double it. That gives you a rough 95% radius. Then ask whether the test had GPS, and you will know more than most of the people repeating the press release.