Modern phased-array radars, by virtue of their thin beams and low side-lobe levels, make detection by the aircraft even more difficult. TVM missiles can be made more accurate by using more sophisticated algorithms for calculating interception than would be possible in the limited processor possible to be accommodated in a missile.

In addition, operators have the option of adjusting the missile’s flight path throughout engagement. Radar-hosting ground station may use radar energy reflected from the target directly and combine this information with the downloaded information from the missile to generate the interception course. This improves resistance to electronic countermeasures.

One disadvantage is vulnerability of the ground radar station to anti-radiation missiles as the radar has to remain active throughout engagement.

Possibility of data link jamming is another shortcoming.

Another potential disadvantage is that the missile will not be able to continue engagement if the target aircraft could manage to put an obstacle such as a hillock between the radar beam and itself, or if it could manage to go out of the radar’s tracking envelope.

Missile guidance and control
A missile’s flight path is generally divided into three separate phases, namely, launching or initial phase, mid-course phase and terminal phase.

During the launch phase, flight controls are locked in a neutral position as the missile does not have aerodynamic stability during this phase. The guidance system takes charge immediately after the initial phase is completed and controls are unlocked. The initial phase lasts for a very short duration.

The main task of the mid-course guidance is to place the missile near the target, enabling the terminal phase guidance system to successfully take over. This phase is the longest in both distance travelled by the missile as well as time duration. During this phase, control may need to be exercised to ensure that the missile follows the desired course and also stays on course. In some cases, mid-course guidance may also additionally perform the task of terminal phase guidance.

Fig. 4: Missile stabilisation
Fig. 4: Missile stabilisation
Fig. 5: AIM-7 sparrow being launched from F-18A Hornet
Fig. 5: AIM-7 sparrow being launched from F-18A Hornet

Terminal phase is the most crucial phase of the missile’s flight path, as it is this phase that leads to target-hit or target-miss. The terminal phase guidance system needs to have high accuracy and fast response to guidance signals. Towards the end of the missile’s journey to intercept the target, when the missile is very close to target, it may not be left with sufficient energy to execute any high-g manoeuvre. Proportional guidance helps address this issue.

In the case of radar guidance, semi-active, active or TVM guidance, guidance signal is generated based on radar energy reflected off the target. Every missile-guidance system consists of an attitude control system and a flight path control system.

The attitude control system controls the attitude of the missile in pitch, yaw and roll parameters on the designated flight path (Fig. 4). The flight path control system guides the missile to its designated target. This is achieved by determining flight path errors, generating necessary commands to correct these errors and sending these commands to the missile’s control sub-system that works on servo principle.

Control units make corrective adjustments to missile control surfaces on receiving an error signal. These units also adjust wings or fins to stabilise the missile in roll, pitch and yaw. Guidance and stabilisation are two separate processes that occur concurrently.

Radar-guided missile systems
Now, we will discuss some prominent surface-to-air and air-to-air radar-guided missile systems.
Missile systems described here include AIM-7 Sparrow, MIM-104 Patriot missile system (both semi-active radar-guided missile systems), AIM-54 Phoenix, RIM-174 ERAM, AIM 120D AMRAAM, MBDA Exocet (all active radar-guided missile systems) and Akash surface-to-air missile.

AIM-7 Sparrow is a medium-range, supersonic-speed semi-active radar-guided air-to-air missile manufactured by Raytheon.

A derivative of this missile is the ship based version called RIM-7 Sea Sparrow used for air defence role. The missile had been in active service from 1950’s till 1990’s. It is being gradually phased out in favour of AIM-120 AMRAAM, an all-weather, day/night operation beyond-visual-range air-to-air missile.

AIM-7 Sparrow has a number of variants designated as AIM-7A, AIM-7B, AIM-7C, AIM-7D, AIM-7E, AIM-7F, AIM-7G, AIM-7H, AIM-7M, AIM-7N, AIM-7P and AIM-7R.

Development of AIM-7B, AIM-7N and AIM-7R was discontinued after initial interest. Table I shows a comparison of different variants of AIM-7 Sparrow. Many AIM-7 variants had ship based versions. These included RIM-7E, RIM-7F, RIM-7H, RIM-7M, RIM-7P and RIM-7R. AIM/RIM-7R programme was abandoned in 1996 due to its high costs.

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