Page 53 - Satellite Communications, Fourth Edition

Orbits and Launching Methods 33

Line of apsides. The line joining the perigee and apogee through the
center of the earth.
Ascending node. The point where the orbit crosses the equatorial
plane going from south to north.
Descending node. The point where the orbit crosses the equatorial
plane going from north to south.
Line of nodes. The line joining the ascending and descending nodes
through the center of the earth.
Inclination. The angle between the orbital plane and the earth’s
equatorial plane. It is measured at the ascending node from the
equator to the orbit, going from east to north. The inclination is
shown as i in Fig. 2.3. It will be seen that the greatest latitude,
north or south, reached by the subsatellite path is equal to the
inclination.
Prograde orbit. An orbit in which the satellite moves in the same
direction as the earth’s rotation, as shown in Fig. 2.4. The prograde
orbit is also known as a direct orbit. The inclination of a prograde
orbit always lies between 0° and 90°. Most satellites are launched
in a prograde orbit because the earth’s rotational velocity provides
part of the orbital velocity with a consequent saving in launch
energy.

Figure 2.4 Prograde and retrograde orbits.