training course on weather radar systems - RTC, Regional Training ...
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MODULE A- INTRODUCTION TO RADAR<br />
3.3. Refracti<strong>on</strong><br />
Electromagnetic waves propagating within the earth's atmosphere do not travel in straight lines<br />
but are generally refracted. The density differences in the atmosphere affect the speed and<br />
directi<strong>on</strong> of electromagnetic waves. In some regi<strong>on</strong>s, a wave may speed up, while in other<br />
regi<strong>on</strong>s it may slow down. This situati<strong>on</strong> is known as refracti<strong>on</strong>. One effect of refracti<strong>on</strong> is to<br />
extend the distance to the horiz<strong>on</strong>, thus increasing the <strong>radar</strong> coverage. Another effect is the<br />
introducti<strong>on</strong> of errors in the measurement of the elevati<strong>on</strong> angle. Refracti<strong>on</strong> of the <strong>radar</strong> waves in<br />
the atmosphere is caused by the variati<strong>on</strong> with altitude of the velocity of propagati<strong>on</strong>, or the<br />
index of refracti<strong>on</strong>, defined as the velocity of propagati<strong>on</strong> in free space to that in the medium in<br />
questi<strong>on</strong>. Now, let us remind some basic parameters regarding refracti<strong>on</strong>.<br />
Refracti<strong>on</strong> Models<br />
N<strong>on</strong>-standard refracti<strong>on</strong> model<br />
(1992)<br />
Figure-3.8<br />
Doviak and Zrnic 2002<br />
3.3.1. Refractive index<br />
The speed of electromagnetic radiati<strong>on</strong> depends up<strong>on</strong> the material through which it is travelling.<br />
In a vacuum such as the nearly empty space between the sun and earth, for example, light travels<br />
at a speed of 299 792 458 ± 6 m/s, according to the Nati<strong>on</strong>al Bureau of Standards (Cohen and<br />
Taylor,1987)<br />
When electromagnetic radiati<strong>on</strong> travels through air or other materials, it travels slightly slower<br />
than in a vacuum. The ratio of the speed of light in a vacuum to the speed of light in a medium is<br />
called the refractive index of the medium and is defined mathematically as<br />
32<br />
TURKEY RADAR TRAINING 1.0 / ALANYA 2005