FORECASTING ENVIRONMENTAL EFFECTS ON ELECTRO-OPTICAL (EO) SYSTEMS

LEARNING OBJECTIVES: Identify how the environment affects EO systems, and state the problems associated with these systems, Explain the lessons learned with EO systems. EO systems are concerned with millimeter-wave, IR optical, and UV wavelengths, As these wavelengths decrease, resolution increases, but at the same time there is a decrease in penetration and range. Typically, these systems are line of sight.

The following problems should be considered when dealing with EO systems:

. You must assess the environments effect on the ability of a line of sight instrument to detect or track a target. The view of the instrument might be obscured by material in the atmosphere or may be distorted by refraction.

. There maybe some limited range over which the EO sensor will work.

. Time-of-day will be a limiting factor, if the sensor relies on reflected sunlight or distinct contrasts (visual or thermal).

. Radiative transfer - as electromagnetic energy travels through the air.

Some of the energy may go unimpeded, directly from the source to the detector,

Some energy may be scattered away (loss); energy not associated with the source may be scattered toward the sensor (noise).

Some energy may be absorbed before it ever gets to the sensor (loss).

Some energy may even be emitted from particles within the path (more noise).

These effects, along with signal loss due to spherical spreading, all contribute to attenuation of the desired signal.

. Spreading - The energy going from the target back to the sensor undergoes further loss due to spreading. This is true even for the return trip (reflection) for an active sensor, although the spreading of the transmitted energy is focused or beamed.

. Contrast - For adequate detection on tracking, sufficient contrast must exist between the intended target and its background. Background might be the sea surface, sky, or terrain. The EO sensor may use thermal, textural, color, light intensity, or pattern contrasts as the method for detection. Insufficient contrast between the intended target and the background causes no acquisition or tracking. Radiative crossover is a key example. The temperature between a metallic object and the ground has different rates of heating and cooling. Twice a day both will be at about the same temperature and will provide no contrast to IR sensors.

. Wavelength dependence - Each of these loss phenomena is dependent upon wavelength. Sensors operating in certain bands have markedly different characteristics. Remember, the compromise is usually between better resolution and less susceptibility to atmospheric phenomena. While resolution increases with decreasing wavelength, so does weather sensitivity. Table 10-2 shows how environmental elements affect wavelengths.

Battle experience in desert areas has shown that the operation of optical instruments suffers greatly in these environments. Rapid changes in the index of refraction can result in the shimmering of images, causing optical instruments to lose lock-on tracked targets.

Mirages and other refractive phenomena also add to the confusion.

Frequent airborne haze, dust, sand, and smoke from both naturally occurring winds and storms, and horn the battle, can hamper guidance or surveillance systems operating in, at, or near optical wavelengths. The effects are more severe at these wavelengths than at microwave, UHF, or VHF.

LEARNING OBJECTIVES: Familiarize yourself with the procedures for obtaining route weather forecast (WEAX) or aviation route weather forecast (AVWX) support. Recognize the standard format of the two products. Consider the obstacles a ship underway in the Pacific in July may have to overcome. The CO feels uneasy; it is tropical storm season and there isnt a weather division on board. Knowledge of this subject area may help the CO accomplish a successful mission. WEAX or AVWX information is useful to ships receiving OTSR support as well as for independent steaming units, since the OTSR service does not include specifically tailored weather forecasts. WEAX forecasts are designed for ships without embarked aviation units, while AVWX is tailored for ships with an embarked aviation unit.

WEAX/AVWX support services are requested in the movement report (MOVREP) in accordance with NWP 10-1-10. Once WEAX/AVWX has been requested on the initial MOVREP, units should continue entering the WEAX/AVWX notation to any subsequent MOVREP to ensure support is continued.

Table 10-2.-How Environmental Elements Affect Wavelengths

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