At sea there is an old rule: if another ship stays on the same bearing while it gets closer, you are on a collision course. A missile does this deliberately: it steers so that the direction to the target does not change (proportional navigation).
In practiceMost guided missiles, from MANPADS to the AIM-120, therefore do not fly to where the target is, but to where it is about to be. A late, sharp turn by the target forces the missile into a large steering correction — that is why a pilot breaks away at the last moment.
All forms of guidance answer the same two questions: who sees the target? and where is the steering solution computed? All the families can be arranged along those axes:
| Family | Who sees the target? | Computation | Does the missile transmit? | Section |
|---|---|---|---|---|
| Beam riding | platform (keeps the beam on the target) | in the missile (stay in the beam) | no | Beam rider |
| Command guidance | platform (tracks target and missile) | on the platform; steering commands via a link | no | Command guidance |
| Semi-active homing | missile sees the reflection of the platform's illumination | in the missile | no (receives only) | Semi-Active Radar Homing |
| Passive homing | missile sees the target's own emission/heat | in the missile | no | Passive homing |
| Active homing | missile with its own radar | in the missile | yes | Active Radar Homing |
| Track via Missile | missile receives, ground computes | hybrid | no (receives and relays) | Track via Missile (TVM) and SAGG |
| Navigation guidance | nobody — a position instead of a target | in the weapon (INS/GPS/terrain) | no (possibly a radar altimeter) | Navigation guidance |
In addition there is a second layer: the guidance law — the mathematics that goes from "where is the target" to "how much to steer". The most important ones:
- Pursuit: always steer straight at the target. Simple, but against moving targets it produces an ever tighter chasing turn — inefficient.
- Lead / collision course: aiming ahead at the point where the target will be.
- Proportional navigation (PN): the standard for homing missiles: steer so that the bearing to the target stays constant (the line of sight does not rotate). A constant bearing with decreasing range means, mathematically, a collision. PN calculates the required lateral acceleration from the rotation rate of the line of sight — efficient and with minimal information.