Fire-control radar

A fire-control radar (FCR) is a radar which is designed specifically to provide information (mainly target azimuth, elevation, range and velocity) to a fire-control system in order to calculate a firing solution (i.e. information on how to direct weapons such that they hit the target(s)). Such radars typically emit a narrow, intense beam of radio waves to ensure accurate tracking information and to minimise the chance of losing track of the target. Some modern radars have a track-while-scan capability enabling it to function simultaneously as a fire-control radar and a search radar. This works either by having the radar switch between sweeping the search sector and sending directed pulses at the target to be tracked, or by using a phased-array antenna to generate two (or more) discrete radar beams and dividing them between both tasks.

Operational phases
Fire-control radars operate in three different phases:


 * Designation or vectoring phase: The fire-control radar must be directed to the general location of the target due to the radar’s narrow beam width. This phase ends when lock-on is acquired.
 * Acquisition phase: The fire-control radar switches to the acquisition phase of operation once the radar is in the general vicinity of the target. During this phase, the radar system searches in the designated area in a predetermined search pattern until the target is located or redesignated. This phase terminates when a weapon is launched.
 * Tracking phase: The fire-control radar enters into the track phase when the target is located. The radar system locks onto the target during this phase. This phase ends when the target is destroyed.

Performance
The performance of a fire-control radar is determined primarily by two factors: radar resolution and atmospheric conditions. Radar resolution is the ability of the radar to differentiate between two targets closely located. The first, and most problematic, is gaining high range resolution. To do this in a basic fire-control radar system, it must operate at smaller pulse width. Bearing resolution is typically ensured by using a narrow (one or two degree) beam width. Atmospheric conditions, such as moisture lapse, temperature inversion, and dust particles affect radar performance as well. Moisture lapse and temperature inversion often cause ducting, in which RF energy is bent as it passes through hot and cold layers. This can either extend or reduce the radar horizon, depending on which way the RF is bent. Dust particles, as well as water droplets, cause attenuation of the RF energy, translating into a loss of effective range. In both cases, a lower pulse repetition frequency makes the radar less susceptible to atmospheric conditions.

Countermeasures
Most fire-control radars have unique characteristics, such as radio frequency, pulse duration, pulse frequency and power. These can assist in identifying the radar, and therefore the weapon system it is controlling. This can provide valuable tactical information, like the maximum range of the weapon, or flaws that can be exploited, to combatants that are listening for these signs. During the cold war Soviet fire control radars were often named and NATO pilots would be able to identify the threats present by the radar signals they received.

Surface based
One of the first successful fire-control radars, the SCR-584, was used effectively and extensively by the Allies during World War II for anti-aircraft gun laying. Since WWII, the U.S. Army has used radar for directing anti-aircraft missiles including the MIM-23 Hawk, the Nike series and currently the MIM-104 Patriot.

Ship based
Examples of fire-control radars currently in use by the United States Navy:
 * Mk 95 — Continuous Wave Illuminator (NATO Seasparrow Surface Missile System)
 * Mk 92 — Combined Antenna System (Mk 75 Gun, formerly SM-1 missiles)
 * AN/SPG-62 — Continuous Wave Illuminator (AEGIS)
 * AN/SPQ-9B — Pulse Doppler (Mk 45 lightweight gun)

Aircraft based
After WWII, airborne fire control radars have evolved from the simpler gun and rocket laying AN/APG-36 system used in the F-86D to the Active Electronically Scanned Array based AN/APG-81 of the F-35.