Introduction

orbit of the satellite. If the orbit of an equatorial orbiting satellite were roughly equal to the distance from earth to the sun, ordinary latitude could be used to determine the elevation of the antenna, which is, of necessity, very finely focused. However since there is a disparity this angle is somewhat less than the latitude for the earth station. See Figure 1-3. Simple logic readily points out that as the latitude increases the 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˚ for a satellite at the earth station’s longitude. Figure 1-3 illustrates why it is not possible to pinpoint a satellite with ordinary navigation.

SUN

A

SATELLITE

45° N

00° (EQUATOR)

EARTH

Figure 1-3: Angle A (the difference at a specific latitude between the angle to the sun and the angle to a satellite) is the reason ordinary navigation techniques cannot be used to find a satellite

Figure 1-4 confirms that the angle from the prime meridian to a satellite will be considerably different than the angle from a North American or European earth station to the same satellite. It should also be noted that the elevation also decreases for a satellite the further east or west from the earth station’s longitude.

Satellite Footprints Even though a satellite, in theory, can communicate with 40% of the earth’s surface from its location, in most cases it will not. The antenna systems onboard the satellites are usually designed to cover a specific area. These areas are referred to as hemispheres, zones, and spots. A hemispherical beam is designed to cover roughly 40% of the earth’s surface, for example, the western hemi- sphere. A zonal beam covers a specific area, for example, the Continental United States, which is usually referred to as the CONUS beam. A spot beam is exactly what it implies, concentrating on a smaller geographical area, such as the western United States. With each of these 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.

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1705A Spectrum Monitor

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Tektronix 1705A instruction manual SUN Satellite, Earth