# In current missile, what is the guidance law used at terminal phase in homing missile guidance? [closed]

In current missile, what is the guidance law used at terminal phase in homing missile guidance?

eg: Proportional navigation, Augumented PN, Optimal Guidance Law, etc

At what distance the terminal phase guidance engage for Air Interceptor Missile(AIM) { eg: sidewinder, Derby, etc }. This is for active seeker(radar). And what is the terminal phase guidance used for active (radar) seeker.

what are information get from seeker to guidance system at terminal phase using active seeker(radar), other than Line_of_Sight(LOS) and Relative range(R) between missile and target.

According to this easy to read brief introduction:

Optimal Homing Guidance Laws

Other Optimal Homing Guidance Laws, many modern guidance laws require an estimate of time-to-go (tgo), which is the time it will take the missile to intercept the target or to arrive at the closest point of approach (CPA). The tgo estimate also is a critical quantity for missiles that carry a warhead that must detonate when the missile is close to the target.

• Time to go is not used for the terminal phase of the engagement. – Tyler Durden Aug 19 '15 at 19:20
• @TylerDurden How do you arrive at that conclusion? Many missiles use TTG during the terminal phase to augment PN to provide the zero-miss vector and CPA. Try this for a start – Simon Aug 19 '15 at 20:42

Normally the terminal phase of trajectory will be computed by some specialized algorithm which is specific to the missile, particularly to the type of sensor and processor used as well as the nature of the propulsion and maneuverability characteristics of the vehicle.

The reason for this is that during the terminal phase only a few measurements are possible. For example, let's imagine the signal train of the weapon's seeker has a 15 millisecond sensor loop time, a 50 millisecond control loop, and the vehicle is traveling 1500 feet per second. In that case you make one measurement every 22 feet and one control change every 75 feet. So, if the weapon is 200 feet away from the target it will get to make only 2 control changes before intercept based on about 8 measurements total. Depending on the type of measurement, there will be a highly specialized algorithm which is designed to optimize the chance of a kill given the limited information available.

• The specialized algorithm that he's asking about (as stated in his question) is: "Proportional navigation, Augumented PN, Optimal Guidance Law, etc". – Rhino Driver Aug 17 '15 at 22:14
• @RhinoDriver By a "specialized" algorithm I am referring to an algorithm SPECIFIC TO THE MISSILE as it says in my post. PN is a generic navigational method which any missile can use. Generic navigation is not used in the terminal phase of missile flight. Generic methods are generally only used in mid-course navigation. The OP asked about terminal phase navigation, not mid-course flight. – Tyler Durden Aug 17 '15 at 22:17
• Terminal guidance references the final guidance stage that the missile uses to guide to the target. In the case of IR missiles that literally only have a single guidance stage, they are terminal the moment they leave the rail. An active missile (like the 120) is terminal as soon as it enters an active state. In either case the missile doesn't make drastic changes to its guidance because its become terminal, it continues the same track. Smarter missiles may adjust to the target using optimal homing guidance, but even then, its still maintaining the same guidance law in the terminal phase. – Rhino Driver Aug 17 '15 at 22:44
• It's also worth noting that pronav ultimately refers to the type of intercept geometry the missile uses. In the case of a non-manoeuvring target, the missile will continue the same profile until impact to preserve maximum energy for the end game. – Rhino Driver Aug 17 '15 at 23:08
• @RhinoDriver The OP does not state what kind of missile he is talking about, however, he specifically refers to those having a terminal phase. As far as the AIM-120 is concerned, it has a terminal phase which is completely different than its search phase. It uses a closed-loop active radar seeker. Closed loop homing algorithms are generally very complicated and are specifically tuned to the sensor (a kind of radar in this case). – Tyler Durden Aug 17 '15 at 23:23