Guided munitions need to have a way to acquire their target. Different types of weapons will require one of several methods of acquiring a target.
Radar guided weapons (usually anti-air or anti-ship missiles) require a radar lock in order to home in on a target. A radar lock can be acquired via one of the vessel's on-board radars, or remotely via data-link.
Radar missiles can be defeated by chaff (passive jamming), active jamming, and manoeuvring; these are obviously more effective when used together.
There are two types of radar guided missiles:
Semi-Active Radar Homing (SARH) Edit
A missile with semi-active radar homing does not have it's own radar transmitter but can detect radar signals. For it to "see" a target the launch vehicle must continuously illuminate the target with it's own radar. If radar lock is broken in any way the missile will be unable to continue tracking.
Any target with a radar warning receiver can detect the powerful radar beam used to illuminate it for the SARH missile; telling the target a radar missile has been launched against it.
The need for the launch vehicle to continuous lock the target while attacking with a SARH missile can leave it vulnerable as it is continuously advertising its position with it's own radar. Radars that lack track while scan function cannot be used to search for targets while using a SARH missile, reducing situational awareness.
This type of homing is used in most older AAMs such as the R-27r and AIM-7 sparrow
Active Radar Homing Edit
A missile with active homing capability has its own radar. The missile's on-board radar has a limited range, so the firing vessel must maintain lock until the missile's radar has acquired the target, at which point the missile guidance is self contained, and it will track the target on its own making the missile fire and forget.
Heat (IR) Edit
Heat-seeking missiles require a heat source in order to track a target. The hotter the target, and the more sensitive the seeker, the further away a heat-seeking missile can lock/track a target. This also means that an IR seeker can see the hot engine exhaust from the rear aspect of an aircraft further away than it can see the same aircraft from the front.
As an IR missile homes in on heat emitted by the target there is no warning from the RWR that one has been launched making them ideal for taking a target by surprise.
There are two ways of locking a target with a heat-seeking missile:
When a heat-seeking missile is selected, a circular reticle will appear in front of it. This shows the field of view of the heat-seeker, and any heat source that passes through this field will be tracked by the seeker. Once a target has been locked, it will be tracked until the target exceeds the missile's maximum off-boresight angle.
If a target is locked via radar, the heat-seeker can track the target even if it is outside of the boresight field of view. This allows for off-boresight lock and launch.
Laser guided weapons require a target to be painted by a targeting pod. Once a targeting pod has acquired and stabilized on a target, the laser point just needs to be within the field of view of the missile. These missiles can also lock onto targets painted by targeting pods on other vessels. These will show up as small green diamonds. The Missile must be pointing within ~40 degrees for it to lock on to the Laser designated area. Otherwise it will just fly away without engaging its targeting systems.
GPS weapons require a target to be designated in the GPS Coordinator.
Anti-Radiation missiles home in on outgoing radar signals. The Radar Warning Receiver must be activated in order for this type of missile to detect and lock on targets.
Currently this targeting type uses the outgoing radar signals only for initial target acquisition. After the missile is launched, it flies to towards the GPS coordinates of said target. Hence, switching off the radar does not defeat the missile. Since GPS coordinates are static (see GPS guidance), it also means that ground vehicles or ships easily defeat the missile by moving away.