§9.7 Homing guidance

With homing guidance the target sensor sits in the missile itself: the seeker sees the target and the missile computes its own steering solution (usually with proportional navigation, Guidance methods). The closer the missile gets, the stronger the signal and the more precise the guidance — exactly the opposite of command guidance, where the error grows with range. That is why homing is the method for the terminal phase.

The three flavours differ in where the "illumination" comes from: from the missile's own platform (semi-active), from the target itself (passive) or from the missile (active).

Semi-Active Radar Homing

With Semi-Active Radar Homing (SARH) the launch platform continuously illuminates the target with its radar (the illuminator, often CW or interrupted CW — Continuous wave radar). The missile itself transmits nothing: its seeker receives the energy reflected from the target and homes on it. A rearward-facing reference antenna receives the direct illuminator signal, so that the seeker can pick the moving target out of the clutter by Doppler comparison.

• Advantage: the missile is relatively simple and the heavy, powerful radar sits on the platform — long detection and illumination ranges.
• Disadvantage 1 — tied-down platform: the illuminator has to keep illuminating the target until impact. The ship or aircraft cannot turn away all that time and can only engage as many targets at once as it has illuminators.
• Disadvantage 2 — a loud warning: the target hears the CW illumination on its RWR and so knows a missile is on its way — with plenty of time for jamming, chaff and manoeuvres.

Classic examples: older/mid-generation air-to-air missiles and many ship- and ground-based SAM systems. The pattern of Phases of the missile intercept returns: many modern systems use SARH only in the terminal phase, or have switched to active seekers.

Passive homing

With passive homing all the energy comes from the target itself; neither missile nor platform transmits anything. Two main forms:

IR homing ("heat-seekers")

The seeker homes on the target's heat (Infrared): exhaust, plume, and with modern seekers the whole airframe heated by friction. Evolution through generations: from spin-scan/con-scan reticle seekers (from directly behind only, easily lured by flares) via cooled all-aspect seekers to imaging IR: a camera image in which the target is recognised by its shape — flares are rejected as point sources. The domain of MANPADS and modern short-range air-to-air missiles.

Anti-radiation homing

The anti-radiation missile (ARM) homes on the target's radio emissions — above all enemy radars. The seeker is a wideband passive receiver that takes a bearing on the target radar and classifies it. The weapon of SEAD (Suppression of Enemy Air Defences): the radar faces the choice of transmitting and being hit, or staying silent and being blind. Countermeasures: emission discipline (transmitting briefly and irregularly), decoy emitters that pull the ARM away, and switching off quickly (to which modern ARMs respond with an INS/GPS memory of the last measured position).

Properties shared by all passive homing:

• Completely silent: no emission at all — often the first warning is the missile itself (with IR, only UV/IR plume detection can still raise the alarm).
• True fire and forget: the platform is free after launch.
• Dependent on the target's signature: if the source goes out (exhaust shielded, radar switched off), the seeker loses its grip; and the countermeasure race (flares/DIRCM, decoys) aims precisely at this.
• No range information (Disadvantages of infrared): the seeker measures direction only; fortunately proportional navigation needs nothing more.

Active Radar Homing

With Active Radar Homing (ARH) the missile carries its own complete radar: transmitter and receiver in the nose. After the midcourse (INS + datalink, Phases of the missile intercept) the seeker starts transmitting itself and tracks the target entirely on its own — from that moment the missile is "autonomous" and the platform free: fire and forget at long range.

• Advantages: the platform can turn away after launch or engage the next target; attacking several targets at once becomes possible; and the seeker keeps getting closer, so its modest transmit power (1/R⁴ now works in its favour) is more than enough in the terminal phase.
• Disadvantages: a complete radar in a missile nose is expensive and small (a limited antenna → limited range and a wide beam, Gain); and as soon as the seeker transmits it is detectable and jammable — DRFM jammers and towed/expendable active decoys are aimed exactly at this.

For the target, the RWR sequence is characteristic: first possibly the platform's tracking radar, then silence during the midcourse (datalink only), and then suddenly a new X/Ku-band emitter that approaches and steps up its PRF — the seeker has gone active. This is the last possible moment for countermeasures.