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The view-direction of a satellite dish depends on the dish position on the (or Elevation ) and Azimuth . Proper combination of these two angles is required to exactly point the desired satellite, from which the dish will receive the Electromagnetic Radiation that it will then convert to Electric Signal sent to a class="copylinks">Decoder , Which Will Eventually Convert It Into An Audio-video Signal And Transmit It To A [[TV set.

Satellites used to broadcast TV signals are positioned in the Clarke Belt , an imaginary "belt" which surrounds the Earth and lies over the Equatorial Plane ; the distance between the belt and the Earth is determined in such a way that any object moving along it will have two main features:

  • it can remain in its position without requiring any engine to defeat the Gravity attraction (apart for minor Orbit correction required to compensate Atmosphere Drag )

  • it appears always still in the same position in the sky to an observer located on the Earth surface.


Being the satellite ''stationary'' with respect to the Earth ("geo" in Greek language), this kind of satellites are called ''geostationary'', and they can then be "viewed" by a parabolic-dish with fixed heading.


GRAPHICAL EXPLANATION OF ELEVATION AND AZIMUTH

First picture shows how the Elevation angle is determined:

The Altitude/Elevation angle (ALT) depends on the observer's s), its Elevation must be 90 Degree s to point the satellite; viceversa, if the dish is at the north pole, its Elevation should be 0 degrees; but actually, a dish positioned at north pole couldn't probably view any geostationary satellite, which would result "shadowed" by horizon line; in theory, a dish at north pole, with elevation = 0, could only see an object in equatorial orbit only if it was located at Infinite distance.

Next picture shows how the Azimuth angle is determined:

If the observer Longitude (LONG) and the satellite longitude (SATLON) are the same, they will lie on the same Meridian , thus the dish must be pointed exactly to the South (azimuth=180 Degree s) to see the satellite. If the longitudes are different, instead, as shown in the picture, the required azimuth will differ from 180 degree, as much as longitudes differ.

Besides, Altitude and Elevation of the dish depends one on the other, so the final formulas needed to determine them are slightly complex:

  • cos(LAT)---cos(LONG-SATLONG)-1

  • sqrt( 1-(cos(LAT)^2) --- (cos(LONG-SATLONG))^2 )

    • ELEVATION = atan(v1/v2)

AZIMUTH = 180 + atan( tan(LONG-SATLONG) / sin(LAT) )
POLARIZATION = -atan( sin(LONG-SATLONG)/tan(LAT) );

  • LONG = dish longitude;

  • SATLONG = satellite longitude;

  • LAT = dish latitude.

  • ''sqrt'' means Square Root



GRAPHICAL EXPLANATION OF DISH POLARIZATION OR ''SKEW''

In case the satellite longitude is very different from dish longitude, it maybe necessary to adjust its ''skew'' too; this adjustment is usually very slight and it's due to polarization plane of the dish being rotated when changing azimuth and elevation of the dish itself, as shown in this picture:

The satellite polarization plane is indeed parallel to Earth axis, and so must be the dish polarization to allow proper signal reception.

This picture shows in detail how the dish should be moved to fix ''skew'':


EXTERNAL LINKS

http://www.sat-sales.com/calculator.php Satellite finder with magnetic north, azimuth & elevation calculator