TARGET SIZETarget size can range from relatively small to relatively large. An individual tank or a battery placement can be considered small, but so can a destroyer when compared to a weapons plant. Larger targets are, for the most part, easier to locate.

DISTANCE FROM THE TARGETThe farther away an aircraft is from a target, the smaller the target appears, and vice versa. Of course, the closer an aircraft gets to its target, the more susceptible the aircraft becomes to enemy fire.

TARGET- TO BACKGROUND-ENERGY CONTRASTThe physical processes that control target- to background-energy contrast are reflection, absorption, and emission.

Reflection is normally the most important process in locating a target or object for the visible and near-infrared wavelengths. Since the same intensity of sunlight falls on all surfaces, the amount of energy reflected by a target and the targets background determine whether enough contrast exists for a sensor to distinguish between the two. Without reflected sunlight or reflected energy from some artificial source, an object could not be detected unless it emitted significant amounts of visible or near-infrared radiation.

Reflection is also important at other infrared and microwave wavelengths. Visible and infrared lasers are used to spotlight targets for sensors that operate on the same wavelengths as the lasers. Similarly, at microwave wavelengths, radar returns from targets on the surface are reflected back to a microwave sensor.

Absorption by objects is dependent on the wavelength of the incident radiation, Elec-tromagnetic energy that strikes a target or its background (incident radiation) and is not reflected is absorbed or transmitted.

Except for fires and other illuminators, such as searchlights, emissions of electromagnetic energy at visible wavelengths by targets and their backgrounds are practically zero in most cases. This is not to say that targets do not emit elec-tromagnetic energy. Targets and backgrounds emit radiation at specific wavelengths dependent on the composition and physical temperature of the target and background.

The wavelengths of surface emissions are in the middle and far infrared ranges, and infrared sensors are used to measure these emissions.

EFFECTS OF THE ATMOSPHEREThe effects of the atmosphere on electromagnetic energy have already been discussed. Reduced risibilities due to the presence of clouds, precipita-tion, haze, smoke, fog, etc., all impact visual detection and the identification of targets.

Learning Objective: Name the various types of target acquisition systems, and identify the most proficient system.

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