The control unit activates the PGMs aerodynamic control surfaces to keep the pattern of energy differences within the field of view as it guides the PGM to its target. The term lock-on is used to describe the activation of the control unit. Once locked on, the PGM is guided toward the target. There are two fundamental sensor-tracker systems: the edge tracker and the centroid tracker.

EDGE TRACKER. The edge tracker locks on the target and guides the PGM toward the area of most intense energy contrast within the sensors field of view.

CENTROID TRACKER. The centroid tracker is a dual-polarity system. It locks on the target and guides the PGM toward the center (centroid) of the most or least intense radiation (emitted or reflected) within the field of view. A polarity switch in the cockpit permits pilots to make the selection to lock on the maximum or minimum energy.

Learning Objective: Identify the advan-tages and disadvantages of PGMs.

ADVANTAGES AND DISADVANTAGES OF PGMs

The advantage of PGMs is improved accuracy in hitting an intended target. The disadvan-tages are weapon cost, sensitivity to the environment, and aircraft exposure time to enemy defenses.

The cost of PGMs is extremely high compared to conventional munitions.

PGMs are sensitive to weather and other environmental factors. There are no all-weather PGMs. The environment influences the effective-ness of PGMs in a highly complex manner. First, there are the effects of Earths atmosphere and surface on the propagation of electromagnetic energy. Second, atmospheric elements such as turbulence, icing, hail, lightning, and electrical charge buildup (triboelectrification) all have the potential for adversely affecting the use of PGMs. The degree of degradation caused by the above elements is not known in detail. However, severe turbulence or greater may be sufficient to break "lock-on." Icing can disturb the aerodynamic flight and coat the sensor cover so that it is no longer useful. Hail and large aerosols can render a sensor inoperative through ablation (deterioration through pitting of the sensor lens). Lightning and triboelectrification have the potential for creating transient currents in the PGMs electronics, which may affect system performance.

Table 6-4-1 lists the major atmospheric and solar effects on PGMs and TA systems. Figure 6-4-1 shows the effect of weather elements and sensor resolution on the various wavelength categories.

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