Modern Warfare Mod 3 brings World in Conflict from the Cold War into the Modern Age. It also ups the ante on realism and authenticity in every role – Infantry, Armor, Support and Air, while trying our best to keep everything relatively balanced for fun and interesting gameplay.

Report article RSS Feed FLINT: Lead-Pursuit Guidance Principles

Flexible Interceptor (FLINT) project is a quest to replace World in Conflict's standard Homing Shooter rockets to improve realism in missile behavior. Ever since videos of AIM-9X missile using WiC FLINT have been published on YouTube, players were fascinated to see how the missile "wiggles" and leads its target constantly while in pursuit. The pursuit is then ended with a mid-air collision where AIM-9X punches the target from its side.

Posted by blahdy on Aug 3rd, 2011

While it is true that missile's flight path in lead-pursuit guidance looks very advanced, the actual guidance concept is quite simple in principle.

Introducing Proportional Navigation and a Brief History

Proportional Navigation (or simply "Pro-Nav") was first discovered during World War II by the US Navy.  It was then utilized in 1951 by William B. McLean (1914-1976) at Naval Air Weapons Station China Lake, during his development of the AIM-9 Sidewinder missile.

After the World War II, both the U.S. Air Force and Navy were on a quest to develop the world's first practically useable guided air-to-air missile to destroy Soviet bombers.  While the first guided air intercept missiles were developed by the Germans during the war, they were manually guided by the crew over radio (MCLOS).  This limited their effectiveness to shorter than gun range and in practical combat scenarios, they were conclusively useless while in the heat of intense combat.  If an air intercept missile was to ever replace the gun, it was clear that it would need an automatic guidance, without much human input.

After the war, two school of thoughts surrounded the development of world's first real air intercept missile: radar-guided and infrared. The DoD and much of the guided missile community opted for a radar-guided missile, resulting in development of AIM-4 Falcon for the Air Force.  The proponents of radar-guided missile argued that infrared guidance does not work well during bad weather.  The Navy opted for development of its own semi-active radar homing missile, called the AAM-N-2.  The AAM-N-2 became what is now famously known as the AIM-7 Sparrow.

William B. McLean, working as head of the Aviation Ordnance Division at the Naval Weapons Center at China Lake, saw the early radar-guided missiles as too complicated and "over-engineered" for what they were trying to achieve.  Dr. McLean believed that the ultimate air-to-air solution involved infrared based "heat-seeking" technology, which would provide a "fire and forget" autonomous guidance system.  This was the beginning of the now famously known AIM-9 Sidewinder missile.

Dr. McLean and his team sought of new innovative idea to automatically guide the Sidewinder missile onto its target. Since rocket's acceleration and fuel are rather limited, if the missile was to hit its target, it would have to constantly lead ahead of the target it is pursuing, which is known as "lead-pursuit" form of "collision homing" guidance.  To achieve this lead-pursuit guidance, one would think that the missile would need an advanced fire control computer with the ability to find the range to its target, measure target's speed and so on and so forth.  Dr. McLean and his team had a much simpler idea: proportional navigation (Pro-Nav).

Pro-Nav guidance was remarkable in that it effectively gave you the world's most accurate fire control computer for free.  You did not need any sophisticated equipment or a computer to achieve Pro-Nav lead-pursuit interception.  In fact, you did not even need to know the range to your target or its speed either.  All you needed to know is that you see the target in a constant sightline with your eyes constantly until impact, while running in same direction.

Constant Bearing, Decreasing Range (CBDR)

The principles of Pro-Nav as implemented by Dr. McLean and his team was based on the naturally occuring phenomenon known as "Constant Bearing, Decreasing Range" or CBDR.  CBDR was first discovered by sailers out at sea.  CBDR phrase refers to when another ship viewed from the deck or bridge of one's own ship, is getting closer and closer but does not appear to "move" (in other words, the other ship looks "frozen" in your eye's field of vision, even though its getting closer and both ships are moving).  Under this condition, a collision at sea was deemed imminent and sailers are constantly trained to watch for this situation, especially when maneuvering in tight spaces such as in a harbour.

The "sight bearing" is an imaginary line from your eye's vision to the target.  This 'sight bearing' line is also called "Line of Sight" or LOS. As the target is moving from left to right in your field of view, you will note that the LOS line's angle is also changing from left to right.  This change of sightline angle is called the "LOS Rotation Rate."

