The following pages will contain information about orbital mechanics, models and properties. It will give an overview over different orbital terms used to describe the orbital motion of a satellite.

To calculate orbital motions and positions in three dimensional space it requires a number of orbital parameters. These are given beforehand, calculated or given by observations, as in case of the Two Line Elements (TLE), which will be described here.
Further it requires the current time and the time at which an orbiting object was in a specific position. This time is also measured and defined in a certain format called Julian Date.

The Coordinate Frames
To define an orbit around Earth, two specific Earth related coordinate systems are given beforehand. They both have their origin in the geometrical center of Earth and are named the Earth Centered Inertial (ECI) coordinate frame and the Earth Centered Earth Fixed (ECEF) coordinate frame.

The ECI Coordinate Frame
The term ECI stands for Earth Centered Inertial and represents a coordinate system with origin in the center of Earth, which is fixed relative to the Earth rotation. Its X-axis is parallel to the Vernal Equinox (The rotational axis of the Earth is tilted relative to its orbital plane) and its Z-axis, which is parallel with the Earth rotational axis.

The ECEF Coordinate Frame
The second coordinate frame is the Earth Centered Earth Fixed (ECEF) coordinate frame. In this frame the X-axis is passing through the zero longitude, also known as Greenwich meridian, and has a Z-axis parallel with the rotational axis. In this way the ECEF frame is fixed to the earth itself and rotates around with it.