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What is meant by polarization of satellite signals? Explain ionospheric and rain depolarization?

### Explain rain depolarization in detail.

Marks: 5 M, 10 M

Year: May 2012, Dec 2012, Dec 2013, May 2013, Dec 2014

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1. Polarization refers to the orientation of the electric vector of the electromagnetic wave in space.
2. Consider the situation where a geostationary satellite is transmitting a linear polarized wave. In the situation the usual definition of horizontal polarization is where the electric field vector is parallel to the equatorial plane and vertical polarization is where the electric field vector is parallel to the earth’s polar axis.
3. It will be seen that the sub satellite point on the equator, both polarization will result in electric fields that are parallel to the local horizontal plane. Care must be taken therefore not the use “horizontal” as defined for terrestrial system.
4. For other points on the earth’s surface within the footprint of the satellite beam, the polarization vector (the unit vector in the direction of the electric field) will be at some angle relative to the reference plane. This reference plane will be taken to be that which contains the direction of propagation and the local gravity direction.

Ionospheric depolarization:

1. When a linear polarized wave traverses the ionosphere, it sets in motion the free electrons in the ionised layers. These electrons move in the earth’s magnetic field and therefore, they experience a force.
2. The direction of electron motion is no longer parallel to the electric field of the wave, and as the electrons react back on the wave, the net effect is to shift the polarization. This is referred to as ionospehric depolarization.
3. The ionospheric depolarization or angular shift in polarization ( referred to as faraday rotation) is dependent on the following factors:
4. The length of the path in ionosphere
5. The strength of the earth’s magnetic field in the ionized region
6. The electron density in the region.
7. Faraday rotation is inversely proportional to the frequency squared and it is not considered to be a serious problem for frequencies about above 10GHz.

Rain Depolarisation:

Larger raindrops can be considered to be oblate spheroids with some flattening underneath as a result of resistance. Also in a realistic situation, aerodynamic forces will cause some tilting of the drops, thus there will be certain randomness in the angle of tilt

A linear polarized wave can be resolved into two component waves, one vertically polarized and the other horizontally polarized. Consider a wave with its electric vector at some angle T relative to the major axis of the raindrop. Here the major axis of the rain drop is shown to be horizontal. The vertical component of the electric field lies parallel to the minor axis of the raindrop and therefore encounters less water than the horizontal component. Hence there will be a difference in the attenuation and phase shift experienced by each of the electric field components. These differences are termed as differential attenuation and differential phase shift and they result in depolarization of wave.

Expression for XPD in dB associated with rain

$XPD=U-V log A$

Where U and V are empirically determined coefficients and A is the rain attenuation. U, V and A are in dB. $V=20; for 8 \lt f \lt 15 GHz$ $=23; for 15 \lt f \lt 35 GHz$ And $U+ 30 log f – 10 log (0.5-0.4697 cos4T) - 40 log (cosEl)$ Where, $f: frequency in GHz$ $El: angel of elevation of the propagation path at the earth station.$ $T: tilt angel of polarization relative to horizontal$ $T: 45° for circular polarization$

When the electric field is parallel to the ground [horizontal], T = 0°

With the electric field vector in the reference plane containing the direction of propagation and the local vertical, T = 90° – El [all angles in degrees]