Pagina 142 - TELE-satellite-1207

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TELE-satellite International — The World‘s Largest Digital TV Trade Magazine
— 06-07-08/2012
defined using varying systems. Unless
exact definitions rule out any ambigui-
ties, one sometimes needs almost inves-
tigative skills to find out without doubt
what is actually mean when reading
about a position.
The equator can be established as the
zero-point for the geographic latitude.
Towards the north the range extends
to 90 degrees north, or +90 degrees,
towards the south it is 90 degrees south
or -90 degrees to the pole(s).
As regards the geographic longitude
an artificial zero-point had to be deter-
mined. The meridian crossing the Green-
wich observatory near London with its
line from the north pole to the south
pole has been accepted as zero meridian
since 1884. From this meridian, the circle
is divided into 360 degrees towards the
East. This system is derived from astron-
omy which also employs a left system
(seen from the north pole) for projec-
tions from the earth’s surface into space.
This way the position of celestial bodies
such as satellites in a geostationary orbit
can be precisely defined.
Fig. 1
features four points with their
positions, which are listed in the table
below with their used designations.
In Europe the relative designations are
predominant, based on the zero meridian
towards west or east. While this is the
most complex and longest designation
it is one that leaves no room for misin-
terpretation. As satellites are positioned
near the equator, it is not necessary to
also give the latitude as it is always zero
When installing a polar mount antenna
the most important parameter – apart
from the exact positioning of the mast
pointing north from its horizontal instal-
lation – is the precise determination of
the southerly direction at the location. In
most cases a compass will be used to find
the exact south. However, the results are
frequently inaccurate and problematic
in principle, as the following sources of
error clearly demonstrate:
1. The magnetic north pole as indicated
by the magnetic needle of a compass is
not the same as the geographic north
pole and shifts from year to year. This
is also the reason way nautical charts –
which have to show the exact geographic
north – have to be redrawn almost every
year and are sold with revised informa-
tion in certain intervals. To give you an
understanding of the deviation look at
the following comparison for a specific
point in time:
The magnetic north pole was situated
north of Canada at
- approx. 100 degrees western longi-
tude and
- approx. 75 degrees northern lati-
- and that was approx. 1,500 km away
from the geographic north pole.
The magnetic south pole was approx.
2,500 km away from the geographic
south pole and was situated at
- approx. 140 degrees eastern longi-
tude and
- approx. 65 degrees southern lati-
The further north we move, the larger
the deviation caused by a compass
becomes (depending on the geographic
longitude, however).
2. Below the surface conditions may
exist that influence the magnetic field.
3. Objects made of metal (such as
metal roofs, railings, concrete rein-
forcements etc.) may be located close
to where the compass is used. Theses
objects may severely interfere with the
magnetic field.
Relative, in use
20/30 degrees
+20/+30 degrees 20 degrees eastern longitude/30 degrees northern latitude
320/30 degrees
-40/+30 degrees 40 degrees western longitude /30 degrees northern latitude
335/-30 degrees -25/-30 degrees
25 degrees western longitude /30 degrees southern latitude
15/-20 degrees
+15/-20 degrees
15 degrees eastern longitude /20 degrees southern latitude
Sat. Astra
19,2 degrees
+19,2 degrees
19,2 degrees east
Sat. Hispasat
330 degrees
-30 degrees
30 degrees west
4. Even the sunspot activity may dis-
tort the earth’s magnetic field.
5. The glass protecting the magnetic
needle may become electrostatically
charged and then causes massive meas-
urement errors if one is not cautious
enough. Plastic coverings are particu-
larly problematic in this regard.
If all these issues are taken care of a
compass may actually deliver quite sat-
isfying results. Still, the most reliable
determination of the southerly direction
is by locating a satellite that is as close
as possible in the exact south when seen
from the given location of the antenna.
As a rule of thumb, satellites are mostly
spread with a 3-degree spacing, which
makes this method very accurate.
Generally however, you will first have
to find out the exact position given in
geographic longitude and latitude of
the location of the antenna. If you need
help obtaining these parameters please
approach your local TV installer, check
the Internet or look at special maps. If
you know somebody owning a handheld
GPS device this would also be a conven-
ient way of finding out where exactly
you are.
The ‘bad habit’ of giving the deviation
of satellites from a southern direction
towards the east or the west (azimuth)
by stating the compass course is gain-
ing ground. This requires that we have
to take into account that a compass has
its zero point in the north, which means
that south is at 180 degrees. The com-
pass rose is divided into 360 degrees
and constitutes a right system:
0 north, 90 east 180 south 270 west.
If we assume that a satellite is posi-
tioned three degrees east of the south-
ern direction, it has a compass reading
of 177 degrees. It would be easier to
simply state 3 degrees east, or azimuth
+3 degrees (east).
There are cases when older march-
ing compasses are being used to deter-
mined the exact south. These also have
their zero point in the north, but the full
circle is divided into 64 segments (with
one segment equalling 5.625 degrees)
and it is a left system:
0 north, 16 west, 32 south, 48 east.