Affects of Weather on Infrared Sensors

Topics: Infrared, Meteorology, Weather forecasting Pages: 10 (3487 words) Published: June 23, 2013
Weather Effects on Aviation Infrared Systems

Embry-Riddle Aeronautical University

Abstract
Infrared (IR) systems utilize electromagnetic energy of specific wavelengths emitting from bodies to accomplish an end goal of the system (Smith, 2005). Examples of these goals in aviation are imaging, detection, tracking, and targeting. Infrared systems are also used in other aviation areas such as weather equipment or air to ground targeting systems, but this paper will focus on systems specifically carried on aircraft. Since IR systems are costly and their goals are not usually associated with getting the aircraft from point A to B, they are not common on general aviation or commercial aircraft. Although only military systems will be discussed, the basic principles presented here still apply to other IR sensors as well. The IR spectrum has been a concern to military aviation since the first heat seeking AIM-9 missile was developed in 1956 (Hept, 2002). Due to their importance, all factors affecting their performance are considered and scrutinized. The technology of the sensors will continue to grow, but the one factor that can still dominate their usefulness is environmental conditions. Engineers will continue to develop better and smarter sensors, but the atmosphere will always be an obstacle for IR systems without a solution in the near future. Like any good pilot knows, the atmospheric conditions must be dealt with by being able to predict, adjust or mitigate their effects. Due to the fact that weather can render these systems ineffective it is important to understand why and how these effects can be avoided or exploited. This paper will give the reader a background on IR energy, a sample of the military aircraft systems that utilize it, followed by an explanation of how weather affects them and how this can be forecasted and quantified.

Infrared Theory
The basics of IR theory can be traced back to the principle that all bodies above absolute zero degrees emit electro-optical radiation. The wavelengths we see are in the visible light spectrum, wavelengths that IR systems collect are slightly longer than visible light and thus are called infrared, meaning near red (Smith, 2005). WAVELENGTHCATEGORIES| VISIBLE0.4 m –0.74 m| NEAR IR0.74 m –2 m| MIDDLE IR2 m –6 m| FAR IR6 m –15 m| FAR-FAR IR15 m –0.1mm| MILLIMETER0.1mm–1cm| MICROWAVE1cm–10cm| RESOLUTION| USUALLY DECREASES WITH LONGER WAVELENGTH|

WEATHER SENSITIVITY| GENERALLY DECREASES WITH LONGER WAVELENGTH|

Figure one. The IR Spectrum.
When the IR sensors in question detect and measure this IR energy it is called radiometry. They take advantage of IR radiation because it is not absorbed quickly by the atmosphere, is emitted day or night, and is emitted in large amounts from objects much hotter than their background. The amount and wavelength of IR energy emitted by an object depends primarily on its temperature, surface reflectivity and material composition (Willardson, 1970). Not all the IR energy emitted from an object will be detected by a sensor. Like any source emitting radiation, the intensity of radiation diminishes inversely by the square of the distance from the object, as shown in figure two. Figure two. Radiation flux in relation to distance.

This is an idealized model not considering the medium the waves are traveling through. In reality the atmosphere and its conditions must be considered to predict the energy that will reach the sensor, affecting overall performance. The absorption and scattering of energy by the atmosphere is called attenuation and is primarily a function of range and wavelength, along with atmosphere conditions. The strongest absorber of IR wavelengths in the atmosphere is water vapor, the reason IR sensors cannot see through clouds (Reusch, 1998). Figure three shows the atmospheric transmission of IR wavelengths and what wavelengths are absorbed by different molecules.

Figure...

References: Air Force Research Laboratory. (1998). Optimizing Infrared and Night Vision Goggle Sensor
Performance by Exploiting Weather Effects
Hept, G.B. (2002). Infrared Systems for Tactical Aviation. (Doctoral dissertation). Retrieved from
http://www.fas.org/man/dod-101/navy/docs/es310/syllabus.htm
Reusch, W. (1999). Infrared Spectroscopy, Michigan State University. Retrieved from
http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm.
Smith, W.J. (2005). Modern Lens Design (2nd ed.). New York, NY: McGraw-Hill
United States (1992)
Willardson, R.K., & Beer, A.C. (1970). Semiconductor and Semimetals (Vol 5, Infrared Detectors). New York, NY: Academic Press.
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