Semi-Major Axis

The semi-major axis (a) is half the longest diameter of an elliptical orbit — essentially the average distance from the orbiting body to the central body. For near-circular orbits, it approximates the orbital altitude plus Earth's radius. It is the single parameter that determines the orbital period via Kepler's third law: T² ∝ a³. Earth's radius is 6,371 km, so a LEO satellite at 400 km altitude has a semi-major axis of 6,771 km. TLEs don't directly contain the semi-major axis, but it can be derived from the mean motion.