Radar Propagation

The ability to predict tropospheric refraction of radar signals, caused by spatial variations of the atmospheric refractive index ($n$), is important for the design and operation of communication systems. Such variations can be conveniently described in terms of the modified refractivity, $M=10^{6}n + z/R,$ which takes into account the curvature of the earth (of radius $R$). Under most atmospheric conditions, modified refractivity increases with height, which causes radar signals to be refracted away from the earth's surface. Figure 1 shows how propagation through a US `standard atmosphere' affects radar transmissions from a 10GHz antenna. The coverage diagram shows the one-way propagation factor, which is a measure of signal strength relative to the attenuation in free space.

Figure 1. Coverage diagram for propagation from a 10GHz antenna 25m above a sea surface, within a US standard atmosphere of $\partial M/\partial z=0.118$M-units m$^{-1}$.

Meteorological conditions can occur in which $M$ decreases with height, at least for some range of heights. This causes downwards refraction, resulting in a ducting region of trapped radar energy. In Figure 2, we show an example of such trapping for a strong, surface-based duct.

Figure 2. Propagation at 10GHz within a strong surface duct.