Two Line Elements
An option to determine a satellites motion after its launch is to use Two Line Elements (TLE), also called NORAD lines.

This information is based on, that every object, which orbits the earth and is bigger than a tennis ball, is monitored and tracked by Radar by NORAD (North American Aerospace Defense Command).
The format has been used since the beginning of space flight and is still in use by NORAD and NASA. The origin of the Two Line Elements is unknown but some reports refer to it as a T-card format.

The Two Line Elements consists of two 69 character strings of data in a format as shown in the below figure.



Updated Two Line Elements of any non-military satellite in orbit can be downloaded over the internet from the following webpage:

http://celestrak.com/NORAD/elements/

The parameters given by the Two Line Elements, which concern the orbital motion of the satellite, are the following:

Time of Epoch:
Time of epoch represents the time when the orbital parameters given in the TLE were obtained. In the TLE format it is given as epoch year (The first two numbers) and the Julian day and fraction of the same day.

1.st Derivative of Mean Motion:
This term represents the change in the mean motion of the satellite.
The first derivative of the mean motion given in the TLE is the half value of the change of the mean motion in revolutions per day squared and is caused by atmospheric drag pulling the satellite into a lower orbit and hereby accelerating it up.

2.nd Derivative of Mean Motion:
This term represents the second derivative of mean motion. However, usually is this term not used and set to zero in the TLE.

Drag Term:
The drag term or radiation pressure coefficient consists of a coefficient describing the effect of drag on the satellite. The Drag Term is based on the satellites surface and mass and it is used in the SGP4 orbit model.