Introduction
1-10 1705A Spectrum Monitor
orbit of the satellite. If the orbit of an equatorialorbiting satellite were roughly
equal to the distancefrom earth to the sun, ordinary latitude could be used to
determinethe elevation of the antenna, which is, of necessity, very finely
focused. However sincethere is a disparity this angle is somewhat less than the
latitude for the earthstation. See Figure 1-3. Simple logic readily points out
that as the latitude increasesthe angle from horizon to the satellite decreases. An
example of this would be that at 45˚North or South latitude the angle above the
horizon is about 40˚fora satellite at the earth station’s longitude. Figure 1-3
illustrates why it is not possibleto pinpoint a satellite with ordinary navigation.
A
EARTH
SATELLITE
SUN
00°
(EQUATOR)
45°N
Figure1 -3:Angle A (t he difference at a specific latitude between the angle to the
sunand t he anglet o a satellite) is the reason ordinary navigation techniques
cannot be used to find a satellite
Figure 1-4 confirms that the anglefrom the prime meridian to a satellite will be
considerably different than the angle from a North American or Eu ropean earth
station to the samesatellite. It should also be noted that the elevation also
decreasesfor a satellite the further east or west from the earth station’s longitude.
Even though a satellite,in theory, can communicate with 40% of the earth’s
surfacefrom its location, in most cases it will not. The antenna systems onboard
the satellites areusually designed to cover a specific area. These areas are
referredto as hemispheres, zones, and spots. A hemispherical beam is designed
to cover roughly 40% of the earth’ssurface, for example, the west ernh emi-
sphere. A zonal beam coversa specific area, for example, the Continental
United States, which is usually referredto as the CONUS beam. A spot beam is
exactly what it implies, concentrating o n as maller geographical area, such as the
western United States. With each of th ese beams there are areas where the signal
strength is greater. Figure 1-5 shows a propagation map for the western spot
beam for one Ku--Band satellite.
Satellite Footprints