training course on weather radar systems - RTC, Regional Training ...

MODULE A- INTRODUCTION TO RADAR
3.3. Refraction
Electromagnetic waves propagating within the earth's atmosphere do not travel in straight lines
but are generally refracted. The density differences in the atmosphere affect the speed and
direction of electromagnetic waves. In some regions, a wave may speed up, while in other
regions it may slow down. This situation is known as refraction. One effect of refraction is to
extend the distance to the horizon, thus increasing the radar coverage. Another effect is the
introduction of errors in the measurement of the elevation angle. Refraction of the radar waves in
the atmosphere is caused by the variation with altitude of the velocity of propagation, or the
index of refraction, defined as the velocity of propagation in free space to that in the medium in
question. Now, let us remind some basic parameters regarding refraction.
Refraction Models
Non-standard refraction model
(1992)
Figure-3.8
Doviak and Zrnic 2002
3.3.1. Refractive index
The speed of electromagnetic radiation depends upon the material through which it is travelling.
In a vacuum such as the nearly empty space between the sun and earth, for example, light travels
at a speed of 299 792 458 ± 6 m/s, according to the National Bureau of Standards (Cohen and
Taylor,1987)
When electromagnetic radiation travels through air or other materials, it travels slightly slower
than in a vacuum. The ratio of the speed of light in a vacuum to the speed of light in a medium is
called the refractive index of the medium and is defined mathematically as
32
TURKEY RADAR TRAINING 1.0 / ALANYA 2005