Page 54 - Satellite Communications, Fourth Edition

34 Chapter Two

Retrograde orbit. An orbit in which the satellite moves in a direction
counter to the earth’s rotation, as shown in Fig. 2.4. The inclination
of a retrograde orbit always lies between 90° and 180°.
Argument of perigee. The angle from ascending node to perigee,
measured in the orbital plane at the earth’s center, in the direction of
satellite motion. The argument of perigee is shown as w in Fig. 2.5.
Right ascension of the ascending node. To define completely the posi-
tion of the orbit in space, the position of the ascending node is speci-
fied. However, because the earth spins, while the orbital plane remains
stationary (slow drifts that do occur are discussed later), the longitude
of the ascending node is not fixed, and it cannot be used as an absolute
reference. For the practical determination of an orbit, the longitude and
time of crossing of the ascending node are frequently used. However,
for an absolute measurement, a fixed reference in space is required.
The reference chosen is the first point of Aries, otherwise known as the
vernal, or spring, equinox. The vernal equinox occurs when the sun
crosses the equator going from south to north, and an imaginary line
drawn from this equatorial crossing through the center of the sun
points to the first point of Aries (symbol ϒ). This is the line of Aries. The
right ascension of the ascending node is then the angle measured east-
ward, in the equatorial plane, from the ϒ line to the ascending node,
shown as Ω in Fig. 2.5.
Mean anomaly. Mean anomaly M gives an average value of the
angular position of the satellite with reference to the perigee. For a

Figure 2.5 The argument of perigee w and the right ascension
of the ascending node Ω.