Despite this shortcoming, semi-active radar guidance is likely to stay because of better radar resolution and higher power possible with ground based or aircraft-mounted radars.
It may be mentioned here that radar resolution becomes better with an increase in transmit antenna size, and the radar’s size and weight have a direct bearing on power output capability of the radar. A large-size transmit antenna and large-size radar cannot be accommodated within the missile as would be the requirement in active radar guidance.
Raytheon’s AIM-7 Sparrow with an operational range of 30km to 50km for different variants uses semi-active radar homing guidance.
Active radar guidance
In the case of active radar guidance, the missile tracks its target by means of emissions that it generates itself with the radar transceiver located onboard the missile. The missile has the required electronics hardware and software to find and track the target autonomously and does not depend upon the services of an external radar transmitter as is the case in semi-active radar guidance.
Active radar guidance is commonly used for terminal homing in anti-ship, surface-to-air and air-to-air missiles. Fig. 2 shows the basic concept of active radar homing guidance.
One advantage of active radar homing guidance is higher tracking accuracy and better kill probability as compared to semi-active radar guidance. This is due to the missile being much closer to the intended target in the terminal phase than would be the launch platform. This also gives the missile higher resistance to electronic countermeasures.
Another major advantage is the much-reduced vulnerability of the launch platform to counter attack. Since the missile is autonomous in the terminal phase, launch platform, ground based or on aircraft need not keep its radar enabled during this phase. In fact, the aircraft in case of being the launch platform can exit the scene or undertake other tasks while the missile homes on to the target providing fire-and-forget capability.
Looking at disadvantages, active radar guided missiles are likely to have increased size and weight as compared to missiles employing semi-active radar guidance. In the case of former, the missile has to accommodate the radar transceiver and electronics for autonomous target tracking.
Also, these missiles are more expensive as sophisticated electronics have only one-time use.
Another shortcoming is that, active radar guided missiles, due to radar emission, are more than likely to be detected by sophisticated radar-warning sensors equipping modern fighter aircraft, enabling these to take an evasive action or deploy countermeasures well in time. Given the fact that even modern aircrafts cannot match the manoeuvrability and agility of state-of-the-art missiles, there is not much an aircraft can do to avoid interception.
Due to restriction on size of the radar transmitter that is linked to power available at radar output and size of transmitting antenna that is linked to beam width, active radar guidance alone cannot provide guidance to long-range missiles. A practical solution can be to use semi-active radar guidance or inertial guidance for midcourse correction and active guidance for terminal homing.
Raytheon’s RIM-174 ERAM, also called standard missile-6 (SM-6), is a state-of-the-art surface-to-air missile in this category, having an operational range of 240km and using a combination of inertial guidance, semi-active radar guidance and active radar homing. It has been in service since 2013.
AIM-54 Phoenix air-to-air missile manufactured by Hughes Aircraft Co. and Raytheon Corp., with an operational range of 190km, also uses a combination of semi-active and active radar homing. It remained in service from 1974 to 2004.
MBDA Exocet anti-ship sea-skimming missile, with an operational range of 70km to 180km, also uses inertial and active radar homing guidance.
Track-via-missile radar guidance
TVM, also called re-transmission guidance, is a combination of semi-active radar homing and radio command guidance. In this case, the target is illuminated by an external radar, and the reflected radar energy is intercepted by a receiver onboard the missile, like it is done in semi-active radar homing.
However, the missile has no onboard computer to process these signals to generate track information. Signals are instead transmitted back to the radar-hosting launch platform over down data link for processing. The missile is then commanded over the up data link to adjust its flight trajectory to intercept the target.
Fig. 3 illustrates the concept of TVM guidance. MIM-104 Patriot missile system is an example of TVM guidance.
Unlike active radar guidance, which alerts the target aircraft about impending danger, in the case of TVM guidance, it may know that it is being illuminated but is never sure whether it is being engaged.