ORBITAL ELEMENTS -- Keplerian Element Set

Six elements define the earth orbiting artificial satellites.

                        

a	Semi major axis	Determine shape of ellipse
e	Eccentricity	Determine shape of ellipse
Mo	Mean anomaly	Gives the position of the satellite in its orbit at a reference time known as the epoch
ω	Argument of perigee	Gives the rotation of the orbit’s perigee point relative to the orbit’s line of nodes in the earth’s equatorial plane
i	Inclination	Relate the orbital plane’s position to the earth
Ω	Right ascension of the ascending node	Relate the orbital plane’s position to the earth

                    

Line of apsides:- The line joining the perigee (point of closest approach to earth)  and apogee (point farthest from earth) through the center of the earth.

Ascending node:- is the  point where the orbit crosses the equatorial plane going from south to north whereas  Descending node:- is 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 is 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 angle from ascending node to perigee, measured in the orbital plane at the earth’s center, in the direction of satellite motion is known as Argument of perigee.

The position of ascending node is required to completely specify the position of orbit in space, the longitude of ascending node is not fixed as the earth spins and therefore it cannot be used as an absolute reference.

So 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. An imaginary line drawn from this equatorial crossing through the center of the sun points to the first point of Aries.

The right ascension of the ascending node is then the angle measured eastward, in the equatorial plane, from the line to the ascending node.

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