This post is part of a series on understanding the meaning behind the values contained in a Two Line Element set, or Keplerian Elements. The main article is here.

### First Time Derivative of the Mean Motion

Engineers like to use long confusing names for some of these fields. This field is sometimes called the “Ballistic Coefficient”.

This value is the daily rate of change in the number of revolutions (orbits) the object completes each day divided by two.

This number is an indication of the drag that an object is experiencing. Note that as the orbit gets lower, atmospheric drag will increase but the object needs to orbit faster to stay in orbit. Eventually atmospheric drag will overcome and the object will reenter (decay).

Note that this value is not used in modern calculations of orbits using the SGP4/SDP4 models, only the simpler SGP algorithm.

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