Viewing angles and orbital disturbances
Satellite communication tutorial
20201120 02:36:10
Viewing angles and orbital disturbances
The earth station will receive the maximum signal level, if it is located directly below the satellite. Otherwise, it will not receive the maximum signal level, and this signal level decreases as the difference between the latitude and longitude of the earth station increases.
So depending on the requirement, we can place the satellite in a particular orbit. Now let's talk about viewing angles.
Viewing angles
The following two angles of the earth station antenna combined together are called viewing angles .
 Azimuth angle
 Elevation angle
Usually the values of these angles change for nongeostationary orbits. While the values of these angles do not change for geostationary orbits. Because, the satellites present in geostationary orbits seem stationaryareas in relation to the Earth.
These two angles are useful for pointing the satellite directly from the earth station antenna. Thus, the maximum gain of the earth station antenna can be directed to the satellite.
We can calculate the viewing angles of the geostationary orbit using the longitude and latitude of the earth station and the orbit position of the satellite.
Azimuth angle
The angle between the local horizontal plane and the plane passing through the earth station, satellite and the center of the earth is called azimuthal angle .
The formula for the azimuth angle ( $ alpha $ ) is
$ $ alpha: = 180 ^ 0 + Tan ^ { 1} left (frac {Tan G} {TanL} right) $$
Where,
The following figure illustrates the azimuth angle.
Measure the horizontal angle between the earth station antenna and the north pole as shown on the figure. It represents the azimuth angle. It is used to track the satellite horizontally.
Angle of elevation
The angle between the vertical plane and the pointing line towards the satellite is called the angle of elevation. The vertical plane is nothing other than the plane, which is perpendicular to the horizontal plane.
The formula of the elevation angle ( $ beta $ ) is
$$ beta = Tan ^ { 1} left (frac {cosG.cosL0.15} { sqrt {1cos ^ 2G.cos ^ 2L}} right) $$
We can calculate the elevation angle using the above formula. The figure following illustrates the angle of elevation.
Measure the vertical angle at the level of the earth station antenna from the ground to the satellite, as shown in the figure. It represents the angle of elevation.
Orbital disturbances
Here are the orbital disturbances due to gravitational and nongravitational forces or parameters.

Irregular gravitational force around the Earth due to 'a nonuniform mass distribution. The Earth's magnetic field also causes orbital disturbances.

The main external disturbances come from the Sun and the Moon. When a satellite is close to these external bodies, it receives a stronger gravitational pull.

Low orbiting satellites are affected by friction caused by colliding with atoms and ions.

The pThe loss of solar radiation affects large GEO satellites, which use large solar arrays.

Selfgenerated torques and pressures caused by RF radiation from the antenna.
Most satellites use a propulsion subsystem in order to maintain correct direction of the axis of rotation and control the 'altitude of the satellite against disturbance forces.