M

The Mean Anomaly indicates where the satellite was located within its orbit at the specified Epoch.

The Mean Anomaly at any time t, M(t), can be determined by adding the last known Mean Anomaly, M_o, to the orbit's Mean Motion multiplied by the time that has elapsed (t - t_o):

M(t) = M₀ + n (t - t₀)

where M(t) = the Mean Anomaly at time t;
           M₀ = the Mean Anomaly at the Epoch time;
           n = the satellite orbit's Mean Motion;
           t = the chosen prediction time; and
           t₀ = the Epoch time.

For a perfectly circular orbit (Eccentricity of 0), the Mean Anomaly is exactly equal to the True Anomaly throughout the orbit.

The Mean Anomaly is related to the Eccentric Anomaly (E) through Kepler's Equation.

The Mean Anomaly can range anywhere from 0 to 360 degrees.

For the ISS TLE:

"249.9177" = A Mean Anomaly of 249^o.9177.