Active radar homing

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Active radar homing is a missile guidance method in which a guided missile contains a radar transceiver and the electronics necessary for it to find and track its target autonomously.

1 Advantages
2 Disadvantages
3 Passive radiation homing
4 Operation

[edit] Advantages

There are two major advantages to active radar homing:

Because the missile is tracking the target, and the missile is typically going to be much closer to the target than the launching platform during the terminal phase, the tracking can be much more accurate and also have better resistance to ECM. Active radar homing missiles have some of the best kill probabillities, along with missiles employing track-via-missile guidance.
Because the missile is totally autonomous during the terminal phase, the launch platform doesn't 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 great advantage that modern air-to-air missiles have over their predecessors.

[edit] Disadvantages

There are two major disadvantages to active radar homing:

Since the missile has to contain an entire radar transceiver and electronics, it was until recently difficult to fit all of this into a missile without unacceptably increasing its size and weight. Even with today's miniaturisation making this possible, it's quite expensive to make these missiles since the sophisticated electronics within the missile are inevitably destroyed upon impact.
There is very little chance that targets with any sort of decent radar warning receiver would be unaware that an incoming missile is approaching them. This gives them sufficient time to take evasive action. However, given the accuracy of this homing method, unless the target is especially maneuverable or the missile is not, there may not be much they can do to avoid being intercepted.

[edit] Passive radiation homing

Many missiles employing this type of guidance have an extra trick up their sleeves; If the target does attempt to jam them using some kind of ECM, they can in effect turn into an anti-radiation missile and home in on the target's radiation passively. This makes such missiles practically immune to ECM, in addition to removing the second disadvantage. Since they already have the radar receiver on board, this should not be a terribly difficult feature to add (at least, it requires extra processing logic but little extra hardware).

[edit] 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. 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.

Examples of missiles which use active radar homing (all in their terminal phase) include: