Keplerian Elements

Johannes Kepler (1571-1630) discovered the laws of orbital motion, now called Kepler's laws. A set of 6 parameters, together with the exact time for which they are valid, is required to describe an orbit that obeys these laws. While there are various ways to choose the parameters, the standard set, used by AMSAT (and not coincidentally, the ones published by NASA), follows:
Epoch Time
Inclination
RAAN
Eccentricity
Argument of Perigee
Mean Anomaly
Mean Motion
Real orbits are not ideal: over time they decay due to atmospheric drag, or are gradually perturbed by tidal effects and the nons-spherical gravitational field of the earth. These deviation from Kepler's Laws is adjusted for by more parameters, including :
Decay Rate
For bookkeeping purposes, some descriptive information about the satellite is also included in the published "element sets":
Catalog Number
Epoch Revolution
An alternate specification for the size of the orbit, sometimes found in published data, is:
Semi-Major Axis

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