Society of Broadcast Engineers



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SBE 24 GeoSat DecSign
Why is the declination angle to a geostationary satellite
POSITIVE in the Southern Hemisphere and
NEGATIVE in the Northern Hemisphere?

Astronomers have defined declination as follows:

  • Any astronomical body north of the Celestial Equator has positive declination.   Example: the star Polaris (the "Pole Star") is at declination +90° (approximately), which puts it almost directly above the North Pole.


  • Any astronomical body south of the Celestial Equator has negative declination.  Example: the star Sirius (the "Dog Star") is at declination -16° (approximately).  It rises in the east and sets in the west, but it's always 16° below the Celestial Equator.
  • The following sketch illustrates the situation.   Note that:

  • The Geostationary Satellite lies in the earth's equatorial plane.   At the scale of this drawing, the declination appears to be 0 degrees.


  • The Sun's declination varies throughout the year.   This, of course, is why we have seasonal variations in temperature.


  • Sirius is always -16°.
  • EARTH, VIEWED FROM A POINT DIRECTLY ABOVE THE EQUATOR
    (FROM A VERY LONG WAY AWAY)  (NOT TO SCALE)

    Now let's zoom in toward the earth.   Note that:

  • An antenna located in the northern hemisphere must be aimed slightly "down" (south) to view the satellite.  The line-of-sight to the satellite crosses the Equatorial Plane and arrives at infinity south of the Celestial Equator.  Hence, its declination angle is negative.


  • An antenna located in the southern hemisphere also must be aimed slightly "down" (north) to view the satellite.  The line-of-sight to the satellite crosses the Equatorial Plane and arrives at infinity north of the Celestial Equator.  Hence, its declination angle is positive.
  • EARTH, VIEWED FROM A POINT DIRECTLY ABOVE THE EQUATOR
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