Circular error probable in the context of "Intercontinental ballistic missile"

The Norden Mk. XV, known as the Norden M series in U.S. Army service, is a bombsight that was used by the United States Army Air Forces (USAAF) and the United States Navy during World War II, and the United States Air Force in the Korean and the Vietnam Wars. It was an early tachometric design, which combined optics, a mechanical computer, and an autopilot for the first time to not merely identify a target but fly the airplane to it. The bombsight directly measured the aircraft's ground speed and direction, which older types could only estimate with lengthy manual procedures. The Norden further improved on older designs by using an analog computer that continuously recalculated the bomb's impact point based on changing flight conditions, and an autopilot that reacted quickly and accurately to changes in the wind or other effects.

Together, these features promised unprecedented accuracy for daytime bombing from high altitudes. During prewar testing the Norden demonstrated a 150 feet (46 m) circular error probable (CEP), an astonishing performance for that period. This precision would enable direct attacks on ships, factories, and other point targets. Both the Navy and the USAAF saw it as a means to conduct successful high-altitude bombing. For example, an invasion fleet could be destroyed long before it could reach U.S. shores.

A guided bomb (also known as a smart bomb, guided bomb unit, or GBU) is a precision-guided munition designed to achieve a smaller circular error probable (CEP).

The creation of precision-guided munitions resulted in the retroactive renaming of older bombs as unguided bombs or "dumb bombs".

A precision-guided munition (PGM), also called a smart weapon, smart munition, or smart bomb, is a type of weapon system that integrates advanced guidance and control systems, such as GPS, laser guidance, or infrared sensors, with various types of munitions, typically missiles or artillery shells, to allow for high-accuracy strikes against designated targets. PGMs are designed to precisely hit a predetermined target, typically with a margin of error (or circular error probable, CEP) that is far smaller than conventional unguided munitions. Unlike unguided munitions, PGMs use active or passive control mechanisms capable of steering the weapon towards its intended target. PGMs are capable of mid-flight course corrections, allowing them to adjust and hit the intended target even if conditions change. PGMs can be deployed from various platforms, including aircraft, naval ships, ground vehicles, ground-based launchers, and UAVs. PGMs are primarily used in military operations to achieve greater accuracy, particularly in complex or sensitive environments, to reduce the risk to operators, lessen civilian harm, and minimize collateral damage. PGMs are considered an element of modern warfare to reduce unintended damage and civilian casualties. It is widely accepted that PGMs significantly outperform unguided weapons, particularly against fortified or mobile targets.

During the Persian Gulf War guided munitions accounted for only 9% of weapons fired but accounted for 75% of all successful hits. Despite guided weapons generally being used on more difficult targets, they were still 35 times more likely to destroy their targets per weapon dropped.