Six numbers that define an orbit
A Keplerian orbit is fully specified by six classical orbital elements:
| Element | Symbol | Meaning |
|---|---|---|
| Semi-major axis | a | Size of the orbit |
| Eccentricity | e | Shape (0 = circular, 1 = parabolic) |
| Inclination | i | Tilt relative to the reference plane |
| RAAN | Ω | Longitude of ascending node — orientation |
| Argument of periapsis | ω | Where periapsis sits in the orbit |
| True anomaly | ν | Where the spacecraft is right now |
Working with TLEs
Two-Line Element sets (TLEs) encode these elements in a compact ASCII format. Mean motion (rev/day) is converted to semi-major axis via Kepler's third law.
Tips
- Inclination controls ground-track coverage. Sun-synchronous orbits hover near 97.4°–98.5° at typical LEO altitudes.
- RAAN precesses due to Earth's oblateness (J2); that is exactly what sun-synchronous orbits exploit.
- Eccentricity 0 makes ω undefined — most circularised LEO sats use "argument of latitude" instead.