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Environmental Effects on PGMs

Environmental effects of weather can influence the success of a mission by affecting the use of Precision Guided Munitions. In order to brief weather conditions that may affect PGMs, an understanding of their operation was important.

Visible Systems. Passive systems which respond to naturally emitted or reflected EMR in the visible spectrum. Environmental limitations are as follows:
(1) Clouds or Fog,
(2) Precipitation, blowing snow or blowing spray,
(3) Poor illumination,
(4) Low sun angle.

Near-IR Passive Systems. A TV-silicon vidacon which senses radiation between .5 and 1.2 microns. The longer wavelength of these systems enhances the sensor's ability to penetrate atmospheric aerosols such as haze. Near-IR systems cause an increase in contrast between natural and painted objects than at visual wavelengths. Environmental limitations are the same as for visible systems except that atmospheric aerosols are less of a problem.

IR Semiactive Systems. These employ a laser designator operating at IR wavelengths. The point of maximum reflected energy was sensed and tracked using a centroid tracker. These systems have the advantage of day or night operations. Environmental limitations:
(1) Clouds/Fog, other than very thin (they absorb IR energy),
(2) Haze and other dry aerosols (for near IR systems only),
(3) High absolute humidity (for far IR and far IR systems only).

Middle and Far IR Passive Systems. These systems respond to naturally emitted EMR from terrestrial objects in the middle and far IR wavelengths. Threshold thermal contrasts must be met for IR lock-on. These systems may be used for day or night operations. Resolution was poor due to long wavelength. Environmental limitations:
(1) Clouds/Fog other than very thin,
(2) High Absolute Humidity (for far IR only).

Millimeter Wave/Microwave Systems. These systems respond primarily to emitted, but also to reflected energy at millimeter wave or microwave wavelengths. Most of these systems are passive, such as the "SHRIKE" anti-radiation missile which has a microwave sensor and homes in on microwave or radar emissions. Day or night operations are possible, but resolution was poor due to long wavelength. Environmental limitations:
(1) Dense clouds of high liquid water content (e.g. CR or TCU).
(2) Precipitation causes attenuation.

Environmental conditions can affect target acquisition anywhere in the cycle:
(1) Severe (or greater) turbulence can break lock-on,
(2) Icing of any kind can coat sensor cover which blinds it or the ice can cause aerodynamic problems (changes shape and adds weight). Icing can also jam the controls of the tracker,
(3) Ablation or erosion of the sensor cover by pitting may occur when the PGM passes through hard atmospheric particles such as hail or sand,
(4) Lightning or Triboelectrification (static charge buildup) can foul electronic circuitry. Lightning can cause detonation of PGM,
(5) Sun angle: (a) Mie scattering (path radiance) causes problems for TV and near-IR systems, (b) Low sun angle can create shadows which may cause false lock-on (c) Thermal contrast was affected as the different parts of the target heat up and cool down at various rates as the sun rises and sets.
(6) Target Size and shape determines the range at which target acquisition and lock-on can be accomplished,
(7) Soil moisture content and precipitation, (a) Change in moisture content of soil can change its color and the inherent contrast. It can also change the soil temperature, and thus affect thermal contrast (moist soil warmer than dry), (b) Rain or snow in the target area will affect the temperatures of targets and backgrounds, affecting thermal contrast, (c) Snow cover has a strong influence on contrast. 1 The inherent contrast can decrease, 2 Path radiance problem for VIS sensors, 3 Patchy snow cover creates alternating hot/cold spots in background, affecting thermal contrast.
(8) Radiative Temperature Crossover was a function of the diurnal temperature cycle. It occurs when the target and background achieve the same radiative temperatures and was caused by the different rates at which the target and background heat up and cool down,
(9) Strong surface winds can decrease thermal contrast,
(10) Thermal clutter was hot or cold spots surrounding the target which can confuse or distract the IR sensor by momentarily alterinq or wipinq out the thermal contrast.