Satellite dish installation

Encyclopedia : S : SA : SAT : Satellite dish installation

Calculating satellite dish direction

Home dish installation

The view-direction of a satellite dish depends on the dish position on the Earth and on the satellite position in the Clarke belt. The view-direction is determined by two angles: altitude (or elevation) and azimuth. Proper combination of these two angles is required to exactly point the dish towards the desired satellite, from which the dish will receive the electromagnetic radiation that it will then convert to electric signal sent to a 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 above the equator in the equatorial plane; the distance between the belt and the Earth is determined in such a way that any object moving along it in an easterly direction 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 for a north-south drift due the gravitational pull of the moon and sun and, to a lesser extent, east-west drift due to the earth having an uneven triaxial gravitational field shape around the equator, which has three peaks towards which satellites are slowly attracted.
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 pointing.

Graphical explanation of Elevation and Azimuth

First picture shows how the Elevation angle is determined for a site approximately on the same longitude as the satellite:

The Altitude/Elevation angle (ALT) depends on the observer's latitude (LAT), e.g. its angular distance from the equator (the horizontal line in this picture). At a first glance, it could look like, from this figure, that the dish should be pointed down rather than up; actually, the "reference line" is not just the equator, but the horizon line: in this picture, it is represented by the line which "touches" the Earth exactly in the point where the dish/observer is. So, the Elevation angle is calculated with respect to this line. If the Latitude is greater, the Altitude will be lower, and vice-versa. If the dish is positioned exactly on the equator (LAT=0 degrees), its Elevation must be 90 degrees to point the satellite directly overhead; vice-versa, if the dish is at the north pole, its Elevation should be 0 degrees; but actually, a dish positioned at north pole couldn't view any geostationary satellite, which would below the horizon; in theory, a dish at north pole, with elevation = 0, could see an object in equatorial orbit only if it were 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 degrees) or to the north (azimuth = 0 deg) to see the satellite, depending on whether the observer is in the northern or southern hemisphere. If the longitudes differ, as shown in the picture, the required azimuth will differ from 180 degree or 0 deg.

Altitude (Elevation), Azimuth and Polarisation angle formulas are as follows:

v1 = 6.612 * cos(LAT)*cos(LONG-SATLONG)-1
v2 = 6.612 * 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 If the satellite longitude is different from dish longitude, it is necessary to adjust the skew. This adjustment is small if the site is approximately on the same longitude as the satellite, but may be as much as + / - 90 deg if the site is near the equator and significantly to the east or west of the satellite. The polarization plane of the receive system is adjusted by rotating the feed/LNB assembly in its mounting clamp or by rotating the entire dish and feed assembly. In the case of a polar mount the entire dish and feed assembly are swung by an actuator across the sky so that the azimuth, elevation and polarisation all vary simultaneously and automatically. A polar mount, provided it is initially correctly aligned, enables any one of all of the visible geostationary satellites to be viewed, using a single actuator movement.

In the case of linear polarisation, the two satellite polarization planes are normally parallel to and at right angles to the Earth's axis, and so must be the dish polarization to allow proper signal reception and to avoid interference from signals on the opposite polarisation. The cross-pol interference null is quite narrow and adjustment to +/- 1 deg accuracy is required for all transmitting antennas and is desirable for receiving antennas.
This picture shows in detail how the dish should be moved to fix the polarisation or skew angle:

External links

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.