A radar guided missile is a type of
anti-aircraft missile that uses
reflected radar energy to guide itself toward its target. Radar guided missiles may be launched from
fighter aircraft,
vehicles, or
ships, and to engage targets at very
long range. They are very popular worldwide, with a great number of designs available.
There are two types of radar guided missile: passive and active:
The first radar guided missiles employed Passive guidance. Passive guided missiles includes the US AIM-7 Sparrow, Russian AA-7 Apex and AA-8 Aphid as examples. A passively guided missile has a radar reciever but no radar transmitter. The radar signal must be transmitted from another source, be it an airplane or surface-based radar. Radar lock must be maintained throughout the entire attack, with the missile's computer performing fast fourier transforms to maneuver itself towards its target. If radar lock is broken, or the distance to target increases, the missile will self destruct, so it does not strike a friendly aircraft. This means that the attacking radar is committed to remain on until the attack has been completed.
That commitment produces certain disadvantages, particularly when the radar source is another airplane, such as the US F-4 Phantom or a French Mirage. The transmitter itself is subject to counterattack by radar homing missiles such as the US HARM or Shrike. The radar itself provides a beacon for the attacking missile, which renders launch platforms vulnerable. Self-preservation may require a radar to shut down, or an aircraft to maneuver during combat, ending the attack. If the source radar damaged or destroyed the attack fails. The increased vulnerability of the launch platform does much to offset the passive missiles signficant advantages in cost and simplicity. However, passive missiles are not expected to become extinct any time soon. In the past, the complexity of good radar transmitters all but mandated passive systems. With modern microelectronics many of those disadvantages have been overcome. Today's missiles can be very smart indeed. But they do not replace a human operator. Nor can any missle-bourne radar match the power and capability of the bigger base radars.
The Army's Patriot Missile operates somewhat differently, with terminal homing. Patriot missiles recieves guidance instructions from their ground base, so it retains the vulnerabilities inherent to the type, The modern tracking system permits a Patriot battery to simutaneously engage multiple targets.
Active guided missiles have grown in popularity since they were introduced. An actively guided radar missile contains its own complete radar, and attack computer. Examples include the US AIM-54C Phoenix and the AIM-120 AMRAAM. The target is located with an external radar, but once launched the missiles own radar takes over and guides itself to the target. For this reason actively guided missiles are considered fire and forget weapons, which improves survival odds for their user.
Radar guided missiles may have a very long range, over 100 miles. However, the earth's curvature reduces that range significantly against low altitude targets, such as cruise missiles. For this reason the Army's Patriot is being fitted with it's own active seeker head. Airborne radars with a "look down, shoot down" capability, such as that on the Panavia Tornado or SU-27 are better for attacking low altitude targets.
All radar missiles share certain disadvantages. The guidance and tracking radars must emit radio waves. Therefore, they can be detected by a radar detector at ranges greater than the radar itself can detect. Using any radar tells the other side you are around. That alerts the target, and a good electronic countermeasures or ECM package may be able to determine whether or not an attack has been initiated. Radar signals can be jammed using air and ground based jammers, or spoofed using chaff. For that reason some missiles, such as the Soviet SA-6, employ an additional infrared tracking capability. Radar missiles are complex compared to their infrared homing brethren. However, these weapons are likely to remain in use for some time to come.