However, if you are also running left to right, in same direction as the target, but you adjust your turn so that this LOS angle never changes -- you will begin to realize that you and the target are on a constant bearing.  In other words, it looks like the target does not appear to "move" in your eye's field of vision.  It looks almost as if the target is running a treadmill, but is getting bigger and bigger as you are getting closer to your target every second.  Maintain this constant bearing to the target, and you and the target will collide together.  It is this phenomenon that Pro-Nav aims to exploit. 

Pro-Nav Operating Principle

Proportional navigation means that as you are chasing your target, your running-path rotation rate is faster than the LOS rotation rate by some constant multiplier.  So eventually your path will be "correct" and your LOS angle won't change.  Now you are on a perfect collision course.  This multiplier number is called the Navigation Constant.  The higher the navigation constant, the faster you correct your path early in the flight and the less correction is needed later in the flight.  This is why when you watch a real dogfight, often times, the missile seems to go wildly way to the left or right out of your view, then several seconds later, missile comes back to the fight and smacks the target from its side.

Because the missile is "proportionally" adjusting its lead based on LOS rotation rate relative to target's movement, it is named as "proportional navigation" guidance.  And because Pro-Nav adjusts the flight proportional to target's movement, it is remarkably efficient in conserving rocket's energy to produce an optimal interception course.  Combine this with the fact that all you need to know is that you run in the same direction as your target, but maintain a constant sightline angle: Now you have the most perfect target leading computation system without having any complicated guidance computers or electronics!

Indeed, the first Pro-Nav implementation by Dr. McLean and his team on the Sidewinder missile was also amazingly simple.  The missile's heatseeking mirror sits on a gyroscope so it will keep itself pointed to the target no matter where the missile is going.  The voltage applied to the mirror to keep it pointing to its target, is amplified (Navigation Constant multiplier) then used to deflect the control surfaces that also steer the missile, therby making missile velocity vector rotation proportional to LOS rotation rate.  Although, this early and extremely simple implementation without much electronics did not result in a perfect rotation rate that is exactly proportional to LOS rotation (which would require constant airspeed), it was equally effective.

This early implementation of Pro-Nav was quite impressive during tests in early 1950's.  A missile without a fire control computer and so simple had scored a direct hit against a moving QB-17 drone.  In fact, it was so impressive and more effective than other missiles that the Air Force eventually dropped the AIM-4 Falcon program during Vietnam War and opted to standardize to AIM-9 Sidewinder and AIM-7 Sparrow missiles.  In fact, Pro-Nav proved to be so remarkably effective and simple to implement, that practically every guided missile in the world since Sidewinder is now using a form of proportional navigation.  Even the most advanced Surface-to-Air Missile (SAM) systems in today's modern times, like the radar-guided Patriot and SM-2 missiles, use a form of proportional navigation to intercept their targets.


The ease and simplicity of Pro-Nav not only makes the missile look cooler and realistic in game, it also makes coding lot easier for devs. The old target leading mechanism used by MW Mod for SAMs and guidance scripts were rather crude and complicated to deal with, especially for semi-active homing rockets.  With Pro-Nav, there is practically no math to deal with other than basic linear algebra.  Simplicity never gets old, indeed.

To learn more about Flexible Interceptor (FLINT) and its capabilities in video games, check out our quick presentation here:

Someone wrote:A common misconception is that FLINT takes lot of CPU time -- this is actually false.  The CPU tax on World in Conflict is due to the way programming API interface works to render customized missiles in game -- but the actual guidance algorithms and missile sensor routines have very little performance impact, if any.

Some videos of FLINT in action:


Post comment Comments
Keksz Aug 3 2011, 9:34pm says:

you love math, right? :P

+9 votes     reply to comment
P3ACE753 Aug 3 2011, 11:47pm says:

The song is Rob Dougan - Furious Angels (no lyrics version)!

One of my all time fav songs of all time!

+1 vote     reply to comment
Heaney Aug 4 2011, 5:50am says:

The bit I understood least was the American football reference...

+3 votes     reply to comment
chema1994 Aug 4 2011, 5:56am says:

Nice work, very interesting

+1 vote     reply to comment
FoxFort Aug 4 2011, 9:42am says:


+1 vote     reply to comment
ikalugin Aug 4 2011, 2:50pm says:

even i did XD

+1 vote     reply to comment
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Released Dec 22, 2011
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