Orbital Parameters of Satellite

 Orbital Parameter

• Apogee : 

The point farthest from earth. Apogee height is shown as ha.

• Perigee :

The point of closest approach to earth. The perigee height is shown as hp.

• 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. It will be seen that the greatest latitude, north or south, 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. below. 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.

• Retrograde orbit :

An orbit in which the satellite moves in a direction counter to the earth’s rotation. 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.

• Right ascension of the ascending node :

To define completely the position of the orbit in space, the position of the ascending node is specified. However, because the earth spins, while the orbital plane remains stationary (slow drifts which 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.

• Mean anomaly :

Mean anomaly M gives an average value of the angular position of the satellite with reference to the perigee. For a circular orbit, M gives the angular position of the satellite in the orbit. For elliptical orbit, the position is much more difficult to calculate, and M is used as an intermediate step in the calculation.

• True anomaly :

The true anomaly is the angle from perigee to the satellite position, measured at the earth’s center. This gives the true angular position of the satellite in the orbit as a function of time.

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