A fire-control system (FCS) is a number of components working together, usually a gun data computer, a director and radar, which is designed to assist a ranged weapon system to target, track, and hit a target. It performs the same task as a human gunner firing a weapon, but attempts to do so faster and more accurately.
An early use of fire-control systems was in bomber aircraft, with the use of computing bombsights that accepted altitude and airspeed information to predict and display the impact point of a bomb released at that time. The best known United States device was the Norden bombsight.
Simple systems, known as lead computing sights also made their appearance inside aircraft late in the war as gyro gunsights. These devices used a gyroscope to measure turn rates, and moved the gunsight's aim-point to take this into account, with the aim point presented through a reflector sight. The only manual "input" to the sight was the target distance, which was typically handled by dialing in the size of the target's wing span at some known range. Small radar units were added in the post-war period to automate even this input, but it was some time before they were fast enough to make the pilots completely happy with them. The first implementation of a centralized fire control system in a production aircraft was on the B-29.
By the start of the Vietnam War, a new computerized bombing predictor, called the Low Altitude Bombing System (LABS), began to be integrated into the systems of aircraft equipped to carry nuclear armaments. This new bomb computer was revolutionary in that the release command for the bomb was given by the computer, not the pilot; the pilot designated the target using the radar or other targeting system, then "consented" to release the weapon, and the computer then did so at a calculated "release point" some seconds later. This is very different from previous systems, which, though they had also become computerized, still calculated an "impact point" showing where the bomb would fall if the bomb were released at that moment. The key advantage is that the weapon can be released accurately even when the plane is maneuvering. Most bombsights until this time required that the plane maintain a constant attitude (usually level), though dive-bombing sights were also common.
The LABS system was originally designed to facilitate a tactic called toss bombing, to allow the aircraft to remain out of range of a weapon's blast radius. The principle of calculating the release point, however, was eventually integrated into the fire control computers of later bombers and strike aircraft, allowing level, dive and toss bombing. In addition, as the fire control computer became integrated with ordnance systems, the computer can take the flight characteristics of the weapon to be launched into account.
By the start of World War II, aircraft altitude performance had increased so much that anti-aircraft guns had similar predictive problems, and were increasingly equipped with fire-control computers. The main difference between these systems and the ones on ships was size and speed. The early versions of the High Angle Control System, or HACS, of Britain's Royal Navy were examples of a system that predicted based upon the assumption that target speed, direction, and altitude would remain constant during the prediction cycle, which consisted of the time to fuze the shell and the time of flight of the shell to the target. The USN Mk 37 system made similar assumptions except that it could predict assuming a constant rate of altitude change. The Kerrison Predictor is an example of a system that was built to solve laying in "real time", simply by pointing the director at the target and then aiming the gun at a pointer it directed. It was also deliberately designed to be small and light, in order to allow it to be easily moved along with the guns it served.
The radar-based M-9/SCR-584 Anti-Aircraft System was used to direct air defense artillery since 1943. The MIT Radiation Lab's SCR-584 was the first radar system with automatic following, Bell Laboratory's M-9 was an electronic analog fire-control computer that replaced complicated and difficult-to-manufacture mechanical computers (such as the Sperry M-7 or British Kerrison predictor). In combination with the VT proximity fuze, this system accomplished the astonishing feat of shooting down V-1 cruise missiles with less than 100 shells per plane (thousands were typical in earlier AA systems). This system was instrumental in the defense of London and Antwerp against the V-1.
Although listed in Land based fire control section anti-aircraft fire control systems can also be found on naval and aircraft systems.
In the United States Army Coast Artillery Corps, Coast Artillery fire control systems began to be developed at the end of the 19th century and progressed on through World War II.
Early systems made use of multiple observation or base end stations (see Figure 1) to find and track targets attacking American harbors. Data from these stations were then passed to plotting rooms, where analog mechanical devices, such as the plotting board, were used to estimate targets' positions and derive firing data for batteries of coastal guns assigned to interdict them.
