T/G Layout 1

eplerian elements

Also known as satellite orbital elements, Keplerian elements are the set of six indepen-

dent constants which define an orbit—named for Johannes Kepler. The constants

define the shape of an ellipse or hyperbola, orient it around its central body (in the

case of environmental satellites the central body is Earth), and define the position of a

satellite on the orbit. The classical orbital elements are:

Keplerian elements

a: semi-major axis, gives the size of the orbit,

e: eccentricity, gives the shape of the orbit,

ι: inclination angle, gives the angle of the orbit plane to the central

b o d y ’s equator,

Ω: right ascension of the ascending node, which gives the rotation of the

orbit plane from reference axis,

ω: argument of perigee is the angle from the ascending nodes to perigee

point, measured along the orbit in the direction of the satellite’s motion,

θ: true anomaly gives the location of the satellite on the orbit.

figure 56.

KE P L E R I A N E L E M E N T S

K

perigee

point on orbit where

satellite is closest to

the central body

geocenter

line of apsides

straight line drawn

from perigee to

apogee

a (1-e)

r

θ

apog ee

point on orbit path

where satellite is

far thest from the

central body

2a*

*2a is the major axis

semi-major axis is 1/2 the longest diameter of an orbital

ellipse, that is 1/2 the distance between apogee and perigee

.