ANOMALOUS PROPAGATION  

Anomalous propagation (AP) refers to abnormal refraction of electromagnetic energy. AP occurs when the atmospheric conditions cause electromagnetic energy to arrive at a destination(s) via a path(s) significantly different from a normally expected path(s). AP is most likely to occur when subrefraction, superrefraction or trapping conditions occur, because these condi-tions create the greatest amount of refraction. When AP is present, strange radar images occur. For example, radar operators report targets at far greater ranges than would normally be expected. They report echoes on radar screens when there is nothing in the atmosphere to produce echoes (such echoes are referred to as "false echoes" or "ghost targets"). They report no contacts (echoes) but visual surveillance indicates otherwise. This latter type of occurrence is due to a "blind spot" or "radar hole" in radar coverage.

A radar hole is an area of the atmosphere through which little or no electromagnetic energy penetrates. These holes or blind spots are produced when energy waves, traveling at different speeds through various layers of the atmosphere, are diverted from their normal path. Radar holes occur at the boundary between different propagation paths. The radar waves do not effectively penetrate these areas, and planes or missiles inside a radar hole can go undetected. Radar blind spots may be compared to the "shadow zones" encountered in sonar searches of the oceans.  

The existence and dimensions of radar holes can be manually determined using graphs and formulas, or they can be computed using computer programs, such as the Integrated Refractive Effects Prediction System (IREPS) and the Refractive Index (RIA). Because of the widespread availability of these computer programs, you will most likely never have to perform the manual calculations. However, I have included them in Appendix IV so that you will have a better understanding of atmospheric refraction and what goes into these computer programs.

Learning Objective: Identify the fastest method of computing refractivity, and describe the capabilities of related refrac-tivity computer programs.

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing