There are two major advantages to active radar homing:
As the missile is tracking the target it is going to be much closer to the target than the launching platform during the terminal phase, thus the missile's tracking can be much more accurate and better resistant to electronic countermeasures. Active radar homing missiles have some of the best kill probabilities, along with missiles employing track-via-missile guidance.
Because the missile is totally autonomous during the terminal phase, the launch platform does not need to have its radar enabled at all during this phase, and in the case of a mobile launching platform like an aircraft, can actually exit the scene or undertake other actions while the missile homes in on its target. This is often referred to as fire-and-forget capability and is a significant advantage that modern air-to-air missiles have over their predecessors.
Disadvantages
Because most missiles are powered by rocket motors, they have no on-board electricity generation capability. This means that active radar-guided missiles usually rely on battery power for the radar transmitter, significantly limiting its power - although this can be mitigated by employing the designs described below.
Because a complete radar system is implemented, an active system will be more expensive than a semi-active system if all other factors are equal.
Passive radiation homing
Many missiles employing passive homing have an additional capability: if the target does attempt to use noise jamming, the missile can home in on the target's radiation passively (home-on-jam). This gives such missiles improved performance against noise jamming targets and allows anti-aircraft munitions to attack targets they would not otherwise be able to fire on effectively.[citation needed].
Operation
Active radar homing is rarely employed as the only guidance method of a missile. It is most often used during the terminal phase of the engagement, mainly because since the radar transceiver has to be small enough to fit inside a missile and has to be powered from batteries, therefore having a relatively low ERP, its range is limited.[2] To overcome this, most such missiles use a combination of command guidance with an inertial navigation system (INS) in order to fly from the launch point until the target is close enough to be detected and tracked by the missile. The missile therefore requires guidance updates via a datalink from the launching platform up until this point, in case the target is maneuvering, otherwise the missile may get to the projected interception point and find that the target is not there. Sometimes the launching platform (especially if it is an aircraft) may be in danger while continuing to guide the missile in this way until it 'goes active'; In this case it may turn around and leave it to luck that the target ends up in the projected "acquisition basket" when the missile goes active. It is possible for a system other than the launching platform to provide guidance to the missile before it switches its radar on; This may be other, similar fighter aircraft or perhaps an AWACS.
