Mean Motion
The average angular speed of a satellite in its orbit, expressed as revolutions per day.
Explanation
Mean motion (n) is a standard orbital element derived from the semi-major axis. It represents the number of complete orbits a satellite makes per day. For example, a typical LEO satellite has a mean motion of about 15-16 revolutions per day (a 90-minute period), while a GPS satellite at 20,200 km has a mean motion of about 2 (12-hour period). Mean motion is one of the six Keplerian elements and is included in TLE data sets. It is directly related to orbital period by n = 86400/T (where T is the period in seconds). Mean motion is used in orbital propagation to predict satellite position over time, and its rate of change (first and second derivatives, also included in TLEs) indicates drag or other perturbations. A steadily increasing mean motion indicates orbital decay due to atmospheric drag.
Why It Matters
Mean motion is the most commonly used shorthand for orbital altitude in operational tracking. It appears in every TLE and is the primary input for most orbital propagation software.
Concept Map
How Mean Motion connects to other glossary terms:
Frequently Asked Questions
How is mean motion used in TLEs?
TLEs include mean motion as revolutions per day, plus its first and second time derivatives to account for drag and orbital changes.
Does mean motion change over time?
Yes. Atmospheric drag increases mean motion (shorter period) as altitude decreases. This gradual change is tracked in TLE elements.
Sources
Last updated: July 1, 2026