The first part of this article covered defensive combat operations, air defence warfare, fighter aircraft and AEW aircraft combo (phase I), and air search radars and fighter aircraft combo (phase II). Read on:

Phase III: Operations: Aegis accompaniments
Destroyers and cruisers associated with the carrier are called Aegis ships, which are equipped with a robust command and control (C2) system called Aegis. These ships are more potent due to the presence of superior radars and C2 systems, which are based on high-performance computers.

Aegis cruisers are well-suited to conduct air defence warfare due to their superior array of radars. These have 2D long-range air-search radar AN/SPN-49, which is also present in the carrier. This radar is followed by passive electronic phased array radar AN/SPY-1.

Aegis destroyers do not have AN/SPN-49 radar. Hence, these cannot detect threats from a long range. Aegis cruisers with AN/SPN-49 radars detect threats at a long range and then Aegis C2 system lets the threats of interest to be tracked by AN/SPY-1 radar.

Fig. 5: SM-2 missile being launched from Aegis cruiser
Fig. 6: Launchers in Aegis destroyer (top) and Aegis cruiser (bottom)

AN/SPY-1 multi-functional radars. These are phased-array radars, which are the latest-generation radars that were introduced in the 1980s and are still state-of-the-art. These have antennae composed of many transmitting and receiving (T/R) elements. Each element radiates and receives RF energy individually, a sort of many tiny antennae arranged into one big antenna. A single source supplies RF energy to all T/R elements through computer-controlled phase shifters.

The radar computer controls the direction of the beam by varying the phase of the RF signal emitted by each T/R element. In time-domain analysis, the signal to each T/R element is delayed for a certain duration by these phase shifters. By suitably varying the phase (suitably delaying), a wavefront of electromagnetic energy is formed in the desired direction. In simple words, a beam is formed. A moving beam is formed by varying the phase accordingly. This technique is called electronically slewing the beam, in contrast to mechanical slewing.

In mechanically-slewed radars, position of the beam is constant with respect to the antenna. So the antenna itself must be slewed for scanning targets. The ability to slew rapidly is called agility. For rapid scanning requirements, the antenna must be slewed rapidly. But there are mechanical limitations beyond which the antenna cannot be slewed.

In AN/SPY-1 phased-array radar, the beam-formation technique provides agility for rapid slewing. Since the phase is varied by a computer, the direction of the beam can be switched in microseconds; there is no need for the antenna to move at all. Because of this, the antenna of this radar is not fixed in any mast but embedded on the superstructure of the vessel itself.

Using this technique, many individual beams can be formed depending upon the requirement for various functions, with each beam performing a specific function like scanning, tracking or providing datalinks. A single radar antenna covers a 90° sector in the azimuth and from the horizon in the front to the zenith overhead in elevation.

There are four antennae for this radar to cover a total of 360°. In this arrangement, a hemisphere of RF energy is maintained around the ship up to a range of around 185km (100-nautical-miles). The radar’s peak power is 4MW to 6MW and average power of 58kW in the S band.

Next in the line are the missiles for engaging the target.

SM-2 SAMs. The surface-to-air missile (SAM) used in Aegis vessels is called Standard Missile-2 (SM-2). Its range is quite long and current versions are capable of engaging targets even up to 200km. The guidance scheme of SM-2 is a semi-active radar-guidance scheme.

Generally, due to the long range involved, majority of long-range SAMs have semi-active radar-guidance schemes. This means that these do not have radar transmitters but only radar receivers. These can only receive radar echoes but not chirp out radar signals like usual radars. These depend on other radars for guiding these to intercept the target.

Due to the longer engagement range, more radar signal strength is required to look for the target. For more signal strength, the missile must have ultra-strong batteries to pump enough amperes into the radar. But ultra-strong batteries are heavy and consume more space in the missile. This extra weight introduces restrictions in the missile’s manoeuvrability.

Due to these reasons, the missile only has the radar receiver section as the target seeker.


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