How a heat seeking missile reaches its target

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How a heat seeking missile reaches its target
Modern man-pads regularly use a heat seeking sensor to home in their target. In the early days, the sensor typically consisted of a single photon-sensitive electronic sensor and a lens. The lens would focus the infra-red radiation on different segments of the photon-sensitive sensor and the missile would try to maintain the heading such that the focused energy reaches the central part of the sensor which had the highest sensitivity. As the craft commences evasive maneuvers, the reflection of the heat source begins to depart from the center of the sensor and this causes the control modules to begin compensation.
This basic approach was not optimal since the trajectory of the missile could enforce very high acceleration and forces to the body and the missile would be easy to out maneuver. The next generation of the navigation algorithms, thus, had to be developed to improve the optimality of the missile trajectory when the target is maneuvering. To do this, new sensor design was implemented to supply additional information to the control module as well as helping the control module to separate the counter measures from the real target.
This second generation sensors exploited an interesting geometric property of a rotating mirror. A rotating mirror will produce a rotating image of the target on the sensor surface. Using combinations of sophisticated optical assemblies and rotating parts, one could design a sensor which could measure the angel of incidence. This angel is the angel between the missile axes and the targets line of sight. Measuring this angel was highly useful because the control module could use a navigation technique called “proportional navigation” using this measurement.
Sailors have long developed a simple yet powerful way to determine whether two vessels would collide or not. They have found that if the incidence angel between two vessels remains constant, they would collide. This basic principle is the foundation of the proportional navigation technique which I in use by almost every missile today. They try not to let the incidence angle to drift. Using this principle, the missile acts like it is going to where the target will be in the future not to the place where the target is. The trajectory can become smoother and thus the extreme forces and accelerations are avoided leading to simpler missile design and manufacture.
To remove the moving parts (optical assembly) the current trend is to use focal plane arrays or FPAs. These sensors provide a thermal map of the area to which the sensor is facing much like imaging sensors in the visible spectrum. Using the FPA, all rotating mirrors are removed and the sensor can be gimbaled even more easily to improve off-bore side performance. Now a day, a modern heat seeking missile can hunt its aggressive targets using optimal trajectories (modified proportional navigation) to reduce the fuel consumption and structural tensions while simultaneously avoiding the countermeasures of the target by processing the thermal map of the target frequently and in different spectrums.

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