U.S. Coast Artillery forts bristled with a variety of armament, ranging from 12-inch coast defense mortars, through 3-inch and 6-inch mid-range artillery, to the larger guns, which included 10-inch and 12-inch barbette and disappearing carriage guns, 14-inch railroad artillery, and 16-inch cannon installed just prior to and up through World War II.
Fire control in the Coast Artillery became more and more sophisticated in terms of correcting firing data for such factors as weather conditions, the condition of powder used, or the Earth's rotation. Provisions were also made for adjusting firing data for the observed fall of shells. As shown in Figure 2, all of these data were fed back to the plotting rooms on a finely tuned schedule controlled by a system of time interval bells that rang throughout each harbor defense system.
It was only later in World War II that electro-mechanical gun data computers, connected to coast defense radars, began to replace optical observation and manual plotting methods in controlling coast artillery. Even then, the manual methods were retained as a back-up through the end of the war.
Land based fire control systems can be used to aid in both Direct fire and Indirect fire weapon engagement. These systems can be found on weapons ranging from small handguns to large artillery weapons.
Modern fire-control computers, like all high-performance computers, are digital. The added performance allows basically any input to be added, from air density and wind, to wear on the barrels and distortion due to heating. These sorts of effects are noticeable for any sort of gun, and fire-control computers have started appearing on smaller and smaller platforms. Tanks were one early use that automated gun laying had, using a laser rangefinder and a barrel-distortion meter. Fire-control computers are not just useful for large cannons. They can be used to aim machine guns, small cannons, guided missiles, rifles, grenades, rockets—any kind of weapon that can have its launch or firing parameters varied. They are typically installed on ships, submarines, aircraft, tanks and even on some small arms—for example, the grenade launcher developed for use on the Fabrique Nationale F2000 bullpup assault rifle. Fire-control computers have gone through all the stages of technology that computers have, with some designs based upon analogue technology and later vacuum tubes which were later replaced with transistors.
Fire-control systems are often interfaced with sensors (such as sonar, radar, infra-red search and track, laser range-finders, anemometers, wind vanes, thermometers, barometers, etc.) in order to cut down or eliminate the amount of information that must be manually entered in order to calculate an effective solution. Sonar, radar, IRST and range-finders can give the system the direction to and/or distance of the target. Alternatively, an optical sight can be provided that an operator can simply point at the target, which is easier than having someone input the range using other methods and gives the target less warning that it is being tracked. Typically, weapons fired over long ranges need environmental information—the farther a munition travels, the more the wind, temperature, air density, etc. will affect its trajectory, so having accurate information is essential for a good solution. Sometimes, for very long-range rockets, environmental data has to be obtained at high altitudes or in between the launching point and the target. Often, satellites or balloons are used to gather this information.
Once the firing solution is calculated, many modern fire-control systems are also able to aim and fire the weapon(s). Once again, this is in the interest of speed and accuracy, and in the case of a vehicle like an aircraft or tank, in order to allow the pilot/gunner/etc. to perform other actions simultaneously, such as tracking the target or flying the aircraft. Even if the system is unable to aim the weapon itself, for example the fixed cannon on an aircraft, it is able to give the operator cues on how to aim. Typically, the cannon points straight ahead and the pilot must maneuver the aircraft so that it oriented correctly before firing. In most aircraft the aiming cue takes the form of a "pipper" which is projected on the heads-up display (HUD). The pipper shows the pilot where the target must be relative to the aircraft in order to hit it. Once the pilot maneuvers the aircraft so that the target and pipper are superimposed, he or she fires the weapon, or on some aircraft the weapon will fire automatically at this point, in order to overcome the delay of the pilot. In the case of a missile launch, the fire-control computer may give the pilot feedback about whether the target is in range of the missile and how likely the missile is to hit if launched at any particular moment. The pilot will then wait until the probability reading is satisfactorily high before launching the weapon.