Radar guidance like infrared (IR) homing guidance is predominantly used in surface-to-air and air-to-air guided weapons. While IR homing guidance is largely employed in short and medium operational ranges, radar guidance is particularly attractive for long-range air-to-air guided missiles. State-of-the-art radar-guided air-to-air missiles using active radar homing guidance technology have operational ranges in excess of 150km. This concluding part of the article focuses on operational aspects of different types of radar-guided weapons. Features and capabilities of major international radar-guided weapon systems are also discussed.

Radar-guided weapons are surface-to-air and air-to-air anti-aircraft-guided missiles in which a radar is used to designate the intended target with electromagnetic energy, and the missiles make use of radar energy reflected from the target to steer itself to intercept and destroy the target. The basic concept is the same as the one used in the case of laser-guided munitions, except for the fact that laser-guided munitions are largely surface-to-surface and air-to-surface weapons and radar-guided weapons are mainly air-to-air and to a limited extent surface-to-air missiles.

Another difference is in their operational ranges, which are much larger in the case of radar-guided missiles, approaching 150km to 200km in the case of state-of-the-art air-to-air missiles than in the case of laser-guided munitions that seldom exceed 20km.

Fig. 1: Semi-active radar homing guidance basic concept
Fig. 1: Semi-active radar homing guidance basic concept
Fig. 2: Active radar guidance basic concept
Fig. 2: Active radar guidance basic concept

AIM-54 Phoenix and AIM-120D AMRAAM have maximum operational ranges of 190km and 180km, respectively. Also, laser is either on a land based platform or aircraft mounted in semi-active homing guidance. It is an integral part of the weapon hardware in the case of active laser homing.

In the case of radar-guided weapons, the radar is also mounted either on a land vehicle or an aircraft in semi-active radar homing guidance. Radar is an integral part of the guided-missile hardware in the case of active radar homing guidance.

A variation of semi-active radar guidance is track-via-missile (TVM) guidance. Like semi-active homing missiles, the ground based radar designates the target with radar energy, which is then reflected off the target and detected by the radar receiver onboard the missile.

However, unlike a semi-active radar homing missile, the missile does not have hardware and software to compute track and intercept information. Instead, data from the reflected radar energy is relayed back to the ground station via a data link. The ground station computes the track information and communicates it back to the missile.

Radar-beam-riding guidance is another technique that was used in earlier days for short-range surface-to-surface and surface-to-air missiles. But because of inherent shortcomings of the beam-riding concept, both laser and radar beam-riding weapons are more or less an extinct entity.

Semi-active radar guidance
In a semi-active radar guidance system, the target aircraft is illuminated by electromagnetic energy emitted by the fire control radar located either on the launch aircraft or at an appropriate ground location. The fire control radar acquires the target and tracks it. A small radar transmitter generating a very narrow beam then selectively illuminates the target using tracking information generated by the fire control radar. A radar receiver onboard the missile receives the radar energy reflected off the target and locks on to the target.

Once the missile is locked on to target, it may be launched. The radar seeker unit provides the angular error information to the flight path control system, which steers the missile to intercept the target through tail fins.

Semi-active radar homing guidance uses a continuous wave doppler radar in bi-static configuration. It uses bi-static because transmit and receive antennae are at different locations with the former being collocated with a fire control radar either on the launch aircraft or on ground, and the latter on the nose cone of the missile.

Doppler shifted receive signal frequency is computed from the closing velocity determined using flight path geometry. Information on the receive signal frequency is used by the missile to acquire the target. Fig. 1 shows semi-active radar homing basic concept when both fire control radar and missile are onboard the launch aircraft.

Semi-active radar guided weapons have the advantages of being low- cost and less complex. Due to the fact that the fire control radar needs to remain committed throughout the duration of the engagement till the target intercept has been achieved, semi-active radar guidance has certain disadvantages, more so when the radar is located on the launch aircraft. It renders the launch aircraft vulnerable to counter attack from the adversary using anti-radiation missiles as the radar itself acts as a beacon for the counter attacking missile.

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